^^^^^^^^^^^^^^^^^— 
 
 
 NOAA Technical Report ERL 376-AOML 22 
 
 A Comparison of 
 Satellite-Observed 
 Sea-Surface Temperatures With 
 Ground Truth in the Indian Ocean 
 
 Ants Leetmaa 
 Matthew Cestari 
 
 August 1976 
 
ATMOSP^ 
 
 r Mtm Of 
 
 NOAA Technical Report ERL 376-AOML 22 
 
 A Comparison of 
 Satellite-Observed 
 Sea-Surface Temperatures With 
 Ground Truth in the Indian Ocean 
 
 Ants Leetmaa 
 Matthew Cestari 
 
 Atlantic Oceanographic and Meteorological Laboratories 
 Miami, Florida 
 
 August 1976 
 
 U.S. DEPARTMENT OF COMMERCE 
 
 £ Elliot Richardson, Secretary ^o^'Q/v 
 
 o 
 
 ° National Oceanic and Atmospheric Administration 
 
 w" Robert M. White, Administrator 
 
 .-§ \ 
 
 o Environmental Research Laboratories "^ - 1916 "- 
 
 » Wilmot Hess, Director Boulder, Colorado 
 
 m, 
 
 a 
 
NOTICE 
 
 The Environmental Research Laboratories do not approve, 
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 material mentioned in this publication. No reference shall 
 be made to the Environmental Research Laboratories or to this 
 publication furnished by the Environmental Research Labora- 
 tories in any advertising or sales promotion which would in- 
 dicate or imply that the Environmental Research Laboratories 
 approve, recommend, or endorse any proprietary product or 
 proprietary material mentioned herein, or which has as its 
 purpose an intent to cause directly or indirectly the adver- 
 tised product to be used or purchased because of this Envi- 
 ronmental Research Laboratories publication. 
 
 n 
 
CONTENTS 
 
 Page 
 
 1. INTRODUCTION 1 
 
 2. THE SATELLITE-OBSERVED SEA-SURFACE TEMPERATURE MAPS 2 
 
 3. SEA-SURFACE TEMPERATURE VARIATIONS ACCORDING TO SATELLITE DATA 3 
 
 4. COMPARISON OF SATELLITE DATA WITH SHIP REPORTS 5 
 
 5. COMPARISON OF SATELLITE DATA WITH 1963 SURFACE OBSERVATIONS 8 
 
 6. SUMMARY 10 
 
 7. REFERENCES 10 
 
 m 
 
Digitized by the Internet Archive 
 
 in 2013 
 
 http://archive.org/details/comparisonofsateOOIeet 
 
A COMPARISON OF SATELLITE -OBSERVED SEA-SURFACE 
 TEMPERATURES WITH GROUND TRUTH IN THE INDIAN OCEAN 
 
 Ants Leetmaa 
 Matthew Cestari 
 
 Daily worldwide sea-surface temperature maps are produced by 
 the National Environmental Satellite Service. For the first half 
 of 1975, sea-surface temperatures recorded on these maps were com- 
 pared with concurrent ship observations in the Indian Ocean. Addi- 
 tional comparisons were made with historical data. These show sys- 
 tematic differences between the satellite and sea-surface observa- 
 tions. The satellite-derived temperatures appear to be too low 
 along the equator and along the East African coast in the vicinity 
 of the equator. Furthermore, in April, May, and June the areas off 
 the equator (and not along the coast) appear to have temperatures 
 that are too high. Although the mean differences are not large 
 (l°-2°C), the fact that the errors vary in time and space made it 
 difficult to apply the satellite data for oceanographic interpre- 
 tations. 
 
 1. INTRODUCTION 
 
 Numerical experimentation has shown that the tropics are an important 
 area for interactions and feedbacks between the ocean and the atmosphere. 
 From present planning, it is clear that during the First GARP Global Experi- 
 ment (FGGE) equatorial regions will receive special attention in the ocean 
 as well as in the atmosphere. The Indian Ocean, because of the monsoons, 
 will also have a special observing period during FGGE, the Monsoon Experi- 
 ment (MONEX). 
 
 Because of the importance of equatorial regions to climatic studies, 
 and because FGGE will provide relatively complete meteorological coverage, 
 a group of oceanographers has started planning an Indian Ocean Experiment 
 (INDEX). The primary goal of INDEX will be to study the transient reponse 
 of a low latitude ocean to a strong regular forcing by the atmosphere. Pilot 
 experiments, whose results will aid in the design of the final experiment, 
 are now taking place. Sea-surface temperature maps from satellite data could 
 be a valuable tool to study the onset of the Somali Current, upwelling along 
 the Arabian coast, and heat budgets in the Arabian Sea. At the present time 
 such maps are available from the National Environmental Satellite Service. 
 However, as with e^jery new product or technique, they have to be examined 
 carefully to ascertain their limits of accuracy and applicability. This 
 study reports on a number of intercomparisons between the satellite-observed 
 sea-surface temperatures and "ground truth" in the Indian Ocean during the 
 first half of 1975. The results suggest that more work has to be done before 
 reliable sea-surface temperatures can be obtained from satellites. 
 
2. THE SATELLITE-OBSERVED SEA-SURFACE TEMPERATURE MAPS 
 
 The National Environmental Satellite Service provides daily worldwide 
 satellite sea-surface temperature (SSST) maps. This product is known as 
 the Global Sea-Surface Temperature Computation (GOSSTCOMP). One form of this 
 is an uncontoured computer printout with sea-surface temperature values for 
 each one-half degree of latitude and longitude. With each numerical value 
 for temperature is a code that indicates the estimated reliability of the 
 data. If the code is "+4" , then the last reading had been taken four days 
 before the date of the map, etc. If the number of days exceeds nine, the 
 code space is blank, and the temperature value given is from historical data. 
 If data are available for the day of the map, a letter appears in the code 
 space. An "+A" indicates that the temperature listed is an average of five 
 readings. A "+B" indicates an average of five to eight values and so on up 
 to "+H" which indicates that over 25 values were averaged. The better maps 
 in our analysis had mostly D's through H's associated with the temperature 
 readings. 
 
 For this s 
 to represent an 
 whole week beca 
 weekly represen 
 start with the 
 selected was co 
 Africa from 6°S 
 the collection 
 month to illust 
 
 tudy, the daily map with the highest code letter was selected 
 entire week. One day was chosen to be representative of a 
 use changes from day to day were observed to be small, and 
 tations were more readily compared than daily maps. They 
 week of January 3-9 and end with June 1-7, 1975. Each map 
 ntoured in the area of the Indian Ocean off the coast of 
 
 to 15°N and 35°W to 65°E in latitude and longitude. From 
 of maps, one was selected from the early portion of each 
 rate any monthly differences (Figs. 1-3). 
 
 50° E 
 
 IO°N 
 
 IO°N 
 
 FiguAz J. SatdUUtz A&a-iuAfiaco. Vaj^vJkI 2. ScuteZtLte. Ada-AuA&acz 
 
 twpoMVbjJio. data {on. JanaaAy 197S. twp&WLtuAn data {ok Vebtiuany 1975, 
 
3. SEA-SURFACE TEMPERATURE VARIATIONS ACCORDING TO SATELLITE DATA 
 
 The seasonal variations of sea-surface temperatures in the Indian Ocean 
 is strongly related to the NE and SW monsoons, the transition periods be- 
 tween them, and the ocean current systems established by the winds. The 
 features observed on the maps must be interpreted in the context of these 
 phenomena. Figure 1 shows that there was not a wide range of temperatures 
 in January. Most of the readings were either slightly greater or less than 
 26°C. On either side of the equator the temperatures are somewhat warmer 
 than at the equator. During February (fig. 2), the sea surface immediately 
 north and south of the equator warms, while temperatures at the equator re- 
 main cool, as in January. North of approximately 8°N the temperatures begin 
 to decline with areas containing temperatures lower than 24°C. This is colder 
 than in January. In March (fig 3) the same pattern persists, but a warming 
 trend is evident. The area of the equator continues to remain cool, and the 
 areas immediately north and south of the equator (5°S-8°N) are warmer. 
 Larger areas of 28°C and higher temperatures are visible, with 30°C tempera- 
 tures reported for some locations. Temperatures for April (fig. 4) show an 
 increased warming trend, with many areas containing temperatures of 30°C and 
 higher. 
 
 50° E 
 
 10° N 
 
 10° N 
 
 VIquaq. 3. SatoJUUX.il i>oji-i>uJi{ i a.c.<i torn- FsLguAe, 4. SatolZltz Aoa-AuAfiace, tem- 
 poAatvAn data ^on. Ucuich 1975. poAatiVm data faofi kphJJL 7975. 
 
Temperatures in the area of the equator, as noted in all previous 
 months, remain between 26°C and 28°C. Areas immediately north and south of 
 the equator have become considerably warmer. In the north there are isolated 
 areas with temperatures higher than 31°C. The warmest month from January 
 to June, 1975, is May (fig. 5). Again the area at the equator remains cool. 
 Large areas of 30°C and higher temperatures are visible north and south of 
 the equator. The north exhibits a slight cooling trend in June (fig. 6). 
 The equatorial band remains cool, and areas to the north and south become 
 cooler. Fewer and smaller areas of 30°C and higher temperatures are still 
 present, and the major portion of the entire region contains temperatures 
 of 28°C or slightly higher. The areas of 30°C and higher temperatures seem 
 to have moved toward the north and south, away from the area immediately 
 north and south of the equator. In all months, temperatures along the East 
 African coast were cooler than those offshore. 
 
 On first glance the seasonal variation in the sea-surface temperature 
 pattern as seen from these maps appears to be reasonable. The transition 
 period between the northeast and the southwest monsoon occurs during March. 
 April, and through the middle of May. A major factor in the heat budget of 
 the surface layers is evaporation, which is proportional to wind speed. Dur- 
 ing the transition, the evaporation decreases and the sea-surface temperature 
 increases. The cooler coastal areas could be related to upwelling or to 
 north-south transport of cooler water by the Somali Current along the coast. 
 A feature that is anomalous, however, is the cool band of water along the 
 equator. In the Pacific and Atlantic Oceans, such a cool band is indicative 
 of equatorial upwelling. However, in the Indian Ocean the winds are not 
 favorable for upwelling, and this feature is rarely present. To examine the 
 validity of this indication and others in more detail, a comparison was made 
 of these satellite data with data from a number of other sources. 
 
 50° E 
 
 IO°N 
 
 IO°N 
 
 T^guAn 5. SateJtZitz tm-AuA&aco. torn- VIqul/lh 6. SatoZtut<i &QjOL-t>vJi{axL2. tem- 
 p&icutuAz data ^on May J 9 75. p&tcutuAd data ^OK June. 7 975. 
 
4. COMPARISON OF SATELLITE DATA WITH SHIP REPORTS 
 
 We can compare the satellite data with actual ship observations obtained 
 at the same time in the same area. For February through May 1975 data are 
 available from a chartered research vessel, La Cu/u.eo4£, in the vicinity of 
 the equator. Bucket thermometer readings (estimated accuracy ±0.2°C) were 
 taken periodically along 55°40'E from 3°S to 2°N. The National Weather Ser- 
 vice also provides information on air temperature, dew point, and sea surface 
 temperature at ship positions through its twice-daily surface-weather maps. 
 
 There were 106 cases in which bucket thermometer readings from La 
 Cu/UeuMi could be compared with data from the satellite. The mean differ- 
 ence for the whole data set was +0.4°C. Temperatures recorded from the 
 ships were, on the average, higher. The standard deviation was 0.9°C. 
 This indicates that the scatter was quite large. The mean difference actu- 
 ally is rather small. However, if the data are studied in more detail, it 
 becomes clear that there are obvious trends. The satellite temperature data 
 for the equator are always lower than the surface observations and the differ- 
 ence becomes greater as time goes on. For example, for all intercomparisons 
 (32) in the region from 0.5°S to 0.5°N the mean difference is +0.9°C. The 
 standard deviation is 0.7°C. Clearly the equator is systematically colder in 
 the satellite data. This conclusion supports our previous speculations. 
 
 The satellite data were also compared with merchant ship reports. Un- 
 fortunately, there is a wery limited amount of ship data available in real 
 time. Also, frequently only the air temperatures are available rather than 
 the sea-surface temperatures. In the tropics this is not a serious problem 
 because the differences between these are usually small. Thus, to maximize 
 the data set, the ship reports were compared three ways. First the Satellite 
 Sea Surface Temperatures (SSSTs) were compared with the reported air tempera- 
 tures. The SSSTs were then compared with the sea-surface temperatures, and 
 finally with air and sea temperatures that were within one degree of each 
 other. Approximately 270 ship reports were available in the period from 
 January to JUne 1975. 
 
 The difference between air temperature from ship reports and satellite 
 sea-surface temperature was determined from three 2-month groups. For Janu- 
 ary-February, the mean difference was +0.85. For March-April it was -0.59, 
 and for May-June it was -0.97. This indicates that the satellite tempera- 
 tures are lower than actual temperature measurements for January and February, 
 but higher than actual for March-April and May-June. These differences also 
 appear to have a geographic dependence. Figure 7 shows the geographical 
 distribution of these differences. For May-June the satellite reads low 
 in the vicinity of the equator and along the Somali coast, and high else- 
 where. March-April shows the same trend. In January-February the satellite 
 reads systematically low almost everywhere. 
 
 The difference between sea-surface temperatures from merchant vessels 
 and SSST was investigated for only May-June because not enough data were 
 available for other months. The geographic distribution of the differences 
 is shown in figure 8. Again the satellite appears to read low in the vicin- 
 ity of the coast and the equator and high elsewhere. 
 
40° E 
 
 5CV 
 
 60' 
 
 
 \l_UW 
 
 
 
 
 
 
 
 
 
 
 
 
 1 
 + 1.4 
 
 3 
 
 + 1.23 
 
 
 
 
 
 
 
 1 
 -0 5 
 
 
 1 
 + 1.0 
 
 
 2 
 
 + 1.08 
 
 
 
 
 4 
 + 0.7 
 
 1 
 
 
 
 2 
 +1.7 
 
 
 
 
 
 
 + 3.8 
 
 
 
 1 
 
 + 1.3 
 
 
 2 
 
 +0.1 
 
 
 + 1.0 
 
 
 
 
 + 28 
 
 2 
 + 2 OS 
 
 -0.1 
 
 +1.4 
 
 2 
 + 1.4 
 
 
 
 
 
 
 
 
 -0.1 
 
 1 
 
 + 1.0 
 
 + 5.0 
 
 
 
 
 
 
 + 1.8 
 
 
 
 
 + 1 
 
 1 
 
 + 1 < 
 
 
 
 -1.5 
 
 
 
 
 
 
 
 
 1 
 
 ♦205 
 
 
 
 -1.0 
 
 1.3 
 
 + OIS 
 
 It 
 +0.84 
 
 
 
 
 IO°N 
 
 ^ 
 
 
 
 1 
 
 • 
 
 
 
 -3.8 
 
 1 
 -3.3 
 
 
 -1 
 
 4 
 
 
 
 'as 
 
 
 
 
 
 
 4 
 -1 
 
 
 
 1 
 + 1.3 
 
 
 
 
 
 
 
 
 
 1 
 + 1.2 
 
 
 
 
 
 
 2 
 
 + 0.2 
 
 
 1 
 +0.2 
 
 
 
 2 
 
 -048 
 
 I 
 -0 
 
 24 
 
 1 -0.73 
 
 1 
 -0.08 
 
 IO°N 
 
 January-February 
 
 March-April 
 
 
 N_UW 
 
 
 
 1 
 -I.I 
 
 
 
 
 
 
 
 
 1 
 -2 « 
 
 
 
 
 
 
 
 
 
 2 
 -1.8 
 
 
 
 
 
 
 
 ft 
 
 4 
 -1.03 
 
 1 
 -1.1 
 
 
 
 
 
 
 
 
 ^+1 7 
 
 1 
 + 1.1 
 
 2 
 
 +085 
 
 
 
 
 1 
 + 03 
 
 +08 
 
 
 
 
 
 
 2 
 
 +2.7 
 
 1 
 
 + 1.3 
 
 1 
 +08 
 
 
 1 
 -1.8 
 
 1 
 -1.3 
 
 1 
 + 3.8 
 
 
 
 
 
 1 
 +2.» 
 
 2 
 -188 
 
 
 
 
 1 
 -0 8 
 
 
 
 
 
 
 8 
 -1.18 
 
 t 
 -2.1 
 
 -03 
 
 1 
 -1.0 
 
 
 
 
 
 
 
 
 i 
 
 -1.8 
 
 4 
 -03 
 
 1 
 
 + 8 
 
 1 
 
 + 1.2 
 
 
 48 
 
 -1.7 
 
 2 
 -1.88 
 
 
 
 1 
 -3.4 
 
 IO°N 
 
 May-June 
 
 flgu/iz 1 '. Gzognaphlc (Lu&UbiitLon ofi di^zAznd-ZA between 
 cua tzmpzAatuAU and batzVLitz bza-i>vJi\adZ tzmpzhatuAZA , 
 Thz uppzA numbzA In za.dk 6quaAz -LndidatzA thz numbzA o{) 
 compasiiAonA . 
 
40° E 
 
 50' 
 
 60' 
 
 
 \ 
 
 
 _ 
 
 u 
 
 | 
 
 
 t 
 -2 2 
 
 
 
 
 
 
 
 
 f- 
 
 ) 
 
 1 
 +0.8 
 
 
 
 
 
 
 
 
 t 
 -2 1 
 
 
 
 
 
 
 
 por 
 
 fro.73 
 
 4 
 -1.28 
 
 1 
 -2 2 
 
 
 
 
 
 
 
 
 
 2 
 ■JO 45 
 
 
 
 
 1 
 + 1.3 
 
 1 
 + 18 
 
 
 
 
 
 
 + 3.1 
 
 -0.7 
 
 
 
 
 1 
 
 +0.7 
 
 1 
 + 08 
 
 
 
 
 
 
 2 
 
 -1.2 
 
 
 
 
 
 
 
 
 
 
 3 
 -2 23 
 
 4 
 -1.2 
 
 -0.3 
 
 1 
 + 1.0 
 
 
 
 
 
 
 
 
 1 
 
 -0.5 
 
 3 
 
 -0.17 
 
 1 
 + 0.8 
 
 
 
 
 1 
 -0.2 
 
 
 
 1 
 -2.4 
 
 IO°N 
 
 V-Iquaq. S. Gtognapkic ttiAfsiibuutLon o& difi&oAmceA bd- 
 £w<i2.n Ae.a-Atin&acn tompeAatuAOA {ftom m&ichawt Akipi 
 and i><JutolZlt<i A&a-iuA&acLd tmpeAcutuA&A {on. Uay-Jun& 
 1975. 
 
 For a final intercomparison, we computed the monthly mean (SSST) for 
 April, May and June, 1975 for the areas: REGION I - 12°-13°N, 54°-56°E; 
 REGION II - 11°-12°N, 54°-55°E; REGION III - 10°-11°N, 53°-55°E (fig. 9). 
 We compared these with the 21-year mean temperatures in these regions as de- 
 rived from all ship reports on file at the National Climatic Center. These 
 were computed by Fieux and Stommel (1975). The results are presented in 
 Table 1. 
 
 It should be noted that the satellite data, except for one comparison 
 always give higher temperatures than the historical data do in this region. 
 In April and June the satellite temperatures are nearly always two standard 
 deviations away from the historical temperatures. 
 
50°E 
 
 55° E 
 
 - I3°N 
 
 - IO°N 
 
 VtauJie. 9. Ro.Qi.ovii, faon. wklck c.ompa/uj>ovu> 
 u)QA£ made, between hatoJUUutQ. i>e.a-t>vJi- 
 fiaae. tempoAatuAe. data, avid kiitotUcal 
 bhip data. 
 
 Table. 7. Compa/vUon ofi AveAage. SSST'6 with HtitotvLcal Skip Data, 
 
 SSST SHIPS 
 Apri 1 
 
 SSST SHIPS 
 May 
 
 SSST SHIPS 
 June 
 
 REGION 
 
 28.49 28.52 29-64 29.51 29-32 27-90 Mean Temp. 
 
 0.48 
 
 0.66 
 
 0.86 Standard Deviation 
 
 REGION I I 
 
 29-25 28.38 30.34 29-50 29.74 27-17 Mean Temp. 
 
 0.60 
 
 0.78 
 
 0.90 Standard Deviation 
 
 REGION I I I 
 
 29-93 28.84 30.82 29.43 29-49 26.65 Mean Temp, 
 
 0.56 
 
 1.06 
 
 1.20 Standard Deviation 
 
 COMPARISON OF SATELLITE DATA WITH 1963 SURFACE OBSERVATIONS 
 
 From the International 
 sea-surface temperature for 
 surface observations from s 
 fig. 10) may 'be compared wi 
 obvious difference is that 
 temperature minimum. Also 
 along the coast sea-surface 
 are in the satellite data. 
 that the International Indi 
 
 Indian Ocean Expedition, data are available on 
 1963 (Wyrtki , 1973). These were accumulated from 
 hips. Maps for January through June 1963 (see 
 th satellite temperature maps from 1975. One 
 in January-June, 1963, there was no equatorial 
 in the 1963 data, there is no indication that 
 temperatures are lower for January-April as they 
 Another difference between the two data sets is 
 an Ocean Expedition maps for April and May contain 
 
 8 
 
50° E 60° 
 
 January 
 
 50° E 60° 
 
 y-s>25 
 
 y^\~^ 
 
 ""Nr 
 
 
 £5 <l — 
 
 c 
 
 
 k^ 
 
 
 I0°N 
 
 February 
 
 I0°N 
 
 March 
 
 April 
 
 
 .29' 
 
 *? 51 
 
 ^ 
 
 ^ 
 
 ^ 
 
 May 
 
 I0°N 
 
 June 
 
 VIqujkl 10. Sza-AuAfacz tmpoxatvJtd. data fan, 1963 obtained fywm the. Inter- 
 national. Indian Ocuan Expedition. 
 
temperatures of 30°C and slightly above as maximum temperatures for the area 
 being studied, while the satellite maps contain areas with temperatures above 
 31°C for April and above 32°C for May. The maximum temperatures are there- 
 fore higher in the satellite readings than in the 1963 International Indian 
 Ocean Expedition data. This, of course, might be related to year-to-year 
 variations. 
 
 The final major difference between 1963 and 1975 data is the north-south 
 temperature gradient for May. The satellite temperatures range from 27°C at 
 the equator to greater than 32°C at about 10°N along 55°E, while the Inter- 
 national Indian Ocean Expedition data range from 29°C to between 30°C and 
 31°C in the same area. Clearly the satellite temperature data have a wider 
 range. Satellite-measured temperatures are higher in warm areas and lower 
 in cool ones than temperatures measured at sea-surface. 
 
 6. SUMMARY 
 
 It is difficult to obtain reliable sea-surface temperature data for the 
 Indian Ocean. Potentially, satellite IR data could satisfy this need. How- 
 ever, the quality of these data has to be assessed before extensive use can be 
 made of them. Since ground truth is difficult to obtain in this region, we 
 tried to evaluate the SSST's by comparing them with data from a number of 
 independent sources. In each case, the intercomparisons showed serious dis- 
 crepancies between the satellite data and the ground truth. Although any one 
 result might be suspect, a clear trend emerged from the analyses. The SSST's 
 appear to be too low along the equator and along the East African coast in 
 the vicinity of the equator for the time period examined. Furthermore, in 
 April, May, and June in the areas off the equator and not along the coast, 
 they appear to be too high. 
 
 On the basis of this study, one would have to conclude that the SSST's, 
 at least for the Indian Ocean, are suspect. The errors appear to vary geo- 
 graphically in time and more work must be done if SSST's are to be of great 
 usefulness to oceanography. Meteorologically, improved SSST's are very impor- 
 tant because during FGGE, SSST's will be used in the numerical models. An 
 enhanced north-south temperature gradient in the models will probably create 
 unrealistic atmospheric circulation patterns in the tropics. 
 
 7. REFERENCES 
 
 Fieux, M. L. and H. Stommel , 1975: Personal communication. M.I.T., Cambridge, 
 Mass. 02139. 
 
 Wyrtki, K. 1973: Oceanographic Atlas of the International Indian Ocean 
 Expedition. NSF-I0E-1, Washington, D.C. '531 pp. 
 
 10 
 
 H U. S. GOVERNMENT PRINTING OFFICE: 1976-677-347/1291 REGION NO. 8 
 
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