Inside Thermals

Abstract

Conditional sampling is used to locate mixed layer thermals and surface layer plumes, as well as their downward moving companions - downdrafts - in a large data set obtained from flights by an instrumented motorglider in connective boundary Iayers over Eyre Peninsula, South Australia.  The high resolution and excellent spatial coverage of the data permits a detailed study of internal structure.  A compositing technique is used to construct average intersections through thermals and plumes from aircraft runs of given heights and directions.  Groups of composites are then combined to form horizontal and vertical cross-sections, revealing the flow patterns and distribution of physical variables within "typical" thermals and plumes and their environment.  Surface layer plumes are found to have strong lateral inflow patterns, in which air from the horizontal plane channels around the sides and then in behind the microfront present at the upstream edge.  Mixed layer thermal towers have a relatively simple form, consisting primarily of large columns of warm, upwards moving turbulent air, which may occasionally be in a state of slow clockwise rotation.  Finally, the results of this study are combined into a schematic composite depicting the highly complex interactions between the convective plumes of the surface layer and the thermal towers of the mixed layer.