The Global Dimming & Brightening based on FORTH radiative transfer model during 1984-2018 and its evaluation against GEBA & BSRN ground-based networks

The Earth’s energy balance and radiation budget, which play a key role in the Earth’s climate system, are driven by the incident solar radiation at surface (surface solar radiation, SSR). Over the past few decades, changes in the SSR (ΔSSR) have been observed that are dependent on the transparency of the terrestrial atmosphere. This phenomenon, called global dimming and brightening (GDB), is a significant factor in climate change and modulates global warming. This study examines the interdecadal variability of SSR based on computations of the FORTH radiative transfer model, using as input data cloud optical properties taken from the International Satellite Cloud Climatology Project H Series (ISCCP-H) and aerosol optical properties and meteorological data taken from the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) Reanalysis. Ground-based measurements of SSR from the Global Energy Balance Archive (GEBA) and Baseline Surface Radiation Network (BSRN) networks are utilized to evaluate the FORTH’S SSR fluxes and GDB. The FORTH RTM computations are made on a monthly basis from 01/1984 - 12/2018 at 51 atmospheric levels and a spatial horizontal resolution of 0.5°×0.625° (with a conversion of the original input data to the same spatio-temporal resolution). Firstly, the FORTH SSR fluxes are evaluated against ground measurements from GEBA and BSRN. This comparison reveals a general underestimation of the FORTH SSR fluxes, with a satisfactory evaluation metrics, such as the relative bias, which is equal to –2.9% and –7.7% against GEBA and BSRN, respectively or the correlation coefficient values, computed using deseasonalized SSR anomalies, being equal to 0.72 and 0.8 against GEBA and BSRN, respectively. Then, the SSR changes (or GDB) for each pixel, also calculated using deseasonalized SSR anomalies, were compared with the GDB from the corresponding GEBA/BSRN station, lying in that pixel, for their common time period. This comparison reveals an agreement between the sign of the FORTH’s pixels and the corresponding stations’ GDB equal to 63.5% for the GEBA and 54.5% for the BSRN sites. Finally, the GDB was also calculated on global (land & ocean), hemispherical and regional scales, either for the entire period and for sub-periods too. The computed GDB for the period 01/1984-12/2018 is equal to –2.22 ± 0.38 W/m² for the Globe, -0.48 ± 0.39 W/m² for the Northern Hemisphere and -2.73 ± 0.54 W/m² for the Southern Hemisphere. Larger GDB values are estimated over oceans than land (-2.56 ± 0.44 versus -1.04 ± 0.47 W/m², respectively), suggesting that the atmosphere over oceans got less transparent than over continents during the 35-year study period. During this period, a brightening has taken place over Europe, Middle East, Mexico against a dimming over India, Maritime Continent, Australia and Southern Ocean.