FLOW OVER MOUNTAINS - CORIOLIS-FORCE, TRANSIENT TROUGHS AND 3 DIMENSIONALITY

Some issues, concerning uniform, stratified flow over a three-dimensio nal mountain, that have not been fully explored are studied using prim itive-equation models. When the Froude number (Fr) is small (e.g. Fr < 0.5), we find that the Coriolis force cannot be neglected for flow ov er small-scale mountains (characteristic length L < 50 km) even though the Rossby number (Re) is large. When the Coriolis force is neglected , a pair of symmetric vortices is induced on the lee side, irrespectiv e of the scale of the mountain, as long as Fr lies roughly between 0.1 and 0.5. The major effect of the Coriolis force is to modify or dimin ish the lee vortices due to the leftward deflection on the upstream si de of the mountain. The Coriolis deflection also reduces the upstream propagation. When Fr is greater than or equal to 1, most of the flow g oes over the mountain and the Coriolis effect is small for the same Ro . Therefore, the importance of the Coriolis effect is determined by bo th Ro and Fr. As the scale of the mountain increases from small-scale to mesoscale (100 km < L < 300 km), the upstream flow is deflected mor e to the left by the Coriolis effect,and a trough develops on the lee side. The length-scale of the trough depends on the scale of the mount ain in the along-flow direction. For smaller-scale mountains, small vo rtices can be generated within the lee-side trough when Fr is small. W hen the mountain scale is increased, the lee-side vortices disappear, and only a lee-side trough remains. Depending on the mean wind speed a nd the mountain scale, the lee-side trough can be advected by the anti cyclonic flow over the mountain to the south, and remain attached to t he mountain for a long time. A train of troughs is generated on the le e side of an elliptic mountain associated with the lee-side gravity wa ves. Simulations of flows over mountains in two-dimensional and three- dimensional models with the same cross-sectional profile are compared. For small-scale mountains (L < 100 km), the two-dimensional model ove restimates the amount of airflow over the mountain and also the lee-si de downslope wind.