ORE Seminar: Toward a Consistent Analysis of Tropical Cyclones Winds and Waves

Recent satellite missions offer a unique opportunity to consistently analyse Tropical Cyclone generated ocean surface waves and associated ocean surface wind field. This includes mostly Synthetic Aperture Radar (SAR) and altimeters missions for waves, and radiometers and SAR for wind. The measurements principles, capabilities and complementary of each mission is presented.

Ocean surface wind measurements from radiometers benefit from very large coverage, high revisit time and are weakly sensitive to rain. They provide medium resolution O(50km) wind measurements that can be used for 34-, 50- and 64-kts wind radii estimates. This provides direct information on the storm size. SAR missions are more limited in coverage but the SAR is the only space-borne instrument able to probe at very high resolution O(10m) and over all ocean basins the sea surface under extreme conditions. This allows direct measurements in the inner core. Although these are sun-synchronous missions, we will show that their combination allows to describe Tropical cyclones properties during their whole lifetime. Significant wave height of ocean waves is provided by altimeters with no information on the waves direction or wavelength. In a very complementary way, SAR can provide the significant wave height but also the swell wavelength and direction. However, these swell parameters cannot be derived when wind speed is too strong. These limitations can be overcome by analysing the hurricane generated waves when they have traveled out of the strongest wind area of the storm. Indeed, well systems are fingerprints of the extreme ocean storms, and can propagate all the way across ocean basins from the area of high winds that generated them. Very long-period swells have been observed to propagate up to halfway around the globe. Space-time analysis of ocean swell parameters measured from space at global scale can thus be used to monitor the same swell field over 3 to 10 days along their initial propagation direction. The proper combination of regularly sampled acquisitions over the ocean surface at different places and times can yield the position of the generating storm and predictions for the arrival time of swells with different wavelengths and directions. In order to consistently analyse hurricane properties (intensity, extent, speed) and Tropical Cyclones (TC) generated waves, a refocusing technique has been developed to precisely study the distribution of the swell wavelengths and directions as measured by Sentinel-1 with respect to the hurricane track far from their generation area. All swell detected systems are back-propagated in space and time using the estimated group velocity. Swell-current interactions are neglected, and back-propagated tracks are then cross-assigned with the hurricane Best Track, i.e. the space-time location of the hurricane center. For a given location, we select all swell systems with trajectories crossing the storm center, e.g. found +/- 3 hours around the track time and within a 2° by 2° box around the storm-center.

Based on these remote sensing data, we analyse the distribution of hurricane generated waves wavelengths with respect to their direction of propagation outside of the TC together with the hurricane characteristics given by SAR and radiometers. Impact of the extended fetch effect is discussed.

Event Sponsor
Ocean and Resources Engineering, Mānoa Campus

More Information
(808) 956-7572, http://www.soest.hawaii.edu/ore/event/seminar_181128/

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