l55./dmNt^^ (a i NOAATR NESS 61 A UNITED STATES DEPARTMENT OF COMMERCE PUBLICATION **-" V NOAA Technical Report NESS 61 U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Environmental Satellite Service The Measurement of Atmospheric Transmittance From Sun and Sky With an Infrared Vertical Sounder W. L. SMITH AND H. B. WASHINGTON, D.C. September 1972 NOAA \l REPORTS National Environmental Satellite Service Series en I Satellite Service (NESS) is responsible for the estab- ■ it ion of the National Operational Meteorological Satellite Svstem and of satellite systems of NOAA. The three principal Offices of NESS are and Research. The NOAA Technical Report NESS series is - to facilitate early distribution of research results, data handling Lnalyses, and other information of interest to NOAA organizations. • of a Report in NOAA Technical Report NESS series will not preclude later in an expanded or modified form in scientific journals. NESS series of NOAA s is a continuation of, and retains the consecutive numbering sequence of 5enes, ESSA Technical Report National Environmental Satellite Center (NESC) ' series, Weather Bureau Meteorological Satellite Laboratory (MSL) Report - to 3; are listed in publication NESC 56 of this series. arts 1 to SO in the series are available from the National Technical Information apartment of Commerce, Sills Bldg., 52SS Port Roval Road, Springfield Va .2151. Price: $3.00 paper copy; $0.95 microfiche. Order by accession number^ when' i of each entry. Beginning with 51, Reports are available through 'the Super- intendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. I NESC 39. NESC 41. - :. NESC 43. NESC 44. C 45. .NESC 46. NESC 49. NESC 51. 52. ESSA Technical Reports Angular Distribution of Solar Radiation Reflected from Clouds as Determined from TIROS IV Radiometer Measurements, I. Ruff, R. Koffler, S. Fritz, J. S. "on, and P. K. Rao, March 1967. (PB 174 729) Motions in the Upper Troposphere as Revealed by Satellite Observed Cirrus rormarion, il. McClure Johnson, October 1966. (PB 173 996) Cloud Measurements Using Aircraft Time-Lapse Photographv, L. F. Whitnev ,Tr and E. Paul McClain, April 1967. (PB 174 728) The SINAP Problem: Present Status and Future Prospects. Proceedings of a Conference held at the National Environmental Satellite Center, Suitland, Md., January 18-20, 1967, E. Paul McClain, Reporter, October 1967. (PB 176 570) Operational Processing of Low Resolution Infrared (LRIR) Data from ES$A Satellites, Louis Rubin, February 1968. (PB 178 123) Atlas of World Maps of Long-Wave Radiation and Albedo -- For Seasons and ths Based on Measurements from TIROS IV and TIROS VII, J. S. Winston and V. Ray Taylor, September 1967. (PB 176 569) Processing and Display Experiments Using Digitized ATS-1 Spin Scan Camera Data, M. B. Whitney, R. C. Doolittle, and B. Goddard, April 1968. (PB 178 4^4) The Nature of Intermediate-Scale Cloud Spirals, Linwood F. Whitney, Jr. and Leroy D. Herman, May 1968. (AD-675 681) Monthly and Seasonal Mean Global Charts of Brightness From ESSA 3 and ESSA 5 igitized Pictures, February 1967-February 1968, V. Ray Tavlor and Jav S Winston, November 1968. (PB 180 717) A Polynomial Representation of Carbon Dioxide and Water Vapor Transmission William L. Smith, February 1969. (PB-183 296) Statistical Estimation of the Atmosphere's Geopotential Height Distribution From Satellite Radiation Measurements, William L. Smith, February 1969. (PB 183 297) Synoptic/ Dynamic Diagnosis of a Developing Low-Level Cyclone and' Its Satellite-^ Viewed Cloud Patterns, Harold J. Brodrick and E. Paul McClain May 1969 (PB 184 612) ' Estimating Maximum Wind Speed of Tropical Storms from High Resolution Infrared Data, L. F. Hubert, A. Timchalk, and S. Fritz, May 1969. (PB 184 611) :ication of Meteorological Satellite Data in Analysis and Forecasting, R. K rson, J. P. Ashman, F. Bittner, G. R. Farr, E. W. Ferguson, V. J. Oliver and A. H. Smith, September 1969. (AD-697 033) Data Reduction Processes for Spinning Flat-Plate Satellite-Borne Radiometers Torrence H. MacDonald, July 1970. hiving and Climatological Applications of Meteorological Satellite Data, John A. Leese, Arthur L. Booth, and Frederick A. Godshall July 1970 :-71-00076) Estimating Cloud Amount and Height From Satellite Infrared Radiation Data P. Krishna Rao, July 1970. (PB-194 685) Longitude Sections of Tropical Cloudiness (December 1966-November 1967) llace, July 1970. NOAA Technical Reports 1 rved Cloud Patterns in Northern Hemisphere 500-mb 1 lysis, Roland E. Nagle and Christopher M. Hayden. April 1971. on ' nsi de back cover) j.oWMOSp r ^ENJ Of C U.S. DEPARTMENT OF COMMERCE Peter G. Peterson, Secretary NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION Robert M. White, Administrator NATIONAL ENVIRONMENTAL SATELLITE SERVICE David S. Johnson, Director NOAA Technical Report NESS 61 The Measurement of Atmospheric Transmittance From Sun and Sky With an Infrared Vertical Sounder W.L.Smith and H.B.Howell t c i 1 > ■■s WASHINGTON, D.C. SEPTEMBER 1972 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. Price 30 cents. UDC 551.501.721:551.508.21:551.521.3:535-1 551.5 Meteorology .501 Methods of observation .721 Radiation observation .508 Instruments .21 Radiometers .521 Radiation .3 Transmittance in atmosphere 535-1 Infrared radiation i i CONTENTS Abstract. 1 1. Introduction 1 2. The ITPR instrument 1 3. Transmittance determination 2 A. From solar radiances 2 B. From sky radiances 3 C. The Mauna Loa experimental set-up and measurement technique . . . 4 D. Supporting conventional observations 7 E. The theoretical transmittance model 10 4. Results of the Mauna Loa observations . . . . 10 5. Conclusion and additional considerations . . . . 12 Acknowledgments 15 References 16 in Digitized by the Internet Archive in 2013 http://archive.org/details/measurementofatmOOsmit THE MEASUREMENT OE ATMOSPHERIC TRANSMITTANCE EROM SUN AND SKY WITH AN INFRARED VERTICAL SOUNDER* W. L. Smith and H. B. Howell National Environmental Satellite Service National Oceanic and Atmospheric Administration Washington, D. C. ABSTRACT. This paper describes the determination of atmospheric transmittance from ground-based sun and sky- radiance measurements with a prototype of the Nimbus E Infrared Temperature Profile Radiometer (ITPR). 1. INTRODUCTION The direct inference of vertical temperature and moisture profiles from spectral radiance observations requires precise knowledge of the atmospheric transmission properties of the atmosphere. The atmospheric transmission functions used in the numerical solution for atmospheric structure are usually modelled by using data obtained from laboratory measurements or by theoretical computation. Experience has shown that, to make meteorological inferences with the required accuracy from remote radiance observations, empirical corrections must usually be made. For spectral regions where molecular scattering is negligible, empirical determinations of atmospheric transmittance can be made from solar radiance and sky-emission radiance measurements. Such measurements, made at various altitudes and viewing angles, can be used to derive the dependence of trans- mittance on pressure, temperature, and gas concentration. This paper describes the determination of atmospheric transmittance from ground based sun and sky radiance measurements with a prototype of the Nimbus E Infrared Temperature Profile Radiometer (ITPR). The observations were obtained during November and December of 1971 from altitude 11,150 feet at the Mauna Loa Observatory, Hawaii. The measurements were conducted to check, and to correct as necessary, transmission models formulated from theoretical line-by-line calculations for application to the spacecraft ITPR measurements. 2. THE ITPR INSTRUMENT The ITPR is a seven-channel sounding radiometer that measures radiance in the 3.7- ym and 11-ym "atmospheric windows", in four different portions of the 15- ym CO2 band, and in an interval near 20ym In the rotational water vapor band. Spectral definition is accomplished using narrow-band interference filters. The spectral response functions for the ITPR are presented in figure 1. Its effective uniform response viewing aperture is 3.57 x 10 - ^ steradians. '^Presented at the International Radiation Symposium (IAMAP/IUGG) Sendai, Japan, May 26 - June 2, 1972. 1 — ' — i — ' — i — ' — i — ' — r ■ i.i.i.i "> — i -1 — r 440 460 480 500 520 540 560 860 880 900 920 940 2400 2500 2600 2700 2800 2900 3000 660 680 700 720 740 760 Wavenumber (cm~l) Figure 1. --Spectral response functions for the seven ITPR channels. 3. TRANSMITTANCE DETERMINATION A. From Solar Radiances The transmittance of the atmosphere can be specified from the radiances measured when viewing, in turn, the solar disc and the sky away from the solar disc, using the relation T(e) - i 3 (8) - i a (9) J o (1) where t(Q) is the transmittance of the atmosphere along the direction 9, I s (@) is the radiance from the sun and the intervening atmosphere measured by viewing towards the sun, I a (@) is the radiance from only the atmosphere measured by viewing away from the sun, and I is the effective solar radiance at the top of the atmosphere. The term 'effective ; is used here to indicate that I is the radiance of an extended blackbody source filling the instru- ment's aperture, which would produce the same response as viewing the solar disc from the top of the atmosphere. From a mountain platform, the trans- mittance is obtained as a function of gas concentration by observing the sun at various solar elevation angles. The effective solar radiance for each ITPR channel was computed from the solar brightness temperatures obtained, from the solar radiance values given by Thekaekara, Drummond et al. (1971). Table 1 summarizes these brightness temperatures, the corresponding Planck radiances, and the average radiances across the instrument's field of view which would be sensed by viewing the sun through a completely transparent atmosphere at Mauna Loa in December (i.e., the solid angle of 7.0 x 10"^ steradians subtended by the sun). Table 1. --Solar brightness temperature, solar radiance, effective solar radiance, and effective solar brightness temperature Channel Central wavenumber T s (°K) I s (mw/m .sr.cm ) I (mw/m .sr.cm ) T e (°K) (cm-1) 1 2679 5,798 242,460 2 899 5,046 29,562 3 747 4,945 20,477 4 714 4,245 15,881 5 690 4,245 14,881 6 668 4,869 16,332 7 507 4,897 9,948 47,541 2,210 5,796 1,435 4,015 1,354 3,114 1,194 2,918 1,183 3,202 1,307 1,951 1,265 B. From Sky Radiances The transmittance of the atmosphere can be derived from measurements of sky emission if the temperature and absorbing gas profiles are known. Given an a priori estimate of the transmittance profile of the atmosphere obtained from a theoretical model, the actual transmittance profile is assumed to b« given by In l-lnT{p, e ) j = in |-lnT(p, e ) +k Q +k 1 i n U (p,e) (2) where t(p» ©) is the true transmittance profile as a function of pressure p along the viewed direction 0, t (p,©) is the transmittance profile given by the theoretical model, and U(p 5 ©) is the absorbing gas concentration profile, The constants k Q and kj_ are parameters that account for the errors of the physical characteristics of the model; k accounts for the error of the logarithm of the absorption coefficient and k^ accounts for the error of the way in which transmittance in the model depends on gas concentration. Equation (2) states that x(p,e) = ;(p,e) a (p ' e) O) with , „, r 1 kl k ° a (p,e) [u( P ,e)j The error coefficients k Q and k-j_ can be related to the measured sky radiance and that radiance calculated with a theoretical model using the differential equation : dl (9) = 9I(k n ,O,0) 9k o dk Q '3l(0,k ls Q) dk ± (4) where I(k o ,0,9) is the radiance computed as a function of k Q using the modification of the theoretical transmittance given by (3) for k-, = 0, while l(0,k, ,9,) is that radiance computed as a function of k-j_ for k = 0. For an upward-viewing instrument the radiative transfer equation relating the emitted sky radiance to the transmission profile is i(e) = f ° b[t(p)1 d T ( P» Q) dp , J Po L J d P (5) where B [T( p )] is the effective Planck radiance profile for the spectral interval of measurement given by the temperature profile T(p). In this study, the error coefficients k Q and kj_ are determined from radiance measurements and calculated radiance values for four zenith angles (Sec = 1, 2, 4, and 8) through the solution of the set of four simultaneous equations given by the numerical form of (4); i(e.) - i(o,o,e.) =) Ko.25,o,e.) i (-0,25,0,9.) ( 0.5 o (6) + 1(0,0. 25, 9 i )- 1(0,-0.25,9,) ( k ( 0.5 \ where I(9j_) is sky emission measured at angle 6 (for Sec 9^ =1, 2, 4, 8) and 1(0,0,9) is the radiance computed using the theoretical transmission model (i.e., k = k]_ = 0). The transmission profiles for initial and resultant values of k Q and k-^ are computed using (3). For spectral channels where both CO2 and h^O absorb, the k Q and k-^ for C0 2 , the major constituent, were determined by assuming that the total transmittance is given by the product of the empirical CO2 transmittance and the theoretical H 2 transmittance. C. The Mauna Loa Experimental Set-Up and Measurement Technique The ITPR, mounted on a precision rotary table, could be leveled to within 1 minute of arc. Figure 2 is a block diagram of the experimental configura- tion. The entire apparatus, including the optical and electronic units, OUTSIDE ui + BLACKBODY r — h~ INSIDE 24 VDC PS HEATER PWR 24VDC OR ■ 110 VAC BENCH CHECK UNIT BLACKBODY PWR. a CONTROL JOSEPH KAY ICE POINT REF — 110 VAC PRINTER STRIP CHART REC TAPE CONTROL UNIT TAPE REC n 24 VDC PS HP DIFF VM Figure 2. — Block diagram of the ITPR Mauna Loa experiment. A-to-D converter, calibration blackbody assembly, and digital output display and printer devices were located on an observation platform that permitted unrestricted viewing of the solar disc from sunrise to sunset. The local zenith angle pertaining to a particular sun measurement is defined for the instrument location (latitude, longitude) by the time at which a maximum radiance was observed as the sun crossed the field of view of the instrument. The scan mirror was set at an angle corresponding to the predicted solar elevation. Small adjustments in azimuth were made during the observation period to place the solar image in the most responsive portion of the field of view. As soon as the maximum radiance was encountered, the instrument was rotated about 10° in azimuth to obtain a measurement of the sky radiance alone. The difference of the radiances obtained by viewing first the solar disc and then the clear sky provided the measure of solar radiance transmitted through the atmosphere along the path of view. The solar radiances measured during several cloudfree days at the Mauna Loa Observatory are presented in figure 3. Independent sky emission measurements were also obtained by scanning from local vertical to the horizon over a period of several minutes. In this case the elevation angle of the measurement was determined from the angle of the scan mirror, which is accurate to within less than 1 minute of arc. Small errors introduced by the uncertainty in the orientation of the scan mirror and the instrument baseplate were alleviated by rotating the instrument 180° en _ 1 1 1 1 1 III/ „ „ K IV - £ u CO "*t ON CM J - 1^ ^3" tr N. < * — m _c - u - - 1 1 1 1 1 ! 1 1 — ^r — o o o o o o o o o o o o o o o o o o o 00 ■o "3 ON o oo o ^r CN o o o o o o K -O *0 o o o o CO o o O O CM — ^UJD - JS - iu/mw) aDUDipDy ( _LUD-JS-jLU/MLU)aDUDipDy o o o o o O O o o o o o o o N o lO ~? ro CM 1 — lO iO iO Lf) ^O LO LO o o o o o o O O CO LO ""S "^ o o o o o o CO •O CM o •o CO CM X- r— r— ( JJLQ-JS-jUJ/MUU) 9DUDipDy ,Q E CD O CD O nd B rd & CD CD > o T3 t3 rd CO f* CM CO H a) c c o Pi PL, ,0 TJ CD co (13 £ n a) a a rd •H TJ Cti u tO H O co i i CO U P M •H and repeating the angular scan by viewing the same atmosphere through the opposite side of the viewing cavity. The average of the two scans provided sky emission radiance as a function of the absolute viewing angle. The radiances obtained in this manner are shown in figure 4. Measurements of solar and sky radiance are not shown for channels 1, 5, and 6. Channel 1 solar radiances could not be observed because the intense incoming energy could not be attenuated adequately or reliably. Useful emission data for channel 1 could not be obtained because the radiances measured when viewing cold space through the atmosphere were close to the channel noise level (0.006 mw/m 2 . sr . cm" 1 ). Channels 5 and 6, which are situated in relatively intense portions of the 15-um CO2 absorption band, did not provide useful sky emission data since the atmosphere is opaque to radiation measured in these channels. D. Supporting Conventional Observations Special vertical temperature and moisture profile measurements were made by radiosonde from the Hilo Weather Station during each radiometric observa- tion period. The portions of these profiles that were above the pressure altitude of the Mauna Loa Observatory (680 mb) are illustrated in figure 5. Surface observations were also obtained from continuous recorders at the observatory. The average values of surface temperature and water vapor mixing ratio adopted for each radiometric observation period are presented in table 2. Table 2. — Surface temperature and water vapor mixing ratio observations, total water vapor concentration, and radiosonde "tie-on" level Total water Date Temperature (°C) Mixing ratio (g/kg) vapor concentration ( pr . cm . ) " Radiosonde tie on level (mb) Nov. 12, 1971 13.1 6.8 0.229 676 Nov. 20, 1971 12.4 5.2 0.134 676 Nov. 21, 1971 12.8 5.3 0.177 679 Nov. 22, 1971 11.8 6.0 0.184 679 Dec. 1, 1971 11.5 7.2 0.544 677 Dec. 3, 1971 12.0 10.0 0.570 668 Dec. 4, 1971 12.1 10.5 0.544 677 "Precipitable centimeters 1-1 1 1 rv 1 1 _ §\\\ - 4 \ \ — - * v - \ - \v \ ^ T E _\ \ V 1 - u IX * t ^ \ • — rv ■<«■ rv •"J. — * \\\ \a U 1 1 1 1 1 * v - \ 1 H V \ — 00 - o — ^r 4 \ 1 1 1 1 i ' s 1 - 1 '. v \ E \ / °^ u -\\\ \ ^~ IV O _ 1 \ \ m :\\ x iv ~ u _ \ \ \ i » \\ \ \ \ — \ \ \ R \ - i\ \ iv \ \ cn 11/12/ \ \ •— ON *\ \ 1 "~ 1 \ \ 1 1 V \ o o O o o o o o o 00 ■O -3 CM o 00 ■o l _UID - JS " t Ul/MUj) SDUDipDy ( _uo - js - uj/mlu) aDUDjpoy 1 1 1 1 1 1 1 IV - *x s CM , — X \ s-\ - _ , — X r \ » - IV ro / N V \ '- CM V CM \ 1 - X \ 1- ► \ » 1 i " E \ \- u i O O w 00 »\ '» CM -C u 1 o CO o C J CD 0) > o G •H & 3 l> X) c ro CM W H a O Ph H CD 3 co rO CD M CD o rB •H TJ rO Pi a o ■H W co , O ^ a> I I CD & M ■H O) -^. E D O) c X Q E Ct> (quu) ajnssajj CD O s: ., i Pn CD /■: -H O co o •H (-' m M C •H X •H E r: ,n co cd U CD D, u 0) -P > CD w o I CD Xi P o CO • O CO •H T3 T3 O rti -H S^ ^ CD « ft •H •H C rd o £ -H rfl +-> K re > O CD H CO •H ,q m O I 1 ,i > CD U txO •H U-t :: There usually are dramatic differences between the surface observations and the radiosonde observations obtained in the free atmosphere at the observatory's 680-mb pressure altitude. These differences are due primarily to severe ground heating induced by the black lava ground cover, the evaporation of water stored in the lava, and the mountain-induced upslope circulation that usually prevails during the day. As a result, the surface observations had to be included as part of the vertical profiles for comput- ing sky radiances. This was done by linear interpolation between some radiosonde "tie on" level values and the surface values. The tie on level was taken as the level that resulted in the minimum r.m.s. difference, for all channels , between the observed sky radiances and those computed for each tie on level using the model adjustment values k and kj_ . The tie on levels are given in table 2. E. The Theoretical Transmittance Model The transmittance model to be used for interpreting the Nimbus E ITPR data has been formulated by Davis (1972) from the technique outlined by Godson (1963). In the Godson technique, the transmittance for a given spectral channel is expressed in terms of a function that represents a combination of the regular and random band models. The model parameters are determined from CC>2 and H2O transmittances obtained by direct line-by-line spectral integrations for homogeneous conditions. The temperature-dependent weighting factors required to convert non-homogeneous paths to equivalent homogeneous paths are based on summaries of line intensities and line half widths for the appropriate spectral intervals. Differences between the atmospheric transmission functions specified by this model and those obtained by "exact" vertical integrations are generally less than 2 percent. 4. RESULTS OF THE MAUNA LOA OBSERVATIONS Figures 6 to 12 show the total atmospheric transmittance derived from all the solar radiance data and from the sky emission data for sec 9 = 1, 2, 4, and 8 (9 - 0°, 60°, 75.5°, 82.8°). The H 2 concentration was computed as the product of the total precipitable water obtained from the conventional data and the appropriate geometric factor for the angle of observation. The CO2 concentration along the zenith direction was computed assuming a uniform mixture of .031% by volume. As shown, there is generally good relative agreement between the two independent measurements of total atmospheric transmission. The excessive scatter of the transmission data in the window channel (fig. 6) is due to the fact that observed radiances usually are far out of range of the to 100°C target radiances used to calibrate the data. Consequently, small errors in the calibration slope induced, for example, by small variations of detector temperature will cause errors in the calibra- ted radiance. The magnitude of these errors is proportional to the deviation of the target temperature from the mid-point of the calibration range. In fact, some of the emission data of channel 2 was disregarded because of negative radiances that resulted from this calibration problem. Some of the scatter also is due to errors in conventional observations of atmospheric water vapor concentration. 1 ] 1 •*t 1 r~ Y~ ~~ I ■ 1 1 1 1 1 1 1 1 — 1 F CO o # — u ii o o 00 * < o CN U Q. X 0) II * ■ ■ 1 /* - « * J «/ < < - - § i DATA 20/71 CN SION 22/71 '3/71 '4/71 - 1 1 \ 1 1 SUN o 11/ ■ £>cn cn~ s-^ - LU D> « * 1 1 1 1 1 -,o 00 ^r o o CN co ■st O cs o CN CO CO i\ K N X O •;. 'i i 13) <1> r-i & o) ■ii a ■i i a) . o co si I > rd PI P 0) -P f.: •H CD t: £j CO 3 C CO in (-1 u CD -p e H CO rd -P H O a> Eh 1 1 PI Hi • rd I> o 1) p^ ?H PL, pi H bO H •H Em PI PI O •H •H bO PI CD O & ■P £ rd o Sn T3 -P P PI •H CD £ o E 1 1 o o u H O '-> H c\ig k. K O -P ■p •H p4 pi g Ph CD Q_ co E-i O PI \— \ rd CD & >, H 1 l rd x) -p o CD -H UD 3 «H a) CO o - ^ rrj cu CD k bf; g a, ■ H Pm 12 The calibration problems of channel 2 do not exist for channels 3, 4, and 7. For channels 3 and 4 (fig. 7 and 8) the total transmittances are plotted against the sec 9, since absorption is due to both CO2 and H2O. The wide range of the emission data shown here is due to the large difference in H^O concentration between November 12 and December 1, 3, or 4 (see table 2). One interesting feature in figure 7 is the apparent change in the stratifi- cation of the atmosphere between the sec 9 values of 1.9 and 2.1 on December 3. This relative drop in transmittance probably was due to the presence of thin cirrus cloud overhead, which is hard to detect visually at high sun. Aside from this feature, excellent relative agreement between the various observation days was obtained for both channels 3 and 4. To isolate C0„ components of total absorption in channels 3 and 4, the CO2 transmittance was estimated by dividing the observed total transmittance by the theoretical- H2O transmittance. The results are displayed in figures 9 and 10. For channel 4 (fig. 9) there is excellent agreement between the CO2 transmittance estimated from the solar observations, sky emission observations and those predicted by the theoretical model. For channel 3 (fig. 10), how- ever, the agreement is quite poor. The agreement between the COj trans- mittances derived from the sun measurements and the emission measurements appears to be poorer than the agreement between the corresponding values of total transmittance (fig. 7); this indicates that the theoretical water vapor transmittances are in error. To verify this, the total transmittances were re-derived from the sky radiances by assuming the model transmittances for CC>2 to be correct; that is, k Q and k-j_ were derived for the h^O trans- mittance model. Then the total transmittance values obtained from both the sun and emission radiance observations were derived by the theoretical CO2 values to obtain observational estimates of the H2O transmittance. The resultant transmittance values derived from the sun and emission observations are shown in figure 11 together with the curve predicted from the theoretical model. Here there is excellent agreement between the sun- and sky-emission data, but drastic disagreement of both with the theoretical estimate. Thus, it is concluded that the model for water vapor transmittance for channel 3 is in error and some parameter adjustment is necessary. Figure 12 shows the results obtained for the water vapor channel. As shown, there is good agreement of both the sun- and sky-emission data with the theoretical model. The small degree of bias between theory and observa- tions for H2O concentrations between 0.5 to 1.5 precipitable centimeters (pr.cm.) is probably within the error of estimation of H„0 concentration. 5. CONCLUSION AND ADDITIONAL CONSIDERATIONS The Mauna Loa experiment has shown that ground-based sun and sky radiance observations with a satellite-prototype sounder can be used to determine atmospheric transmission for its spectral intervals of observation. Such observations can be used in conjunction with theoretical values or with empirical data to construct accurate analytical models of atmospheric trans- mission functions for application to the spacecraft soundings. Reliable radiances could not be observed at Mauna Loa for the atmospheric window spectral regions because the observed radiances were outside the 13 — 1° _ o O z - < s< *■< '1 o- <* CM CO cm -C — CM 2 = 7 ^ CM CM CM Q y 4*ga *** _* 9*« J I I I I I I L J_ oo o -O O o CM o C Q c o U o u z ODl o; i < u o Mh cu o CI n1 H P •H t: w s 4 ' CN o o a; > •H !m Q • i d- • H cr> a) QJ C U (ti 3 rG W) O •H E o U M-i Tl a) & U cu 4h C •H il •H a; 6 & w 3 t: r;l m m & (U +-> H H Hr (Tl •P H O id E-i 1 a l m • .r: cx> o a> (>; p. fl, -J Eh hn M •H U, 14 o o u~> O l H "J _ o O Id o 0) Eh O Mh 4-> •H E M id to p. +-> o CM CD > ■H & 0) Q I H en C ■p ■H E GO c rd U 4-> CM O o 0) > •H P. Q) Q I I CO O rH H CD C CD C P. id faO O •H v> V-> .2- "I I n r SUN DATA 1 1 1 — i — i i Ch 7 (507 cm" 1 ) l i i i i i i r EMISSION DATA 1/12/71 ■ 11/21/71 a 12/3/71 * 12/4/71 Model '-N-J . -.4 -.3 5.0 Precipitable Water (cm) Figure 12.— EUO transmittance inferred from ITPR channel 7 measurements. dynamic range of these channels. Instrument gain reduction and a larger calibration blackbody temperature range should overcome this difficulty. ACKNOWLEDGEMENTS This experiment was funded by the National Aeronautics and Space Admini- stration. The authors convey their thanks to Harry Press of NASA for his support . We also thank J. Kapsch, F. Mignardi and J. Fischer for their technical support given in arranging and conducting the Mauna Loa Experiment. P. Pellegrino of this Laboratory deserves recognition for taking and reducing the supporting radiosonde observations. We also thank him, L. Mannello, W. Kohri and R. Ryan for their assistance in the reduction and analysis of the radiance data. Our special thanks go to R. Pueschel and his staff at the Mauna Loa Observatory and Mr. Busniewski and his staff at the Hilo Weather Station for their cooperation and warm hospitality. 16 REFERENCES Davis, Paul A., "Transmittance Representation for Application for ITPR Measurements," Final Report , NOAA Contract 2-35136, SRI Project 1385, Stanford Research Institute, Menlo Park, California, 1972, 45 pp. Godson, Warren L. , "Infrared Transmission by Water Vapor," Archiv. fur Meteorologie, Geophysik, und Bioklimatologie , Series B. Band 12, Heft 2, 1963, pp. 196-223. Thekaekara, Matthew P., Drummond, Andrew J., Murcray, D. G. , Gast , P. R. , Laue, E. G. , Willson, R. C, "Solar Electromagnetic Radiation," NASA Report SP-8005, GSFC, Greenbelt, Maryland, May 1971, 33 pp.