Theory and measurement of oceanic wind vector using a dual-frequency microwave airborne radiometer

The near-surface wind speed and direction create a rough ocean surface consisting of a variety of waves and foam. Consequently, the microwave emissivity of the ocean surface is not only a function of the sea skin temperature and salinity but also of the surface roughness and foam distribution. A sensitive microwave radiometer can measure the surface emissivity (brightness temperature) variations and the data can be used to infer the near-surface wind speed and direction. This paper examines the inferred wind vector measurements from data taken by an airborne dual-frequency microwave radiometer using real ocean and atmospheric measurements. Furthermore, the inferred wind vector is computed using both ocean surface and atmospheric models specific to the parameters of this radiometer. The experimental and theoretical results are compared.; An expression for the total brightness temperature observed by an airborne radiometer viewing the ocean is derived. The effects of the cosmic background radiation, the atmosphere, the rough ocean surface, and the radiometer's antenna power patterns are included. Atmospheric emission is calculated by a unique radiative transfer equation (RTE) algorithm. The rough ocean surface is modeled as a two-scale surface. Unlike many two-scale formulations, this one allows for the small-scale roughness to be similar to the incident electromagnetic wavelength which is necessary to model the capillary-ultragravity wave region more realistically. In addition, a realistic surface slope profile is incorporated into the large-scale features. New developments for the complex dielectric properties of sea water at microwave frequencies and the sea skin temperature measurements are incorporated into this model.; A suite of atmospheric and oceanographic measurements were taken during a 1993 remote sensing experiment of the ocean surface. The in situ wind vector measurements are compared with the experimental and theoretical brightness temperature results. This comparison shows that this microwave radiometer can measure the wind vector for certain atmospheric and oceanic conditions. Furthermore, both theory and measurements produce an upwind/downwind asymmetry in the brightness temperatures results. This feature has recently been measured by passive microwave radiometers. The comparison may provide some insight into the small-scale features.