The exchange of sensible and latent heat over the sea

Sammanfattning: Analysis of the neutral exchange coefficients for latent and sensible heat, CEN and CHN, based on measurements from the Östergarnsholms site in the Baltic Sea has been made. The data set used for the CHN study was based on the very accurate MIUU-instrument. These data showed that CHN increase with increasing wind speed where the largest values originated from the periods with the smallest air-sea temperature difference.The data used to study CEN was collected from a LI-7500 instrument. In contrast to the result from the CHN study, CEN was found to be constant with wind speed during periods with an air-sea temperature difference larger than 3 K. However, for smaller T it was demonstrated that CEN increased dramatically with wind speed.Through calculations with a spray-flux model it was shown that spray-effects on the turbulent fluxes are relatively small. It was concluded that the increase of the exchange coefficients is due to dynamic effects.With the aid of spectral analysis it was demonstrated that there is a link between the strong increase of the exchange coefficients and the turbulence structure. During high winds and small T, surface layer scale eddies gain importance in the turbulent transport of latent and sensible heat, at the expense of the larger scale eddies. During these conditions, it was also found that MO-relations for the exchange of sensible heat broke down.The increase in CEN and CHN was found to be closely connected to a similar increase in the respectively roughness lengths, z0q and z0t. It was demonstrated that these values deviated strongly from the z0q and z0t values calculated with surface renewal theory. This result was taken as an indication of the failure of surface renewal theory for wind speeds above 10 ms-1. It is suggested that this is a result from the increasing importance of wave breaking with a possible link to changes in the turbulence structure.

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