Validating global horizontal irradiance retrievals from Meteosat SEVIRI at increased spatial resolution against a dense network of ground-based observations

Abstract. Accurate and detailed retrieval of global horizontal irradiance (GHI) has many benefits, for instance, in support of the energy transition towards an energy supply with a high share of renewable energy sources and for validating high-resolution weather and climate models. In this study, we apply a downscaling algorithm that combines the High-Resolution Visible and standard-resolution channels onboard MSG-SEVIRI to obtain cloud physical properties and GHI at an increased nadir spatial resolution of 1 x 1 km² instead of 3 x 3 km². We validate the change in accuracy of the high-resolution GHI in comparison to the standard-resolution product against ground observations from a unique network of 99 pyranometers deployed during the HOPE field campaign in Jülich, Germany, from 18 April to 22 July 2013. Over the entire duration of the field campaign, a small but statistically significant reduction in root-mean-square error (RMSE) by 2.8 W m^-2 is found for the high-resolution GHI at 5-minute scale. The added value of the increased spatial resolution is largest on days when GHI fluctuates strongly: for the ten most variable days a significant reduction of the RMSE by 7.9 W m^-2 is obtained with high- versus standard-resolution retrievals. In contrast, we do not find significant differences between both resolutions for clear-sky and fully overcast days. The sensitivity of these results to temporal and spatial averaging scales is studied in detail. Our findings highlight the benefits of spatially dense network observations as well as a cloud-regime resolved approach for the validation of GHI retrievals. We also conclude that more research is needed to optimally exploit the instrumental capabilities of current advanced geostationary satellites in terms of spatial resolution for GHI retrieval.