A LABORATORY EXPERIMENT ON THE DEVELOPMENT OF CYCLOGENESIS IN THE LEEOF A MOUNTAIN

One of the most intriguing problems concerning the interaction of subs ynoptic and synoptic atmospheric flows with topographic features is or ographic cyclogenesis. A fully satisfactory prediction theory is not y et available, but a lot of efforts have been made by theoreticians to implement reliable numerical models simulating the different phases of this complex phenomenon. An attempt to perform a laboratory experimen t to simulate physically this kind of interaction has been made by us, through the generation of a baroclinic frontal system in the rotating hydraulic platform of the <<Coriolis Laboratory - LEGI-IMG - Grenoble >>. The adopted technique consists in a device which produces, at the interface separating two water layers of different density (rho(1) and rho(2)), a stream of stratified fluid whose density has an intermedia te value rho(1) < rho(m) < rho(2). This stream is generated at the hei ght of the interface between the two layers; due to the rotation of th e platform, the attainment of geostrophic equilibrium brings about an intermediate-water flow cunning along a wall, giving rise to a three-l ayer baroclinic structure which can represent some of the main outstan ding features of an atmospheric frontal system impinging on a mountain . In a well-defined range of the Rossby and Burger numbers, the instab ility of this current gives rise to a couple of persistent cyclonic an d anticyclonic vortices, whose horizontal dimensions and vertical exte nts reproduce quite faithfully the synoptic situation supporting the o nset of the orographic cyclogenesis, with its characteristic cold fron t stretching between the two vortex structures. It is enough to place an obstacle of a suitable size in the proper geographic position, to m ake the cyclogenesis start. The first results of our simulations have been encouraging, showing the occurrence of lee cyclogenesis when the stream conditions in our model correspond to the synoptic features whi ch have been recognized as the precursors of orographic cyclogenesis i n the lee of the Alps.