VHAMP: Vertical and Horizontal Aeolus Measurement Positioning - Executive Summary

Aeolus aims to measure wind profiles from the surface up to about 30 km altitude. In 2010 it was
decided to change the Aeolus laser operation from pulsed burst mode to continuous mode, to ensure
sufficient measurement stability during flight. A further major adjustment is the change in pulse
repetition frequency from 100 to ~50 Hz. Based on these changes to the Aeolus measurement
sampling, there was a need to reinvestigate which would be the optimized horizontal and vertical
sampling and measurement averaging strategy for the Aeolus mission, ensuring maximized impact of
the data in Numerical Weather Prediction (NWP) and general atmospheric circulation modeling. The
underlying VHAMP study addresses this question taking into account atmospheric dynamical and
optical characteristics and their interaction with the Aeolus measurement system. The main conclusion
from the study are
1. Along track integration of 85-100 km is needed to meet the mission requirement for wind quality.
Bin sizes should be at least 1 km for the Rayleigh channel in the lower troposphere increasing to
1.5 and 2 km in the upper troposphere and lower stratosphere. The Mie channel is expected to be
most relevant for zero wind calibration with 250 m bins near the surface. It is advantageous to
change the vertical sampling along track, i.e., positioning Mie bins up to 11 km over the Poles and
up to 18 km over the tropics to sample tropical cirrus.
2. The effective horizontal resolution of global models is 200-500 km, i.e., substantially lower than
the typical model grid size of 10-20 km. Along track averaging over 85-100 km of Aeolus
measurements will yield kinetic energy spectra similar to global model spectra, i.e., the resulting
wind observations are in agreement with model resolved turbulence and the representativeness
error of Aeolus winds will be negligible. The effective vertical resolution of the ECMWF model is
1.7 km based on an intercomparison study with radiosonde winds.
3. Wind error biases exceeding 0.5 ms-1 and wind error correlations exceeding 0.1 (corresponding to
a random error increase of 0.2 ms-1) are detrimental for Aeolus impact in NWP.
Aeolus will have a substantial beneficial impact for NWP, comparable to radiosondes. Reducing laser
power from 110 mJ to 80 mJ marginally reduces Aeolus impact for NWP. However, this assumes that
laser power reduction does not affect calibration procedures and quality control and classification
procedures of the level-2 processor. These aspects were out of scope of this study.