Assessment of the ECMWF Data Assimilation System Using Radiosonde Profiles During Atmospheric River Events Over Western Europe

Atmospheric Rivers (ARs) are one of the key elements in explaining the occurrence of extreme precipitation and floods in midlatitudes. Despite the recognised importance of ARs, few studies have systematically evaluated how a weather forecast model captures AR conditions affecting western Europe. Using radiosonde observations from different sites across western Europe, this study evaluates the specific humidity, wind speed and moisture flux in the data assimilation system of the ECMWF Integrated Forecasting System (IFS) for the extended winter of 2020/2021 with a particular focus on landfalling ARs. Results indicate a systematic underestimation of specific humidity, wind and moisture flux in the IFS, particularly during more intense moisture flux events. The most significant negative biases of the IFS were observed in the boundary layer (1000–900 hPa) and lower free troposphere (900–700 hPa), coinciding with the core region of moisture flux. Wind speed was generally underestimated, with the largest errors occurring in the boundary layer at locations with complex topography. In addition, as expected, the assimilation of radiosonde profiles reduced short-range forecast errors. The results highlight the current limitations in forecasting AR intensity and location and show the importance of in situ observations to improve the forecast of high-impact weather events.