Oceanic wind vector determination using a dual-frequency microwave airborne radiometer theory and experiment

Abstract

Passive ocean-viewing microwave radiometers produce promising results for measuring the near-surface wind vector by measuring the thermal emission from the combined rough surface and atmosphere. The brightness temperature variations of the rough ocean surface provide the necessary information for determining the near ocean surface wind speed and direction. Ocean wind direction measurements have only recently been obtained by microwave radiometers. These recent wind direction measurements showed that ocean brightness temperature variations with respect to azimuth angle (wind direction) varied by a few Kelvin. All these radiometers measured the azimuthal dependence of vertically- and horizontally-polarized ocean brightness temperatures. Brightness temperature measurements at other polarization states (four Stokes parameters) are also sensitive to near surface ocean wind direction. This paper focuses on a nonpolarimetric aircraft-based dual-frequency dual-polarized microwave radiometer that was developed at the Environmental Technology Laboratory (ETL) of the National Oceanic and Atmospheric administration (NOAA). The lower frequency at 23.87 GHz is horizontally-polarized with respect to the aircraft's flight direction and the 31.65 GHz frequency is vertically-polarized to that direction. This radiometer was one of several instruments deployed in ETL's first Advanced Sensor Applications Program (ASAP) experiment. This experiment was named the San Clements Ocean Probing Experiment (SCOPE). The main purpose of the airborne microwave radiometer was to determine if it could measure azimuthal-dependent brightness temperature variations from a rough ocean surface, and thus infer the near-surface wind vector. Furthermore, in order to gain a better understanding on the potential of the NOAA dual-frequency radiometer for near-surface wind vector determination, the SCOPE radiometer measurements are compared with a new combined atmospheric and two-scale ocean surface model.