Effects of sea spray on tropical cyclone intensity

The intensity of tropical cyclones is sensitive to the rates at which entha lpy and momentum are transferred between sea and air in the high-wind core of the storm. Present models of the wind dependence of these transfer rates suggest that the effective drag coefficient is more than twice the effecti ve enthalpy transfer coefficient at wind speeds above 25 m s(-1). Using thi s ratio in numerical models, however, makes it impossible to sustain storms of greater than marginal hurricane intensity. Some other physical process must, therefore, enhance enthalpy transfer at very high wind speeds. This p aper suggests that re-entrant sea spray explains this enhanced transfer. Wh en a spray droplet is ejected from the ocean, it remains airborne long enou gh to cool to a temperature below the local air temperature but not long en ough to evaporate an appreciable fraction of its mass. The spray droplet th us gives up sensible heat and returns to the sea before it has time to extr act back from the atmosphere the heat necessary to continue its evaporation . Microphysical modeling, combined with data from the Humidity Exchange ove r the Sea Experiment (HEXOS), makes it possible to derive an expression for the net enthalpy transfer of re-entrant spray. This spray enthalpy flux is roughly cubic in wind speed. When this relation is used in a numerical sim ulation of a hurricane, the spray more than compensates for the observed in crease in the ratio of drag and enthalpy transfer coefficients with wind sp eed. The momentum flux associated with sea spray is an important energy sin k that moderates the effects of this spray enthalpy flux. Including a param eterization for this momentum sink along with wave drag and spray enthalpy transfer in the hurricane simulation produces results that are similar to o nes based on equal transfer coefficients.