The composite shape and structure of coherent eddies in the convective boundary layer

Conditional sampling is used to locate coherent structures in a large data set obtained from flights by an instrumented light aircraft in convective boundary layers over Eyre Peninsula, South Australia. The high resolution and excellent spatial coverage of the data enable a detailed study of the internal structure of surface-layer plumes and mixed-layer thermals. A compositing technique is used to construct averaged traverses through coherent structures located within aircraft data runs of different altitudes and directions. Groups of composites are combined to form horizontal and vertical cross-sections which describe the internal flow patterns and the distribution of physical variables associated with "typical" coherent structures and their environment. In addition to the well-known along-wind features of surface-layer plumes, a strong, consistent inflow/entrainment pattern is evident in the lateral direction. Air from the horizontal plane channels around the sides and then in behind the microfront present at the upstream edge. Forces set up by the driving instability in the along-wind and vertical directions are counter-balanced by organised flow in the across-wind direction. It is found that mixed-layer thermal towers have a relatively simple form, consisting primarily of large columns of warm, upward-moving turbulent air, which may occasionally be in a state of slow rotation. An analysis of possible geometrical distortions within the results is performed, leading to a comparison of δw/δz estimated from the horizontal velocity convergence field inside plumes/thermals, with that computed from the slope of partitioned w profiles.