In this report we evaluate normalisation corrections to the demonstration L1b ASCAT backscatter data as provided by EUMETSAT and called“one transponder”data set. Based on the OSI SAF cone visualisation tools at KNMI and the CMOD5 wind sensitivity, calibration of the ASCAT scatterometer is checked. Improved consistency in the KNMI corrections is found, suggesting improved L1b calibration. 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 experimental ASCAT wind product shows similar characteristics to the demonstration ASCAT scatterometer wind product and meets the wind product requirements.
Deviations between scatterometer and Numerical Weather Prediction wind derived backscatter show a significant improvement. Without correction the difference ranges from +0.3 dB to -0.8 dB going from the inner side to the outer side of the swaths. After that the scaling correction is applied the difference ranges from -0.2 dB to +0.3 dB and the experimental L1b data show smaller interbeam differences.
The demonstration ASCAT level 2 wind product stream run at KNMI using the pre-validated ASCAT level 1b stream at 25 km sampling as input may be maintained without any significant effects on product quality. The new L1b σ0 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 ASCAT products become available.
J Verspeek, M Portabella, A Stoffelen, A Verhoef. Calibration and Validation of ASCAT Winds, version 2.1