In MAXSS added-value products will be developed in terms of radial extent and vortex structure information.
Tropical Cyclone (TC), Extra-tropical Cyclone (ETC) and Polar Low (PL) wind structures are provided in terms of the maximum radial extent. For example, for TC’s this is 34, 50, and 64 knots (kt; 1 kt 5 0.51 m s 21 ) or gale force, damaging, and hurricane force winds in quadrants surrounding the TC. These are collectively referred to as wind radii.
Low and medium resolution sensors will be used for the Multi-Mission Wind Products and the SAR data will be used to provide the wind radii corresponding to radius of maximum wind in each of the four geographical quadrants.
SAR high resolution data will be used to produce a collection of vortex structure analyses including wind speed and direction estimates as well as TC (PL or ETC) wind speed decomposition in harmonics and rain bands location. This product will be made available in the vortex referential and used both in the scientific applications and to build/validate the Multi-Mission wind products of this project.
the storm wake composites
Wake composites will be derived based on the storm Atlas ocean and atmospheric data. Starting from the storm track and associated spatial domains and the period of analyses around each type of storm (TC, ETCs, PLs). A tropical cyclone wake is generally characterized by a surface cold anomaly, possibly accompanied with nutrient blooms. Moreover, governed by intense isopycnal displacements (Geissler, 1970), a tropical cyclone can also leave prominent sea-surface height anomalies in its wake. Resulting surface depressions can reach 0.3-0.5 m, depending upon the forcing intensity, size, translation speed, and ocean stratification conditions (Kudryavtsev et al., 2019).
Building on the actual satellite altimeter constellation (presently up to 6 satellites are available, see Table 3), satellite sea surface height estimates may more likely cross such trenches. Using both sea surface height, temperature and salinity observations, a consistent view of the tropical cyclone characteristics and oceanic impact can thus be obtained. Composite anomalies left after the storm passage will be built and statistically combined as a function of distance to storm track, storm translation speed, storm intensity and upper ocean pre-storm conditions. For a given storm these signatures can then be analysed with the ocean surface wind forcing from our new SWS product and the additional support of a semi-empirical 2D model.
For ETCs and PLs, the ‘wake’ composites will be developed using an advanced methodology involving the coordinate transform of cyclones into a non-dimensional azimuthal coordinate system and the further collocation of fields following Rudeva and Gulev, . As for the TCs, ETCs and PLs composite radial/azimuthal analyses will be performed for air–sea turbulent fluxes, heat content, precipitable water, and precipitation, surface wind, SST, SSS, SSH, ocean color, and upper-ocean pre-storm hydrological conditions. These composites will be used to infer statistical oceanic response to storm passage for different environmental conditions.
the storm wind and wave structure parameters
These added-value products will serve to assess the capability and limitation of our SWS product to reproduce the storm structure.
In the case of Tropical Cyclone, TC parameters such as wind radii ( gale-force (34 kt), damaging (50 kt), and destructive wind (64 kt) radii), maximum wind radii Rmax and intensity Vmax will be evaluated and compared to best-track (except Rmax). SAR data will be used to assess the expected impact of resolution on Rmax estimate. In addition, integrated parameters such as Accumulated Cyclone Energy (ACE) or the Wind Power Index (WPI) will be derived. Both ACE and WPI are relevant measures of the TC-induced wake and TC-generated as they integrate the maximum wind speed in time, allowing one to account for the fact that slower storms transfer more momentum to surface currents and hence trigger more mixing in the ocean subsurface. Moreover, they can also be used for seasonal analysis.
In the case of Polar Low and Extra-Tropical Cyclones, the criteria to describe the structure still need to be fully defined. We plan to rely on existing metrics such as the maximum effective radius for ETCs (typically approached at the moment of the minimum SLP) and the cyclone asymmetry ratio h (which will be estimated as the ratio between the smallest and the largest cyclone diameters with the smallest one lying within ∓ 20° of the diameter orthogonal to the largest (Rudeva and Gulev 2007)).
Wave structure metrics will be used as added-value products as well to define wave generation sources to be compared to spatio-temporal characteristics of the MM winds. The proper combination of SAR data at different places and times can yield the position of the generating storm (Heimbach and Hasselmann, 2000, Husson et al., 2012), and predictions for the arrival time of swells with different wavelengths and directions. An ensemble of data acquired at different times and locations can thus be gathered to document a given extreme event. This approach shall be applied to ETC (see Firework illustration from CMEMS product further below) and TC systems, to collect swell observations from SAR, SWIM, and also in situ buoys, outside of the generation area (where wind and rain are expected to be less intense) and also in the generation area from altimeter (TC-adapted editing) to analyse the waves field from a given storm and compare wavelength, direction and significant wave height distribution with parametric wave models.