A two-scale model to predict C-band VV and HH normalized radar cross section values over the ocean

Abstract

This paper presents a two-scale model that can predict C-band, VV polarized (C-VV), normalized radar cross section values generated from the CMOD4 model and calibrated normalized radar cross section values derived from C-band, HH polarized (C-HH), RADARSAT-1 synthetic aperture radar imagery with coincident buoy observations of the local wind. The model is based on standard composite models that incorporate tilt modulations, with a new approach for incorporating hydrodynamic modulations. It is shown that inclusion of the hydrodynamic term does not significantly impact the fit to normalized radar cross section (NRCS) values, but does allow the model to accurately predict the upwind to downwind C-VV ratios and improves the model fit to simultaneous C-VV and C-HH NRCS observations. The model uses a new wave height spectrum that is a linear combination of the Apel and Romeiser spectra, weighted heavily toward the Apel form, and has modified values within the C-band Bragg wavenumber regime which are close to midway between the two spectra. The final model can represent the CMOD4 C-VV NRCS results with a root-mean-square error (RMSE) of 0.47 dB and can represent the RADARSAT-1 C-HH NRCS observations, without any change to the model parameters, with an RMSE of 1.9 dB. The model can accurately reproduce the upwind to downwind C-VV NRCS ratios from CMOD4 (RMSE = 0.15 dB) and fits simultaneous C-VV to C-HH ratios derived from aircraft observations to within 1 dB (RMSE = 0.65 dB). If the C-HH hydrodynamic term is allowed to be scaled differently than the C-VV hydrodynamic term, it lowers the RMSE for the simultaneous C-VV to C-HH aircraft observations to 0.49 dB while not affecting the fit to C-VV and C-HH NRCS values. Compared to other C-HH models published in the literature, this model provides a better fit to the RADARSAT-1 C-HH NRCS data across a larger range of conditions than any other single model, provides a better fit to simultaneous C-VV and C-HH NRCS data at an incidence angle of 20°, and reproduces the decreasing trend in the C-VV to C-HH ratio with increasing wind speed observed in data.