CMOD5.N: A C-band geophysical model function for equivalent neutral wind.

TitleCMOD5.N: A C-band geophysical model function for equivalent neutral wind.
Publication TypeMiscellaneous
Year of Publication2008
AuthorHersbach, H
Secondary TitleTechnical Memorandum
Number554
Abstract

This document describes the evaluation of a C-band geophysical model function. This model function, called CMOD5.N, is to provide an empirical relation between C-band backscatter as sensed by the space-born ERS-2 and ASCAT scatterometers, and equivalent neutral ocean vector wind at 10-metre height (neutral surface wind) as function of (scatterometer) incidence angle. CMOD5.N embodies a refit of CMOD5, a C-band model function which was previously derived to obtain non-neutral surface wind, in such a way that its 28 tuneable coefficients lead, for given backscatter observation, to an enhancement of 0.7ms-1 in wind speed. The value of 0.7ms-1 is chosen to be independent on wind speed and incidence angle, and incorporates the average difference between neutral and non-neutral wind (approx 0.2ms-1) and for a known bias of CMOD5 (approx 0.5ms-1)when compared to buoy wind data. The quality of the CMOD5.N fit is tested for the AMI scatterometer on ERS-2 and ASCAT instrument on METOP-A for July 2007 and January 2008. From this it is found that winds inverted with CMOD5.N are on average 0.69ms-1 stronger than winds determined from CMOD5. As function of wind speed and incidence angle, fluctuations are well within 0.05ms-1 . Differences in wind direction are small. ASCAT and ERS-2 wind speed obtained from CMOD5.N compares on average well with operational neutral wind from the European Centre for Medium-Range Weather Forecasts (ECMWF). In comparison with non-neutral wind, local, seasonally dependent biases between scatterometer and ECMWF model are reduced. Besides effects introduced by orography and ocean currents, a residual stability-dependent bias between scatterometer and neutral wind remains, which is likely connected to a previously reported non-optimality in the ECMWF boundary layer formalism by Brown et al. in 2006.