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

Abstract

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 Cₜ, 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 Cₜ from the NASA Total Ozone Mapping Spectrometer (TOMS) is compared with ground-based Cₜ estimations from the Canadian network of Brewer spectrometers for the period 1989–1998. For snow-free conditions the TOMS derived Cₜ at 324 nm approximately agrees with Brewer data with a correlation coefficient of ∼0.9 and a standard deviation of ∼0.1. The key source of uncertainty is the different size of the TOMS FOV (∼100 km field of view) and the much smaller ground instrument FOV. As expected, the standard deviations of weekly and monthly Cₜ averages were smaller than for daily values. The plane-parallel cloud method produces a systematic Cₜ 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 Rₛ is known from outside data. Since Rₛ varies on a daily basis, using a climatology will result in additional error in the satellite-estimated Cₜ. The Cₜ error has the same sign as the Rₛ 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 Cₜ. Both effects reduce Cₜ compared to that estimated assuming conservative cloud scattering. The error increases if polluted clouds are over snow.