Aircraft observations of the ekman layer during the joint air‐sea interaction experiment

Abstract

AbstractResults are presented from an observational study of the mid‐latitude atmospheric boundary layer over the sea. the data were obtained mainly by instrumented aircraft as part of the Joint Air‐Sea Interaction Experiment (JASIN).The eight occasions chosen for study are characterized by small surface buoyancy fluxes and the conditions are well described as near neutral and barotropic. In the absence of low‐level inversions, a well‐mixed Ekman layer is observed on each occasion but is limited to a depth of approximately 0.2u */f. Accurate measurements of horizontal gradients (including pressure) together with improved wind observations and turbulent flux measurements enabled each of the terms of the momentum balance to be evaluated throughout the depth of this layer. These terms were found to balance quite closely, were well described by Ekman scaling and were consistent with the requirements of the measured turbulent kinetic energy balance. the latter suggests that the main shear production terms are dissipated locally and that little is exported to upper levels in the Ekman layer to enable significant deepening by entrainment although only a slight increase in instability appears to be needed to alter this balance. Conditions in which such boundary layer structure might be observed are suggested. Values of various coefficients used in schemes for relating surface fluxes to mean quantities (CD, CE, Cg, A, B) are derived and compared with previous measurements.Spectral analysis reveals that most of the turbulent transport is confined to a distinct high wavenumber region whose characteristics vary as a function of Ekman layer depth and stability parameters. This is superimposed on larger‐scale fluctuations which do not vary appreciably within the Ekman layer and which therefore dominate the variances in the upper regions as the intensity of the smaller‐scale turbulence decreases strongly with height.Finally, further implications of this interpretation are discussed with particular reference to the heat and water vapour balance. These imply that removal of water vapour from the Ekman layer is accomplished by transfer related to cloud activity, marking a significant change in the mechanism of turbulent transport at the top of this layer. the relationship between these and similar results obtained concurrently by different methods is also discussed.