Laboratory experiments on mountain induced rotors

Stratifed flows over obstacles have received considerable interest in the last decades, especially within the context of air flow over mountains. For the case of mountain lee waves it has been known for some time, that flow reversal can occur under the wave crests under special environmental conditions. These flows are called rotors and are characterised as very turbulent ,quasi two dimensional vortices with horizontal axis parallel to the mountain. Although many laboratory experiments have been performed for stratified flows over obstacles (e.g. Eiff,2005), not much laboratory work has been done on the rotor problem. This might be due to the fact, that conditions for rotor formation have been revealed only recently by numerical simulations (e.g. Vosper, 2004 or Doyle and Durran, 2007). In fact it has been found, that rotor formation is favoured by an elevated inversion above the mountain top, which has not been considered in earlier laboratory experiments with continuous stratification.

We have performed laboratory experiments in the stratified towing tanks of CNRM at Meteo France at Toulouse. We used the same set up for a medium tank of 7 m length and a large tank of 22 m length as already used for the lee wave experiments by Eiff (2005).The main difference is the introduction of a neutrally stratified lower fluid layer capped by a sharp density jump under a continuous stratified upper layer. By varying the towing speed of the obstacles and the position of the inversion with respect to the mountain top we were able to find proper flow configurations with rotor formation. The results of more than one hundred laboratory experiments fitted quite well into the regime diagram on flow phenomena (lee waves, rotors, hydraulic jumps) obtained for similar density profiles in the numerical simulations by Vosper (2004). We will present qualitative results on rotor formation by streak line photographs and short video clips. The flow fields within rotors and their wave environment have been analysed from PIV measurements and will also be presented .

Eiff, O. et al. Dyn.Atmos.Oceans 40,71 (2005)

Vosper,S. Q.J.Roy.Meteorol.Soc. 130,1723 (2004)

Doyle, J.D. and Durran, D.R. J.Atmos.Sci. 64,4202 (2007)