Calibration and Validation of ASCAT Winds, version 5.1

Based on the OSI SAF cone visualisation tools at KNMI and the NOC corrections, calibration of the ASCAT scatterometer is checked. In this report we describe and evaluate level 1b corrections to the operational L1b ASCAT backscatter data version PPF740 as provided by EUMETSAT based on their three transponder calibration campaign. For the left mid antenna an offset in ocean calibration results has disappeared with respect to the former version, suggesting improved L1b calibration as anticipated. For all antennas the “wiggles” have reduced in amplitude. In the outer swath consistent large departures remain, which may be included in an updated version of the geophysical model function, i.e., CMOD5na. Indeed, still the ASCAT wind product based on L1b version PPF740 shows very similar characteristics to the ASCAT scatterometer wind product based on L1b version PPF730 and meets the wind product requirements.

Deviations between scatterometer and Numerical Weather Prediction wind derived backscatter still show a significant improvement after correction. Without correction the difference ranges from +0.4 dB to -0.8 dB going from the inner side to the outer side of the swaths. Also, the PPF 740 L1b data show smaller interbeam differences and the wiggles in the antennas have been reduced. After the NOC correction is applied the difference ranges from -0.12 dB to +0.26 dB and is almost identical for the PPF740 and the PPF730 L1b data.

The operational OSI SAF ASCAT level 2 wind product stream runs at KNMI using the validated ASCAT level 1b stream at 12.5 km and 25 km sampling as input, and may be maintained without any significant effects on product quality. The new L1b sigma0 stream will be corrected using the new linear scaling factors in the transformed z domain, which correspond to addition factors in the logarithmic domain (dB). These changes correspond to slightly resetting the ASCAT instrument gain per beam and per Wind Vector Cell (WVC) in order to maintain the backscatter data consistency and wind product quality.