The fallacy of the enthalpy transfer coefficient in high winds

            In a widely cited 1995 paper published in Journal of the Atmospheric Sciences, Kerry Emanuel demonstrated the delicate balance in tropical cyclones between storm energy lost through drag on the sea surface and energy gained through enthalpy transfer from the ocean.  In his hurricane simulations, Emanuel modeled the air-sea drag with the coefficient C_D and the enthalpy transfer with the coefficient C_K.  He reported that hurricane models perform best when C_K/C_D is in the range 1.2–1.5 and further noted that hurricanes do not form at all (in models), even in ideal conditions, when C_K/C_D is less than 0.75.

            This paper was important for air-sea interaction research because it focused attention on the need to better understand the physics of air-sea exchange in high winds:  The data and flux parameterizations available in 1995, which treated winds up to only about 20 m/s, did not satisfy Emanuel's stated constraints on C_K/C_D.  Observations in moderate winds found C_K/C_D to be much less than 0.75 and, thus, spurred researchers to understand the discrepancy.

            But Emanuel's paper also had an adverse effect on our discipline:  Many have inferred from it that the enthalpy transfer coefficient, C_K, is the best way to model how ocean heat influences storms and, furthermore, that the enthalpy transfer coefficient is a single-valued function of wind speed.  These assumptions have impeded progress on storm modeling because both are fallacious.

            In high winds, when copious amounts of sea spray are present, two routes exist for enthalpy to cross the air-sea interface:  through exchange controlled by molecular processes right at the air-sea interface (the interfacial route), and through spray-mediated processes (the spray route).  Because the two transfer processes scale differently with wind speed and other mean meteorological quantities, the enthalpy transfer coefficient is not single-valued and, thus, cannot be used to accurately predict the air-sea enthalpy flux in high winds.  I will demonstrate these shortcomings of the enthalpy transfer coefficient by using a state-of-the-art bulk flux algorithm (that treats both interfacial and spray routes) to generate total air-sea enthalpy fluxes for a wide range of environmental conditions.  The C_K values inferred from these fluxes increase with wind speed but also become increasingly scattered with increasing wind speed because of the spray-mediated transfer.  That is, C_K is demonstrably not single-valued.  Storm models, thus, must explicitly treat both the interfacial and spray routes by which enthalpy crosses the air-sea interface.