Aeolus + Processes, Final Report

The report presents the results of ESA Contract No. 4000139422/22/I-NS, offering in-
sights into processes and model features influenced by the assimilation of Aeolus horizontal
line-of-sight (HLOS) winds into the ECMWF system.
Using observing system experiments (OSEs), the project provides strong evidence that
Aeolus’ impact on tropical easterlies and westerlies is closely linked to the background
wind shear. The assimilation of HLOS winds enhances the vertical shear of the zonal-
mean zonal wind in the tropical upper troposphere and lower stratosphere (UTLS). This
enhancement arises from the effect on the zonal-mean balanced flow, which is strengthened
by the assimilation of HLOS wind profiles. These effects on the zonal mean state were most
pronounced during the disruption of the quasi-biennial oscillation (QBO) in the winter of
2019/2020. During this period, Aeolus assimilation led to significant changes in vertical
momentum fluxes in the lower stratosphere compared to experiments without Aeolus data.
The assimilation of Aeolus winds amplifies the zonal wind amplitudes of equatorial
Kelvin and n = 1 Rossby waves in the tropical tropopause layer (TTL). This impact is
asymmetric, reflecting the stronger background shear in easterlies, and underscores the
importance of observing shear zones within the UTLS to reduce tropical analysis uncer-
tainties. Through 4D-Var, Aeolus also affects the meridional winds associated with mixed
Rossby-gravity (MRG) waves. The largest effects on equatorial Kelvin and n = 1 Rossby
waves occur at planetary scales, while for MRG waves, they appear at synoptic scales. Ad-
ditionally, Aeolus assimilation systematically reduces the amplitude of divergent circulation
(inertia-gravity modes) and associated vertical velocity at subsynoptic scales.
The strong coupling between Aeolus’ effects and background shear highlights the im-
portance of vertical resolution in follow-on missions. It is recommended to maximize the
vertical resolution of wind profile measurements, especially within the TTL.
Aeolus and COSMIC2 exhibit complementary effects across several evaluated metrics
and processes. Both datasets influence the zonal-mean zonal wind by sharpening the verti-
cal shear zones associated with the QBO. The 4D-Var system generates pronounced effects
on meridional winds from both Aeolus and COSMIC2 data, emphasizing uncertainties in
cross-equatorial flow. However, differences in the balance properties of Aeolus and COS-
MIC2 analysis increments − Aeolus assimilation tends to reduce unbalanced meridional
wind in the stratosphere, whereas COSMIC2 assimilation tends to increase it − require
further investigation.

A special long-term OSE using an ERA6 prototype demonstrated positive impacts
of Aeolus assimilation on analyses and forecasts, with no detrimental effects from data
gaps or variations in data quality. Consequently, Aeolus winds will be assimilated into
the forthcoming European reanalysis, ERA6, ensuring the enduring scientific value of the
Aeolus dataset for climate applications.
The project also developed a novel method to quantify the predictability impact of
observing systems across scales. Applying this method to a set of forecasts from December
2022 revealed that Aeolus data increased predictability by approximately three hours within
the TTL at the 3-day forecast range.