¶Global, diffuse, and horizontal direct (beam) irradiances have been evaluated for 13 stations in Germany where the time series vary between 11 and 48 years. Global irradiance has decreased significantly at two stations and increased at four stations. The mean trend in global is an increase of 1.94 Wm−2 or 1.83% per decade. Diffuse irradiance has decreased at five stations, with a mean reduction of 2.44 Wm−2 or 3.46% per decade, while horizontal direct irradiance has increased an average of 4.86 Wm−2 or 10.40% per decade at five stations. Increases in global and direct are most common at stations in the southwest region of Germany, decreases in global were observed in the southeast, and there was an absence of spatial homogeneity in the diffuse trends. Spatial variability in irradiance over Germany is higher in the direct component compared to variability in global and diffuse.
Trend analyses of concomitant time series of radiation, bright sunshine duration, and modeled estimates of Ångström’s turbidity coefficient suggest that long-term decreases in aerosols are the most likely cause of increases in global irradiance observed at Mannheim, Norderney, and Trier; decreases in diffuse at Hohenpeissenberg, Kassel, Mannheim, and Trier; and increases in direct irradiance at Bocholt, Kassel, Mannheim, and Trier. An increase in sunshine duration at Freiburg likely contributed to an increase in global and direct irradiance observed at that station.
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Power, H. Trends in solar radiation over Germany and an assessment of the role of aerosols and sunshine duration. Theor Appl Climatol 76, 47–63 (2003). https://doi.org/10.1007/s00704-003-0005-8