Evaluation of clear sky models to estimate the surface direct aerosol radiative effect over Germany

The radiation budget of the earth and its climate system is driven by the solar radiation, which interacts with gases, aerosol particles and clouds. Focusing on aerosol, a fundamental measure is the radiative forcing resulting from aerosol-radiation interactions (RFari) which is also known as the aerosol direct radiative effect. Quantifying the surface RFari on regional scales aids the understanding of the role of aerosol in the climate system and is important for the planning of solar energy systems.

This study is based on a one year dataset (2015) of shortwave broadband global and diffuse horizontal irradiance measured with shaded and unshaded pyranometers at 26 station across Germany within the German Weather Service (DWD) observational network. A variety of clear-sky models are utilized to quantify RFari with a clear sky fitting technique. Clear sky models used are MMAC, MRM v.6.1, METSTAT, ESRA, Heliosat-1, CEM and the simplified Solis model. As these models have not been designed to estimate the clear sky irradiance without the presence of aerosol, we evaluated the accuracy of RFari with an reference simulation.

The reference RFari is simulated using the TROPOS (Leibniz Institute of Tropospheric Research) Cloud and Aerosol Radiative Simulator (T-CARS) utilizing the offline version of the ECMWF radiation scheme (ecRad) with input data of meteorological state of the atmosphere, trace-gases and aerosol from CAMS reanalysis.

The clear sky fitting approach for this set of clear sky models agrees well with T-CARS, showing an RMSE of 6.7 Wm^-2 and an correlation of 0.75. The annual mean of surface RFari over the observation stations in Germany shows a value of -13.2 Wm^-2 as an average over all clear sky models, compared to -13.4 Wm^-2 from T-CARS. Out of this set of clear sky models, best performance is shown by the ESRA and MRM v6.1 models. Although, the accuracy of the annual mean RFari from the clear sky fitting approach is strongly depended on the number available clear-sky irradiance measurements and its distribution over the year. Therefore, this approach is not recommended for climatological studies, but may serve as valuable information for e.g. the evaluation of power generation and the influence by aerosol of photo-voltaic power plants.