Semi-collapsed and advective stable boundary layers

Several classes of stable boundary layers are examined including surface-based turbulence, turbulence generated primarily above the surface inversion layer and turbulence in stable boundary layers forced by warm advection over a cooler surface. These analyses are based on data collected in CASES99 and four months of eddy correlation data collected over a snow-covered grass field and a nearby sage brush community. The differences between the three types of stable boundary layers are discussed in terms of intermittency, vertical structure, vertical transport of turbulence energy and performance of Monin-Obukhov similarity theory.

The grass site includes a 34-m tower with 7 levels of eddy correlation data. Thin daytime stable boundary layers are generated as the air flows from the heated brush to the snow-covered grass. Often, a stable internal boundary layer is well defined in terms of vertical profiles of the buoyancy flux over the snow-covered grass. The stable internal boundary layer is generally embedded within a deeper layer of heat flux divergence corresponding to a deeper advected convective boundary layer. The cooling due to vertical divergence of the heat flux appears to be balanced primarily by warm air advection.