Operational assimilation of Aeolus winds in the Meteo-France global NWP model ARPEGE

The European Space Agency's Aeolus satellite was launched in August 2018. Measurements of wind profiles are provided for the first time from space using an onboard Doppler wind lidar. The quality of Aeolus Level-2B (L2B) wind products has been found suitable for data assimilation in the Meteo-France global model ARPEGE since April 2020, in particular, when applying a suitable bias correction method. This article describes a series of Observing System Experiments (OSEs) conducted in April-May 2020 to assess the impact of Aeolus horizontal line-of-sight winds (HLOSW) on Meteo-France's global numerical weather prediction analyses and forecasts. Innovation statistics and a posteriori diagnostics from a period of July-August 2019 were used to scale the random observation errors provided by the L2B processor (mostly for Rayleigh-clear winds). Although the HLOSW data represent only 0.42% of the total amount of all observations assimilated in ARPEGE, their contribution to the reduction of the global analysis variance is up to 2.3% (measured by the Degree of Freedom for Signal). The assimilation of HLOSW showed improvement in 6 hr short-range forecasts which is demonstrated by an overall reduction of innovations statistics for various operational observing systems. From a Forecast Sensitivity to Observations impact (FSOi) study Aeolus is found to be the third most effective observing system (per individual observation) at reducing global 24-hour forecast errors. For longer forecast ranges, the largest positive impacts are noticed over the tropics, particularly in the lower stratosphere up to 102 hr ahead (with up to 2% root-mean-square error reduction for wind and temperature), but also in the troposphere up to 72 hr ahead. To a lesser extent, a similar improvement is observed over the Southern Hemisphere. This positive impact of Aeolus HLOSW in OSEs has led to their operational assimilation at Meteo-France starting in June 2020.