Angular Distributions of Downward Longwave Radiance and Their Meteorological Applications

Abstract

Measured thermal radiances (? > 3 ?m) from clear and cloudy skies are averaged over azimuth angles to obtain average sky radiances at various zenith angles. The observed variation of sky emissivity with zenith angle can be explained through the application of the Goody random model to water vapor absorption in the window regions (e.g., ? = 8?13 ?m). This application forms the basis of a technique for extracting cloud base temperature and vertical optical thickness of water vapor. The importance of background temperature used in the analysis is emphasized. Earlier models of atmospheric thermal emission by comparison do not appear as useful as the model presented here.