Towards a unified theoretical model of ocean backscatter for wind speed retrieval from SAR, scatterometer, and altimeter

We discuss the development of a theoretical model of radar backscatter from the ocean surface which is applicable to radar altimeters, synthetic aperture radars (SAR), and scatterometers. The ultimate objective of such a model is to allow the accurate retrieval of wind speed by including all the physically important factors. Our formulation combines alternative models of the interaction between the wind and the sea surface, different descriptions of the ocean wave height spectrum and two alternative scattering theories, namely the composite-surface model and the integral equation method. The effects of swell, limited fetch, air-sea temperature difference, and rain are included. The model predictions are compared against empirical scatterometer model functions at C band, VV polarisation, where the observational behaviour has been most thoroughly studied. Published results from our model at nadir (see companion paper by Anderson et al. 2002, in this issue) agree well with the empirically observed dependence on wind speed, and the dependence on swell is consistent with recent studies. Here, results at off-nadir incidence angles show a residual discrepancy in the upwind direction which has the dimensional dependence predicted by Phillips for the radar backscatter from breaking waves. However, systematic discrepancies in the upwind to crosswind ratio indicate that the directional spread of waves is still not well understood in the theoretical modelling. Similar behaviour is also observed at Ku band, VV polarisation. The implications for the retrieval of wind speed from individual scatterometer or SAR data-takes are discussed.