The Impacts of Assimilating Aeolus Horizontal Line-of-Sight Winds on Numerical Predictions of Hurricane Ida (2021) and a Mesoscale Convective System over the Atlantic Ocean

On 22 August 2018, the European Space Agency (ESA) launched the first spaceborne wind lidar, the Aeolus satellite, measuring horizontal line-of-sight (HLOS) winds globally. The assimilation of Aeolus HLOS winds has been proven to improve numerical weather predictions (NWPs). Still, its influences on forecasts of tropical cyclones (TCs) and tropical convective systems have yet to be examined in detail. This study investigates the impacts of assimilating Aeolus HLOS winds on the analysis and forecasts of Hurricane Ida (2021) and mesoscale convective systems (MCSs) embedded in an African easterly wave (AEW) over the Atlantic Ocean (AO) during NASA CPEX-AW and CPEX-CV field campaigns. The mesoscale community Weather Research and Forecasting (WRF) model and the NCEP-GSI based three-dimensional ensemble-variational (3DEnVAR) hybrid data assimilation (DA) system are used. Mie-cloudy and Rayleigh-clear winds are assimilated. The results for Ida (2021) show that assimilating Aeolus HLOS winds leads to better track predictions. The intensity forecasts are improved in some cases, even with limited coverage of Aeolus HLOS winds within the inner core region of Ida (2021). In addition, the structure of heavy precipitation associated with Ida (2021) is refined after assimilation of Aeolus HLOS winds. Further diagnosis demonstrates that the improved intensity and precipitation forecasts result from enhanced divergence in the upper level of the troposphere after assimilation of Aeolus HLOS winds. Additional results from the MCS associated with an AEW indicate that assimilating Aeolus HLOS winds enhances forecasts of its precipitation structure and the associated low-level divergence. In short, this study demonstrates the potential of assimilation of Aeolus HLOS winds to improve forecasts for TCs and tropical convective systems.