Regimes of orographic flow and direction of the drag couple on the rotating earth

Numerical simulations of vertically uniform flow past an idealized mountain ridge show that the Coriolis force increases the orographic drag for blocked flows at high values of the nondimensional mountain height, while for nonblocked flows the rotation acts to reduce the drag, as predicted by linear theory. Furthermore, numerical calculations show that by lowering the Rossby number, blocked flow may break out of the blocked state and into a high-drag nonblocked state.

The spatial distribution and temporal variations of the drag in flow on a rotating plane is influenced by the flow regime: in blocked flows, the right part of an isolated mountain gives greater drag than the left part and this difference increases for decreasing Rossby number, as long as the flow remains blocked. On the other hand, in nonblocked flows, the left side of the mountain gives greater contribution to the total drag, but here the drag has much greater temporal oscillations. The assymetry of the drag can be explained by accumulation of low level dense air in the lee of the left side of the mountain in the case of blocked flows, while for nonblocked flows the asymmetry is related to increased wave activity on the left flank of the mountain.

Simulations of real atmpospheric flow from PYREX give similar results on the sensitivity of the drag to the Rossby number and observations around Iceland also confirm the results from the idealized flows.