On SAR hurricane wind speed ambiguities

Radar backscattered signals over the ocean are dampened in extremely high winds, which leads to a wind speed ambiguity problem during the process of wind retrieval from synthetic aperture radar (SAR). This problem was firstly studied by Shen et al. (2007), where we proposed a wind speed ambiguity removal scheme for the two wind speed solutions that may exist for any given normalized radar cross section (NRCS) and wind direction. This approach is based on the operational geophysical model function (GMF) CMOD5. Recently, new C-band GMFs for high wind have been developed, among which, a RH polarized GMF was established for the first time. In this study, the wind speed ambiguity problem will be studied within the context of the available high wind GMFs, which are, CMOD5, CMOD4HW, HWGMF_V and HWGMF H. For the wind retrieval from HH polarized SAR images, a hybrid empirical polarization ratio is generally adopted. To compare the different behavior of various GMF models, this polarization ratio is used to transform the HH polarized GMF into a VV field. Although the wind speed ambiguity problem is found in most GMFs, the saturation wind speed where radar backscattered signals start to decrease is different for the various GMFs. We show that consideration of the wind speed ambiguity problem is important for high wind retrieval from SAR images, especially for category 5 hurricanes.