Tropical Cyclone Wave Products

Tropical Cyclone Wave Products

Introduction

Wave spectra in tropical cyclones vary strongly per quadrant and provide information about the current and past state of the wave field.

However, inside TCs, waves measurements including the wave system direction, energy and wave- length are rare and difficult to obtain with in-situ and remote sensing technics. The first documentation of the sea surface directional wave spectrum in all quadrants of a hurricane in open water was obtained on 24 August 1998, when NOAA and NASA combined their resources to carry a Scanning Radar Altime- ter into Hurricane Bonnie (Wright et al., 2001). However, airborne radar cannot be used to characterize TC-generated waves at global scale. Indeed, most of the NOAA airborne data remain very limited in the vicinity of north America. Historically, in-situ observations of extreme events like tropical cyclones have been achieved large, moored buoy platforms (e.g. those from the NDBC). However their deployement is limited and in the past 37 years less than 100 observations made by moored NDBC buoys with winds > 25 m/s have been obtained (Tamizi and Young, 2020). To note, recent efforts to deploy drifting buoys measuring 2D ocean wave spectra are on-going and will certainly increase this number.

Synthetic Aperture Radar (SAR) instruments on board satellite and in particular the SAR series developed since ERS-1 by ESA and now ESA/Copernicus with Sentinel-1 are good candidate to provide these ocean waves systems characteristics thanks to the dedicated acquisition mode : the so-called Wave Mode. Wave spectra can be derived from modulations of the backscatter in SAR images. The effective azimuth resolution (cutoff) is determined by the radial velocity variance of the surface, which is high in TCs and therefore limits the use of SAR backscatter spectra for two-dimensional wave-spectra retrieval to the outer bands of intense storms.

Under the wind forcing waves grow and waves propagation velocity increase accordingly to possibly escape from the wave generation area and then propagate in the ocean as swell. Far enough from their source, satellite acquisitions are expected to be able to observe these ocean swells during more favorable met-ocean conditions for waves retrieval inversion.

The analysis of swell measurements far from their area of generation to locate the storm source has been firstly applied to data from one single in-situ wave station (wave energy with frequency and direction) collected 2 miles off shore from San Clemente Island, California by Munk et al. (1963) . Such approach has been extended to a network of several wave stations by Snodgrass et al. (1966).

 

The gathering of swell system observed far from a storm to characterize the waves properties across the ocean has already proven to be efficient in the case of extra-tropical storms (Collard et al., 2009; Ardhuin et al., 2009; Delpey et al., 2010) [Delpey, Collard, Snodgrass]. This method is now operationally applied to Level-2 Sentinel-1 wave measurements to provide swell corresponding to storm in the Level- 2 Copernicus wave product (CMEMS, 2013).

Yet, such analysis is not adapted to Tropical Cyclone whose size is much smaller and the Level-4 Copernicus wave product does not allow for an accurate monitoring of the tropical cyclones swells. The Sentinel-1 tropical cyclone waves product intends to fill this gap and opens for an alternate way of estimating tropical cyclone waves properties over all ocean basins and for all tropical cyclones.

Example

the above Figure presents an example of waves characteristics generated by tropical cyclone SURIGAE on 2021-04-19 at 18:00 ± 3 hours as obtained with the processing method presented in the previous chapters. This analysis is done with the data stored in the Level-3 TC WV product : WP022021_SURIGAE_S1WV_l3_TCWAV.nc.

In fact, in this particular case, waves whose back propagation trajectory focused on SURIGAE on 2021-04-19 at three successive track time step 15:00, 18:00 and 21:00 UTC are considered. On the top-left corner, 3 of the main parameters of the TC evolution are presented as function of time in days since the start of the track: the maximum wind speed at each step, the translation speed, and the maximum wind radius. The vertical bar indicates the time of analysis for the TC wave presented

here. On the right hand-side, a synthetic figure of all wave partitions that focused on SURIGAE at 18:00 (± 3 hours) are displayed on a map. The TC track as well as the location of wave partition measurements and their associated back propagation trajectories are respectively plotted with colored circles and colored lines. The color of wave propagations and TC track represents time in days since the start of the track (colorbar on the left of the map). Regarding wave partitions, the size of circles at measurement locations depends on the local significant wave height of the partition (legend on the top left of this map). The color of these circles depends on the wavelength of the partition (colorbar on the bottom right of the map). Regarding the TC track, the black star represents the position of the tropical cyclone on the examined date and time (here 18:00 on 2021-04-19). The black circle is the Rhw of the cyclone at this track step and the black arrow provides its direction of translation.

The polar plot on the bottom left of this figure is the so-called ”wave rose” obtained for this TC track time step. All wave partitions presented on the map are also on this diagram. Radial axis represents their wavelength and polar angle is the direction of propagation relatively to TC direction of translation at the focusing time. The upwards black arrow on the middle of the diagram represents TC direction of translation reference.

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