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 surface UV irradiance from satellite ozone and reflectivity data from a clear-sky case to a cloudy atmosphere and snow-covered surface. Two methods are compared for the estimation of cloud-transmission factor C-T, the ratio of cloudy 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 from 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 standard deviation of similar to0.1. The key source of uncertainty is the different size of the TOMS FOV (similar to 100 km field of view) and the much smaller ground instrument FOV, As expected, the standard deviations of weekly and monthly C-T averages were smaller than for daily values. The plane-parallel cloud method produces a systematic C-T bias relative to the Brewer data (+7% at low solar zenith angles to -10% at large solar zenith angles). The TOMS algorithm can properly account for conservatively scattering clouds 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 additional 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, clouds polluted with absorbing aerosols transmit less radiation to the ground than conservative clouds for the same satellite reflectance and flatten spectral dependence of C-T. Both effects reduce C-T compared to that estimated assuming conservative cloud scattering. The error increases if polluted clouds are over snow.