Numerical Analysis of the Influence of Jets, Fronts, and Mountains on Alpine Lee Cyclogenesis

Abstract

Recent diagnostic and numerical studies have been shown that cyclogensis events in the lee of the Alps occur when: 1) an upper-level trough is upstream: 2) a low-level frontal system impinges on the Alps and, 3) an upper-level streak on the west side of trough moves into the northern Mediterranean. Three case studies focusing on the rapid development stage of Alpine lee cyclogensis are investigated by performing a set of numerical experiments, with emphasis on the above mentioned factors. In order to create slightly different initial fields, we have used a two layer smoothing technique, alternatively reducing low-level available potential energy testing (2)], or reducing an upper-level wind maxima [testing (3)]. Once this is done we readjust the mass and momentum fields using a variational initialization scheme with weak geostrophic constraints. Based on the results of these cases of lee cyclogensis, the weaker lee developments were significantly reduced by decreasing the low-level frontal intensity (2), which implied a greater influence of the low level dynamical processes (frontal impingement) relative to jet streak processes (geostrophic adjustment). In the case of relatively strong cyclogensis, dynamical processes associated with the upper-level jet streak become more important factor than low forcing. An overall inhibitory effect of the Alps was obvious in all three cases. particularly in the case of strong cyclogensis. However, these experiments did show localization of development as manifested by a high-low dipole structure of the mountain induced pressure perturbations.