ASCAT calibration and validation, version 1.0

In this report we present normalisation corrections to the current experimental L1B backscatter data as provided by EUMETSAT during the commissioning phase of MetOp-A. Based on the OSI SAF cone visualisation tools at KNMI and the CMOD5 wind sensitivity, improved calibration of the ASCAT scatterometer is attempted. Indeed, the ASCAT wind product shows similar characteristics to the ERS 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 +0.4 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.

An experimental ASCAT level 2 wind product stream is set up at KNMI using the pre-operational ASCAT level 1b stream at 25 km sampling as input. The level 1b σ0 stream is corrected using linear scaling factors in the transformed z domain, which correspond to addition factors in the logarithmic domain (dB). These changes correspond to resetting the ASCAT instrument gain per beam and per Wind Vector Cell (WVC).

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.