Effects of wind variability on scatterometry at low wind speeds

The dependence of the normalized radar cross section of the sea on wind variability within the resolution cell is examined by considering probability distributions of cross sections and wind vectors. If a threshold wind speed exists below which backscatter is negligible for steady winds, variability of the wind over the resolution cell is shown to cause significant backscatter at mean wind speeds below the threshold. In fact, if the variability is sufficiently high, cross sections become essentially constant at very low wind speeds. The viability of this model is tested by comparing its predictions based on the NASA scatterometer 2 (NSCAT2) model function with probability distributions obtained from NSCAT cross sections that are collocated with buoy measurements. Both the overall probability distribution of cross sections and the probability of negative cross sections obtained from the NSCAT data are shown to be in good agreement with the predictions. A means of improving the accuracy of low wind speed scatterometer measurements is suggested when wind variability is not too high.