A Case Study of the Effects of Topography on Cyclone Development in the Western United States

Abstract

A mesoscale model has been used to simulate an observed trough system which crossed the Rocky Mountains between 24 and 27 March 1983. Numerical simulations have been conducted with and without topography to isolate the effects that the mountains have on the cyclone and the subsequent lee cyclogenesis that occurs in eastern Colorado. The applicability of two theories to describe processes occurring in the cyclone as it crosses the mountains have been investigated: 1) superposition or masking of the cyclone by a topographically induced anticyclone, and 2) upper-level forcing coupled with low-level blocking. In this case study, the low-level absolute vorticity of the cyclone over the region of the Rocky Mountains is less in the simulations with topography than in the simulations without. However, later in the simulations as the cyclone moves away from the mountains, vorticity differences between the simulations decrease markedly. In association with decreased vorticity, higher geopotential heights are found at all tropospheric levels over the mountains in the runs with topography. These height differences are similar in magnitude and character to the anticyclone that develops when the zonally averaged mean flow is allowed to impinge on the topography until a quasi-equilibrium is reached. An upper-tropospheric jet streak and associated indirect circulation are present in this March 1983 case and are simulated by the model. However, comparison of the mountain and no-mountain simulations indicates the presence of topography does not result in significant blocking of the low-level flow or alter the magnitude of the indirect circulation in the lee region. This lack of sensitivity may be a function of the relatively smooth topography employed in the model.