Cross-polarization geophysical model function for C-band radar backscattering from the ocean surface and wind speed retrieval

The wind speed sensitivity of cross-polarization (cross-pol) radar backscattering cross section (VH) from the ocean surface increases toward high winds. The signal saturation problem of VH, if it exists, occurs at a much higher wind speed compared to the copolarization (copol: VV or HH) sea returns. These properties make VH a better choice over VV or HH for monitoring severe weather. Combined with high spatial resolution of the synthetic aperture radar (SAR), the development of hurricane wind retrieval using VH is advancing rapidly. This paper describes a cross-pol C-band radar backscattering geophysical model function (GMF) with incidence angle dependence for the full wind speed range in the available data sets (up to 56 m/s). The GMF is derived from RADARSAT-2 (R2) dual-polarization (dual-pol) ScanSAR modes with 300 and 500 km swaths. The proposed GMF is compared to other published algorithms. The result shows that the simulated VH cross section and the retrieved wind speed with the proposed GMF is in better agreement with measurements. With careful treatment of noise, the VH-retrieved wind speeds may extend to mild or moderate conditions. The higher fraction of non-Bragg contribution in VH can be exploited for analysis of surface wave breaking.