Quality and forecast impact of Aeolus wind lidar observations over West Africa and the tropical Atlantic

Aeolus, the first Doppler wind lidar in space was launched by the European Space Agency in 2018, and acquired three-dimensional atmospheric wind profiles around the globe, until it stopped operation in April 2023. Especially in the tropics, these measurements compensated for the currently limited number of other wind observations. This contribution presents the results of an assessment of the Aeolus wind product quality using radiosondes launched as part of the Joint Aeolus Tropical Atlantic Campaign during August-September 2021, and explores the impact of Aeolus data on analyses and forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) and Deutscher Wetterdienst (DWD), focusing specifically on the notoriously challenging to forecast West African Monsoon (WAM) circulation during the boreal summers of 2019 and 2020.

To evaluate the Aeolus L2B wind product quality across the tropical Atlantic Ocean, 20 radiosondes corresponding to Aeolus overpasses were launched from the islands of Sal, Saint Croix, and Puerto Rico, characterized by a complex environment with a variety of cloud types in the vicinity of the Intertropical Convergence Zone (ITCZ) and aerosol particles from Saharan dust outbreaks. We compute average random and systematic errors for the Rayleigh-clear and Mie-cloudy channels that are largely consistent with the official mission requirements. Below clouds or within dust layers, however, the quality of Rayleigh-clear measurements can be degraded and the estimated error is too small. Gross outliers, which we define as large deviations from the radiosonde but low error estimates, account for less than 5% of the data. They appear at all altitudes and under all environmental conditions, and their root-cause remains unknown. We also found an orbital-dependent bias of up to 2.5 ms^–1 relative to both radiosondes and ECMWF model equivalents. The data impact studies show that assimilating Aeolus generally improves the prediction of zonal winds in the DWD and ECMWF forecasting systems, especially for lead times above 24h. These improvements are related to systematic differences in the representation of the African Easterly Jet North (AEJ-North) and Tropical Easterly Jet (TEJ), with the former weakened at its southern flank in the western Sahel in the ECMWF analysis, while no obvious systematic differences are seen in the DWD analysis. In addition, the TEJ core is weakened in the ECMWF analysis and strengthened on its southern edge in the DWD analysis. The regions where the influence of Aeolus on the analysis is greatest correspond to the ITCZ region for ECMWF and generally the upper troposphere for DWD.

The results of this study demonstrate the value of Aeolus for forecasting in the West African / tropical Atlantic region and provide valuable information for further improvement of the Aeolus wind retrieval algorithm in different atmospheric conditions. Such information can support the design of the planned Aeolus-2 mission in the coming years.