Diurnal Radiance Patterns of Finite and Semi-Infinite Clouds in Observations of Cloud Fields

Abstract

One of the important radiative effects of cloud Shape is to modify diurnal radiance patterns observed from satellites. Theory predicts a diurnal radiance pattern nearly symmetric about local noon for both semi-infinite and finite clouds situated on the equator at the equinox with a satellite directly overhead. For a geostationary satellite (SMS-1) located to the west of the cloud, the semi-infinite cloud still products a pattern nearly symmetric about local noon while finite cubic clouds produce a distinctly different pattern which peaks during the afternoon. Simulated diurnal satellite observations of a finite cubic and semi-infinite cloud were compared with actual diurnal satellite observations of cloud fields with cloud cover varying from less than 30% to greater than 90%. The results for 4 n mi resolution data from the two observed cloud fields demonstrate that the diurnal radiance patterns of both semi-infinite and finite clouds exist in satellite observations. Degrading the resolution to 16 n mi did not significantly alter the semi-infinite or finite diurnal radiance pattern of either observed cloud field. The effects of cloud shape on satellite observations have potential application to problems in data interpretation, cloud cover determination, albedo calculations and identification of cloud fields.