Influence of atmospheric circulation patterns on local cloud and solar variability in Bergen,Norway

In a previous paper, we have shown that long-term cloud and solar observations (1965–2013) in Bergen, Norway (60.39°N, 5.33°E) are compatible with a largely cloud dominated radiative climate. Here, we explicitly address the relationship between the large scale circulation over Europe and local conditions in Bergen, identifying specific circulation shifts that have contributed to the observed cloud and solar variations. As a measure of synoptic weather patterns, we use the Grosswetterlagen (GWL), a daily classification of European weather for 1881–2013. Empirical models of cloud cover, cloud base, relative sunshine duration, and normalised global irradiance are constructed based on the GWL frequencies, extending the observational time series by more than 70 years. The GWL models successfully reproduce the observed increase in cloud cover and decrease in solar irradiance during the 1970s and 1980s. This cloud-induced dimming is traced to an increasing frequency of cyclonic and decreasing frequency of anticyclonic weather patterns over northern Europe. The changing circulation patterns in winter can be understood as a shift from the negative to the positive phase of the North Atlantic and Arctic Oscillation. A recent period of increasing solar irradiance is observed but not reproduce by the GWL models, suggesting this brightening is associated with factors other than large scale atmospheric circulation, possibly decreasing aerosol loads and local cloud shifts.     

Mean bias deviation of the Heliosat algorithm for varying cloud properties and sun-ground-satellite geometry

Summary Reflectances measured by the visible channel of Meteosat are converted to global radiation at the ground by the Heliosat-algorithm. This algorithm is based on the inverse relationship between reflectance and transmittance. This relationship is, however, complicated by two factors: 1) the absorptance of the atmosphere is varying, and 2) the reflectivity is different in different directions. These two factors are again depending on the state of the atmosphere (mainly clouds) and the sun-ground-satellite geometry. The performance of the Heliosat-method is here tested against ground data from Bergen (Norway), Geneva (Switzerland) and Lyon (France), and analysed in light of cloud properties as observed from ground, and different sun-ground-satellite geometries. Situations where modelled and measured radiation differ are identified, and possible causes are suggested. Some of the suggestions are supported by calculations with a radiative transfer model.