A Theoretical Model of Orographically Modified Cyclogenesis

Abstract

The modification of baroclinic instability by low-level orographic blocking of cold advection is considered within a simple model. In this model, the meridional flow in the lower layer of a quasi-geostrophic model is blocked by a semi-infinite knife edge barrier extending eastward from the origin. The stability problem for a vertically sheared current parallel to the barrier is solved by means of the Wiener-Hopf technique, and a number of properties of the eigenmodes are determined. The barrier is found to have an inhibitory effect on cyclogenesis, in the sense that the magnitude of the pressure perturbation in the lower layer is reduced in the vicinity of the barrier. However, the barrier also distorts the flow in such a manner as to lead to the formation of a low-level cutoff low south of the barrier, which is characterized by intense easterly winds on the barrierward side of the low. It is also found that at certain phases of the development, the lower-layer velocities have square toot singularities near the tip of the barrier, leading to a discontinuous turning of air flowing around the tip. Air impinging on the barrier from the northwest develops a split jet pattern; the splitting point moves eastward along the barrier with time. The upper layer flow is almost unaffected near the tip of the barrier, but the pressure perturbations in both layers decay algebraically to zero sufficiently far downstream of the tip. Despite the extreme simplicity of the model, some of the features of the solution were found to be suggestive of features found in observations and numerical simulations of cyclogenesis in the lee of the Alps.