A new Geophysical Model Function (GMF) is being developed for C-band scatterometers. CMOD7 will be used for ERS and ASCAT wind retrievals and is valid for their combined incidence angle range. An intermediate GMF (CMOD6.2) is used as a starting point. For CMOD6.2 there exists a wind vector cell (WVC) dependency in the scatterometer wind speed probability distribution functions (pdf). Also local maxima arise in the wind speed pdf at wind speeds of 2 and 5 m/s.
A higher-order calibration (HOC) correction is applied for each WVC to match the scatterometer wind pdf to a predefined wind speed pdf. The predefined wind speed pdf is a combination of a theoretical function with fitted parameters on the ascending side, and a mean scatterometer pdf on the descending side. The HOC largely reduces the WVC-dependency of the wind speed pdf and also reduces the local maxima in the wind speed pdf.
Wind speed biases for the various scatterometer products, ERS-1, ASCAT-A and ASCAT-B (for 25 km WVC size and for the coastal product of 12.5 km WVC size) with respect to NWP stress-equivalent wind are compared to each other. NWP Ocean Calibration (NOC) is applied for ASCAT (using operational EWMF stress-equivalent winds) and for ERS (using ERA-interim stress-equivalent winds). Difference in bias caused by these differences in used NWP wind field and differences in the ECMWF software version, are compensated for.
The resulting CMOD7 shows good results on all relevant parameters, wind speed and direction, wind speed bias, cone distance and quality control rejection rate for both ERS and ASCAT scatterometers. Triple collocation with NWP and buoy winds also show good results.
JA Verspeek, A Stoffelen. ASCAT/ERS higher order calibration