Estimating the potential impact of the <i>EPS‐Sterna</i> satellite constellation in regional numerical weather prediction

Abstract The European Space Agency (ESA) launched the Arctic Weather Satellite ( AWS ) into a sun‐synchronous low Earth orbit in August 2024. The purpose of the launch is to demonstrate the feasibility of new low‐cost satellite technology in provision of continuous microwave sounding data for the use in numerical weather prediction (NWP) and nowcasting applications. AWS is intended as a predecessor of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Polar System – Sterna ( EPS‐Sterna ), which is a small‐satellite microwave‐sounding constellation that EUMETSAT envisions as one of its future operational programmes. Planned to operate from 2029 to the 2040s, EPS‐Sterna would maintain a constellation of six satellites in orbits complementary to the baseline polar satellites of EUMETSAT, the National Oceanic and Atmospheric Administration (NOAA), and the Chinese Meteorological Administration (CMA). We present an estimate of the potential forecast impact that can be expected from the assimilation of EPS‐Sterna microwave radiance data into regional NWP systems at high northern latitudes. The estimate is based on running state‐of‐the‐art limited‐area NWP systems and quantifying the impact obtained from the heritage microwave sounders of EUMETSAT, NOAA, and CMA. The results indicate that the expected forecast impact will be greater in winter than in summer and that the greatest benefit will be in the forecast of humidity‐related parameters near the surface. In terms of the reduction in root‐mean‐squared forecast error, the impact expectation at short‐range forecast lead times (up to 24 hours) is up to 5%–10% in humidity and cloudiness and up to 1%–2% in temperature.