Estimating Bimodal Frequency-Direction Spectra From Surface Buoy Data Recorded During Tropical Cyclones

Abstract

Techniques for computing wave frequency-direction spectra are evaluated using wave buoy data recorded on the North West Shelf of Australia. Frequency-direction spectra are computed by each of four techniques and compared with the wave directionality expected at the measurement location, based on the meteorological conditions prevailing at the time. Particular emphasis is placed on the wave conditions occurring during tropical cyclones. Such sea states show strong directional bimodality, consisting largely of a cyclone-generated swell component and a local wind sea. The cyclone swell can be expected to have a narrow beam width when the cyclone is several hundred kilometers from the location. It is found that the bimodality is well indicated by a unimodal/bimodality parameter; and of the four techniques for computing the frequency-direction spectrum, the maximum entropy method is best at reproducing the directional bimodality while maintaining the narrow directional spreading in the cyclone swell.