U.S. DEPARTMENT OF COMMERCE 
 
 NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION 
 NATIONAL WEATHER SERVICE 
 
 INTRODUCTION TO 
 WEATHER RADAR 
 
 MARCH 1974 
 
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 INTRODUCTION TO WEATHER RADAR 
 
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 U.S. DEPARTMENT OF COMMERCE 
 Frederick B. Dent, Secretary- 
 National Oceanic and Atmospheric Administration 
 Robert M. White, Administrator 
 
 National Weather Service 
 George P. Cressman, Director 
 
 Silver Spring, Md. 
 March 19 74 
 
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PREFACE 
 
 This study material is designed for employees at Weather Service offices 
 that have local warning radars, or have a remote radar display from 
 Weather Service or military weather radar sets. The information contained 
 herein is considered the minimum necessary to make intelligent use of 
 radar weather information. Federal Meteorological Handbook No. 7, 
 Weather Radar Observations, Parts A, B, and C provides much of the 
 course material, and it is considered essential that local radar users and 
 remote display users become familiar with that manual. A bibliography 
 is provided in the manual, and Weather Service personnel may obtain 
 desired additional reading materials from the Atmospheric Sciences 
 Library at Weather Service Headquarters. This course of study is 
 intended to be dynamic, with improvements and additions incorporated 
 from time to time. 
 
 It is anticipated that the various regional headquarters may wish to require 
 additional study by personnel in their respective regions. In such case, 
 this volume should be considered as Part I of the study guide, and any 
 material added by a region as Part II. 
 
 111 
 
TABLE OF CONTENTS 
 
 Page 
 
 Preface iii 
 
 1. Introduction 1 
 
 2. Fundamentals of the Equipment 5 
 
 2.1 Components 5 
 
 2.2 Determining Distance and Direction 5 
 
 2.3 Pulse Repetition Frequency and Pulse Length 5 
 
 2.4 Maximum Range 7 
 
 2.5 Resolution 7 
 
 2.6 The Radar Beam 9 
 
 2.7 Multiple-Trip Echoes 9 
 
 2.8 Scope Displays 12 
 
 2.8.1 Plan Position Indicator (PPI) 12 
 
 2.8.2 Range Height Indicator (RHI) 12 
 
 2.8.3 A- and R-Scopes 13 
 
 2.9 Weather Bureau Radar Remote (WBRR) 13 
 
 2.10 Video Integrator and Processor (VIP) 16 
 
 2.11 Required Study 18 
 
 3. Fundamentals of Radar Propagation and Detection 19 
 
 3.1 Propagation 19 
 
 3.1.1 Ducting 19 
 
 3.1.2 Anamalous Propagation 23 
 
 3.1.3 Fine Lines and Angels 23 
 
 3.1.4 Chaff 23 
 
 3.1.5 Required Study 29 
 
 3.2 Detection 29 
 
 3.2.1 Large Targets 29 
 
 3.2.2 Ground Clutter 29 
 
 3.2.3 Meteorological Targets 31 
 
 3.2.3.1 Clouds 31 
 
 3.2.3.2 Rain 31 
 
 3.2.3.3 Snow and Ice 34 
 
 3.2.4 Required Study 34 
 
 3.3 Synoptic Scale Weather Systems 34 
 
 3.3.1 Winter Cyclones 36 
 
 3.3.2 Summer Cyclones 36 
 
 3.3.3 Hurricanes 36 
 
 iv 
 
Page 
 
 3.3.4 Fronts 
 
 3.3.5 Squall Lines 
 
 3.4 Severe Local Storms ■ — 
 
 3 . 5 Attenuation 
 
 3.5.1 Attenuation by Pre cipitation 
 
 3.5.2 Range Attenuation ■ 
 
 4. Radar Measurements and Reports 
 
 4 . 1 Measurements 
 
 4.1.1 Direction 
 
 4.1.2 Distance 
 
 4.1.3 Movement •-- 
 
 4.1.4 Echo Height------ 
 
 4.1.4.1 Height Finding Limitations- 
 
 4.1.4.2 Height Finding Technique - - 
 
 4.1.4.3 Required Study 
 
 4.1.5 Intensity of Echoes ■- 
 
 4.1.6 Intensity Trend -■ 
 
 4 . 2 Reports 
 
 4.2.1 RAREP (Radar Report) 
 
 4.2.1.1 Cells 
 
 4.2.1.2 Lines 
 
 4.2.1.3 Areas 
 
 4.2.1.4 Layers 
 
 4.2.2 Narrative Radar Weather Reports 
 
 4. 3 Required Study 
 
 Appendix A Radar Echo Intensities 
 
 Appendix B Reporting a Typical Weather Situation 
 
 LIST OF TABLES 
 
 Table I Characteristics of Weather Radars 
 
 Table II VIP Intensity Levels 
 
 Table III Comparison of Hydrometeors ■ 
 
 Table IV Rainfall - Echo Intensities 
 
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LIST OF ILLUSTRATIONS 
 Figure Page 
 
 The WSR-57 Radar 
 
 Basic Components of a Radar 
 
 Beam Resolution 
 
 Beam Geometry 
 
 PPI Scope 
 
 Convective Rain Cells on RHI 
 
 The WBRR System 
 
 Refraction of Radar Beam 
 
 Heights Indicated on RHI 
 
 Nonstandard Refraction 
 
 Second Trip Echoes 
 
 Anomalous Propagation- ■ 
 
 Fine Line, Sea Breeze Front, and Precipitation 
 
 Angels and Smoke 
 
 G round Clutte r ■ < 
 
 Chaff 
 
 Rain Detection vs. Cloud Detection 
 
 Fog and Low Stratus 
 
 Radar Detection vs. Satellite Pictures 
 
 Bright Band on the RHI - — 
 
 Convective Echoes on the PPI 
 
 Stratif or mi E choe s on the PPI 
 
 Snow Echoes on the PPI 
 
 Typical Hurricane Echoes ■ 
 
 Tornado Hooks ■ 
 
 Pre cipitation Attenuation 
 
 Weather Echoes on Radars of Different Wavelengths 
 
 Range Attenuation on the PPI 
 
 Beam Filling ■ 
 
 Sensitivity - Time Control ■ 
 
 Movement of Lines 
 
 Antenna Tilt and PPI Display 
 
 Attenuation and PPI Display 
 
 RAREP Code 
 
 Grouping of Echoes for Reporting 
 
 PPI Display 
 
 Annotated PPI Display ■ 
 
 Encoded Echoes 
 
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 VI 
 
1. INTRODUCTION 
 
 The first general use of radar was to detect aircraft and surface ships 
 during World War II. With this use in mind, weather was considered a 
 nuisance because it sometimes produced on the radar scopes large echo 
 patterns that made detection of aircraft or ships difficult or impossible. 
 While every effort was made to minimize weather echoes on the military 
 sets, it was early recognized that the weather information attainable by 
 radar would be an important supplement to standard weather observing 
 techniques provided we could properly analyze and define the weather 
 echoes. 
 
 In 1947 the Weather Service began installing converted military radars, 
 primarily in that section of the Nation most frequently subject to tornado 
 activity. Because of their low power, relatively poor beam characteristics, 
 and lack of objective intensity measuring capability, these radars were 
 considered an interim substitute to be used until a radar specifically 
 designed for weather detection became available. In 1959 the WSR-57 
 (Weather Surveillance Radar-57) became available, and a nationwide 
 radar network is being developed around this powerful, sensitive radar. 
 Provided with circuitry modifications as new technology develops, this 
 fine radar should be the "standard" for many years. These radars are 
 manned 24 hours a day by radar observers trained to operate the sets, 
 evaluate the echo display, and disseminate weather information obtained 
 from the displays. The WSR-57 radars serve the tornado country of the 
 Midwest, the hurricane-vulnerable Gulf and Atlantic Coasts, and the flash 
 flood areas of the Appalachian Mountains. Additional limited observations 
 are made by NWS personnel using radar displays at four FAA Air Control 
 Centers in the mountainous regions of the West. 
 
 The NWS operates local warning radars used on an as-needed basis to 
 detect and track severe local storms over areas not adequately covered 
 by the basic network radars. Many of these radars are obsolete, modified 
 World War II surplus equipments. Modern local warning weather radars 
 are being installed to provide more accurate measurements of severe 
 local storms. These new, low-maintenance equipments also reduce 
 operating costs and ensure availability when needed. 
 
 Currently, dissemination of the radar data is made using teletypewriter 
 circuits, a composite radar summary facsimile chart and radar remoting 
 equipment (WBRR). The WSR-5 7 console and antenna are shown in figure 1. 
 
INTRODUCTION 
 
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INTRODUCTION 
 
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2 . FUNDAMENTALS OF THE EQUIPME NT 
 
 2.1 COMPONENTS 
 
 An understanding of how radar works is essential to an understanding of 
 what is displayed on a radar scope. A radar consists of four main parts: 
 (1) a transmitter, (2) an antenna, (3) a receiver, and (4) an indicator. The 
 transmitter converts power supplied to the set into radio energy of a given 
 frequency and transmits it to the antenna. A single antenna reflector 
 serves for both transmitting and receiving the energy waves. The antenna 
 reflector, usually simply referred to as the "antenna," focuses the waves 
 into a narrow beam that can be rotated freely or pointed in a specific 
 direction. The waves are reflected by the target as shown in figure 2, 
 and sent to the receiver for detection. The receiver amplifies the signal 
 from the antenna, and feeds the amplified signal to the indicator. The 
 indicator converts the signal into a form that can be interpreted, usually 
 a visual display on a cathode-ray tube. 
 
 2.2 DETERMINING DISTANCE AND DIRECTION 
 
 A radar is essentially a sounding device that emits a short burst, or 
 pulse, of electromagnetic energy and "listens" for the return of the pulse. 
 The electromagnetic energy travels at 161, 800 nmi per second and is 
 scattered in all directions by objects it encounters, provided the objects 
 are of sufficient size in relation to the wavelength of the energy. Only a 
 small portion of the scattered energy returns to the antenna. Since distance 
 is a product of speed and time, we can easily determine the distance of a 
 reflecting object by measuring the time required for a pulse of energy to 
 return to the radar. Direction is determined by focusing the radar energy 
 into a narrow cone such as a flashlight beam. 
 
 2.3 PULSE REPETITION FREQUENCY AND PULSE LENGTH 
 
 Through the use of a very rapid electronic switch a single antenna can be 
 used for both the transmitter and receiver, and in practice hundreds of 
 pulses are emitted every second, with a "listening" period between pulses. 
 Pulse repetition frequency (PRF) is an important characteristic, since it 
 determines the maximum range of the radar. A low PRF is required for 
 long-range detection. Another important characteristic is pulse duration. 
 A pulse of long duration will detect a weaker precipitation target than a 
 short pulse, but the short pulse provides better resolution. Weather 
 Service radars have two modes of operation, providing one combination 
 of PRF and pulse length for long-range detection of weak targets, and 
 another combination for improved definition. Ordinarily the radar is 
 operated in the long-pulse mode, but it may be operated on short pulse at 
 the radar observer's discretion. Table I shows the PRF and pulse 
 duration, as well as other pertinent information, for various radars 
 commonly used for weather detection. 
 
FUNDAMENTALS OF THE EQUIPMENT 
 
 
 
 
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FUNDAMENTALS OF THE EQUIPMENT 
 
 2.4 MAXIMUM RANGE 
 
 The maximum range of a radar, aside from the geometric considerations 
 to be discussed later, is determined by the pulse repetition frequency of 
 the radar. The energy return from a number of pulses is required in order 
 to display an echo on the scope, so it is desirable that as many pulses as 
 possible reach the target. However, since each pulse must travel to the 
 target and back to the antenna before the next pulse is emitted, too high a 
 PRF would severely limit the maximum range of the radar. The choice is 
 a balance between these considerations. When pulse lengths are short for 
 greater definition, it is necessary to get more hits on the target in order 
 that a detectable signal is returned to the radar, and a high PRF is used, 
 limiting the radar's range. In the case of a WSR-57 operating in short- 
 pulse mode, the pulse has only sufficient time to reflect from a target up 
 to 148 nmi range and return to the antenna before the next pulse is 
 emitted. In long pulse mode, there is sufficient time for the pulse to travel 
 493 nautical miles and return before the next pulse is transmitted. Because 
 of the curvature of the earth, the radar beam is usually well above any 
 weather targets at such extreme ranges, and scope displays are therefore 
 limited to the practical value of 250 nmi. 
 
 2.5 RESOLUTION 
 
 Resolution describes the ability of the radar to show discrete targets 
 separately. There are two distinct resolution problems: (1) range 
 resolution (the ability to distinguish between two targets at the same 
 azimuth but at different ranges) and (2) beam width resolution (the 
 ability of the radar to distinguish between two targets at the same range 
 but at different azimuths). Since it can be shown that the range resolution 
 of a radar is half the pulse length, the WSR-57 cannot distinguish between 
 objects less than 600 meters apart in range when in the long-pulse mode, 
 of less than 75 meters apart when in the short-pulse mode. Beam width 
 resolution is a function of the size and shape of the radar beam, and 
 therefore, the beam width resolution capability of the radar varies 
 inversely with range of the targets. Any target, regardless of how much 
 of the beam it fills, that reflects to the antenna a detectable signal will 
 be displayed on the scope as being at least as large as the beam width 
 in one dimension, and as large as the pulse length in the other dimension. 
 Since the beam width increases at greater ranges, targets at greater 
 ranges must be separated by greater distances in order to appear as 
 separate targets on the scope. An important implication here is the 
 increased beam width distortion, or stretching at right angles to the 
 beam, as distance from the radar increases. Figure 3 illustrates 
 resolution. 
 
FUNDAMENTALS OF THE EQUIPMENT 
 
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 Figure 3. This figure is exaggerated in order to easily illustrate beam- 
 width resolution and beam-width stretching. The targets at range A would 
 both appear on the scope only if the beam were rotating. Neither would 
 appear on the scope if the beam were stationary as shown. The targets 
 at range B are the same distance apart as those at range A, but appear on 
 the scope below as one echo because they are not separated by a beam 
 width. The targets at range C are also the same distance apart, but the 
 echo appears larger because of beam width stretching. 
 
» 
 
 FUNDAMENTALS OF THE EQUIPMENT 
 
 2.6 THE RADAR BEAM 
 
 The shape of a radar beam is determined by the shape and size of the 
 antenna reflector as shown in figure 4. The WSR-57 antenna reflector is 
 a paraboloid 12 feet in diameter, concentrating the radar energy in a 
 conical beam with an angular width of 2 degrees. The reflectors on WSR-1 
 and WSR-3 radars are 6 feet in diameter, and produce a 4-degree beam. 
 Radars designed for other purposes may have a differently shaped beam, 
 as illustrated in figure 4. The beam of a radar set is difficult to define 
 precisely and an arbitary, but very useful, definition is agreed upon. 
 That locus of points around the axis of the beam where the beam energy 
 has decreased by 50 percent from the theoretical maximum along the beam 
 axis is considered the edge of the beam. This does not mean, however, 
 that any given target will always appear on the scope if within the beam as 
 defined here, or will never appear on the scope if outside the beam. An 
 easy, although not exact, illustration of this can be made with a flashlight 
 beam. Many flashlights produce a visible narrow conical beam, yet also 
 illuminate objects outside this cone. A small object is easily identifiable 
 over a wide angle at very close range but must be within the beam to be 
 seen at greater distances, and as the range increases even further the 
 object is not identifiable even though still obviously near the axis of the 
 beam. In the case of radar as with a flashlight, it has been found that there 
 are secondary maxima of energy radiated outside the primary beam, and 
 targets illuminated by these secondary lobes sometimes return enough 
 energy to be detected and displayed on the scope. The radar, however, 
 cannot display except along the beam axis, and will present the target 
 along that axis at a range corresponding to elapsed time between pulse 
 emission and echo return. Thus an echo may appear on the scope in a 
 position that corresponds to a geographical location where there is 
 actually no target. These side-lobe echoes are usually at close range and 
 frequently are masked in the ground clutter around the center of the scope. 
 
 2.7 MULTIPLE-TRIP ECHOES 
 
 A radar always displays echoes as if any signal return is assumed to be 
 from the last emitted pulse. Therefore, if a reflected signal from a 
 previous pulse is detected it will be erroneously displayed on the scope 
 as a target much closer to the radar than is the actual case. This can 
 happen fairly easily in the case of short pulse operation, where maximum 
 displayable range is 125 nmi (WSR-57), because the signal energy is still 
 strong enough to illuminate targets beyond that range, and the beam is 
 still low enough to intercept numerous targets. Multiple-trip echoes occur 
 infrequently when the radar is in long-pulse mode. Such echoes may occur 
 during conditions of super-refraction, and ordinarily disappear if the 
 antenna is pointed slightly upward. The range to a target producing a 
 multiple-trip echo may be determined by R=n(mr)+ r, where n=number 
 of pulses removed from latest (usually one), mr=maximum range as 
 determined by PRF, and r=range of echo as displayed on the scope. 
 
10 
 
 FUNDAMENTALS OF THE EQUIPMENT 
 
 Figure 4. Air traffic control radar is used to detect all aircraft regardless 
 of altitude. It must scan all airspace within its range with each rotation of 
 the antenna. It has a beam narrow in horizontal dimension but wide in the 
 vertical dimension (fan shaped), as shown in the top picture. The bottom 
 picture illustrates what a weather radar might see. A height-finding radar 
 on the other hand, which requires accurate vertical angle measurements, 
 also has a fan- shaped beam but with the narrow dimension being vertical. 
 
FUNDAMENTALS OF THE EQUIPMENT 
 
 11 
 
 Figure 5. PPI scope. The center of the display represents the location of 
 the antenna. A sweep line, labeled "S" is synchronized to rotate with the 
 antenna. As the antenna and sweep line rotate, a picture, or map, of the 
 targets is displayed on the scope. The markings around the edge of the 
 PPI are bearings, or azimuth markings, in true degrees from the antenna. 
 On this scope the echo in the upper right is at an azimuth of 40 degrees 
 and a range of 45 nautical miles. Range markers, labeled "R" are added 
 electronically to the PPI, denoting distance from the antenna in nautical 
 miles. 
 
12 FUNDAMENTALS OF THE EQUIPMENT 
 
 The radar observer will omit multiple-trip echoes from radar weather 
 reports. This is one of several reasons why it is necessary to use 
 RAREPs in conjunction with a repeater scope display. Second trip echoes 
 will appear on a repeater display, and maybe difficult to distinguish without 
 other information. See figure 11. 
 
 2.8 SCOPE DISPLAYS 
 
 The viewing scopes of a radar are cathode-ray tubes similar to an ordinary 
 television tube. The major difference is in the phosphers of the mapping 
 scopes, which have greater retention qualities that allow the echoes to 
 remain visible until the next sweep of the antenna. 
 
 2.8.1 PLAN POSITION INDICATOR (PPI) 
 
 The Plan Position Indicator plots direction versus distance on polar 
 coordinates, with the radar location at the center. Image retention is on 
 the order of 20 seconds. Several different range settings are available, 
 at the option of the operator, and the WSR- 5 7 has an additional off-centering 
 device that allows any portion of the PPI display to be selected for centering 
 and enlargement. Evenly spaced range marks appear on the PPI when the 
 scope is normally centered and operating in standard range increments. 
 Figure 5 illustrates features of the PPI. The image of the PPI scope is the 
 one transmitted on the WBRR but the rotating sweep will not be apparent. 
 Also, the WBRR transmission will not include off-centering display, and it 
 will have only three possible scope range displays, i.e., 50 nmi, 125 nmi, 
 and 250 nmi. WBRR users should be aware that scope display versatility 
 at the radar site is greater than at the WBRR display. 
 
 2.8.2 RANGE HEIGHT INDICATOR (RHI) 
 
 The Range Height Indicator displays range versus height. Since the 
 antenna tilt remains constant for normal search sweeping, this scope 
 usually does not display a picture. The antenna can be made to tilt, 
 however, and if horizontal rotation is stopped and a complete vertical 
 sweep made a vertical profile of echoes will be displayed on the RHI. 
 The WSR-1 has no RHI, but does have a vertical angle indicator to help 
 determine heights. The WSR-3 PPI can be switched to display RHI 
 instead of its usual picture. The maximum range that can be displayed 
 on the WSR-57 RHI has been selected to be 125 nmi because of increasing 
 beam width and altitude at greater ranges, and because non-standard 
 atmospheric refraction of the beam introduces uncertainties in indicated 
 echo altitude. Image retention is on the order of 20 seconds. Note that 
 the range and height scales are not the same, and thunderstorms have 
 the appearance of being tall and thin when actually they are shaped 
 somewhat like a rosebud. The RHI image is not transmitted on the WBRR, 
 but RHI information is included in WBRR scope annotations. RHI scope 
 display is shown in figure 6. 
 
FUNDAMENTALS OF THE EQUIPMENT 
 
 13 
 
 Figure 6. Convective rain cells as shown on RHI. Horizontal lines 
 indicate altitude in thousands of feet. Vertical lines are range markers, 
 in this case 20 miles apart. 
 
 2.8.3 
 
 A- AND R-SCOPES 
 
 The A- scope plots range versus intensity of signal return. It does not give 
 a picture of the size and shape of the target, but is very useful to the radar 
 observer in determining exact ranging and strength of the return, and very 
 importantly, in determining the character of the return. Objects such as 
 buildings, aircraft, and precipitation can be identified by the signatures 
 they present on the A- scope. On the WSR-5 7, any portion of the A- scope 
 can be selected for expanded presentation on the cathode-ray tube, and 
 this usage is called the R-scope. 
 
 2.9 
 
 WEATHER BUREAU RADAR REMOTE (WBRR) 
 
 The WBRR is designed to provide annotated PPI display at locations 
 remote from the radar position. It scans the picture slowly so that the 
 many information bits in the picture can be transmitted over regular 
 telephone line, thus avoiding the expense of coaxial cable or a microwave 
 transmission system. Tests have shown that randomly selected telephone 
 lines are suitable for transmission of the WBRR picture, making it possible 
 to "dial-up" a picture from any radar properly equipped, regardless of the 
 distance. Thus, radarscope data will be immediately available to not only 
 those selected weather service offices under the radar umbrella that have 
 a direct hookup, but to any interested forecast office. Figure 7 illustrates 
 the WBRR system. 
 
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FUNDAMENTALS OF THE EQUIPMENT 15 
 
 In 1965, the first NWS radar remoting system utilizing telephone lines as 
 the transmission medium was developed. This system, designated RATTS- 
 65 1 and later WBRR-65 2 , was designed to provide near real time 
 
 weather radar data to WSFO's and WSO's within the effective range of the 
 WSR-57 radar (nominally 125 nmi). The first system was installed at 
 Galveston- Houston in December 1965. Essentially, the radar picture is 
 transformed from rho-theta to a rectilinear display with a vidicon TV 
 camera, transmitted over 3-kHz telephone lines at a rate of 1 frame every 
 100 seconds and, in the RATT-65 or WBRR-65 system, displayed on a 
 storage tube at the receiver location. From each receiver location, the 
 picture can be transmitted to as many as six 14-inch CRT display monitors. 
 A unique feature of the system is that the radar observer can add remarks 
 pertaining to echo features which are superimposed, with a map of the 
 area, on the radar display using a data insertion device. A telephone hot 
 line is also provided linking the radar operator to the user office. Most 
 of the WBRR systems are connected to radars which have the Video 
 Integrator and Processor (VIP). This device provides up to six contours 
 of echo intensity (rainfall rate). In 1968, the system was modified to 
 utilize a facsimile recorder at the receiving end, which would provide a 
 record of the display rather than the perishable TV picture. The facsimile 
 system is designated WBRR-68. 
 
 There are two primary types of WBRR transmitter-to-receiver links: 
 dedicated line and dial-in. The dedicated line provides continuous access 
 to the radar and is mandatory where the radar remote is used extensively 
 in the severe local storm forecast and warning program. Recorders 
 connected to dedicated lines are usually well within 75 nmi of the radar to 
 provide good coverage at reasonable cost. The dial-in capability is used 
 when occasional pictures from different radars are useful; for example, 
 at a WSFO or a centralized facility, such as the FAA's Central Flow 
 Control Facility. All transmitters are now equipped with multiple access 
 devices (MAD) which permit simultaneous calls to one transmitter. There 
 is no mileage limit to the call, which is charged at the usual long-distance 
 rate. Two complete pictures are received before the transmitter 
 automatically disconnects. If the radar picture is unavailable (due to 
 stoppage of antenna rotation, for example), the transmitter will not answer. 
 All NWS recorders are now equipped with dial-in capability. In addition, 
 there are other government and many non- government users. 
 
 *Radar Telephone Transmission System 
 Weather Bureau Radar Remote 
 
16 FUNDAMENTALS OF THE EQUIPMENT 
 
 2.10 VIDEO INTEGRATOR AND PROCESSOR (VIP) 
 
 INTRODUCTION 
 
 The Video Integrator and Processor (VIP) is an adjunct of the WSR-57 
 radar system. It has been developed to meet both present and anticipated 
 requirements for continuous quantitative data output for (1) console 
 presentation, (2) dissemination by remoting systems such as the WBRR-68, 
 (3) photographic data recording, and (4) further processing using computer 
 techniques. The VIP automatically processes the output of the radar's 
 logarithmic receiver to produce up to six levels of intensity data corre- 
 sponding to pre- selected categories of estimated rainfall rates. These 
 levels may be displayed individually or simultaneously on the radarscope. 
 This will permit the constant monitoring of echo intensities, within these 
 categories, with each rotation of the antenna. 
 
 MODES OF OPERATION 
 
 There are two basic modes of operation designed into the VIP: (1) the 
 contoured log (C-log) and (2) log. The C-log mode can be used to present 
 up to six levels of intensity according to table II. 
 
 The log mode presents the output of the logarithmic receiver without 
 contours. This mode may be usee 
 presently limited to 125 nmi or less 
 
 contours. This mode may be used on all ranges; however, the C-log is A 
 
 Whenever the linear receiver of the radar is used, the VIP does not affect 
 the scope displays in any way. 
 
 VIP CONTROL FUNCTIONS 
 
 The VIP is designed to be operated full time. The OFF-ON switch is located 
 on the back of the cabinet. In order for the VIP calibration to be valid, the 
 radar must be operated in the log receiver mode at 3 rpm, in long pulse, 
 with STC OFF. The mode of operation is selected by using either the log or 
 C-log push button switch on the right side of the VIP cabinet face. In the 
 C-log mode, combinations of levels may be selected by using the level 
 selector push button switches on the left of the cabinet face. Each of these 
 six switches has an indicator light just above the switch. These lights are 
 energized as the signal passes each level and remains on for 20 seconds 
 (one rotation of the antenna at 3 rpm). A quick glance at these lights near 
 the end of each rotation will reveal the highest level signal for that rotation 
 of the antenna. 
 
FUNDAMENTALS OF THE EQUIPMENT 17 
 
 Table II. VIP Intensity Levels 
 
 Rainfall Rainfall Rate 
 
 Level Display# Category (in./hr., lower level) LOG Z Pr* 
 
 1 
 
 gray 
 
 light 
 
 <0.1 
 
 
 < 
 
 -99.5 
 
 2 
 
 white 
 
 moderate 
 
 0.1-0.5 
 
 3.0 
 
 
 -88.0 
 
 3 
 
 black 
 
 heavy 
 
 0.5-1.0 
 
 4.1 
 
 
 -77.0 
 
 4 
 
 gray 
 
 very heavy 
 
 1.0-2.0 
 
 4.6 
 
 
 -72.0 
 
 5 
 
 white 
 
 intense 
 
 2.0-5.0 
 
 5.0 
 
 
 -67.5 
 
 6 black extreme >5.0 5.7 -61.0 
 
 #Refers to radar PPI. White and black are reversed on WBRR. 
 * Equivalent received power using the LIN receiver. The actual signal 
 generator values used to set up the VIP thresholds are 2.5 dB lower than 
 these values, for example -90.5 dBm rather than -88 dBm in the case of 
 level 2. 
 
 OPERATIONAL REQUIREMENTS 
 
 Camera repeater scopes and WBRR transmitters should normally present 
 contoured displays. This means that the radar must be operated in long 
 pulse, and the VIP must be in C-log mode with all six levels selected for 
 display. The radar STC must be turned off or disabled since range normal- 
 ization is provided by the A- 10 board which should have been installed.* 
 The radar antenna must be allowed to rotate at 3 rpm, with tilt adjusted for 
 precipitation detection over the optimum range. The rotation speed should 
 be checked and adjusted during the periodical calibration check, and more 
 frequently if necessary. If this "normal" operation is interrupted for 
 lengthy periods, because of equipment malfunction or for any other reason, 
 appropriate notations should be included on WBRR and camera scope 
 displays. Whenever the normal operation is interrupted briefly, such as for 
 echo top measurement, notations are not necessary. In either case, the 
 normal operation should be resumed as soon as possible. 
 
 *A11 levels except level 1 are range normalized. Level 1 is not range 
 normalized so that all detectable light precipitation will be displayed as 
 level 1. 
 
18 FUNDAMENTALS OF THE EQUIPMENT 
 
 OBSERVATION TECHNIQUES 
 
 The VIP facilitates the recording of radar reports (SD) and narrative 
 reports because it displays on a single sweep a contoured picture which, if 
 constructed by attenuators and grease pencil, would be several minutes in 
 the making. The following observational procedure is suggested: 
 
 After determining that the radar is delivering standard performance, set 
 the radar console PPI to 250-nmi range (C-log cannot be used for this 
 display)* but do not interrupt C-log VIP mode service to remote displays. 
 If there are echoes beyond 125 nmi, locate and record the areas, lines, and 
 cells as necessary on this range setting. Switch then to the shortest range 
 that will display all the echoes within 125 nmi, and switch the main PPI to 
 .he VIP C-log display. Manipulation of the level selector switches will 
 allow determination of maximum intensities and characteristic intensities 
 as reported in SD's and narrative summaries. Note that the normal 
 antenna rotation is not interrupted, and that remote displays can be 
 served with C-log VIP mode while the main PPI is adjusted to the 250-nmi 
 range. It will be necessary to stop the normal antenna rotation in order to 
 determine vertical configuration. This should be done as necessary, but on 
 a scheduled basis as much as possible. Top measurements may be made 
 within the lin receiver, or with the VIP in C-log mode. 
 
 *A modification is being developed to permit C-log VIP operation on 
 250-nmi range; contours would only appear to 125 nmi, and normal echo 
 presentation beyond 125 nmi. 
 
 2.11 REQUIRED STUDY 
 
 FMH No. 7, Part B, Chapter 1. 
 
 FMH No. 7, Part C, Section III. 
 
3. FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 3.1 PROPAGATION 
 
 Electromagnetic waves emitted by a radar travel in a straight line when in 
 a vacuum or in a medium homogeneous in density and composition. The 
 more dense the medium, the more slowly the waves travel. This is very 
 significant when considering the propagation of radar waves, because of 
 the great density variations with height in our atmosphere. Since the waves 
 travel faster in the less dense air at greater heights the motion of a wave 
 front is not straight, and the resultant radar beam is curved as shown in 
 figure 8. It has been found that the amount of curvature depends on 
 temperature and humidity lapse rates in the atmosphere, and because of the 
 many variables involved we can never say just how great the curvature is 
 at any given moment. In order to define a reference, a standard atmosphere 
 is assumed such that the radar beam is considered to have a radius of 
 curvature four times that of the earth, and the radar is calibrated to 
 display echoes as if this were always the case. Any atmospheric variation 
 from standard causes a nonstandard beam curvature resulting in erroneous 
 positioning of the echo on the radar scope. The error in horizontal distance 
 is small, but the error in height as displayed on the RHI can be consider- 
 able, especially at long range (see fig. 9). This error is not an apparent 
 one from scope considerations alone, and without definitive information 
 about the prevailing refractive index an accurate correction cannot be 
 made. Because of great diurnal and areal variations with our present 
 observing networks we cannot accurately define the horizontal and vertical 
 variations of refractive index at any given time, but we can make gross 
 estimates. One can imagine many possible variations in beam refraction, 
 with the curvature changing at different altitudes and different azimuth 
 bearings. When the beam has a curvature greater than standard, thereby 
 remaining closer to the earth than normal, "Superrefraction" exists. If the 
 beam is straighter than standard, "Subrefraction" exists. Figure 10 
 illustrates nonstandard refraction. 
 
 3.1.1 DUCTING 
 
 "Ducting" is an intense form of superrefraction occurring when the beam 
 is trapped below an inversion. This is often the case during early morning 
 hours when there has been good surface cooling by radiation and there is 
 an increase in temperature and a decrease in moisture with height in the 
 lower atmosphere. Ducting can occur in all directions from the radar, or 
 within limited angular boundaries. Cold air outflow associated with 
 thunderstorms sometimes causes ducting. During ducting conditions, the 
 scope will show an enlarged ground clutter pattern and may present ground 
 targets at great distances. Multiple-trip echoes of terrain features 
 hundreds of miles from the radar have been identified on many occasions. 
 
 19 
 
20 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 REFRAftTIOM i>t 
 RADAR BEAM 
 
 RADAR 
 BEAM 
 
 NATURAL 
 HORIZON 
 
 EARTH 
 
 Figure 8. Since the radar beam usually is not straight, but has a radius 
 of curvature four times that of earth (in standard atmosphere), radar can 
 "See" beyond the visual horizon. 
 
 OBSERVED TOP 
 ACTUAL TOP 
 
 ACTUAL 
 BEAM 
 
 NORMAL 
 BEAM 
 
 R HI SCOPE 
 
 Figure 9. Heights of echo bases and tops, as displayed on the radar, are 
 based on normal refraction. Since the radar has no way of detecting the 
 true path of the beam, indicated heights may be erroneous. 
 
FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 2i 
 
 NORMAL REFRACTION 
 
 NORMAL 
 RANGE 
 
 STORM 
 
 EARTH 
 
 SUB REFRAfcTION 
 4 RADAR BEAM 
 
 
 
 EARTH 
 
 ^S^ 
 
 ^v 
 
 SUPER REFACTION 
 4 RADAR BEAM 
 
 Figure 10 
 
22 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 I 
 
 SUPER REFRACTION oU 
 SEftOM) TRIP ECHO 
 
 APPARENT 
 STORM 
 
 NORMAL 
 RANGE ACTUAL 
 
 STORM 
 
 Figure 11. A combination of strong distant echoes and superrefraction may 
 cause radar sets to detect these targets beyond the normal range. Echoes 
 received after the next pulse is transmitted, are called "second trip echoes" 
 (more generally "multiple-trip echoes"). These echoes are "squeezed" 
 when displayed on the PPI because the beam is more narrow at closer 
 ranges. 
 
I 
 
 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 2 3 
 
 3.1.2 ANAMALOUS PROPAGATION 
 
 Extraordinary display of ground targets is often termed "AP" (Anomalous 
 Propagation), and while it is easily identified at the radar console it may be 
 mistaken for precipitation echoes if viewed only on the PPI. AP can appear 
 on the scope at the same time as precipitation and may even be inter- 
 mingled with precipitation, providing a time-consuming task in separating 
 the two in radar reports. If radar reports show precipitation at locations 
 where it is apparent through visual observation that none exists, the radar 
 observer should be notified immediately. Figure 12 shows exceptional 
 anomalous propagation, while figure 15 shows a more normal ground return. 
 
 3.1.3 FINE LINES AND ANGELS 
 
 Radio waves may actually reflect from layers of steep refractive index 
 gradient, returning to the antenna either directly or after striking the 
 ground or other objects. Thus echoes that represent neither precipitation 
 nor ground features may appear on the scope. When the reflecting 
 discontinuity has line form, such as at the density discontinuity at a 
 thunderstorm wind shift line (fig. 13), the echo is called a "fine line," and 
 is reported in the RAREP. Many times the echoes from discontinuities 
 do not have a line form, however, and are difficult to characterize. They 
 then usually fall into the broad category of "angels," a term used to describe 
 echoes of unknown origin. Angels usually appear as weak and amorphous 
 echoes, with a continually changing structure, but may at times present 
 identifiable patterns. The term also includes return from birds and 
 insects. Figure 14 is a good illustration of angels on the PPI. Fine lines 
 and angels appear much more frequently on the WSR-5 7 than on other 
 Weather Service radars, because of its power and sensitivity. 
 
 3.1.4 CHAFF 
 
 Chaff consists of a great many strips of metallic foil released in the 
 atmosphere in order to create a target or interference to radars. The 
 strips are usually cut to a length equal to one-half the wavelength of the 
 radars concerned. The shape and size of the echo from chaff depends on 
 the altitude at which it is released, the pattern flown by the aircraft, the 
 amount of chaff released, and the winds in the atmosphere below the 
 release altitude. It usually first appears as a thin band, and then spreads 
 as it falls and drifts with the wind. It is frequently released in several 
 parallel bands. While it can be mistaken for a weather echo, there is a 
 difference in the characteristics of the two as seen on the A scope, and 
 chaff usually can be observed to fall slowly. Each operator should, at the 
 first opportunity, closely study chaff echoes so as to be able to distinguish 
 them from precipitation. Figure 16 illustrates chaff as it appears on tne 
 PPI. 
 
24 
 
 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 Figure 12. An extraordinary "AP" situation as photographed at the Miami 
 WSR-57. A widespread atmospheric inversion has trapped the radar beam 
 near the surface of the earth northeast of the radar through the south to 
 west of the radar, and as a consequence some of the Bahama Islands, 
 Cuba, and the Florida Keys present a detectable return to the radar. 
 Surface heating has destroyed the inversion over most of the Florida 
 peninsula, however, and refraction northwest of the radar appears near 
 normal. Some of the point ranges south and east of the radar are ships, 
 some are small islands. The scattered straight line in the northeast 
 quadrant is a line of rain showers. The "needlework" that extends from 
 the radar to maximum range between 100 degrees and 140 degrees azimuth 
 is caused by radio interference in the atmosphere, likely from another 
 radar. The range marks are 50 nmi apart. Compare this with figure 15. 
 
FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 25 
 
 I 
 
 > 
 
 Figure 13. Fine lines as seen by the WSR-57 at Daytona Beach, Florida. 
 The line identified by "A" is associated with the sea-breeze front existing 
 at the time, and line marked "B" is the precursor line associated with the 
 thunderstorm seen in the southern quadrant. Note the small showers 
 inbedded in the ground clutter and angels between 250 degrees and 260 
 degrees azimuth. Aircraft targets appear double because the camera 
 shutter was left open for two rotations of the scope sweep, thus spotting 
 the aircraft at two different locations 20 seconds apart in time. 
 
26 
 
 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 Figure 14. Echoes of unknown origin, or "Angels," appearing over the 
 water east of the Miami WSR-5 7. Many of the small targets may be 
 birds. Compare with figure 15. The "fuzzy" triangular shaped target at 
 335 degrees, 25 nmi range (edge of ground clutter) is characteristic of 
 the echo from smoke, the fire being located at the acute angle of the echo. 
 In this case northerly winds are indicated in the low levels. Range is 50 
 nmi with range marks 10 nmi apart. 
 
FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 27 
 
 I 
 
 Figure 15. Ground clutter display representative of ^Normal" conditions 
 at the Miami WSR-5 7. Note the outline of the southern part of Biscayne 
 Bay, and the northernmost of the Florida Keys. Terrain is very flat in 
 south Florida, so most of the echoes in the ground clutter are from man- 
 made objects and trees. The straight lines west and northwest of the radar 
 are echoes from trees and powerlines along highways and canals. The 
 isolated point targets near the ground clutter consist of aircraft, boats, 
 isolated tall structures, and possibly even birds and insects. The more 
 distant point targets are aircraft. There are two very small showers 
 displayed on the scope; can you find them? Range is 50 nmi with range 
 marks 10 nmi apart. 
 
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 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
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FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 29 
 
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 3.1.5 REQUIRED STUDY 
 
 FMH No. 7, Part B, Chapter 2. 
 3.2 DETECTION 
 
 3.2.1 LARGE TARGETS 
 
 We must now consider just what a radar "sees." Obviously if the energy 
 waves from a radar strike an object such as a building or a metal aircraft 
 they will not penetrate but will be reflected, with some of the reflected 
 energy returning to the radar antenna. In such a case the radar will 
 display on the scope a target corresponding to the location of the reflecting 
 object, but can give no further information such as areal extent and shape 
 of the object, since the reflection is from the nearest side only. If the 
 object subtends an angle smaller than the radar beam width, the echo will 
 be indicated on the PPI as an arc, the dimensions of which are determined 
 mostly by the beam width and pulse length of the radar. If the object is 
 significantly larger than the beam width, the shape of its edge nearest the 
 radar will be mapped on the scope. Thus terrain features, especially 
 those close to the radar, are often displayed but are sometimes distorted 
 in shape and size. Only the side of a mountain facing the radar is detected, 
 and intensities may vary in the display because of differing slope angles 
 and terrain cover. Echoes from coastlines may be reflections from tree 
 lines and high ground features and do not necessarily show the actual 
 coastline shape. 
 
 3.2.2 GROUND CLUTTER 
 
 Terrain features such as hills, buildings, trees, and powerlines located 
 very near the radar set may produce permanent or semipermanent echoes 
 on the scope, depending on their distance, relative height, and the prevail- 
 ing meteorological conditions. During periods of ducting and super- 
 refraction the extent of this ground clutter is often greater than under 
 normal conditions. Each radar has a characteristic ground clutter display 
 around the center of the PPI, and each radar station should have recent 
 photographs of the normal ground clutter as well as photographs of the 
 ground clutter as seen during unusual propagation conditions. Also, each 
 WBRR user should have photographs of the ground clutter from the parent 
 radar to assist him in proper interpretation of the display (see fig. 15). 
 
30 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 Table III 
 
 COMPARISON OF HYDROMETEORS* 
 
 TERMINAL 
 
 PARTICLE 
 
 RADIUS 
 
 CONCENTRATION 
 
 FALL VELOCITY 
 
 Typical 
 
 
 
 condensation 
 
 0.00001 cm 
 
 1,000, 000/liter 
 
 . 0001 cm/sec 
 
 nucleus 
 
 
 
 
 Typical 
 
 
 
 
 cloud 
 
 0.001 cm 
 
 1, 000, 000/liter 
 
 1 cm/sec 
 
 drop 
 
 
 
 
 Large 
 
 
 
 
 cloud 
 
 0.005 cm 
 
 1, 000/liter 
 
 27 cm/sec 
 
 drop 
 
 
 
 
 Borderline 
 
 
 
 
 drop 
 
 0.01 cm 
 
 
 70 cm/ sec 
 
 (cloud to rain) 
 
 
 
 
 Typical 
 
 
 
 
 raindrop 
 
 0.1 cm 
 
 1/ liter 
 
 650 cm/sec 
 
 *B . J. Mason, Clouds, Rain, and Rainmaking (Cambridge, 1962), p. 76 
 
 Figure 17. Weather Service radars do not normally detect cloud particles. 
 As shown here, the weather echoes represent only those drops large enough 
 to fall as precipitation. 
 
FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 31 
 
 > 
 
 3.2.3 METEOROLOGICAL TARGETS 
 
 Raindrops, cloud droplets, and other hydrometeors return radar energy to 
 the antenna, and while there is but little energy returned from each of 
 these small targets a concentration of the hydrometeors will produce a 
 detectable echo. While it is convenient to consider radio energy returned 
 to the radar from a target as having been reflected, this is not entirely 
 accurate, especially in the case of targets small compared to the wavelength 
 of the energy. The energy incident on the small particle causes an 
 oscillation of the electric charge in the particle, and this in turn affects a 
 radiation of energy by the particle at the wavelength of the incident 
 radiation. In the field of radar meteorology, both this phenomena and true 
 reflection are generally referred to as "scattering." The term 
 "reflectivity" is often used to describe intensity of signal return by a 
 target. 
 
 3.2.3.1 CLOUDS 
 
 Weather Service radars (10 cm) normally do not detect cloud droplets, 
 showing rainshafts but not clouds (fig. 17). There are possible exceptions, 
 especially at close ranges. However, many targets identified as clouds 
 very likely consist of rain size drops that either evaporate before reaching 
 the ground or are being suspended in updrafts. 
 
 Radars with a shorter wavelength are more likely to "see" cloud droplets, 
 since received power is inversely proportional to the fourth power of the 
 radar wavelength. For this reason, a powerful 3 cm radar, such as a CPS-9 
 (see Table I), may detect clouds more often than a 10-cm radar. Radars 
 designed specifically as cloud detectors have wavelengths on the order of 
 one centimeter. See figures 18 and 19. 
 
 3.2.3.2 RAIN 
 
 It is assumed that the WSR-57 detects all water drops of precipitable 
 size, subject to range and power limitations. However, as a consequence 
 of scattered energy being dependent on the sixth power of drop diameter, 
 large drops will have a greater reflectivity than a greater concentration of 
 smaller drops. This has turned out to be a rather fortunate circumstance 
 for those wishing to estimate rainfall rate with radar intensity measure- 
 ments, because it has been found that there is a relationship between 
 drop size and rainfall rate. This makes radar a very useful tool for 
 hydrology and local warning purposes. Furthermore, it has been found 
 that the most severe turbulence in thunderstorms is usually associated 
 with the strongest echo, and therefore radar is very useful to pilots and 
 air traffic controllers. 
 
32 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
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FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 33 
 
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 SATELLITE 
 
 > 
 
 Figure 19. A weather system over Florida and the eastern Gulf of Mexico, 
 as seen by satellite and by 10-cm radar. The upper left is a photo of the 
 cloud system as seen by satellite, and the upper right is a composite 
 photo of the rainfall, made from WSR-57 radars at Apalachicola and 
 Daytona Beach. In the sketch below the two views are superimposed, the 
 larger black areas being the satellite view, and the white cells in the 
 black areas the radar view. 
 
 > 
 
34 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 3.2.3.3 SNOW AND ICE ™ 
 
 Since scattering by an ice crystal is about one-fifth as great as scattering 
 by a water sphere of the same mass, radar echoes from snow are weaker 
 than echoes from rain having an equivalent water content. However, ice 
 with an outside coating of water, as is found during melting, will have a 
 scattering coefficient near that of a water drop of the same diameter. 
 Since this diameter is greater than that of a pure water drop of the same 
 mass, reflectivity figures from water-coated ice can be very high. This, 
 along with the large size of hailstones, is thought to account for the 
 extremely high reflectivities sometimes found with hail, and to account 
 for the "bright band" sometimes seen in more stable precipitation 
 systems. The freezing level, or more accurately the melting level, is 
 often observed by radar because as snow begins to melt it will have a 
 coating of water, and its scattering efficiency will increase, making an 
 enhanced echo on the scope. When completely melted the drop will be 
 smaller and will have a greater fall speed, resulting in a lesser concentra- 
 tion of smaller drops and a consequent weaker signal at lower altitudes. 
 The bright band is fairly common in the winter months, and is usually 
 associated with synoptic scale weather features. It sometimes can be 
 found in the decaying stages of large thunderstorms. A radar sweeping 
 normally will of course not detect a bright band overhead, but will 
 display echoes at those ranges where the beam intersects the melting 
 snow. In the extreme case a relatively narrow echo will completely M 
 
 encircle the center of the PPI, with the circle diameter dependent on ^ 
 
 the freezing- level altitude. In order to comprehensively map an elevated 
 echo such as this, it is necessary to sweep vertically with the antenna at 
 numerous azimuth headings or to make small changes in the antenna tilt 
 angle over several consecutive horizontal sweeps. Bright band RHI 
 display is illustrated in figure 20. 
 
 3.2.4 REQUIRED STUDY 
 
 FMH No. 7, Part B, Paragraphs 3.4 and 3.7, and Chapter 5. 
 
 3.3 SYNOPTIC SCALE WEATHER SYSTEMS 
 
 Complete precipitation fields associated with synoptic scale systems are 
 almost always too large to be detected and displayed by a single radar. 
 Experience has shown, however, that we can usually infer much about the 
 system from radar display. The different types of weather systems have 
 characteristic radar "signatures," and by observing the variation in these 
 signatures with time and distance, as shown by radar, we can make valid 
 conclusions about the size, intensity, and movement of the systems (see 
 figs. 21 through 25). By continually observing a large area, radar very 
 quickly detects changes in precipitation intensity, coverage, and movement, 
 that might take several hours to detect from the conventional observing 
 network. Chapter 5, Part B, of FMH No. 7 gives more detailed descriptions £ 
 
 of radar echoes in synoptic scale systems. 
 
FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 35 
 
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36 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 3.3.1 WINTER CYCLONES W 
 
 Cyclonic storms in the northern and central parts of the United States 
 often exhibit a large, almost solid, mass of stratiform echo that shows 
 some vertical development within about 150 miles of the low center. In 
 some cases the echo mass breaks into showers along the cold front and 
 cuts off rather abruptly in the cold air mass. The high-level cirrostratus 
 and snow virga often seen far out ahead of an advancing winter cyclone 
 are frequently detected by radar, and elevated echoes may be the first to 
 appear on the scope as a winter storm approaches. Storms over southern 
 states only infrequently exhibit the "standard" winter cyclone pattern. 
 Figures 22 and 23 are PPI presentations rather typical of early phases 
 of the approach of a winter cyclone. 
 
 3.3.2 SUMMER CYCLONES 
 
 Summer cyclones and low-altitude winter cyclones are more difficult to 
 recognize than high-altitude winter cyclones because the echoes associated 
 with the summer storms are normally convective and cellular in nature. 
 Since the cyclone system is usually far too large to be viewed by a single 
 radar, any single radar will generally display afield of scattered to broken 
 echoes with a random, or at least difficult to recognize, pattern. Exceptions 
 will be, echo lines associated with fronts and squall lines and large, fairly 
 solid echo areas close to the center on northern sides of well organized A 
 
 summer cyclone systems. Movement of echoes associated with summer ^ 
 
 cyclones is usually relatively easy to measure because of their cellular 
 nature, but care should be used in using this cell movement to infer cyclone 
 motion. Very often a single radar cannot see enough of the echo field to 
 identify field motion. 
 
 3.3.3 HURRICANES 
 
 Hurricanes are the most spectacular of the large scale echo patterns. A 
 well-developed storm is recognizable even when the center of the storm 
 is a great distance from the radar, and when the center gets close enough 
 that the wall cloud is detected, the hurricane is usually unmistakable 
 (fig. 24). The first echo indicators of hurricanes are not so easily 
 recognized, however. Outer rain bands often appear well ahead of the 
 large rain shield, and while they are curved in a large spiral shape, the 
 radius of curvature is usually so great as to be difficult to recognize. 
 These outer bands often contain squalls of greater severity than normally 
 associated with tropical thunderstorms. 
 
FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 37 
 
 The eye of a well-developed hurricane is a very important feature, and its 
 movement is sometimes easy to track by radar. It is necessary to be 
 cautious in attempting to predict motion from such observations, because 
 experience has shown that the eye does not advance at a steady pace, but 
 rather has variations in both speed and direction. Eye paths have been 
 known to zigzag, loop, stall, and even appear discontinuous, with the eye 
 apparently collapsing and reforming at another location. The resultant 
 track has short-term variations about the mean track of the hurricane, 
 and these variations could be very misleading if used to extrapolate the 
 position of the storm. The position and movement of hurricane centers, 
 as stated by hurricane advisories and bulletins, result from the analysis 
 of data from several sources and represent the best information available. 
 
 3.3.4 FRONTS 
 
 Precipitation associated with fronts does not always indicate the position 
 of the front, as placed on weather maps, nor does it follow any set pattern. 
 The widespread overrunning generally found with warm fronts often, but 
 not always, produces a large smooth echo resembling somewhat the 
 approach of a winter cyclone. As the front approaches, the base of high- 
 level echoes gradually reaches the ground, and bands may appear in the 
 larger echo near the surface position of the front. A bright band is 
 commonly seen in the precipitation associated with a warm front. Radar 
 is particularly useful in locating the thunderstorms occasionally imbedded 
 in warm front weather, and operators should be alert to cores of high 
 reflectivity in otherwise large formless echoes. 
 
 Cold fronts are even less inclined to radar signature than warm fronts. 
 The echoes associated with cold fronts are usually cellular in nature, 
 and not always arranged in lines along the front. Often when a line forms, 
 it will not coincide with the surface position of the front, but the motion 
 of any such line can often be associated with motion of the front. Radar 
 is particularly useful in following the movement of precipitation associated 
 with cold fronts, and in quickly detecting changes in precipitation patterns. 
 The early detection of developing thunderstorms along a front, or developing 
 squall lines out ahead of the front, can add important minutes or even hours 
 to a warning service. 
 
 3.3.5 SQUALL LINES 
 
 Although squall lines are defined as "non-frontal" they often resemble a 
 violent cold front, and are usually found in the warm air ahead of a cold 
 front. They consist of a line of convective cells that can on occasion 
 produce squalls, hail, or tornadoes. Squall lines vary considerably in 
 length, from less than 100 miles to several hundred miles. Their large- 
 scale motion is fairly conservative, but close inspection of the squall-line 
 echoes usually will show that small-scale motion is irregular because of 
 growth and decay of individual cells in the line. Sometimes the formation 
 
38 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 of new cells is dramatic, with almost simultaneous appearance on the PPI 
 of many cells in a completely new line near the old one. The old line will 
 usually then deteriorate or merge with the new one, but on rare occasions 
 the new development is short lived. Although there may be great variation 
 in the appearance, intensity, and even position of a squall line, tracking a 
 line by radar over several hours shows that the activity remains in an 
 instability zone that has a fairly constant size and motion. Squall lines 
 exist because of unusual instability, and therefore the change of the 
 occurrance of severe weather increases tremendously in the squall line 
 complex. Individual cells of high reflectivity or with unusual height, 
 motion, or longevity, should be considered probable generators of hazardous 
 conditions. Aircraft are particularly vulnerable to the extreme instability 
 in and near squall lines, and should plan to give all parts of the line a wide 
 margin of safety. When a squall line is on the scope it should be watched 
 continuously for indications of severe weather. A rather large-scale 
 development on squall lines that sometimes indicates severe weather is 
 a wave, resembling the early stages of a wave along a frontal surface. This 
 is called Line Echo Wave Pattern (LEWP) and is reported in the RAREP. 
 
 3.4 SEVERE LOCAL STORMS 
 
 Because weather radar views all points in its area of surveillance, it 
 provides information on storms that might easily be missed because they 
 lie between visual observation points, and it can easily provide information 
 on size, shape, and changes in storms, that can seldom be determined from 
 the visual observation network. Over most of the country, the network of 
 weather observers is not sufficiently fine to view all weather occurrences, 
 especially small-scale severe storms. By observing a radar scope we can 
 usually determine not only the places where precipitation is occurring at 
 any given time, but the intensity and movement of the precipitation and the 
 characteristics of the weather system. Of major importance in interpreting 
 radar display is continuity of observation. Observing an instantaneous 
 display is much like viewing one frame of a movie film. You may be able 
 to identify the characters, provided their faces are not turned from the 
 camera, but you can't tell much about the action. In addition to closely 
 watching the PPI, measurements of height and intensity are necessary to 
 properly interpret echoes. Occasional sweeps at varying elevation angles 
 and receiver gain settings, and vertical scanning of echoes, are important 
 functions in observing weather by radar. This is especially true in the 
 case of unstable weather situations that may lead to severe weather. 
 Whenever thunderstorms appear on the scope the observer should give 
 undivided attention to the radar and should become intimately acquainted 
 with each echo, so that any changes are quickly noted. In general, the more 
 intense the storm, as measured by reflectivity, height, and speed, the 
 greater the chance of severe weather. Local severe storms sometimes 
 present "signatures," by which they can be identified. Chapter 5, Part B 
 of FMH No. 7 discusses these signatures in some detail, but we will give a 
 brief description here. Be sure to note the summary of severe storm 
 
FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 39 
 
 criteria on page 5 - 24 of FMH No. 7. It should be well understood that 
 although certain echo patterns have been identified in connection with 
 severe weather, they are by no means infallible indicators. 
 
 Local severe storms have been noted in every season of the year, with 
 single thunderstorms as well as with squall lines, and with nearly stationary 
 thunderstorms as well as with rapidly moving ones. However, storms 
 associated with intense squall lines or with a LEWP, or having extreme 
 height or rapid speed, are particularly suspect. Rapidly moving thunder- 
 storms are usually associated with sharp wind shifts and relatively high 
 gusts at the surface, and depending on their reflectivity, with heavy rainfall 
 and possible hail. Severe weather has been observed on several occasions 
 near the junction of two thunderstorms. This junction is most common 
 when one or both of the storms moves quite rapidly. When two storms 
 approach one another, the wind shear that results can be quite large, due 
 not only to the difference in their movements, but to the differing wind 
 components in and near each storm. Time lapse film shows that many 
 intense thunderstorms rotate about a vertical axis, as seen on the PPI. 
 
 The height of a storm relative to the height of the tropopause, appears to 
 be a more important factor in the production of severe weather than the 
 absolute height of the storm. Those storms that reach the height of the 
 tropopause should be considered as probable producers of severe weather, 
 and by the time they penetrate to 10,000 ft above the tropopause, a severe 
 storm is likely occurring at the surface. Hail producing storms are found 
 to be the highest in a given field. Thunderstorms with tornadoes nearly 
 always penetrate the tropopause, often reaching over 50,000 ft. However, 
 funnel clouds that sometimes reach the surface in southern Florida and 
 around the Gulf of Mexico coastline often extend from cumulus clouds that 
 have not reached the thunderstorm stage and are on the order of 20,000 ft 
 in height. While these are not in the same class with the powerful 
 midwestern tornadoes, they can cause local damage. Tornadoes are often 
 associated with storms of high reflectivity, and in some cases the tornado 
 itself may be highly reflective. The associated hook that has been observed 
 on the radar PPI is found in the trailing half of the storm echo. Often the 
 hook is masked in the large echo as seen by a powerful radar such as a 
 WSR-57, but may become evident by adjustment of the attenuator control. 
 Less powerful radars usually fail to detect the weaker echo around the hook, 
 and present a picture of a hook extending from the main echo body even at 
 full gain. Operators of WSR-1 and WSR-3 radars should be alert to the 
 possibility of masked hooks, however, and should study suspicious storms 
 at various gain settings. Tilting the antenna at various angles is also 
 helpful, because sometimes the hook may appear aloft but not at the surface. 
 Hooks have been observed to swirl from the parent cloud, and would appear 
 to be associated with a micro- scale cyclonic circulation. The circle 
 ascribed by the main part of the hook is usually from 4 to 12 miles in 
 diameter. The surface position of the tornado is usually near the small 
 end of the hook. Hook echoes are illustrated in figure 25. Although hooks 
 
40 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 appear sometimes where there is no visual evidence of a tornado at the 
 surface, their association with tornadoes is certainly strong enough to 
 warrant immediate warnings when one is definitely identified on the scope. 
 Be careful of "false hooks" which usually are short lived. Always keep in 
 mind that the absence of a hook echo does not preclude the presence of a 
 tornado. 
 
 Hailstones are highly reflective, and will generally produce an intense echo. 
 This will appear as a hard core within a thunderstorm echo, or may extend 
 as one or more "fingers" protruding from the edge of a thunderstorm echo. 
 These fingers may be evident at full gain, or may appear only after the 
 signal is attenuated. Vertical sweeps have produced, on the RHI, columns 
 of high intensity that weaken near the ground. These possibly indicate hail 
 that is melting before reaching the ground. Thunderstorms that produce 
 hail are particularly violent, with intense updrafts and tops that usually 
 penetrate the tropopause. 
 
 As discussed previously, stratiform precipitation presents a featureless 
 echo, usually relatively large, that changes intensity and shape rather 
 slowly and is not associated with violent weather. Such rainfall can cause 
 flooding, however, and high reflectivity or lengthy sojourn over any 
 watershed should be considered an indication of possible excess 
 precipitation. One difficulty in dealing with large apparently stratiform 
 echoes, is the possibility of overlooking imbedded cells of high reflectivity 
 that indicate heavy rainfall, and may indicate localized convective activity 
 particularly dangerous to aircraft because it is unexpected and hidden. 
 While the turbulence associated with such cells is usually not severe, 
 icing on aircraft can be very heavy in them because the vertical motion of 
 the air suspends large quantities of moisture, both liquid and frozen. The 
 heaviest icing will occur just above the level of the bright band that is often 
 evident with stratiform precipitation. 
 
 3.5 ATTENUATION 
 
 Any process that reduces power density within the radar beam is called 
 "attenuation," and for meteorological radar work range attenuation and 
 atmospheric, or more commonly, precipitation attentuation are considered. 
 
 3.5.1 ATTENUATION BY PRECIPITATION 
 
 A certain amount of absorption occurs even in an atmosphere free of clouds 
 and precipitation, but at frequencies used for weather radar this is 
 negligible. Cloud droplets and precipitation may cause more severe 
 attenuation, however, and must be taken into account. 
 
FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 41 
 
 Figure 21. Typical PPI appearance of echoes from convective type rainfall. 
 Note the irregular but generally sharply defined edges on the precipitation. 
 Range marks are 20 nmi apart. 
 
42 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 Figure 22. Typical PPI appearance of echoes from stratiform type 
 rainfall. Note the maximum range of precipitation defection is roughly a 
 semicircle. This may indicate that the area of rainfall is low level and 
 extends to greater areal coverage than shown, but the radar beam is 
 entirely above the rainfall at ranges beyond the edge of the semicircle. 
 Range marks are 20 nmi apart. 
 
FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 43 
 
 > 
 
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 Figure 23. Typical PPI appearance of echoes from snow. Since reflectivity 
 of snow is low, it may not be detected at great ranges. The area of falling 
 snow is probably larger than shown on the scope. Range marks are 20 nmi 
 apart. 
 
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44 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
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46 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 Attempts at quantitive measurements of atmospheric attenuation have been 
 made, but these have no operational value at this time. In the operation of 
 weather radar we can measure the difference in strength of the signal as 
 transmitted and as returned from the target, but we do not know exactly 
 how much of the signal strength depletion is due to target backscatter 
 inefficiency and how much is due to attentuation between the target and the 
 antenna. Unfortunately, a good target by its very nature is also an excellent 
 attenuator, absorbing and scattering the radar energy in all directions. 
 Attenuation, then, is like backscatter in that it is inversely proportional to 
 the fourth power of the wavelength. The longer the wavelength the smaller 
 the attenuation, if drop size distribution remains the same. This was an 
 important consideration in selecting the wavelength of Weather Service 
 radars. The relatively long 10 cm wavelength selected assures a minimum 
 of attenuation by hydrometeors, and in the operation of these radars 
 atmospheric attenuation is considered negligible although return from 
 distant targets may be attenuated a slight but unknown amount by more 
 nearby precipitation. In the case of radars with a shorter wavelength, a 
 "V" shape indentation in the distant side of a heavy cell is a characteristic 
 indicator of precipitation attenuation. Such an indicator may appear on the 
 PPI of a low-powered 10-cm radar, such as a WSR-1, in the case of a large 
 area of hail or extremely heavy rain (see fig. 2 7). 
 
 3.5.2 RANGE ATTENUATION 
 
 Range attenuation is the dissipation of radar beam intensity due to spreading 
 of the beam. As the cross-section area of the radar beam increases with 
 range, the beam energy is spread ever thinner, just as a flashlight beam 
 becomes dimmer at greater distances from the flashlight. Figure 28 shows 
 how a target might appear to the radar when at different ranges. Since the 
 beam cross section has two dimensions, the energy falling on a point in the 
 beam is proportional to 1/r^, where r is range. 
 
 Consider now how this affects weather surveillance radar. Since we make 
 the very important assumption that meteorological targets completely fill 
 the radar beam (fig. 29), all of the energy (assuming no atmospheric 
 attenuation) transmitted by the radar is intercepted by the target, regardless 
 of the beam cross- section area. Thus, all of the energy is scattered by the 
 target, some of it in the direction of the antenna. But the antenna does not 
 intercept all of the energy scattered by the target, and therefore the 
 1/r^ relationship must be applied to the backscattered energy. One can 
 easily understand that the actual range correction applicable to each target 
 would vary greatly if the targets only partly filled the beam. Our assumption 
 that all meteorological targets fill the beam is a good one for most targets, 
 as long as the top of the beam does not extend to an altitude greater than that 
 of the target. This was a most important consideration in limiting routine 
 intensity measurements to those echoes within 125 nmi of the radar (75 nmi 
 for WSR-1 and WSR-3 radars). 
 
> 
 
 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 47 
 
 PRECIPITATION ATTENUATION 
 
 Area blacked out 
 by attenuation 
 
 ^SSN^N^SS' 
 
 \ / 
 
 OBSERVED 
 ECHO 
 
 Figure 26. The nearby target absorbs and scatters so much of the out- 
 going and returning energy that the radar does not detect the distant target, 
 
 I 
 
 WAVE LENGTH AND ATTENUATION 
 
 10 Cm. 
 
 3 Cm. 
 
 Figure 27. Weather targets as they might appear on radar sets with 
 different wavelengths. Note the bright near edges and V notch shape of 
 the attenuated echoes. 
 
 > 
 
48 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 
 
 DISTANT TARGET 
 WEAK ECHO 
 
 CLOSE TARGET 
 STRONG ECHO 
 
 Figure 28. A rain cell as it might appear at different ranges. The 
 difference in intensity is due to range attenuation. 
 
 LARGE TARGET 
 
 ALL ENERGY STRIKES TAR0ET 
 
 Figure 29. Precipitation targets that completely fill the beam receive total 
 transmitted energy minus intervening attenuation, regardless of range. 
 
> 
 
 FUNDAMENTALS OF RADAR PROPAGATION AND DETECTION 49 
 
 RANGE 
 
 25 50 75 100 125 150 
 
 HAN6E- NAUTICAL MILES 
 
 
 > 
 
 Figure 30. Range normalization compensates electronically for range 
 attenuation. 
 
 > 
 
• 
 
 Intentionally Left 
 Blank 
 
> 
 
 4. RADAR MEASUREMENTS AND REPORTS 
 
 4.1 MEASUREMENTS 
 
 Radar's usefulness as a tool to detect and describe precipitation over a 
 wide area is due in large part to the objective information made available 
 by the radar. Location, size, and movement comprise the basic information 
 that first made radar attractive for weather observations. The addition of 
 height and intensity data has, of course, greatly increased the value of radar 
 weather reports. Subjective information based on operator judgment is also 
 an important and necessary part of radar reports, but will not be considered 
 in this section. We shall deal here with the objective information only. 
 
 4.1.1 DIRECTION 
 
 In order that echoes will be displayed on the PPI scope in their proper 
 direction (azimuth) from the radar site, the PPI sweep line is synchronized 
 with the antenna rotation. On both the console PPI and the WBRR display 
 north direction is at the top, but the console display provides means for 
 accurate direction measurements that are not available to the WBRR user. 
 The azimuth ring associated with the PPI is graduated in one-degree 
 increments and lighted to be always visible. The direction the antenna is 
 pointing can be read at the end of the sweep line, and on the WSR-5 7, the 
 direction of an arbitrary point on the scope can be read from a manually 
 operated pointer. In addition, a number counter on the face of the WSR-57 
 console reads the direction, to the nearest degree to which the antenna is 
 pointing. In the RAREP, directions are reported in whole degrees, relative 
 to true north (see fig. 5). 
 
 4.1.2 DISTANCE 
 
 Radar energy travels at the speed of light, approximately 161,800 nmi/ 
 second. Since we can measure electronically the time elapsed between 
 emission and reception of a pulse of energy, the distance it has traveled 
 is easily computed. Distance between the antenna and reflecting target 
 is, of course, half the total distance traveled by the pulse. 
 
 (161,800/2) (Number of seconds) = echo range in nautical miles. 
 
 Five evenly spaced range marks are displayed on the various radar scopes, 
 their separation depending on the range being displayed. On the WSR-5 7, 
 an electronic cursor associated with the A and R and PPI scopes can be 
 manually moved to pinpoint the distance to arbitrary locations, the value 
 being read to the nearest one-tenth mile from a number counter on the face 
 of the console (see JT2.2 - 2.4). Distances are reported in whole nautical 
 miles, as measured from the antenna location. 
 
 51 
 
52 RADAR MEASUREMENTS AND REPORTS 
 
 4.1.3 MOVEMENT 
 
 Movement of radar weather echoes is measured by consideration of 
 successive positions of the echo, or group of echoes. No effort is made to 
 distinguish between translation and development, nor to associate the 
 movement with any existing wind field. The radar observer simply acts as 
 an umpire and "calls 'em as he sees 'em." He measures the distance and 
 direction between two successive positions of the echo, reporting speed in 
 knots and the direction from which the echo has moved. The reflection 
 plotter mounted on the PPI of the WSR-57 greatly facilitates such measure- 
 ments by minimizing the error of parallax and providing a measuring 
 surface directly over the scope. The time increment between successive 
 points is normally 15 minutes in the case of cells and small elements, and 
 1 hour in the case of lines and areas. 
 
 4.1.3.1 When plotting the successive position of cells, it is usual to select 
 the center of the cell and measure the distance and direction between the 
 positions occupied by the center at different times. This means that, in the 
 case of a cell that is rapidly changing size or shape, the resultant movement 
 of any side or edge of the cell may not be the same as that reported for the 
 cell. A user should be aware that in determining such things as onset of 
 rainfall at a particular location it may not be sufficient to make a simple 
 extrapolation based on reported cell movement. 
 
 4.1.3.2 The movement of areas is also generally measured from centroid 
 to centroid, although this is true to a somewhat lesser extent than in the 
 case of cells. It is not always possible to determine the centroid of an 
 area with any degree of certainty, because the area may be large enough 
 to extend beyond scope range, or the radar beam may overshoot part of the 
 rain area. In addition to determining the movement of the area as a whole, 
 an attempt is made to measure the movement of the cells or elements 
 which comprise the area. This information is reported directly after the 
 area movement. 
 
 4.1.3.3 The movement of lines is defined as the component of motion 
 perpendicular to the axis of the line, and is so reported. It is also 
 important to report the movement of individual cells in the line, as 
 indicated in figure 31, if such movement can be determined. This is 
 especially true if the line and cell movements are considerably different. 
 In the case of sharply curved lines, or of lines that are moving with different 
 speeds along their length, the movement at two or more significant points 
 will be reported. 
 
RADAR MEASUREMENTS AND REPORTS 
 
 53 
 
 MOVEMENT OF LINES 
 
 NEW CELLS 
 
 LINE 
 
 7 NEW CELLS 
 
 MOVEMENT 
 
 FORWARD EDGE 
 MOVEMENT 
 
 Figure 31. The reported movement of the line is the component of 
 
 motion perpendicular to the line, measured center line to center line. 
 Reporting both line motion and cell motion, along with growth and decay 
 information, gives an accurate description of weather action. The line 
 illustrated here has increased in width and length between observations. 
 
54 RADAR MEASUREMENTS AND REPORTS 
 
 4.1.4 ECHO HEIGHT 
 
 It is essentially a geometric problem to determine the height of an echo, 
 since slant range and tilt angle can be easily measured. It is necessary 
 to apply certain corrections to the geometric results, however, and these 
 are discussed below. 
 
 ANTENNA HEIGHT. Since height reports refer to the heights above mean 
 sea level, the height of the antenna above mean sea level is added to the 
 geometric results. 
 
 EARTH CURVATURE. Earth curvature over radar ranges is of sufficient 
 magnitude that a correction must be made for this curvature. Since the 
 earth curves "downward" in relation to a straight- line tangent at the 
 surface, the earth curvature correction is added to the geometric results. 
 
 ATMOSPHERIC REFRACTION. Refraction in the atmosphere causes the 
 radar beam to curve in the same sense as the earth rather than travel in a 
 straight line (see section 3.1). A straight line projected from the antenna 
 is thus at a greater altitude than the actual radar beam, and it is necessary 
 to subtract the refraction correction from the geometric results. The 
 U. S. standard atmosphere is assumed for the purpose of this calculation. 
 
 BEAM WTDTH . The radar presents echoes on the scope as if the beam were 
 a single line extending from the antenna. The beam is actually a narrow 
 cone and its width at any point depends on the angular width of the beam and 
 the distance from the antenna. In the case of the WSR-57, with a two-degree 
 beam width, the beam has a cross sectional diameter of 21,000 ft at a 
 range of 100 nmi. Any target just barely touched by the edge of the beam, 
 that returns a detectable signal to the radar will therefore be displayed on 
 the scope as if it were at the center of the beam --an error of 10,500 
 ft. This error is, of course, less at shorter ranges and greater at longer 
 ranges. If the radar operator tilts the antenna upward until he can just 
 barely "see" the top of the target, the bottom edge of the beam is touching 
 the target but the center of the beam is half a beam- width higher. 
 Therefore, beam width correction is subtracted from the geometric result 
 for determining echo tops. For finding the height of the base of those 
 echoes that do not extend to the ground, the beam-width correction is 
 added to the geometric result. 
 
RADAR MEASUREMENTS AND REPORTS 
 
 55 
 
 ANTENNA TILT 
 
 Figure 32. The radar views a different slice of the atmosphere when the 
 antenna tilt angle is changed. These photographs were taken in a 2-minute 
 period with the radar gain setting remaining constant. At an eight degree 
 elevation angle the radar beam is overshooting all but the very nearby 
 echoes. 
 
56 
 
 RADAR MEASUREMENTS AND REPORTS 
 
 ATTENUATION BY GAIN REDUCTION 
 
 18db 
 
 c 1 db 
 
 Figure 33. This is the same weather situation as shown in figure 32, but 
 here the antenna tilt is held constant and the receiver gain is reduced in 
 three steps. The "hard cores" of greater reflectivity are displayed when 
 weaker parts of the echoes are blocked out by the step attenuators. 
 
RADAR MEASUREMENTS AND REPORTS 57_ 
 
 4.1.4.1 HEIGHT FINDING LIMITATIONS 
 
 A combination of these correction factors places a limitation on the 
 effectiveness of the radar as a height finder at great ranges, and a practical 
 range limit is established for each type of radar. The RHI on the WSR-57 
 has a maximum range of 125 nmi. In addition to the maximum range 
 limitations, echo tops cannot be determined accurately at very close ranges, 
 primarily because of antenna tilt limitations, extraneous echoes at very 
 close ranges, and the necessity to add a component of the pulse length 
 when the beam path through the echo has considerable vertical component. 
 The WSR-57 has a minimum range for height determination of 5 nautical 
 miles. 
 
 4.1.4.2 HEIGHT FINDING TECHNIQUE 
 
 The height of a target is usually determined by stopping the horizontal 
 antenna rotation when the beam illuminates the target, and rotating the 
 antenna vertically. On the WSR-57, the sweep line on the RHI scope will 
 then rotate about the lower left corner of the scope and will paint a vertical 
 cross section picture of the target. Height lines are marked on the scope 
 in 5,000 -ft increments to a height of 70,000 ft. Since the distance 
 represented along the horizontal scale is much greater (125 nmi), echoes 
 are considerably distorted, being stretched vertically (see fig. 8). The 
 same distortion exists in the RHI display of the WSR-3 and WSR-4 radars, 
 although the scope is different than that of the WSR-5 7. On the WSR-3 
 and WSR-4, RHI display is accomplished on the PPI scope by the use of 
 a manual switch. Elevation angles are marked on the face of the scope, 
 and the vertical sweep originates at the center of the scope, with the 
 zero-degree elevation marker extending toward the 90-degree azimuth 
 mark. Radars without RHI display can be used to determine height by 
 noting the elevation angle of the beam at the top of the echo. Heights are 
 reported in hundreds of feet above mean sea level, in 1,000-ft increments. 
 
 4.1.4.3 REQUIRED STUDY 
 
 FMH No. 7, Part B, Section 4.3. 
 
 4.1.5 INTENSITY OF ECHOES 
 
 Sophisticated electronics in modern weather radars make it possible to 
 measure the strength of the received signal, and from this we can infer 
 the intensity of reflectivity from the target. It is only one more step, then, 
 to associate the intensity of reflectivity with intensity of rainfall. Many 
 studies have been made comparing radar reflectivity with measured rainfall 
 rates; and while there is some variability in the results all investigators 
 develop equations of the same form, and when the various curves given by 
 the equations are plotted together the majority fall in a narrow band. This 
 encourages use to accept radar reflectivity from precipitation as a valid 
 
58 RADAR MEASUREMENTS AND REPORTS 
 
 measurement of the rainfall rate, and adds greatly to the usefulness of 
 weather radar. While the results of radar rainfall rate measurements 
 are variable about the rate as caught by a rain gage, for some consider- 
 ations the radar measurements are possibly the most useful because they 
 are made over a large area almost simultaneously, while rain gage 
 measurements are point values.* It has been found that an overwhelming 
 majority of the rainfall from any given rain system falls in the area of 
 strong radar reflectivity, even though this area may represent only a 
 small fraction of the total area of rainfall. Furthermore, there is a 
 positive correlation between severe turbulence and strong radar echoes. 
 
 4.1.5.1 While we do not propose to develop the equation of signal power 
 return and reflectivity here,t the signal power received by the radar 
 
 (P r ) depends upon the characteristics of the radar (R c ) (i.e., power 
 output, antenna gain, beam width, pulse length, and wavelength) the distance 
 to the target (r), and the reflectivity of the target (Z). For those radars 
 that have a wavelength shorter than 10 cm, atmospheric attenuation 
 should also be considered. Disregarding attenuation, the power received 
 is related to the other variable by: 
 
 P = R c Z 
 
 r — 2 
 
 r 
 
 Two important assumptions should be noted here. First, the Rayleigh 
 scattering law is used, which states simply that the drops are spherical 
 in shape and that their radius is not larger than about one-tenth the wave- 
 length of the radar energy. This will be approximately true for rain 
 drops, although it is known that their shape is not always spherical, 
 especially in the case of the larger ones. Hail may exceed the 1-cm 
 radius size limit (10 cm radar) in which case its reflectivity becomes 
 great. The second and most important assumption is that the cross- 
 section area of the radar beam is completely filled by the targets. 
 Whenever the beam is not completely filled, the power received is reduced 
 by a factor equal to that portion of the beam free of echo. This lends a 
 range bias to intensity measurements, because the beam gets larger and 
 higher as the range from the radar increases. For the WSR-57, a rain 
 cell at 100 nmi must be some 20,000 ft in both vertical and horizontal 
 size to completely fill the beam, and beyond that range the top of the beam 
 
 *McCallister, et al. "Operational Radar Rainfall Measurements," Twelfth 
 Conference on Radar Meteorology, (1966), 208-215. 
 
 tSee Appendix A. 
 
RADAR MEASUREMENTS AND REPORTS 59_ 
 
 overshoots many weather echoes. The beams of the WSR-1 and WSR-3 
 are twice as wide as that of the WSR-57, with the consequence that beam 
 filling is a greater problem for these radars. Since we generally do not 
 know what portion of the beam is filled, we can make no correction for 
 this variable. 
 
 4.1.5.2 Inspection of the radar storm equation shows that there is an 
 easily defined relationship between the target reflectivity and the power 
 received by the radar, since the radar characteristics are constant and 
 the range is easily measured. This leaves us with two problems; (1) 
 defining the relationship between reflectivity (Z) and rainfall rate (R), and 
 (2) measuring the signal strength received by the radar. We have already 
 mentioned that the first of these problems is solved empirically. 
 
 The reflectivity varies with drop size and drop density, and consequently 
 the various studies ^hpw a rough latitudinal and even seasonal variation. 
 The value Z = 200R ' , with Z in mm /m and R in mm/hr, is considered 
 representative for a wide variety of rains, and is the value used in arriving 
 at radar rainfall rates as measured by Weather Service radars. The 
 second problem, measuring the signal strength received by the radar, is 
 solved by varying receiver gain settings while viewing target representation 
 on an A-scope. In the case of the older radars such as the WSR-3, not 
 designed for objective intensity measurements, a method has been devised 
 that allows classification of echoes into intensity categories with a fair 
 degree of confidence. The WSR-57, along with other modern weather 
 radars, has a greater dynamic range and precision attenuators, allowing 
 for precise measurements over a wide range of intensities. 
 
 4.1.5.3 While the power transmitted by the radar is large, ranging from 
 60 kW for the WSR-3 to 410 kW for the WSR-5 7, the power received by the 
 radar from the target is quite small and is, in fact, measured in terms of 
 reference to a milliwatt (10 watt). We see that there is a factor of one 
 million between the two. Definitions of signal attenuation or amplification 
 are usually made by comparing one power level with another, using the 
 decibel (dB) as the unit of comparison. The decibel can be defined as 
 
 P 2 
 dB = 10 log — 
 
 1 
 
 where Pi and P2 are the two power levels. When one milliwatt is used as 
 the reference power level, the term is 
 
 n , Power received (watts) 
 dBM = 10 lQ g 10-3 (watts) 
 
60 RADAR MEASUREMENTS AND REPORTS 
 
 If we substitute simple figures into the equation, we see that a signal with 
 half the power of the reference (1/2 milliwatt) produces -3 dBm, and a 
 signal that has 1/1000 the power of the milliwatt reference corresponds 
 to -30 dBm, or is "30 dB below a milliwatt." The receiver in the WSR-57 
 normally detects signals down to 108 dB below a milliwatt (-108 dBm), 
 which is very good sensitivity. The important feature is, of course, the 
 calibrated attenuator stepping device which allows us to make a good 
 measurement of signal strength. . The received signal is attenuated in 3 dB 
 steps, halving it each time, to a maximum of -99 dBm (WSR-57). The 
 number of decibels necessary to decrease the signal amplitude to an 
 established reference level is a measure of the power of the signal, hence 
 a measure of the reflectivity of the target. This is converted to a rainfall 
 intensity classification for reporting purposes. Figure 33 illustrates the 
 change in the appearance of echoes as attenuation is applied to the echo 
 signal. 
 
 4.1.5.4 The various intensity classifications, with their theoretical rainfall 
 rate values, are shown in table IV, compared with surface observation 
 reporting criteria. It is very important to note the differences between 
 "radar intensity" and surface observation rainfall intensity criteria. The 
 words used to describe the radar intensities actually refer to signal 
 intensity rather than rainfall rate, but the symbols adopted for code 
 purposes are those commonly used in reporting observed rainfall 
 intensities. It should be noted that the symbols do not in all cases refer 
 to the same values. It is important to keep this in mind when reviewing 
 radar reports. 
 
 4.1.5.5 The choice of intensity to be reported for a single weather echo, 
 such as a rain cell, is fairly obvious. The most intense return to be found 
 from any part of the cell is considered to characterize the cell, and is so 
 reported. The same reasoning is applied in the case of a group of 
 convective echoes. The strongest cell in the group is considered to 
 characterize the known potential of the cells in the group at the time of the 
 observation. Convective cells often change their characteristics very 
 rapidly, with the entire life cycle of individual cells possibly occurring 
 between observations. Any group of convective cells likely contains, at 
 any given time, cells ranging from new and increasing, through mature 
 at maximum strength, to decaying and weak. With present day techniques 
 it would be impossible to acquire any sort of meaningful median value by 
 measuring individual cells in a group. Besides, our interest in such a 
 case is not in the mean or average intensity to be found in the area, but 
 rather in the intensity which represents the maximum potential of any 
 cell in the group. For systems other than convective type, a category 
 of intensity that is predominant throughout the area is more readily 
 determined, and is used as the characteristic intensity. Intensities 
 usually change less rapidly in stratiform systems and are more likely to 
 be fairly uniform over large areas. Intensity measurements are not 
 reported for frozen precipitation because a satisfactory reflectivity- 
 
RADAR MEASUREMENTS AND REPORTS 
 
 61 
 
 Radar Observations 
 
 Table IV. Rainfall - Echo Intensities 
 Rainfall Rate (In/Hr) 
 
 Surface Observations 
 
 Weak 
 
 <0.10 
 
 Light 
 
 
 Moderate 
 
 Trace - 0.10 
 
 
 0.10 - 0.50 
 
 Moderate 
 
 
 Strong + 
 
 0.10 - 0.30 
 
 
 0.50 - 1.00 
 
 + Heavy 
 
 
 Very Strong + + 
 
 1.00 - 2.00 
 
 
 Intense X 
 
 2.00 - 5.00 
 
 
 Extreme XX 
 
 >5.00 
 
 
 
 >0.30 
 
 
 snowfall relationship has not yet been developed. Intensity values are 
 not reported for drizzle because some of the drizzle drops may be too 
 small to be detected by radar, and because drizzle is usually confined 
 to low levels where it may lie below the radar beam. 
 
 4.1.6 
 
 INTENSITY TREND 
 
 The intensity trend as reported in the RAREP is intended as an indicator 
 of significant change. If the intensity of an echo changes by an amount 
 approximately equal to, or greater than, the range of an intensity category 
 (such as weak, which is approximately 16 dB at 125 nmi), the trend will 
 be reported as increasing (+) or decreasing (-). If a change is less than a 
 category range, it is reported as "no change." Note that with this definition 
 it is possible for the intensity category to change from report to report, 
 while the intensity trend is reported as "no change." 
 
62 RADAR MEASUREMENTS AND REPORTS 
 
 4.2 REPORTS 
 
 The radar operators at national network stations generally have two basic 
 types of reports to make on a routine basis. One is the RAREP (Radar 
 Report) designed for use mainly by Weather Service offices and the FAA 
 (fig. 34), and the other is a narrative weather description designed for 
 direct reporting by local news media and the FAA. Both should be available 
 to remote display users, and should always be used to complement the 
 remote display. Appendix B gives an example of a weather situation and 
 associated RAREP and narrative reports. Another means of radar data 
 dissemination is the transmission of maps of radar echoes via facsimile 
 circuits. 
 
 4.2.1 RAREP (RADAR REPORT) 
 
 The RAREP includes all the weather echoes detected by the radar, 
 supplemental information concerning severe or unusual conditions, notations 
 of fine lines, and remarks concerning extraordinary phenomena. Weather 
 echo information ordinarily includes type, configuration, amount, intensity, 
 intensity trend, location, movement, height of echo tops, and height of 
 bases, where applicable. The regular RAREP is completed once each 
 hour, at H plus 35 minutes, provided there are weather echoes on the 
 scope. Special observations are taken at other times when required. 
 All weather echoes are grouped into one of four major forms (areas, 
 lines, layers, and cells), and are described in accordance with the RAREP 
 code. Lines and cells may appear within areas, and large precipitation 
 areas may contain smaller areas of a significantly different type of echo 
 (see fig. 35). The next few paragraphs discuss elements of the RAREP 
 that are determined largely subjectively, and that have not been discussed 
 previously in this chapter. 
 
 4.2.1.1 CELLS 
 
 When cells, either isolated or in a group, are reported individually, the 
 report will include the type of precipitation, intensity, net intensity change 
 during the 15 minutes just prior to reporting time, location of the center 
 of the cell, movement of the cell when known (representing the 15-minute 
 period just prior to reporting time), height of echo top if the echo is 
 within 125 nmi of the antenna, size of the cell, and any pertinent qualifying 
 remarks. If the cell intensity qualifies as very strong or greater its 
 location will be determined by stopping the antenna rotation at a point 
 where the sweep line illuminates the echo, placing an electronic cursor at 
 the center of the echo, and then reading azimuth and distance from dial 
 counters on the face of the radar console. Location then will be reported 
 to the nearest whole degree of azimuth and to the nearest whole nautical 
 mile in range. For cells of lesser intensity, a mechanical pointer over 
 the PPI scope is used to determine direction, and distance is estimated 
 with the aid of range marks. Direction in this case is reported to the 
 
RADAR MEASUREMENTS AND REPORTS 
 
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64 RADAR MEASUREMENTS AND REPORTS 
 
 nearest whole degree of azimuth, as before, but range will generally be 
 reported in increments of 5 nautical miles. Echo location on the PPI 
 scope of the WSR-57 is greatly aided by a reflection plotter which is 
 mounted over the scope. A grease pencil mark on the reflection plotter 
 appears to have been made directly on the scope face, thereby avoiding 
 parallax errors. 
 
 4.2.1.2 LINES 
 
 Lines are located by indicating end points, and any other points along the 
 axis necessary to describe the shape of the line. The average width of 
 the line is reported, with qualifying remarks where applicable. Areal 
 coverage is reported to the nearest tenth part by the use of one or two 
 digits directly after the configuration contraction. Example: LN8TRW 
 etc... The intensity ascribed to the line is usually the greatest found in 
 the line (characteristic intensity), because lines are ordinarily comprised 
 of convective type precipitation. Intensity trend reported represents 
 change in the maximum intensity during the 1 hour period preceding the 
 observation time. The movement of a line is the component of motion 
 perprendicular to the major axis of the line. That is, a line extending 
 from north to south would be reported as moving only east or west. 
 Line motion reported is representative of a period 1 hour prior to the 
 reporting time. In addition to line motion, movement of individual cells 
 in the line may be reported, and this cell motion will not necessarily be 
 perpendicular to the major axis of the line. The cell motion is measured 
 over a 15-minute period. The maximum echo altitude noted along the 
 line will be reported as "MAX TOP," to the nearest thousand feet in 
 hundreds of feet. Other tops may be reported when significant. All top 
 reports use mean sea level as base. 
 
 4.2.1.3 AREAS 
 
 Weather echoes associated geographically and by physical causes are 
 grouped and reported as "AREAS" in the RAREP. The location and size 
 of an area are described in one of three ways: (1) by reporting the azimuth 
 and range of the center of the area along with the diameter of the area, 
 (2) by azimuth and range of end points of a line through the middle of the 
 area, accompanied by area width, and (3) by the azimuth and range of 
 significant points on the area boundary. In the latter case the points will 
 be listed in order, clockwise around the area. Areal coverage is reported 
 to the nearest tenth part by the use of one or two digits directly after the 
 configuration contraction. Example: AREA8R- etc... If the area is 
 composed of convective echoes, the maximum echo intensity noted in the 
 area will be considered the characteristic intensity, and will be reported 
 as area intensity. This does not mean that an isolated thunderstorm in a 
 large area of light showers will necessarily establish the characteristic 
 intensity of the area. If the radar observer considers isolated areas of 
 cells as not characteristic of the whole area, he will report them separately. 
 
RADAK MEASUREMENTS AND REPORTS 
 
 65 
 
 If the area is composed of stratiform precipitation, the predominant 
 intensity over the area will be reported as characteristic. Intensity- 
 tendency and area movement will represent a time increment of 1 hour, 
 as with lines. Movement of cells or elements within the area may be 
 reported separately, the time increment being 15 minutes. The highest 
 echo top in the area will be reported, and other tops may be reported in 
 remarks if thought significant. 
 
 4.2.1.4 LAYERS 
 
 The radar sometimes detects stratified echoes, the bases of which do not 
 reach the ground. These are reported as layers (LYR), and position, shape, 
 type, intensity, movement, and tops are reported as in the case of areas. 
 In addition, the height of the base is reported in hundreds of feet. Layer 
 thickness is sometimes difficult to determine because of beam width 
 distortion, and in some cases information may be so nebulous that the 
 layer is simply reported in remarks rather than in standard code format 
 
 Figure 35. Grouping of echoes for reporting purposes. Classification 
 involves, in addition to geographical location, consideration of configuration, 
 coverage, continuity of pattern, and meteorological processes. 
 
66 RADAR MEASUREMENTS AND REPORTS 
 
 4.2.2 NARRATIVE RADAR WEATHER REPORTS 
 
 The narrative radar weather report produced at Weather Service radar 
 stations is not intended for long-line distribution, and is tailored to meet 
 the specific needs of the radar locale. Narrative reports are generally 
 issued once each hour, but the frequency may vary with the time of day 
 and the weather coverage and type. The report covers an area of assigned 
 station responsibility, such as several counties, the area usually being 
 about 200 miles across. Severe or particularly significant weather at 
 greater ranges is often reported. Azimuth and range readings are not 
 used to identify locations, but rather geographical and political features 
 generally recognized by the public are used. Cities, towns, highways, 
 state and county boundaries, rivers, lakes, mountain ranges, and coastlines, 
 are typically mentioned as references in describing weather locations 
 and movements. Acetate scope overlays showing these features are used 
 at the radar site, and remote display users may desire a similar tool 
 tailored to fit their display. Descriptions of weather types, intensities, 
 and tendencies are usually not so specific as in the RAREP, with terms 
 used in general public weather forecasts preferred. The radar observer 
 must be constantly aware that his report will be delivered to the public 
 as valid and timely until such time as he gets a subsequent report into 
 the hands of the news media. For example, a thunderstorm may move 
 20 miles or more between regular hourly reports, and this may be 
 significant in some localities. Unscheduled special reports are not 
 necessarily effective unless flexible and highly effective communications, 
 including necessary manpower, exist between the Weather Service office 
 and the various news outlets. 
 
 The remote display user can gain a broader and more complete 
 understanding of the scope display by reference to the narrative report 
 as well as the RAREP. Generally speaking, the type of commerce and 
 agriculture predominant in the radar surveillance area will be the same 
 as the remote display user will be concerned with, so the emphasis in 
 the report may be directly applicable to the use of the remote display user. 
 However, user stations should be aware that with their observation and 
 communication facilities they are uniquely fitted to provide the radar 
 station with much needed verification and amplification information. Remote 
 display users should be alert to necessary corrections and additions to 
 both RAREP and narrative reports, and should bring these to the attention 
 of the radar operator at the radar station. 
 
 4.3 REQUIRED STUDY 
 
 FMH No. 7, Part A, and Chapter 4, Part B. 
 
> 
 
 RADAR MEASUREMENTS AND REPORTS 
 
 67 
 
 Figure 36. When the PPI display is the only information available, it is 
 sometimes very difficult to identify and evluate the weather phenomena 
 depicted. In order to evaluate such a PPI display as this one, the radar 
 observer makes use of calibrated intensity attenuation, the A- and R- scope 
 for ranging and character, and the RHI scope and angle tilt for vertical 
 extent and configuration. He must use continuity considerations, and 
 peruse all pertinent weather reports available. 
 
68 
 
 RADAR MEASUREMENTS AND REPORTS 
 
 Figure 37. Through the use of the data insertion device (DID) available 
 on the WBRR, the radar observer can annotate the scope display that is 
 transmitted, providing the user with readily available necessary information 
 about the scope display. Shown here is the same scope picture as in 
 figure 36, but with annotations that very greatly enhance the usefulness 
 of the picture. Without this additional information the user could give 
 only the geographical location of echoes and make only very gross estimates 
 of their character and intensity. 
 
APPENDIX A 
 
 RADAR ECHO INTENSITIES 
 
 The intensity assigned to a radar echo is determined by the radar observer 
 employing an objective method based on an empirical relationship between 
 radar reflectivity and rain intensity. In order that the user may make most 
 effective use of these data it is necessary to have some idea of the physical 
 principles involved. Basically, the radar transmits a very short burst of 
 energy, then listens for that small portion of the energy backscattered 
 from the target. The power received from the target is a function of the 
 radar design parameters and the reflective qualities of the target. In the 
 case of precipitation targets, it has been shown theoretically that the 
 power which is backscattered to the radar antenna is proportional to the 
 summation of the sixth power of the drop diameter. This is expressed 
 mathmaUcally in the form: Z = D , where Z is radar reflectivity 
 
 mm m .* The average power received by a radar system from a 
 precipitation target is given by the following equation: 
 
 RCZ 
 
 P = -J (1) 
 
 r 2 
 
 r 
 
 This equation states that the average power received (P ) is directly 
 proportional to the power transmitted (Pt), the radar constant (C) and 
 the precipitation reflectivity (Z) and is inversely proportional to the 
 square of the range (r). Included in the radar constant (C) are the radar 
 design parameters such as antenna gain, vertical and horizontal beam 
 width, pulse length, wavelength, and a term for the complex index of 
 refraction. 
 
 Solving for radar reflectivity, equation (1) becomes: 
 
 p 2 (2) 
 
 P r^ 
 
 r 
 
 Z = ■ 
 
 P t C 
 
 *mm - millimeter 
 m - meter 
 
 69 
 
7 APPENDIX A 
 
 The final step is to relate the radar reflectivity derived from equation (2) 
 to rainfall rate (R). Over the past 15 years, numerous studies have been 
 conducted to establish the relationship of Z to R. Many factors are known 
 to affect the Z-R relationship, such as drop size and type of precipitation. 
 For example, a few large drops can produce reflectivity values as great 
 or greater than many small droplets. 
 
 Equation (3) proposed by Marshall and Palmer is considered to be 
 representative for most rains and is used by the National Weather Service. 
 
 1.60 
 Z = 200R (3) 
 
 where Z is in mm /m and R in mm/hr. 
 
 In the theoretical derivation of radar echo intensity a number of assumptions 
 are necessary. It is not within the scope of this discussion to mention all 
 of them, but we should point out the more important ones and how they 
 may affect the interpretation of these data. In equation ( 1) the power 
 received (P r ) is inversely proportional to the square of the range (r ). 
 This is an important assumption since it implies that the meteorological 
 target completely fills the radar sampling volume. In actual practice the 
 meteorological target does not always fill the beam, particularly at 
 extended ranges (see fig. 3). The range attentuation factor in the radar 
 equation for a point target would be 1 . Therefore, for a precipitation 
 
 r* 
 target, partially filling the radar beam might conceivably result in an 
 attenuation factor of 1 or some other variation of the exponent. The 
 
 r 3 
 net result is generally an underestimation of echo intensity at extended 
 ranges, particularly when the echoes are of limited vertical extent. 
 Because of this intensity error at extended ranges, radar intensity 
 measurements are not made at ranges beyond 125 nautical miles. Often 
 the effects of overshooting or partial beam filling occur well within the 
 125-mile limitation; the user should be cognizant of reported storm heights 
 and exercise his judgment accordingly. For example, winter storms 
 approaching the west coast of California are often quite shallow, having 
 tops to 12,000 or 16,000 feet resulting in a radar intensity classification 
 of weak when in fact it may be moderate. While the Z-R relationship 
 expressed in equation (3) is probably representative of most rains, it is 
 based on assumptions which do not always occur in nature. For example, 
 the target is assumed to be comprised of liquid, spherical rain drops of 
 average size and distribution. Actually the sampled volume may be 
 comprised of large flattened droplets or water coated ice spheroids or 
 any combination of these. The reflectivity or scattering properties vary 
 considerably with the precipitation state. Snow reflectivity is very 
 complex and is not clearly established. Therefore, intensities from snow 
 echoes are not determined and are reported in the RAREP with no intensity 
 symbol. 
 
APPENDIX A 
 
 71 
 
 In spite of these problems, the radar can and does give us a reasonably 
 good estimate of storm intensity to a range of 125 nautical miles. The 
 present method of classifying radar intensity in six categories (weak, 
 moderate, strong, very strong, intense, and extreme) has been fairly 
 well correlated with weather types and precipitation intensities. 
 
 Particular emphasis has been placed on the correlation of radar storm 
 reflectivity with the occurrence of severe weather. Radar echoes reported 
 in the "very strong" category correlate rather highly with the occurrence 
 of thunderstorms, hail, and damaging winds. 
 
 The use of the radar intensity data often leads the user to some conclusions 
 or assumptions that are not necessarily true. We should like to call your 
 attention to them: 
 
 1. The radar intensity classifications, while related to theoretically 
 derived rainfall rates, do not correspond directly with precipitation 
 intensity reported in regular surface observations. The criteria 
 for surface observations and radar are shown below: 
 
 SURFACE OBSERVATIONS 
 
 RW- 
 
 LIGHT 
 
 TRACE TO .1 IN/HR 
 
 RW- 
 
 LIGHT 
 
 LESS THAN .1 IN/HR 
 
 RW 
 
 MODERATE 
 
 . 1 TO .3 IN/HR 
 
 RW 
 
 MODERATE 
 
 .1 TO .5 IN/HR 
 
 RW + 
 
 HEAVY 
 
 GREATER THAN . 3 
 
 RW + 
 
 HEAVY 
 
 .5 TO 1.0 IN/HR 
 
 
 
 IN/HR 
 
 RW + + 
 
 VERY HEAVY 
 
 1.0 TO 2.0 IN/HR 
 
 
 
 
 RWX 
 
 INTENSE 
 
 2.0 TO 5.0 IN/HR 
 
 
 - 
 
 
 RWXX 
 
 EXTREME 
 
 GREATER THAN 5 . 
 IN/HR 
 
 The probability that the radar intensity will correspond with 
 rain-gage data directly under a given echo is not high. There 
 are several reasons for this: (1) The radar beam is sampling 
 at some distance above ground level. Due to fall velocity, 
 coalescence, trajectory^ and evaporation, there can be quite a 
 difference between the radar estimation and the precipitation 
 caught at ground level. (2) The radar estimate is an instantaneous 
 reading and is integrated over a large sample volume while the 
 rain gage samples about a square foot over many hours. 
 
72 APPENDIX A 
 
 In summary, ^adar can provide an estimate of storm intensity, based on 
 theoretical and empirical relationships, in six categories, operationally 
 useful to the user. Recent work by researchers show that point rainfal] 
 estimates are within a factor of 2 when compared to rain gage 
 measurements. Hopefully the day will come when radar will provide 
 precise rainfall measurements; however, at present a more realistic 
 view is to use radar to augment rain gage networks. 
 
APPENDIX B 
 REPORTING A TYPICAL WEATHER SITUATION 
 
 SI TUATION: 
 
 The Florida Peninsula and the shallow waters north of the Keys are 
 covered with scattered convective echoes. Some of the echoes over land 
 have been attaining moderate intensity during the past 2 hours, but they 
 have remained relatively small and there have been no reports of thunder. 
 The echoes over water are generally smaller in diameter and height, 
 and none have been observed to reach moderate intensity. Although the 
 life span of individual echoes is less than 1 hour, the outline of the area 
 covered has not changed significantly in either size or location during 
 the past hour. Movement of representative cells, checked over a 15- 
 minute period, is from 120 degrees at 11 knots. Some of the echoes 
 south of Lake Okeechobee have increased in size, height, and reflectivity 
 during the past hour, and some of them are now strong. The larger 
 cells move from 250 degrees at 13 knots, but tend to dissipate as they 
 move the east of the lake. New cells form and grow rapidly as the old 
 ones move out. The result is that the area covered by the stronger cells 
 remains in about the same location. 
 
 73 
 
74 APPENDIX B 
 
 EXPLANATION: 
 
 1. The strong echoes are reported first because they are considered 
 the most important (FMH No. 7, Part A, J5.2.2). 
 
 2. The rectangular area that incloses the strong echoes as well as 
 closely associated cells that are not necessarily strong, is judged 
 to have .6 of its area covered by echoes, hence the system and 
 coverage are reported as "AREA6 M (FMH No. 7, Part A, 315.2 
 and 5.3). 
 
 3. The intensity, shape, and size of some of the cells in the 
 rectangular area indicate they are thunderstorms (FMH No. 7, 
 Part A, JF5.4.1, 5.5, and 5.6). Although not all the cells in the 
 area are strong, the character and intensity of the strongest 
 echo in the area are reported as the characteristics of the 
 area. Since the intensity has increased by more than one 
 intensity category during the past hour, the intensity trend is 
 reported as increasing (FMH No. 7, Part A, JT5.6). 
 
 4. End points and width of the rectangle are reported as the most 
 economical means of describing the area (FMH 'No. 7, Part A, 
 316.1 and 6.5). 
 
 5. Since the rectangular area is greater than 50 miles in extent, the 
 location of the maximum top height is reported (FMH No. 7, 
 Part A, 318.3). 
 
 6. The moderate and weak cells comprise a system of echoes 
 probably related by meteorological processes, and can be outlined 
 by a fairly simple irregular shape (FMH No. 7, Part A, 3T6.1, 
 6.2 and 6.6). It is not necessary to make a note of the fact that 
 the area of strong echoes is entirely enclosed by the area of 
 moderate and weak echoes (FMH No. 7, Part A, 55.2.1). The 
 echoes over the water could, of course, be reported separately 
 as another area, since their characteristics are somewhat 
 different, but in this case the information can easily be conveyed 
 by a remark (FMH No. 7, Part A, 319.1). The northernmost 
 defining point is reported first and the others are reported in 
 clockwise order. 
 
 7. The maximum top was determined to be 28,000 ft, and other 
 increasing tops over land are nearing that figure (FMH No. 7, 
 Part A, Section 8). 
 
APPENDIX B 
 
 75 
 
 
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 Wl I MIA 7 ~ 
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 Cp • 
 
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 ftgs 
 
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 RAREP: 
 
 MIA 1735 AREA6TRW + /+ 337/65 282/50 60W 0000 CELLS 2513 MT 430 AT 320/69 
 AREA2RW/NC 341/165 6/70 212/70 231/110 321/175 0000 CELLS 1211 
 MT 280 AT 294/87 CELLS OVR WATER ALL WK WITH TOPS BLO 200 
 
 NARRATIVE REPORT: 
 
 MIAMI WEATHER SERVICE 061255E 
 
 AT ONE PM THE MIAMI RADAR SHOWED A WIDE BAND OF THUNDERSHOWERS 
 EXTENDING FROM THE SOUTHERN TIP OF LAKE OKEECHOBEE SOUTHWARD 
 TO NEAR THE TAMIAMI TRAIL AND SOUTHWESTWARD TO WITHIN TWENTY 
 MILES OF EVERGLADES CITY. ALTHOUGH THE ATLANTIC COAST WAS FREE 
 OF PRECIPITATION SMALL SCATTERED SHOWERS CONTINUED OVER THE 
 REST OF SOUTH FLORIDA AND A CROSS FLORIDA BAY TO THE KEYS . 
 
 » 
 
 Figure 38 
 
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PENN STATE UNIVERSITY LIBRARIES 
 
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