NWP Ocean Calibration for the CFOSAT Wind Scatterometer and Wind Retrieval Evaluation
SCAT onboard CFOSAT is the first rotating fan-beam scatterometer in orbit since Oct 2018. The
unique rotating fan-beam feature leads to varying geometries across the swath and furthermore
leads to varying wind retrieval performance across the swath. The Wind Vector Cells (WVCs) are
classified into three categories across the swath, which are outer, sweet and nadir. The sweet
WVCs contain the largest number of views and the most diverse geometries, which lead to the best
wind retrieval performance among these three types. In the outer-swath 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 for these WVCs. The azimuth angles
of the nadir WVCs are mainly around 0° and 180°, but the nadir WVCs contain the large range of
incidence angles. 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), which
has been successfully implemented on fixed fan-beam and rotating pencil-beam scatterometers.
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. Both methods are able to correct the
wind speed bias and improve the wind retrieval effectively. However, the NOCant correction
results in better fits of the Geophysical Model Function (GMF) 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
(PDF) per WVC. Moreover, NOCant reduces the average wind direction bias and the relative wind
direction (relative to the satellite motion direction) biases, as compared to NOCinc correction. The
relative wind direction biases at nadir WVCs reduces the most with the NOCant correction,
whereas it stays relatively similar in sweet and outer swath WVCs. This is likely because the
NOCant provides a better calibration at the nadir azimuth angles of 0 and 180 degrees. 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.
