A laboratory experiment on the development of cyclogenesis in the lee of a mountain

Summary One of the most intriguing problems concerning the interaction of subsynoptic and synoptic atmospheric flows with topographic features is orographic cyclogenesis. A fully satisfactory prediction theory is not yet 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 experiment 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 (ϱ₁ and ϱ₂), a stream of stratified fluid whose density has an intermediate value ϱ₁ < ϱ_m < ϱ₂. This stream is generated at the height of the interface between the two layers; due to the rotation of the platform, the attainment of geostrophic equilibrium brings about an intermediate-water flow running along a wall, giving rise to a three-layer baroclinic structure which can represent some of the main outstanding features of an atmospheric frontal system impinging on a mountain. In a well-defined range of the Rossby and Burger numbers, the instability of this current gives rise to a couple of persistent cyclonic and anticyclonic vortices, whose horizontal dimensions and vertical extents reproduce quite faithfully the synoptic situation supporting the onset of the orographic cyclogenesis, with its characteristic cold front stretching between the two vortex structures. It is enough to place an obstacle of a suitable size in the proper geographic position, to make 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 which have been recognized as the precursors of orographic cyclogenesis in the lee of the Alps.