Aeolus + Processes, Technical Note 2

The report presents results of the Work package 2 (WP2) of ESA Contract No.
4000139422/22/I-NS which focuses on Aeolus effects on the vertical momentum fluxes
(VMFs) in the tropics. The VMFs are affected by Aeolus through changes in the horizontal
winds and indirectly through vertical motions. The vertical velocity and VMFs are crucial
variables to be realistically represented by high-resolution weather and climate models.
In particular, the VMFs drive the stratospheric quasi-biennial oscillation (QBO) which
experienced the second disruption on the record during the Aeolus operations.
The results show that the assimilation of Aeolus winds leads to a systematic, albeit
small, reduction of the vertical velocity in the ECMWF system. It is related to the sys-
tematic reduction of the horizontal wind divergence reported in Technical Note 1. A
significant impact of Aeolus in the kinetic energy spectra of vertical motions was found for
the large-scale Kelvin waves with the strongest effect during the QBO disruption in early
2020.
During the QBO disruption, corrections of the background zonal wind in the lower
stratosphere by Aeolus were particularly large. Changes in the analyses that increased the
vertical shear of the zonal wind were aligned with changes in the VMFs. Average forecast
improvements by Aeolus of a few % with respect to NoAEolus increased to about 15%
improvements in day 1 forecasts in the lower stratosphere early in 2020 (see the figure). In
normal QBO conditions such as summer 2020, Aeolus winds improved the transition from
the easterly to westerly phase of the QBO. This shows as an alignment of wave forcing
(i.e. the vertical gradient of the VMFs) with the changes in the background flow. In
summer 2020, the forcing was largely due to the Kelvin wave and eastward-propagating
inertia-gravity waves.
When measured by differences in zonally averaged fluxes within 10°S and 10°N, the
effects of Aeolus winds on the VMFs are relatively small (below 1%). However, effects
at individual scales can be large depending on the background flow and wave motions.
In early 2020, the period of the QBO disruption, the assimilation of Aeolus winds in the
ECMWF system resulted in up to about 20% change in stratospheric VMFs with respect
to NoAeolus.