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Air-Sea Interactions at Strong and Extreme Wind Speeds

Invita OCEANOGRAFIA FISICA
Fecha 2019-12-06, 11:30:00 hrs
Lugar: Auditorio Pedro Ripa - Edificio de Oceanologa
Ponente(s): Alexander V. Babanin
Professor, Ocean Engineering
Department of Infrastructure Engineering
Melbourne School of Engineering
The University of Melbourne

Resumen:  The presentation will outline recent advances in understanding and modelling Metocean conditions at strong winds. These include dynamics of surface waves, Wave Boundary Layer in on the atmospheric side of the interface, and upper ocean mixing. This will lead us to conclusions on the most essential uncertainties and urgently needed measurements at high winds, potentially those capable to improve Metocean forecast in such conditions.

Furthermore, we will discuss two topics of wave-air-sea interactions at extreme wind speeds. The first topic is dedicated to the change of regime for such interactions, by comparison to benign, moderate and even strong winds. As a reference point for the extreme Metocean conditions, the hurricane-scale classification is often used: that is a tropical storm becomes a hurricane if the wind speed reaches U~33m/s (120 km/h). We argue that such classification is not arbitrary, and indeed signifies change of the physics (dynamics and thermodunamics) in all environments near the air-sea interface: in the atmospheric boundary layer, at the surface, and through the upper ocean. This threshold is approximately the wind speed at which the drag coefficient was found to saturate in the field and laboratory observations (U10≈32-33m/s), which saturation has received a lot of attention lately. Less known are the observations that below the surface, change of the upper-ocean mixing mechanism and of bubble dynamics occurs at U10>35m/s. Directly at the surface, wave dynamics also undergoes essential transformations, from wave breaking (dissipation) being driven by evolution of nonlinear waves, to the breaking being forced directly by the winds, at U10≈34m/s. Linked with this, remarkable enhancement of the interface gas-transfer rates occurs at these wind speeds.

The second topic will highlight potential differences between air-sea interactions at extreme winds in tropical and powerful extra-tropical cyclones. It is argued that possible reasons for such behaviour may be due to different physics of wave evolution in compact hurricanes and in large storms.

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