NWP Ocean Calibration for the CFOSAT wind scatterometer and wind
retrieval evaluation
Abstract
The unique rotating fan-beam feature of SCAT onboard CFOSAT leads to
varying geometries across the swath and furthermore leads to varying
wind retrieval performance across the swath. The Wind Vector Cells
(WVCs) across the swath are classified into outer, sweet and nadir. The
sweet WVCs contain the most diverse geometries, which lead to the best
wind retrieval performance. In the outer WVCs the azimuth and incidence
angle diversity is very limited and the number of views is smallest,
which makes the wind retrieval the most ambiguous and difficult to
improve. Secondly, in order to improve the wind retrieval, two kinds of
NWP Ocean Calibration (NOC) are applied. One is a NOC as a function of
incidence angle (NOCinc). The other one is a newly developed NOC as a
function of incidence angle and antenna azimuth angle (NOCant), which
takes the rotation angle into account. The NOCant correction results in
better fits of the Geophysical Model Function than the NOCinc correction
does, except for the outer WVCs, where the limited diversity of views
determines retrieval quality. The results show that NOCant correction
improves the wind speed Probability Distribution Function per WVC and
reduces the average wind direction bias and the relative wind direction
(relative to the satellite motion direction) biases, as compared to
NOCinc correction. In conclusion, the rotating fan-beam feature of SCAT
leads to unique and various data characteristics across the swath.
Overall the performance of the proposed NOCant correction is better than
NOCinc and improves the wind statistics.