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.

On Surface Waves Generated By Extra-Tropical Cyclones. Part II: Simulations

“…Nevertheless, shape of the scaled total spectrum (where swell dominates) is again surprisingly close to the JONSWAP spectrum. This observation is similar to what was reported by [42] and [43] under hurricane conditions, -shape of the dominant wave spectra, regardless of whether they are wind waves or swells, is very close to the shape of the JONSWAP spectrum.…”

“…Spectrum outside the swell front exhibits a wave system of remarkably shorter waves (as compared with swell in box III) with a wide angular spread. Similar to the case with east-north swell, the scaled omnidirectional spectra, k p S(k)/e, on different sides of the swell front are very similar and well consistent with the shape of the scaled JONSWAP spectrum, again confirming the experimental finding by [42] and [43] for waves in hurricanes. The histograms of distribution of the model energy of wave-trains over wavelength and directions shown in the third column, are also well consistent with 2D SWIM spectra.…”

On Surface Waves Generated by Extra-Tropical Cyclones—Part II: Simulations

“…Nevertheless, shape of the scaled total spectrum (where swell dominates) is again surprisingly close to that of the JONSWAP spectrum. This observation is similar to what was reported by [42,43] under hurricane conditions; the shape of the dominant wave spectra, regardless of whether they are wind waves or swells, is very close to the shape of the JONSWAP spectrum.…”

“…The spectrum outside the swell front exhibits a wave system of remarkably shorter waves (as compared with swell in box III) with a wide angular spread. Similar to the case with northeast swell, the scaled omnidirectional spectra, k p S(k)/e, on different sides of the swell front are very similar and consistent with the shape of the scaled JONSWAP spectrum, again confirming the experimental finding by [42,43] for waves in hurricanes. The histograms of distribution of the model energy of wave-trains over wavelength and directions, shown in the third column of Figure 12, are also consistent with 2D SWIM spectra.…”