An observational study of the mid-latitude, marine, atmospheric boundary layer

This thesis presents the results of a study of the atmospheric boundary layer over the sea typical of mid-latitude conditions. The observations were obtained mainly using instrumented aircraft as part of the Joint Air-Sea Interaction Experiment. Following a brief account of the present state of knowledge concerning boundary layers of the type encountered, the rationale for the type of measurement strategy employed is explained and details of the field experiment are presented. The aircraft instrumentation and data processing methods are then described in detail with particular reference to improvements in accuracy of the measurements and error analysis. Comprehensive intercomparisons between the aircraft and between ships and aircraft confirm that accuracy requirements are satisfied.The large scale horizontal and vertical structure is defined (including the pressure gradient), which together with the turbulent flux measurements enables the momentum balance to be investigated throughout the depth of the mixed layer. This is found to be well described by Fkman scaling and to be consistent with the requirements of the turbulent kinetic energy balance. These results are compared with previous experimental data and theoretical predictions. A spectral analysis follows showing that most of the turbulent transport is confined to a high wavenumber region whose characteristics vary as a function of mixed layer parameters. Finally, further implications of the interpretation of the results are discussed with particular reference to the heat and water vapour balance. These imply that removal of water vapour from the mixed layer is accomplished by transfers related to cloud activity, marking a significant change in the mechanism of turbulent transport at the top of the mixed layer.