This study describes a method for determining the reflection of sunlight to space and absorption by the earth and atmosphere, using low-resolution radiometer data from earth satellites. The method has been used with TIROS IV data together with radiation measurements at the ground to determine the reflection and absorption of sunlight over the United States during the spring of 1962.
The results indicate that for this region and time, 40 percent of the incident sunlight at the top of the atmosphere was reflected to space, 13 percent was absorbed by the atmosphere and clouds, and the remaining 47 percent was absorbed at the earth's surface. Atmospheric absorption of sunlight varied from over 20 percent in the moist air in southeastern United States to less than 10 percent over much of the dry mountainous west and northern plains.
When atmospheric absorption values determined from this study are compared with earlier studies of absorption in a cloudless atmosphere, there is good agreement at low values of atmospheric water vapor; however, the present study gives significantly higher absorption at high values of water vapor.
Based on this study, an empirical relationship is determined for fractional absorption of sunlight in an atmosphere with clouds as a function of optical pathlength of water vapor: qa=0.096+0.045(u*)½logeu*. The fractional absorption of sunlight, qa, is the fraction of the total amount incident at the top of the atmosphere. The optical pathlength, u*, is given in cm.: u*=u.sec ζ. Here, u is total precipitable water in a vertical column, given in cm., and ζ is the solar zenith angle.
National Environmental Satellite Center, ESSA, Madison, Wis.