4. 3 Atmospheric Attenuation 



The radiation emanating from the earth's surface or clouds is attenuated by 

 atmospheric absorption; hence, the satellite sensor measures radiation with an 

 equivalent black body temperature, T_,, less than the temperature, T, of the 

 radiating surface. The difference, AT = T-T_, depends on the amount of absorbing 

 gases between the radiating surface and the top of the atnnosphere.and so, in general, 

 increases with increasing nadir angle of view of the satellite sensor. 



The absorbing gases in the atmosphere which affect the measurement from a 

 satellite of terrestrial radiation are water vapor, carbon dioxide and ozone. Water 

 vapor has a strong vibration-rotation band centered at 6. 3|J., and rotation bands 

 beyond 20fi; CO^ has strong bands centered at 4. 3|j. and at 15|a.; O- has a strong but 

 relatively narro'w band centered at 9. 6|J.. The region 8 to 13|j., to which Channel Z 

 of the TIROS satellites is sensitive, is termed a "window'' region, because of 

 relatively little atmospheric absorption. However, it is a "dirty" window at best 

 because of absorption by O,, and because of the residual absorption by H^O due to 

 many small bands scattered throughout the region. 



It is possible to calculate the amount of atmospheric absorption if the distri- 

 bution of absorbing gases in the atmosphere is known. The problem is complicated 

 because the absorption varies with pressure and temperature in a non-linear fashion. 

 The calculations utilize experimental measurements of absorption by atmospheric 

 gases. However, because environmental conditions in the experiments are different 

 from those in the atmosphere, theory must be applied. Good results have been ob- 

 tained by use of the Curtis approximations except in the case of ozone. 



Computations of the outgoing radiation for 106 model atmospheres have been 



13 

 made by Wark et al. For clear conditions, water vapor was the primary variable. 



A correction for an average ozone distribution was made; the error due to different 



ozone distributions is likely to be relatively small. The results for clear sky 



conditions are shown in Fig. 4-2 for nadir angles of and 45 . In these diagrams, 



the ordinates are the amount of precipitable water in the atmosphere. Horizontal 



lines indicate "average" conditions (w = 2. 25 cm) and "moist" conditions (w = 3. 9 cm). 



The nearly vertical lines are labeled T , the equivalent black body radiation as 



measured by the Channel 2 sensor. Thus for a nadir angle with T = 290 the 



correction to be added to obtain the temperature of the radiaj;ing surface is 6. 8 C 



for average humidity and 9 C for moist conditions; at 45 nadir angle the corrections 



are 9. 5 and 12°C. 



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