Elevated residual layers and their influence on surface boundary-layer evolution

Elevated mixed layers (EMLs) are an important factor in the development of springtime thunderstorms over the United States. EMLs can be considered a subset of a larger class, called residual layers, since the mean 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 where terrain gradients exist. A simple slab mixed-layer model is used to examine how idealized ERL potential temperature profiles influence surface boundary-layer development. In addition, several regionally generated ERLs were observed over Phoenix, Arizona, during the Southwest Area Monsoon Project. These ERLs appear to have produced a change from moistening to entrainment-drying surface boundary-layer regimes. The thermodynamic structure of an ERL is determined by the processes that form the boundary layer, the timing and vertical extent of boundary-layer detachment from the elevated terrain relative to the diurnal heating cycle, and the vertical motion field (if any) accompanying the horizontal advection of the ERL away from the elevated terrain. Results suggest that the creation and evolution of ERLs may be important aspects of surface boundary-layer development in regions near and downstream of elevated terrain.