Comparisons between ground measurements of broadband ultraviolet irradiance (300 to 380 nm) and total ozone mapping spectrometer ultraviolet estimates at Moscow for 1979 to 2000

Abstract

We show the comparisons between ground-based measurements of spectrally integrated (300 to 380 nm) UV irradiance and satellite estimates from the total ozone mapping spectrometer (TOMS) total ozone and reflectivity data for the whole period of TOMS measurements (1979 to 2000) over the Meteorological Observatory of Moscow State University (MO MSU), Russia. Several aspects of the comparisons are analyzed, including effects of cloudiness, aerosol, and snow cover. Special emphasis is given to the effect of different spatial and temporal averaging of ground-based data when comparing low-resolution satellite measurements. The comparisons in cloud-free conditions with different aerosol loading have revealed positive TOMS bias with interannual variations within +5 to +20%. Reprocessing the TOMS data by accounting for boundary layer aerosol absorption (single scattering albedo ~0.9) removes the bias for all-sky conditions, except for completely overcast situations, where the TOMS bias remains large (15 to 17%). The single scattering albedo was independently verified using sun photometer (CIMEL Electronique) sun and sky-radiance measurements at MO MSU in September 2001. The analysis of interannual UV variations shows quite similar behavior for both TOMS and ground measurements (correlation coefficient r0.8). No long-term trend in the annual mean bias was found for all conditions. Both TOMS and ground data show positive trends in broadband UV irradiance (300 to 380 nm) between 1979 and 2000. The UV trend is attributed to decreases in both cloudiness and aerosol optical thickness during the late 1990s over the Moscow region. However, if the analyzed period is extended to include the preTOMS era (1968 to 2000 period), no trend in ground UV irradiance is detected.