THE EFFECT OF BAROTROPIC SHEAR ON UPPER-LEVEL INDUCED CYCLOGENESIS - SEMIGEOSTROPHIC AND PRIMITIVE EQUATION NUMERICAL SIMULATIONS

Idealized numerical experiments within the frameworks of semigeostroph ic and primitive equation dynamics were performed to study the effect of barotropic sheer on idealized upper-level induced cyclogenesis. Loc alized finite-amplitude potential temperature anomalies were used as i nitial perturbations, and the atmosphere was considered as a dry frict ionless fluid of uniform quasigeostrophic potential vorticity on an f- plane. It is demonstrated that the main features of the numerical simu lations are in essence unaffected by the choice of the dynamical frame work. They comprise, for instance, the development of elongated cold f ronts under anticyclonically sheared conditions, a ''T-bone'' shaped f rontal palette in the unsheared case (cf. Shapiro and Keyser), and a B ergen-type occlusion process in the simulations with cyclonic shear. T his confirms the profound dynamical influence of lateral shear in the background environment upon die resulting surface cyclone and frontal structures (and the accompanying evolutions at upper levels) that has been found in previous normal-mode experiments. This sensitivity is sh own to be related to the different orientation of the additional defor mation field associated with the background shear. The differences bet ween surface cold and warm fronts are analyzed in more detail using a combined Eulerian and Lagrangian approach. Consideration is also given to the shortcomings of the present approach and to a possible strateg y for further idealized model investigations.