Retrieving significant wave height in the Indian Ocean near Visakhapatnam using Jason-2 altimeter data

Five-parameter model (BETA5) has been used to derive significant wave height (SWH) using 20-Hz Jason-2/OSTM Ku band waveforms in the Bay of Bengal basin of the Indian Ocean near the Indian city of Visakhapatnam for the years 2010-2012. Majority of the waveforms are traditional ocean-like waveforms based on Brown model. Nevertheless, quite a few of them have shapes differing significantly from the Brown model. Historically, two standard methods have been used for waveform retracking. One is maximum likelihood estimator (MLE) with 4 parameters, i.e. MLE4 (OCEAN retracker), which uses analytical second-order Brown model and the second one using first-order Brown model is MLE with three parameters, i.e. MLE3. It has been found in the study that standard MLE retrackers are not appropriate for retracking the nontraditional waveforms. Estimated SWHs have been validated against SWH measured by a buoy located in the close proximity of Visakhapatnam. SWH from BETA5 retracker matches buoy-derived SWH quite well except for a positive bias. Also, the bias is less for SWH retrieved using BETA5 than using MLE retrackers. Quantitatively, at a distance of 8km from the coast correlation coefficient between BETA5 retracked SWH and buoy measured SWH is 0.85 and the unbiased root mean square error (RMSE) of the retracked SWH is 0.26m. These are better than the corresponding values for MLE3 and OCEAN retrackers.