Air-Sea Interaction and Sea-spray in Typhoons (ASIST)

The exchange of heat and moisture between the ocean and atmosphere depends upon turbulent mixing in the lower atmosphere; this depends strongly on the wind speed. At very high wind speeds the sea surface becomes dominated by breaking waves, whitecaps, and sea spray. The water droplets lofted into the lower atmosphere by turbulent air motions will undergo two interactions with the air around them. First, if they are at a different temperature from the air, they will cool or warm until their temperature matches that of the air; secondly, they will start to evaporate, contributing water vapour to the air. Evaporation requires energy, which is obtained initially by a cooling of the droplet, and then a transfer of heat from the air to the droplet. The net result of these processes is a transfer of heat and moisture between ocean and air at some altitude above the surface. The simplified mathematical descriptions of of heat and moisture exchange used within weather forecast and climate models assume that all the heat and moisture exchange takes place at the surface, thus they may provide the wrong values at high wind speeds. The difficulty of making appropriate measurements under severe storm conditions at sea means that there are very few measurements available with which to evaluate whether the spray generated actually has a significant impact or not. Theoretical studies disagree strongly about the importance of the effect. In this study we will instrument autonomous buoys, specifically designed to operate in extreme conditions, to measure the turbulent exchange of heat, moisture, momentum, and sea-spray between the atmosphere and ocean, along measurements of wave state, turbulence in the surface layer of the ocean, the extent of wave breaking, and mean conditions in the surface layers of the ocean and atmosphere. The data obtained will be used to evaluate the air-sea exchange, and determine the contribution of the sea spray to the total exchange of heat and moisture. The measurements will also be used to better define the rate of sea-spray aerosol generation under high winds, and the influence of wave state on the production rate, both of which are poorly defined. Improving understanding of air-sea interaction is particularly important for high wind conditions, because the development and track of severe storms - hurricanes and typhoons - is critically dependent on the heat and moisture input and frictional drag at the surface. This study will ultimately lead to improved prediction of severe tropical storms. The study will be carried out in close collaboration with the University of Miami, Florida, who operate the buoys from which the measurements will be made. The buoys will be deployed in the East/South China Sea in June 2010 and recovered during October 2010.