A Study of Wind Direction Signals in WindSat Linearly Polarized Channels

The atmosphere has a significant effect on Wind- Sat measured brightnesses, particularly the linearly polarized channels. Due to the large size of these effects, measurement of the small azimuthal wind direction signal in the linearly polarized channels can be extremely difficult, and these channels are largely excluded or de-emphasized in current wind direction retrieval algorithms. In this work, attempts to determine the amplitudes of wind direction signals in WindSat linearly polarized channels are reported through a simple data binning procedure. Knowledge of these amplitudes as a function of environmental parameters will provide a first step in developing future wind direction retrieval algorithms that improve use of the linearly polarized channels. Several choices of the ground-truth data used in the binning process are examined, and the statistical significance of the harmonics obtained with each is quantified. Azimuthal harmonic coefficients using ground truth information from WindSat EDR 1.8.1 are found to be the most significant for moderate water vapor cases. The resulting dependence of the azimuthal harmonics on atmospheric parameters is investigated, and attempts to express this dependence in terms of atmospheric attenuation are described.