The Structure and Evolution of a Simulated Rocky Mountain Lee Trough

Abstract

This paper describes the life cycle of a lee trough associated with the passage of a baroclinic wave over the Rocky Mountains based on two overlapping simulations by the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model. The evolution of the lee trough may be summarized as follows. A surface cyclone and lee trough developed and intensified as an upper-level short-wave trough approached and moved over the Rockies. Near the low center,.the lee trough was found to be frontogenetical, with the frontogenesis being forced by 1) confluent deformation associated with the wind fields of the developing cyclone and lee trough and 2) differential vertical motion along the eastern slopes of the Rocky Mountains and the adjacent sloping terrain of the High plains. In turn, the frontogenesis drove an ageostrophic circulation that sharpened the lee trough. The lee trough represented a break in trajectory origin, with trajectories ending ahead of the lee trough originating over the Great Plains, and trajectories ending behind the trough beginning over the high terrain of the Rocky Mountains. The lee trough developed a warm occlusion-like structure as it moved eastward across the Great Plains and was overtaken by an upper-level short-wave trough and associated upper-level baroclinic zone. The upper-level baroclinic zone provided the ?elevated cold front? of the occlusion, while at the surface, an occlusion-like thermal ridge formed as the warm tongue and baroclinity of the lee trough were overtaken by a zone of low-level cold advection. This zone of cold advection could not be traced to a preexisting surface-based frontal zone, but rather developed from the convective mixing of baroclinity front aloft to the surface. A conceptual model is presented that describes the influence of the Rocky Mountains and sloping terrain of the High Plains upon the evolution of die cyclone and lee trough. Primarily due to the complex terrain of the region, this evolution deviates drastically from the Norwegian cyclone model. For example, near the Rockies, neither a classical warm sector in which warm air is advected from the south nor a classical surface cold front separating warm-sector air from polar air are evident. Well downstream of the mountains, however, the cyclone begins to develop a more classical appearance.