Aeolus + Processes, Technical Note 3

Aeolus measurements were not used in the ERA5 climate reanalysis. This technical note (TN) presents<br> the results of the Work Package 3 (WP3) of the ESA-funded project “Aeolus+Processes” (project No.<br> 4000139422/22/I-NS), which evaluates the impact of Aeolus observations in a prototype of the ERA6<br> (ECMWF Reanalysis v6) reanalysis system, with the goal of informing the decision of whether to use<br> Aeolus in ERA6.<br> The assessment is based on a 17-month long observing system experiments (OSEs) using the best<br> available Aeolus L2B winds reprocessed datasets, including the 4th reprocessing.<br> The Aeolus impact during this period, in which the instrument changed laser and there were two data<br> gaps, showed no issues in terms of mean state discontinuities, and the Numerical Weather Prediction<br> (NWP) impact on random forecast errors was impressive. Therefore, it was decided that Aeolus L2B<br> winds should be assimilated in ERA6 from September 2018 to April 2023, noting that operational NWP<br> use at ECMWF only began in January 2020. This study helps ensure that the full Aeolus mission dataset<br> value will continue to be exploited for climate reanalysis applications.