Azimuthal variations of the sea surface brightness temperature: comparison of the model spectra calculations with aircraft observations

One of the important applications of microwave radiometry of the ocean is remote sensing of the near-surface wind vector. Due to anisotropy of wind-generated waves, the brightness temperature exhibits azimuthal variations. Knowledge of model functions, that describe the dependence of the azimuthal harmonics versus wind speed and other parameters, is critical for the retrieval of a wind vector from the radiometric signal. In moderate wind conditions, surface roughness is a major factor determining thermal microwave radiation. The sea wave spectrum is a key parameter relating the observed brightness temperature with wind speed and wind stress. During recent years, models of the sea wave spectrum were intensively developed to explain the features observed by microwave radars. The authors study how such models agree with radiometric observations and, in particular, with the amplitude of the second azimuthal harmonic of the brightness temperature contrast. The azimuthal variations of the brightness temperature at vertical and horizontal polarizations are described by the first and second harmonics T_b=A₀+A₁Cos(φ)+A₂ Cos(2φ) where φ is the angle between the wind direction and the observation plane. The authors consider only the second harmonic A₂ which is directly related with the sea wave spectrum