Long-term variations of available solar radiation on seasonal timescales in 1955–2006 at Tartu-Tõravere Meteorological Station, Estonia
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The time series of the daily sums of global and direct irradiance recorded at Tartu-Tõravere Meteorological Station site (58°16′N, 26°28′E, 70 m a.s.l.) in 1955–2006 have been analyzed in seasonal timescales. The average daily ratio G/G clear of available global irradiance to its local climatic clear-sky value in the summer half-year corresponds to 65.5%, while that of the direct irradiance on the horizontal surface I′/I′ clear was 41% of the climatic clear-sky value. In the case of dry Rayleigh atmosphere as a reference, these ratios are 53.5% and 28%, respectively. The time series of the summer season totals reveal a longer interval of reduced values in 1976–1993 as well as two periods of frequent sunny summers in 1967–1975 and since 1994. The probability density distribution of the summer season totals during the observed period is strongly asymmetric; in spring, however, it is close to the normal distribution. In winter, there is a moderate negative correlation between the G/G clear and the North Atlantic Oscillation as well as the Arctic Oscillation indices.
KeywordsAerosol Optical Depth North Atlantic Oscillation Sunshine Duration Cloud Amount Probability Density Distribution
The work has been supported by grant number 7137 of the Estonian Science Foundation. Part of the work is related to the EC COST 726 action. The authors thank the Estonian Meteorological and Hydrological Institute for the radiation data. The authors also thank two anonymous reviewers for the useful comments and suggestions.
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