Aeolus + Processes, Technical Note 3

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