The impact of Aeolus observations on wind and rainfall predictions

Abstract. Previous studies showed substantial improvements in upper-level wind and mass field forecasts from assimilating Aeolus wind observations. This study extends those analyses using the improved reprocessed Aeolus dataset (version B16) in experiments with the global ECMWF forecasting system spanning more than three years. Results show that zonal wind forecasts improve through most of the troposphere during the first forecast week, with smaller gains in Northern Hemisphere midlatitudes. The impact extends into the stratosphere, reducing Root Mean Square Errors by about 0.5 %, up to 1.5 % in the tropical upper troposphere. These improvements lead to more accurate rainfall forecasts in some regions and seasons, as measured by the Fraction Skill Score (FSS) and the Stable Equitable Error in Probability Space (SEEPS). They are largest during the winter half-year in the extratropics, particularly in the Southern Hemisphere, and appear primarily at the grid scale. The largest FSS improvements, reaching several percent, occur for 5–10 d leads and heavy rainfall categories, while SEEPS indicates modest but consistent gains in categorical precipitation skill. These results show co-occurring improvements in wind and precipitation, particularly in winter, which may reflect an influence of Aeolus winds on the large-scale circulation that governs the organization of cyclones, fronts, and heavy rainfall.