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Enhancements in the determination of ocean surface wave height from grazing incidence microwave backscatter

M.D. Henschel et al. (1995) described a procedure for estimating ocean surface wave height from the shadowing statistics of a grazing incidence marine radar image of the sea surface. Their model, based on the geometrical optics approach of L.B. Wetzel (1990), related the normalized lengths of the regions of illumination ("islands") and shadow ("troughs") to the significant wave height through a parameter they called the "threshold probability of illumination" (P/sub 0/). The present authors have carried out several field data collection exercises in the past few years in which a very large number of grazing incidence microwave images of the sea surface were recorded. They have used these images to calibrate and to validate the M.D. Henschel et al. model. Preliminary results, have shown that the model performed well for wave heights close to those for which it was calibrated, but significantly underestimated larger wave heights. Analysis of this failure mode, and of much more data from high sea state situations, has shown that the normalized island lengths do not decrease as quickly as the model predicts they should. The appearance of these larger islands implies that the P/sub 0/ is not a constant, but decreases with increasing wave height. A significant correlation also exists between the calculated P/sub 0/ and the mean non-shadowed NRCS of the images in the upwave direction. Recalculation of the significant wave height, based on a varying P/sub 0/ estimated from the above-mentioned NRCS value, produces a much better estimation of the measured significant wave height over a wider range of wave heights than did the original estimates based on a fixed P/sub 0/.