Turbulence Structure and Fluxes over Sea

Sammanfattning: Understanding the processes that control air-sea exchange of momentum, sensible heat and moisture (latent heat) is of great importance for atmospheric and oceanic modelling. In the atmosphere, for instance, cyclone intensity and track depends on these processes. Combined effects of wind drag and surface convection affects the mean wind and turbulence structure near the surface, with additional indirect effects on the upper parts of the atmospheric boundary layer (by for example entrainment). For the oceanic side the atmospheric forcing generates waves, provides mixing and drives ocean currents.The most reliable way to estimate exchanges between the ocean and the atmosphere are through accurate direct measurements. The standard method used for measuring turbulent fluxes is the eddy covariance method. When using this method to measure scalar fluxes, there is often a spatial separation between the instruments measuring the scalar and the wind. This leads to an error in the calculated scalar flux if the separation is not accounted for. This source of error is investigated for the first time over sea, in addition the measured two-point covariance can also be used to indicate a changed turbulence structure over sea in comparison to over flat terrain.We show the impact of a process that changes the turbulence structure over sea using numerical simulations. Long fast swell waves on the ocean surface are shown to induce turbulent wind eddies in the flow at the scale of the dominant wave and thereby alter the turbulence structure and momentum exchange in the marine atmospheric boundary layer. Analysis of turbulence structure and momentum exchange is carried out for a range of conditions that have previously been shown to be prevalent at lower latitudes, where the impact of swell is expected to be higher than at mid and high latitudes.