Daytime Boundary-Layer Evolution over Mountainous Terrain. Part 1: Observations of the Dry Circulations

Abstract

This paper presents the boundary layer structure which accompanies the development of daytime local wind systems in a broad mountain valley, as revealed by cross sections of potential temperature. It describes how this structure leads to the occurrence of a region of convergence to the downwind side of mountains. Previous studies, based primarily on one-dimensional sounding of potential temperature and horizontal winds, have shown that profiles of static stability and the presence of winds aloft have an important effect on the manner in which daytime, thermally-forced wind systems develop. In the present study, two-dimensional cross sections obtained from aircraft data, vertical soundings and surface mesonet data show several relevant features. In mid to late morning near the surface, for example, upslope winds form in a shallow mixed layer at the underside of the nocturnal inversion layer (cold pool); at elevations above the top of this cold pool, convectively-mixed surface winds exist. The spatial arrangement of these features in two dimensions is such that a region of convergence forms near the surface on the leeward side of mountains or mountain ranges. This convergence region, called the leeside convergence zone, thus occurs at the upwind edge of the cold pool in a mountain valley. Evidence suggests that it is an important mechanism for the initiation of mountain- generated cumuli and their continued growth into cumulus congestus and cumulonimbus clouds.