Aeolus + Processes, Technical Note 1

The report presents results of Work package 1 (WP1) of ESA Contract No. 4000139422/22/I-
NS, focusing on the quantification of the impact of the assimilation of Aeolus winds on
tropical circulation in the ECMWF system.
The analysis is based on a two-year long observing system experiment (OSE) using
Aeolus winds on top of all operationally assimilated observations in the ECMWF 4D-Var
system. The Aeolus experiment is contrasted with an experiment which did not use Aeolus
data (NoAeolus). In addition to the Kelvin, MRG and n = 1 Rossby waves, effects of
Aeolus winds are studied in n > 1 Rossby modes and in unbalanced circulation made
of the eastward- and westward-propagating inertia-gravity modes (EIG and WIG modes,
respectively). Physical-space analysis is complemented by wave space statistics.
Strong evidence is presented that the assimilation of Aeolus winds increased the ver-
tical wind shear within the tropical upper troposphere and lower stratosphere (UTLS)
in the ECMWF analyses. The assimilation of HLOS profiles increased the Kelvin and
n = 1 Rossby wave amplitudes in the UTLS with respect to the background flow with the
strongest effect in 100 − 180 hPa layer. The largest effects on the Kelvin wave
are found at planetary scales. The effects on Rossby waves are largest at synoptic scales
(figure), and similar applies to the MRG waves with the impact produced by the 4D-Var.
Globally, the n = 1 Rossby is a wave signal with the greatest benefit from the assimilation
of Aeolus winds. Systematic effects of the assimilation of Aeolus winds are found in the
divergent circulation projecting onto the WIG and EIG modes with Aeolus systematically
attempting to reduce the amplitude of EIG and WIG modes in the tropics. This will be
further studied in WP5.