Tuesday, 13 January 2009: 2:15 PM
Spray-mediated enthalpy flux to the atmosphere and salt flux to the ocean in high winds
Room 128A (Phoenix Convention Center)
Emanuel (1995, J. Atmos. Sci.
, 3969–3976) identified the delicate balance in tropical cyclones between the enthalpy that the ocean gives up to the storm to energize it and the mechanical losses through drag on the ocean surface that weaken it. He quantified this balance as the ratio of the enthalpy transfer (Ck
) and drag (CD
) coefficients, Ck
. Only when Ck
is at least 0.75 do modeled storms reach hurricane strength. The catch, though, is that when measurements of Ck
made in moderate winds are extrapolated to hurricane wind speeds, Ck
is much smaller than 0.75. Andreas and Emanuel (2001, J. Atmos. Sci.
, 3741–3751) therefore proposed that air-sea enthalpy transfer mediated by sea spray could augment the usual interfacial enthalpy transfer (parameterized as Ck
) and deduced an algorithm to predict this spray enthalpy flux using data from HEXOS, the experiment to study Humidity Exchange over the Sea.
Here I update that spray flux algorithm by adding flux data from FASTEX, the Fronts and Atlantic Storm-Tracks Experiment, to our previous analysis. Andreas and Emanuel introduced the idea that re-entrant spray droplets accomplish this enthalpy transfer. These droplets are flung into the near-surface atmosphere, give up sensible heat (i.e., they cool) and water vapor to the atmosphere, and then fall back into the sea, thereby cooling the ocean and completing the enthalpy transfer from sea to air. But because these re-entrant droplets have also lost pure water, they are saltier than the surface water when they return and, thus, also produce a salt flux at the sea surface. No one has ever considered this effect; I will therefore evaluate that spray salt flux here and present an algorithm to estimate it. This salt flux to the surface will enhance ocean mixing and, therefore, is a crucial buoyancy term in coupled ocean-atmosphere models.