Satellite estimation of spectral surface UV irradiance 2. Effects of homogeneous clouds and snow

This paper extends the theoretical analysis of the estimation of the surfac e UV irradiance from satellite ozone and reflectivity data from a clear-sky case to a cloudy atmosphere and snow-covered surface. Two methods are comp ared for the estimation of cloud-transmission factor C-T, the ratio of clou dy to clear-sky surface irradiance: (1) the Lambert equivalent reflectivity (LER) method and (2) a method based on radiative transfer calculations for a homogeneous (plane parallel) cloud embedded into a molecular atmosphere with ozone absorption. The satellite-derived C-T from the NASA Total Ozone Mapping Spectrometer (TOMS) is compared with ground-based C-T estimations f rom the Canadian network of Brewer spectrometers for the period 1989-1998. For snow-free conditions the TOMS derived C, at 324 nm approximately agrees with Brewer data with a correlation coefficient of similar to0.9 and a sta ndard deviation of similar to0.1. The key source of uncertainty is the diff erent size of the TOMS FOV (similar to 100 km field of view) and the much s maller ground instrument FOV, As expected, the standard deviations of weekl y and monthly C-T averages were smaller than for daily values. The plane-pa rallel cloud method produces a systematic C-T bias relative to the Brewer d ata (+7% at low solar zenith angles to -10% at large solar zenith angles). The TOMS algorithm can properly account for conservatively scattering cloud s and snow/ice if the regional snow albedo R-S is known from outside data. Since R-S varies on a daily basis, using a climatology will result in addit ional error in the satellite-estimated C-T. The C-T error has the same sign as the R-S error and increases over highly reflecting surfaces. Finally, c louds polluted with absorbing aerosols transmit less radiation to the groun d than conservative clouds for the same satellite reflectance and flatten s pectral dependence of C-T. Both effects reduce C-T compared to that estimat ed assuming conservative cloud scattering. The error increases if polluted clouds are over snow.