Wave Field Properties in Tropical Cyclones From the Spectral Observation of the CFOSAT/SWIM Spaceborne Instrument

Abstract

Tropical cyclones (TCs) are extreme events that generate, because of their movement, complex wave fields. The Surface Wave Investigation and Monitoring (SWIM) instrument is a real aperture radar that provides unprecedented detailed information about the waves with dominant wavelength between 70 and 500 m in all directions at the global scale. In this study we collocated 3 years of SWIM data with 67 TCs in the Northern Hemisphere to analyze the impact of the TC characteristics on the wave field. TCs have been classified into three different classes (slow, moderate speed, and fast) estimated based on the ratio between the maximum sustained wind and the displacement velocity. In order to analyze the characteristics of the wave field in the space domain, the observations have been separated according to the distance with the TC center and the quadrant. The results show that the characteristics of the TCs impact the wave field: the more favorable conditions for trapped wave phenomenon appears to be under moderate speed TC conditions. In slow and moderate speed TCs, close to the center the directional spectra are mono-modal and tend to become bi- or multi-modal when the distance to the center increases whereas in fast-moving TCs, the directional spectra are always bi- or multi-modal. Omni-directional spectra show similarities with fetch-limited spectra in slow and moderate speed TCs whereas in fast-moving TCs, because of the presence of mixed-sea, the decrease of energy with frequency is less steep than in fetch-limited conditions.