performance of Aeolus in heterogeneous atmospheric conditions using high-resolution radiosonde data

The European Space Agency Aeolus mission aims
to measure wind profiles from space. A major challenge is
to retrieve high quality winds in heterogeneous atmospheric
conditions, i.e. where both the atmospheric dynamics and optical
properties vary strongly within the sampling volume. In
preparation for launch we aim to quantify the expected error
of retrieved winds from atmospheric heterogeneity, particularly
in the vertical, and develop algorithms for wind error
correction, as part of the level-2B processor (L2Bp).
We demonstrate that high-resolution data from radiosondes
provide valuable input to establish a database of collocated
wind and atmospheric optics at 10m vertical resolution
to simulate atmospheric conditions along Aeolus’ lines
of sight. The database is used to simulate errors of Aeolus
winds retrieved from the Mie and Rayleigh channel signals.
The non-uniform distribution of molecules in the measurement
bin introduces height assignment errors in Rayleigh
channel winds up to 2.5% of the measurement bin size in the
stratosphere which translates to 0.5ms−1 bias for typical atmospheric
conditions, if not corrected. The presence of cloud
or aerosol layers in the measurement bin yields biases in Mie
channel winds which cannot be easily corrected and mostly
exceed the mission requirement of 0.4ms−1. The collocated
Rayleigh channel wind solution is generally preferred because
of smaller biases, in particular for transparent cloud
and aerosol layers with one-way transmission above 0.8.
The results show that Aeolus L2Bp, under development,
can be improved by the estimation of atmosphere optical
properties to correct for height assignment errors and to identify
wind solutions potentially detrimental when used in Numerical
Weather Prediction.