Physical simulations in rotating tank of lee cyclogenesis

Some results of physical simulations of interactions between atmospheric baroclinic flows and a tridimensional mountain, carried out in a rotating tank, are presented and discussed. Complex dynamic and thermodynamic processes occur when the so called lee, or secondary, cyclogenesis can develop; in this case, potential energy of the basic flow is converted into kinetic energy of its perturbation field, giving rise to strengthening and deepening of its pressure troughs. The presence of an obstacle can exert a blocking on the colder lower layers of the impinging stably stratified airflow, forcing it to deflect horizontally round its borders. In this way, these layers get the lee region with a delay. These two effects, of blocking and delay, are the responsible for the initial pressure decrease downwind of the mountain and for the subsequent proper downstream baroclinic development. Following this rather simple scheme, the lee cyclogenesis has been simulated in the rotating tank of the IMG of GRENOBLE. The experiments were observed by three TV cameras continuously recording the trajectories of dye tracers at three levels of the stratified fluid. In order to measure the vertical density profiles, a rectangular mesh of samplers was rapidly moved up and down in the tank.