Based on the OSI SAF cone visualisation tools at KNMI and the CMOD5 wind sensitivity, calibration of the ASCAT scatterometer is checked. In this report we describe and evaluate normalisation corrections to the pre-operational L1b ASCAT backscatter data version PPF6.3.0 as provided by EUMETSAT based on their three transponder calibration campaign. For the left mid antenna a “wiggle” in ocean calibration results has disappeared with respect to the former version, suggesting improved L1b calibration as anticipated. In the outer swath consistent large departures remain, which need checking against other ancillary geophysical data sources to gain confidence in their validity. Indeed, still the ASCAT wind product based on L1b version 6.3.0 shows very similar characteristics to the ASCAT scatterometer wind product based on L1b version 6.2.0 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 +0.5 dB to -0.4 dB going from the inner side to the outer side of the swaths. Also, the PPF 6.3.0 L1b data show smaller interbeam differences and the wiggle in the left mid beam antenna has disappeared. After the scaling correction is applied the difference ranges from -0.1 dB to +0.5 dB and is almost identical for the PPF6.3.0 and the PPF6.2.0 L1b data.
The pre-operational OSI SAF ASCAT level 2 wind product stream runs at KNMI using the pre-validated ASCAT level 1b stream at 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.
In concert with EUMETSAT more detailed aspects of the ASCAT scatterometer L1b product and L2 product are currently being tested as more calibrated ASCAT products become available.
J Verspeek, M Portabella, A Stoffelen, A Verhoef. Calibration and Validation of ASCAT Winds, version 4.0
2008, EUMETSAT, 2008