Gridded daily European solar cloud modification factors derived from ERA‐40 information and pyranometer observations

Abstract

The long‐term UV climatology and trend analysis in the COST‐Action 726 requires daily solar cloud modification factors (SOL‐CMFs) as input of algorithms, transforming them into UV‐CMFs. A CMF is the ratio of all‐sky to clear‐sky downwelling irradiation. A complete spatial and temporal coverage is achieved by calculating daily SOL‐CMFs on the 1° × 1° COST‐726 grid (31°N to 80°N, 25°W to 35°E) using the ERA‐40 shortwave net all‐sky and clear‐sky irradiation. Known deficiencies in ERA‐40 SOL‐CMFs (especially the bias due to clouds) are corrected using SOL‐CMFs derived from pyranometer observations. These are determined based on measured daily sums of solar global irradiation from up to 152 European sites from several data sources. An analysis of clear‐sky days during 1981–1993 and a comparison with results from high‐quality data enabled the selection of appropriate sites in the Mediterranean and southeast Europe from data that has not been classified as that of the group of best quality. For these sites and the period 1964–1980, a homogenization is performed. A cross‐validation of all daily SOL‐CMFs from observations is performed prior to their gridding. Gridding uses ordinary Kriging. Bias‐corrected ERA‐40 and SOL‐CMFs from observations are merged using a distance‐dependent weight derived from overall structural analysis. The COST‐726 database offers daily SOL‐CMF fields of complete spatial coverage from 1958 to 2002. They are unbiased and of known quality. The aerosol direct radiative effect included in the SOL‐CMFs from observations is retained and accounts for long‐term aerosol trends in agreement with the trends of dimming and brightening.