The VIERS radar backscatter model development for wind field retrieval

Traditionally, the operational retrieval algorithms for scatterometers relating the radar backscatter measurements to surface wind vectors have been empirical. Since, however, the backscatter reflects in some way the state of the high frequency waves, the assumption that the backscatter only depends on the local wind speed may be questioned. It is known that the spectrum of gravity-capillary waves not only depends on the wind but is determined by a number of physical processes, namely wind input, nonlinear interactions, viscous dissipation and dissipation due to slicks. Thus, for strong winds when the waves are steep, nonlinear interactions may become so important that the state of the gravity-capillary waves is determined by the longer gravity waves. The backscatter then depends on the history of the wind field. On the other hand, for low wind speed dissipation due to slicks may be relevant in determining the shape of the gravity-capillary spectrum, again suggesting that not only the local wind speed determines the backscatter. The above considerations prompted the VIERS-1 group to perform an extensive investigation into the dependence of the radar backscatter on geophysical parameters. As a result, a scatterometer algorithm was developed based on the present understanding of backscatter and of the relevant processes governing the shape of the gravity-capillary spectrum. The scatterometer algorithm was validated thoroughly against laboratory measurements. The VIERS algorithm was also applied to field conditions. Assuming that the ECMWF wind and wave fields are accurate the backscatter was calculated with the VIERS algorithm and compared the results with the backscatter as obtained from the ERS-1 satellite. It is concluded from this comparison that the VIERS-1 algorithm is performing well, even compared to CMOD4.