Flow over mountains: Coriolis force, transient troughs and three dimensionality

Some issues, concerning uniform, stratified flow over a three‐dimensional mountain, that have not been fully explored are studied using primitive‐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 over small‐scale mountains (characteristic length L < 50 km) even though the Rossby number ( Ro ) is large. When the Coriolis force is neglected, a pair of symmetric vortices is induced on the lee side, irrespective 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 diminish the lee vortices due to the leftward deflection on the upstream side of the mountain. The Coriolis deflection also reduces the upstream propagation. When Fr is ≥ 1, most of the flow goes over the mountain and the Coriolis effect is small for the same Ro . Therefore, the importance of the Coriolis effect is determined by both 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 more 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 mountain in the along‐flow direction. For smaller‐scale mountains, small vortices can be generated within the lee‐side trough when Fr is small. When the mountain scale is increased, the lee‐side vortices disappear, and only a lee‐side trough remains. Depending on the mean wind speed and the mountain scale, the lee‐side trough can be advected by the anticyclonic flow over the mountain to the south, and remain attached to the mountain for a long time. A train of troughs is generated on the lee side of an elliptic mountain associated with the lee‐side gravity waves. 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 < 100km), the two‐dimensional model overestimates the amount of airflow over the mountain and also the lee‐side downslope wind.