Synthetic Aperture Radar Observations of Extreme Hurricane Wind and Rain

Abstract

Over the last decades, data from spaceborne Synthetic Aperture Radar (SAR) have been used in hurricane research. However, some issues remain: (1) many SAR images capture incomplete hurricane core structures; (2) the radar signal is attenuated by the heavy precipitation associated with hurricane; (3) wind directions retrievals are not available from the cross-polarized SAR measurements. When wind is at hurricane strength, the wind speed retrievals from co-polarized SAR may have errors because the backscatter signal may experience saturation and become double-valued. By comparison, wind direction retrievals from cross-polarization SAR are not possible until now. In this study, we develop a two-dimensional model, the Symmetric Hurricane Estimates for Wind (SHEW) model based on the mean wind profile in all radial directions, and combine it with the modified inflow angle model to detect hurricane morphology and estimate the wind vector field imaged by cross-polarization SAR. By fitting SHEW to the SAR derived hurricane wind speed, we find the initial closest elliptical-symmetrical wind speed field, hurricane center location, major and minor axes, the azimuthal (orientation) angle relative to the reference ellipse, and maximum wind speed. This set of hurricane morphology parameters, along with the speed of hurricane motion, are input to the inflow angle model modified with an ellipse-shaped eye, to derive the hurricane wind direction. A one-half modified Rankine vortex (OHMRV) model is proposed to describe the hurricane wind profile, particularly for those wind profiles with a wind speed maximum and an inflection point possibly associated with the degeneration of the inner wind maximum in the hurricane reintensification phase. The proposed method works well in area with significant radar attenuation by precipitation. Moreover, five possible mechanisms for the rain effects on the spaceborne C-band SAR observations are investigated: (1) attenuation and (2) volume backscattering for the microwave transfer in atmosphere; as well as (3) diffraction on the sharp edges of rain products, and (4) rain-induced damping to the wind waves and (5) rain-generated ring waves on the ocean surface.