CMOD5.n - the CMOD5 GMF for neutral winds

At the OSI SAF 3rd user workshop in Amsterdam (September 2007), several users requested the OSI SAF to produce neutral winds rather than real 10m winds.

Currently, the CMOD5 Geophysical Model Function (GMF) is used to retrieve ASCAT winds, but it is modified such that 0.5 m/s is added to the wind speeds. This is done since we know from ERS scatterometer wind statistics that CMOD5 winds underestimate the real 10m winds by approximately 0.5 m/s. The statistical difference between the current CMOD5 + 0.5 and the CMOD5.n (neutral) winds is another 0.2 m/s in all conditions studied. As such, the OSI SAF plans to add 0.2 m/s to their ASCAT winds by implementing CMOD5.n.

Following the above recommendation, ECMWF has fitted new CMOD5.n coefficients according to a desired CMOD5 + 0.7 m/s behaviour. KNMI subsequently produced a CMOD5.n Lookup Table and tested the retrieved Maximum Likelihood Estimator (MLE) and winds against the CMOD5 + 0.7 m/s, hereafter referred to as CMOD5.7, retrieval outputs.

This report describes the validation of the CMOD5.n coefficients and shows that the winds retrieved with the new GMF closely compare to those retrieved by CMOD5.7.