The Derivation of Cloud Parameters from Satellite-measured Radiances for Use in Surface Radiation Calculations

Abstract

A simple scheme is developed to infer cloud amount, optical thickness, and height from satellite-measured radiances for use in surface radiation calculations. The essence of the cloud scheme is to specify a cloud reflectivity threshold for identifying pixels that are likely to be totally filled with clouds. Area-averaged values for the three cloud parameters are derived from the reflectivities of these cloudy pixels and the satellite-measured radiances in the visible and IR window channels. By applying the cloud scheme to the ISCCP (International Satellite Cloud Climatology Project) B3 radiance data and using a radiation routine, the surface radiative fluxes are computed for the tropical and subtropical western Pacific regions (30°S-30°N, 100°E?180°). It is found that the three cloud parameters are quite sensitive to the cloud reflectivity threshold, but the surface radiation is rather insensitive to the set of cloud parameters inferred by a scheme that is directly dependent upon the satellite radiance measurements. Over a broad area in the tropical and subtropical western Pacific regions, the difference in the net surface radiative fluxes is <2 W m?2 for the cloud reflectivity thresholds of 0.3 and 0.4. This study further strengthens the view of other investigators that the net solar flux at the surface can be linearly related to the net solar flux at the top of the atmosphere. For a given amount of solar radiation absorbed by the earth?atmosphere system, the partition of the absorption between the surface and the atmosphere is affected by the solar zenith angle. As a result, the solar zenith angle has a significant effect on the relationship between the fluxes at the surface and at the top of the atmosphere.