ANALYSIS OF THE TURBULENCE STRUCTURE OF A MARINE LOW-LEVEL JET

Four aircraft measurement sets made in late May 1989 within low level jets over the Baltic Sea have been analyzed to estimate the turbulence energy budget. It is concluded that the jets had the same origin as f ound in an earlier study from the same general area: inertial oscillat ion caused by frictional decoupling when relatively warm air flows out over much colder water. In order to combine budget estimates from the four flights to form a representative average, self-preservation simi larity was assumed. When the terms were made nondimensional with the p roper scale combination, the largest terms in all four runs were of or der one, indicating that the scaling is physically sound.Three terms w ere found to dominate the turbulence energy budget: shear production, dissipation and pressure transport. The latter was obtained as remaind er term, since local time rate of change and advection terms were foun d to be of negligible magnitude. Shear production was found in a narro w layer above the jet core and in a much deeper layer below it. The pr essure transport term was a gain in this layer as well, helping to kee p the layer below the jet well mixed. This is in agreement with result s from aircraft measurements in the low level jet and monsoon boundary layer over the Arabian Sea. It is concluded that development of the i nertial jet downwind of a coastline is of fundamental importance for e xchange of momentum at the sea surface in conditions when relatively w arm air is advected over cold water. The jet produces turbulence that promotes mixing in the lower layers, which sharpens the shear below th e jet core, so that mixing becomes even more effective. Turbulence bro ught down to the surface by the pressure transport term is likely to b e of the 'inactive' type, which does not produce shear stress. Through the above-mentioned process it is, however, instrumental in promoting the mechanism that eventually produces 'active turbulence', the carri er of momentum.