30 years of observations of incoming radiative fluxes at the Lindenberg BSRN site: methods, applications and results

The BSRN site was established at the Meteorological Observatory Lindenberg – Richard Aßmann Observatory (MOL-RAO) of the German Meteorological Service (DWD) in 1994. Since then, incoming shortwave and longwave irradiances have been observed continuously and with high accuracy. We will discuss the methods of operations and the uncertainties of the observations.

The primary applications of the BSRN data are the i) validation of climate model projections and surface products derived from satellite data, and ii) monitoring of the shortwave and longwave radiative components and their changes with the best methods currently available. We will present such results from the radiation records. While the observed increase in the incoming longwave radiation of 3.5 Wm^-2 per decade due to increasing air temperature, humidity and greenhouse gas concentrations is consistent with the corresponding projections from climate models, the interpretation of the continuous increase in total solar irradiance of 3.5 Wm^-2 per decade since the 1990s - referred to as brightening - is more complex. The first period of this brightening may be solely attributed to the decrease in the aerosol load. However, aerosol loads have then stabilized at low levels at the beginning of the 21^th century and thus the direct aerosol effect might have been less dominant in recent years. Instead, changes in cloudiness may have become more important. Indeed, calculations of the cloud radiative effect indicate a decrease in the magnitude over the past 30 years, which may imply a decrease in cloud cover, a shift towards a different cloud type and/or changes in microphysical cloud properties. However, these results are related to substantial uncertainties and thus are not conclusive.