The impact of clouds and aerosols on the radiative fluxes in Germany over the past 70 years from a surface perspective

The German Weather Service (DWD) initiated the operational observation of solar global and diffuse radiation at the Observatory Potsdam in 1937. Nowadays, these components are observed at 35 stations using thermopile-based pyranometers. Most of the stations are also equipped with a pyrgeometer to record the downwelling long-wave radiation.

The solar radiation experienced major decadal variations in Germany over the past 80 years. While a significant decrease between 1950 and 1985 occurred, a continuous increase can be observed since 1985 with some of the highest annual means ever recorded in the past five years. The decrease in the second half of the 20^th century with the subsequent increase is commonly referred to as dimming and brightening.  While these variations were most likely due to anthropogenically caused variations in the aerosol load, the recent increase is not yet fully understood. Since aerosol loads have stabilized at low levels at the beginning of the 21^th century, the direct aerosol effect might be less prominent at present. Instead, changes in cloudiness may have become more important.

In order to disentangle the observed trends in the radiative fluxes and to determine the contribution from aerosols and clouds separately we also use radiation data from the BSRN station at the Meteorological Observatory Lindenberg. The station was established in 1994 and high accuracy measurements of downwelling long-wave radiation and all three components of downwelling short-wave radiation (global, diffuse and direct) have been recorded continuously since then. We use a 1-dim. Radiative Transfer Model (RTM) to calculate the short- and long-wave cloud-free fluxes in order to quantify the cloud radiative effect. Similar, we estimate the direct aerosol effect. Preliminary results indicate a decrease in the cloud radiative effect over the past 20 year, which can imply a decrease in cloud cover but may also include a shift towards a different cloud type and/or a change in microphysical cloud properties. Finally, we present methods which can be used to determine the spectral AOD and the cloud-radiative effect for sites or periods when no direct observations of the spectral AOD and no information about the vertical temperature and humidity structure of the atmosphere for thermal RTM calculations are available.