ELEVATED RESIDUAL LAYERS AND THEIR INFLUENCE ON SURFACE BOUNDARY-LAYER EVOLUTION

Elevated mixed layers (EMLs) are an important factor in the developmen t of springtime thunderstorms over the United States. EMLs can be cons idered a subset of a larger class, called residual layers, since the m ean state variables are the same, at least initially, as those of the boundary layers in which EMLs are formed. It is possible, however, for boundary or residual layers that are not necessarily well mixed to be advected off regions of elevated terrain and overrun boundary layers forming over lower terrain. These layers are called elevated residual layers (ERLs); ERLs may form frequently in regions near mountains wher e terrain gradients exist. A simple slab mixed-layer model is used to examine how idealized ERL potential temperature profiles influence sur face boundary-layer development. In addition, several regionally gener ated ERLs were observed over Phoenix, Arizona, during the Southwest Ar ea Monsoon Project. These ERLs appear to have produced a change from m oistening to entrainment-drying surface boundary-layer regimes. The th ermodynamic structure of an ERL is determined by the processes that fo rm the boundary layer, the timing and vertical extent of boundary-laye r detachment from the elevated terrain relative to the diurnal heating cycle, and the vertical motion field (if any) accompanying the horizo ntal advection of the ERL away from the elevated terrain. Results sugg est that the creation and evolution of ERLs may be important aspects o f surface boundary-layer development in regions near and downstream of elevated terrain.