How sea spray can affect the intensity of tropical cyclones

The intensity of tropical cyclones is sensitive to the rates at which enthalpy and momentum are transferred between sea and air in the high-wind core of the storm. The best current estimates of the wind dependence of these transfer rates show that the effective drag coefficient is more than twice the effective enthalpy transfer coefficient at wind speeds above 25 m/s. Using this 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. We suggest that re-entrant sea spray accomplishes this enhanced transfer. When a spray droplet is ejected from the ocean, it cools to a temperature below the local air temperature in a time that is short compared to typical droplet residence times but long compared to the time required for it to evaporate an appreciable fraction of its mass. The spray droplet thus gives up sensible heat and returns to the sea before its continued evaporation can extract its price in latent heat from the atmosphere. From microphysical modeling, we derive an expression for the net enthalpy transfer mediated by re-entrant spray as a function of the spray generation rate for each droplet size. Summing this enthalpy transfer over all the droplet sizes and combining that result with data from HEXOS (the Humidity Exchange Over the Sea experiment) implies a function for the spray enthalpy flux that depends very strongly on wind speed. When this relation is used in a numerical simulation of a hurricane, the spray more than compensates for the observed increase in the ratio of drag and enthalpy transfer coefficients with wind speed. The momentum flux associated with sea spray is an important energy sink that moderates the spray enthalpy flux. Including a parameterization for this momentum sink along with wave drag and spray enthalpy transfer in the hurricane simulation produces results that are similar to ones based on constant and equal transfer coefficients.