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Air-Sea Interaction under Stormy and Hurricane Conditions: Physical Models and Applications to Remote Sensing

Final Report Summary - ASIST (Air-Sea Interaction under Stormy and Hurricane Conditions: Physical Models and Applications to Remote Sensing)

The ASIST project “Exchange Programme Air-Sea Interaction under Stormy and Hurricane Conditions: Physical Models and Applications to Remote Sensing (ASIST)” comprises four EU groups from four countries (Keele University (KU), Mediterranean Institute for Oceanography (MIO) and Heidelberg University (HU)), the Finland Meteorological Institute (FMI) and a team from the Institute for Applied Physics Russian Academy of Sciences (IPF RAS). The project aimed to radically advance our understanding of air-sea interaction under hurricane conditions and to develop a new scientific basis for better modelling and remote sensing of storms and hurricanes. The partners assets included had three truly unique large wind-wave facilities. The project was built on deep multilayered collaboration between the consortium partners manifested in an intense exchange of ideas, people, techniques, data and know-how. The exchange was in no way confined to the secondments (14.1 person-months from the EU and 19.8 person-months to the EU). The partners had regular working group meetings and performed joint experiments in the third countries throughout the duration of the project, the intense exchange of expertise was continued via email, skype and other electronic channels. The partners were able to generate significant funding from national funding agencies, which enabled the participants to accomplish their ambitious research programme.

Under stormy conditions, the interacting boundary layers of atmosphere and ocean become multiphase: the layer of the atmosphere adjacent to the water surface is saturated with spray, foam areas cover substantial part of the water surface, while the near-surface water layer is saturated with air bubbles. This leads to radical changes in the ocean-atmosphere energy and gas exchange. Very little was known about the specifics of the air sea interaction physics under such conditions. Empirical data obtained in field conditions were sparse and of poor quality (e.g. the difference in concentration of spray in the near the water surface layer of the atmosphere given by different authors reaches 3 orders of magnitude). There were no reliable models of spray generation. Crucially, the existing methods of retrieving wind remotely from satellite microwave observations cease to work at winds exceeding 27 m/s, which makes remote sensing of wind speeds in the hurricanes impossible. The challenge was to understand, quantify and model the specifics of the air-sea interactions under such conditions and to develop the scientific basis for new microwave methods enabling satellite monitoring of very strong winds over the sea.

The strategy adopted was first to investigate the key air-sea interaction processes in the controlled laboratory environment, then, to model or parametrise them and, finally, check the modelling or parameterisation in field conditions by employing available observations. To this end the consortium extensively used three large wind-wave facilities with unique complementary capabilities: Thermostratified High Speed Wind-Wave Tank IAP RAS (N-Novgotod), AELOTRON (Heidelberg) and LASIF (Marseille). The project produced many first rate scientific results briefly described in the report and reflected in more than 60 publications, here we mention just two.

1. The main physical mechanisms of the anomalous air-sea interactions at high wind have been identified and attributed to spray and foam. Employing high-speed video recording the dominant mechanism of spray generation under strong winds has been revealed and shown to be caused by the phenomenon of “bag-breakup” fragmentation, in which thin-walled "membranes" are blown by wind from the water wave crests. These membranes then “explode” with the formation of a large number of droplets (see Fig. 1). The droplets have been shown to be the dominant mechanism of the heat exchange. A quantitative model of spray generation in the storm wind, supported by direct numerical simulations, has been developed. The presence of foam has been found to decrease roughness and, hence, to affect the momentum transfer by decreasing the drag. These findings enabled us to put forward new models of energy and momentum transfer between the ocean and the atmosphere under storm conditions, which explain the observed anomalies of energy and momentum exchange.

2. In operational weather forecasting the preferred satellite remote sensing methods for monitoring the speed and direction of winds over the sea are relying upon the use of electromagnetic waves of the microwave band, since for waves of this band the intense cloud cover, typical of storm conditions, is transparent. However, the available wind speed recovery algorithms cease to work under conditions of intense storms, since in the dependence of microwave scattering wind speed there is a saturation if either horizontal or vertical polarization is used. To address this challenge new methods of microwave remote sensing of the ocean surface have been considered, in particular, the approach based upon processing the scattered cross polarisation signal, which does not have the above-mentioned drawback (the signal does not saturate for very high winds). New methods for microwave diagnostics of the sea surface and wind require knowledge of poorly understood specifics of microwave scattering on the complex water surface (wave braking, foam, spray) and multiphase flow in the boundary layer of the atmosphere. The consortium has made a major advance in addressing this gap. In particular, the wind speed dependence of the cross section of the X-band microwave co- and cross-polarisation backscattering has been established (Fig. 2a). On its basis and the found empirical drag law, a novel geophysical model function (GMF) relating the scattering cross section to the wind speed has been proposed. The obtained GMF can be used to retrieve the wind speed from the scatterometry data. On this basis a new effective algorithm for retrieving the hurricane wind speed from microwave backscattering data has been developed and tested on field data for Hurricane Earl (Fig. 2b).

The results obtained in the project significantly advance our understanding and provide a solid scientific basis for better modelling and remote sensing of storms and hurricanes.