ANALYSIS OF THE TURBULENCE STRUCTURE OF A MARINE LOW-LEVEL JET ANN-SOFI SMEDMAN, MICHAEL TJERNSTROM and ULF HOGSTROM Department of' Meteorology, Uppsala University, Uppsala, Sweden (Received in final form 8 January, 1993) Abstract. 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 found in an earlier study from the same general area: inertial oscillation 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 similarity was assumed, When the terms were made nondimensional with the proper scale combination, the largest terms in all four runs were of order one, indicating that the scaling is physically sound. Three terms were found to dominate the turbulence energy budget: shear production, dissipation and pressure transport. The latter was obtained as remainder term, since local time rate of change and advection terms were found to be of negligible magnitude. Shear production was found in a narrow layer above the jet core and in a much deeper layer below it. The pressure transport term was a gain in this layer as well, helping to keep the layer below the jet well mixed. This is in agreement with results from aircraft measurements in the low level jet and monsoon boundary layer over the Arabian Sea. It is concluded that development of the inertial jet downwind of a coastline is of fundamental importance for exchange of momentum at the sea surface in conditions when relatively warm air is advected over cold water. The jet produces turbulence that promotes mixing in the lower layers~ which sharpens the shear below the jet core, so that mixing becomes even more effective. Turbulence brought down to the surface by the pressure transport term is likely to be of the 'inactive' type, which does not produce shear stress. Through the above-mentioned process it is, however, instrumental in promot- ing the mechanism that eventually produces 'active turbulence', the carrier of momentum. 1. Introduction Observations of boundary-layer structure at various sites in the Baltic Sea area during the last 15 years indicate frequent occurrence of low-level jets during spring and summer. In connection with plans for possible large-scale off-shore wind energy exploitation in the Kalmarsund area (A in Figure 1) near the Swedish coast, a systematic pilot balloon study was launched during April - June 1991. The result shows that, for this particular period, low-level jets occur about 50% of the time. In this paper, data from a series of aircraft measurements were used to study the structure and turbulence budget of this jet in detail. As a working hypothesis, the so-called 'self preservation principle' was employed. This idea, which is standard in the analysis of laboratory free shear flows (see Townsend, 1976) means that the transverse distributions in such a flow retain their functional form but differ from one type of flow to another. It is shown that this is a reasonable model also for the atmospheric jet studied here. Boundary-Layer Meteorology 66: 105-126, 1993. 9 1993 Kluwer Academic Publishers. Printed in the Netherlands.