On off-nadir wind retrieval over the sea surface using APR-2 or similar radar geometry

Abstract

ABSTRACTIn some cases, nadiral wind retrieval algorithms are not applicable for wind measurements due to the specificity of a radar instrument measuring geometry or its mounting on aircraft, and so off-nadir backscattering measurements can only be performed. In this paper, a method for off-nadir wind retrieval over the sea based on the sea-surface backscattering measurements with the Airborne Precipitation Radar-2 or similar radar measuring geometry has been proposed. The study is based on a theory of the quasi-specular microwave backscattering from the sea surface. An anisotropic water surface approach is considered. The wind vector is retrieved from the system of equations for measured normalized radar cross sections (NRCS) by using the relationships between the speed of the sea-surface wind and the standard deviations of the sea-surface slopes along the up- and cross-wind directions. A wind vector recovery with the cross-track scanning measurement geometry has been simulated. The maximum errors of the wind speed and direction estimations are within typical errors of wind measurements by a scatterometer. A maximum altitude of the method applicability for this geometry has been defined. Recommendations to perform the measurements have been described in detail.