A satellite altimeter model for ocean slick detection

Abstract

About 5% of Ku-band altimeter ocean data are degraded by the occurrence of high radar return cross sections (σ0), usually called σ0 blooms. During blooms, which occur during no or low wind conditions, the mean altimeter waveform can significantly depart from the expected shape. In about 60% of the cases the waveforms are distorted to such an extent that either the range tracker loses lock or the off-nadir angle estimate becomes unrealistic. The analysis of high data rate altimeter waveforms during bloom events reveals the presence of V-shaped patterns similar to the ones observed during rain events. These patterns trace small-scale (i.e., smaller than the altimeter footprint) changes in surface backscatter. Such variations of surface roughness are commonly observed in SAR images under low wind conditions. On the basis of the experience gained through the analysis of high-resolution altimeter waveforms in the presence of rain cell, a model is developed to analyze the altimeter response to phenomena whose length scale is smaller than the altimeter footprint. The model is applied to simple patterns (linear slicks and circular patches) as well as to realistic surface σ0 estimated by SAR. It is also used to analyze bloom events in terms of surface slicks. The model results shows that the small-scale σ0 variations explain the behavior of altimeter waveforms in bloom events. The results also show that a good proportion of data during bloom events are still valid for estimating geophysical parameters as the Brown model remains valid. Use of high-resolution altimeter waveforms may also offer an interesting mean to study marine slick occurrence rates and type. Copyright 2006 by the American Geophysical Union.