Lee Cyclogenesis. Part I: Analytic Studies

Abstract

The growth of synoptic scale cyclones imbedded in a baroclinically unstable zonal flow over a long straight mountain range is investigated. Two different analytical models of the phenomenon are used. The first model uses the linearized quasi-geostrophic equations. It allows a simple superposition of a steady state mountain forced solution and a transient Eady wave. There is no dynamic interaction between the two solutions, but the time evolution of the combined solution reproduces many characteristics of a disturbance passing over the Rocky Mountains. The semigeostrophic equations are used in the second model. These equations allow a linear solution in transform space, but the transformation of the solution to physical space is nonlinear. This allows an interaction between the mountain forced and transient solutions. The minimum pressure developed by the semigeostrophic system is the same as that of the quasi-geostrophic system. However, the shape of the wave is distorted. This effect is caused by the divergent part of the mean flow over the mountain ridge.