Long-term UV irradiance changes over Moscow and comparisons with UV estimates from TOMS and METEOSAT

Abstract

We analyzed long-term variations of UV irradiance 300-380 nm over Moscow 55.7N, 37.5E since 1968 using a complex dataset that includes ground-based UV measurements, UV retrievals from two satellites, and the results of a previously developed empirical model. Long-term interannual changes of UV irradiance, 300-380nm, during 1968-2003 show the absence of any linear trends although an increase is detected in the late 90-s due to cloud amount and aerosol content decrease. The ground-based data are compared with UV satellite retrievals from two independent methods as well as with the results of an empirical model that accounts for the physical dependence of UV on cloud parameters (amount and optical thickness), surface albedo, total ozone, and aerosol properties of the atmosphere. UV datasets over Moscow obtained from different satellite instruments: from the Total Ozone Mapping Spectrometer (TOMS) data (version 8) since 1979 and from METEOSAT/MVIRI since 1984. The original METEOSAT processor, using visibility observations at a nearby meteorological station to quantify the aerosol load, leads to a significant underestimation of the UV daily doses (-23% in warm period and -31% in cold period). Substituting the visibility observations by in situ monthly mean aerosol optical depth improves significantly the agreement in both warm and cold periods (respectively, -9% and -10%) but the bias still remains. The difference between TOMS UV retrievals and ground-based data has different signs in warm (+6%) and cold (-15%) periods. Applying off-line absorbing aerosol correction in TOMS UV retrievals eliminates the positive bias in warm period. The negative bias during the cold period can be due to the application of minimum Lambertian effective reflectivity (MLER) approach to determine the surface albedo especially in conditions with non stable snow cover (end of February- March, and November-December). Model reconstruction of UV variability demonstrates high correlation with aerosol corrected satellite UV retrievals (0.83-0.94) as well as with ground data (0.82) during warm period. During cold months the correlation between satellite UV retrievals and ground-based measurements is much worse.