A Diagnosis of Alpine Lee Cyclogenesis

A diagnostic analysis of two cases of lee cyclogenesis in the Alps region is presented. Data used consisted of routinely available rawinsonde, satellite and aircraft data. The analysis employed a three-stage variational objective scheme which allowed assimilation of satellite retrieved temperatures, imposition of a weak geostrophic constraint consistent with quasi-geostrophic flow, and a method to satisfy continuity while combining kinematic and dynamic estimates of vertical motion. Evolution of vorticity, vertical motion and energy are computed along with frontogenesis and Q-vectors (Hoskins et al., 1978) as diagnostic tools. The analyses indicate that lee cyclogenesis is a multi-stage process involving interaction between the flow and terrain. The developing cyclone passes through three stages: a shallow, terrain-induced barotropic phase; a rapid development phase where barotropic and baroclinic processes act in concert; and a phase where the mature storm is dominated by baroclinic processes. From a quasi-geostrophic point of view the most important process associated with mountains is the increase in strength of a baroclinic zone as it passes over the mountain range. The secondary flow responds by producing upward vertical motion in the lee of the mountain at middle levels, superimposed over low-level descending air on its slope. The stretching thus realized rapidly increases vorticity.