Abstract's details

Ocean wave fields under tropical cyclone conditions as evidenced by SWM/CFOSAT

Eva Le Merle (CNR-ISMAR, Italy)

CoAuthors

Daniele Hauser (LATMOS (CNRS, UVSQ, Sorbonne Université), France); R Santoleri (CNR-ISMAR, Italy); C Yang (CNR-ISMAR, Italy)

Event: 2022 Ocean Surface Topography Science Team Meeting

Session: CFOSAT

Presentation type: Type Forum only

Several studies have shown in the past that the wave fields generated by TCs are asymmetric with higher significant wave height (Hs) in the right front quadrant (with respect to the TC track) (Young, 2006, Kudryavstev et al, 2015). Simulation studies indicate that this asymmetry increases with the TC displacement speed (Kudryavstev et al., 2015, 2021) up to a certain limit. Correlatively, almost in all quadrants of the TC, the shape of the wave spectra at frequencies about the peak look like those for fetch limited conditions (Young, 2006; Hu and Chen, 2011) even when the wave spectra tend to become bi- and tri-modal in direction (Young 2006; Hu and Chen, 2011; Esquivel-Trava et al., 2014; Hwang & Walsh 2018). Young (2006) suggested that in TCs, the nonlinear interactions are the dominant process that controls the shape of the omni-directional spectra even if the wave energy is contained in several wave components with different directions.

The SWIM instrument on-board the CFOSAT mission provides directional ocean wave spectra every 70 km along-track for waves with dominant wavelengths between 70 and 500 m. SWIM provides an extremely interesting data source to further extend the previous analyses carried out with satellite data (only Hs information from altimeter were used), extend those obtained from buoy observations, and assess analytical or modeling studies.

During the conference, we will report on a study carried out by investigating wave parameters estimated from the SWIM instrument in 46 events of the Northern hemisphere in 2019 to 2021. For this study we have classified TCs in three different categories following the suggestion made by Kudryavtsev et al. (2021): slow, medium and fast moving TCs. This has been done by using to the ratio of the maximum wind speed to the translation speed of the TC, provided by the TC data base IBTRACS. Furthermore, the wave parameters estimated from SWIM along its tracks, have been placed in a common reference frame, which takes into account the displacement direction of the cyclone and the distance of the observations with respect to the radius of maximum wind.

Several parameters have been investigated as the significant wave height, the dominant wavelength, the wave age, the frequency spread or peakedness of the omni-directional spectrum, the directional spread, the direction of the dominant waves, the ratio of energy and difference in direction between the first and second partitions. We will show that the assymetry in the fields of several of these parameters depends of the TC class (slow, medium or fast displacement speed). This seems to qualitatively confirm the analytical and numerical approach proposed by Kudryavtsev et al (2021), with however some differences which will be discussed during the conference. With the SWIM data set, we also find that although most regions of the cyclones are characterized by waves not any more forced by the wind, the omni-directional spectra follow adimensional laws which look like those established in the past for fetch-limited situations.

 
Eva Le Merle
CNR-ISMAR
Italy
eva.lemerle@artov.ismar.cnr.it