Observations and Numerical Modeling of an Elevated Mixed Layer

Abstract

Observations and a numerical model have been used to investigate the structure of an elevated mixed layer (EML) that formed in the lee of the Rocky Mountains over eastern Colorado. The EML formed as a dry convective boundary layer over the higher mountainous terrain of western Colorado, and then advected eastward, producing upper-level warming over the eastern plains. This upper-level warming generated a strong capping inversion at the top of a surface-based plains convective boundary layer that formed concurrently with the EML. A model trajectory analysis indicated that air from the plains boundary layer was detrained into the EML in a zone of convergence along the foothills of the Front Range of the Rocky Mountains. Specific physical processes responsible for meso-?-scale EML development were examined using a two-dimensional version of the mesoscale numerical model. Blocking of the plateau-level winds by the plains breeze proved to be the primary constraint on advection of the EML and its underlying lid over the adjacent plains. Such blocking was minimized for large-scale conditions that prevented the plains mixed layer from growing to elevations significantly above the plateau surface. Both greater plateau height and increased plains surface moisture availability contribute to keeping the plains boundary layer below the plateau height, and thus favor generation of a meso-?-scale EML and lid.