The impact of flow regimes on asymmetry of orographic drag at moderate and low Rossby numbers

The change of flow regimes from 'flow over' (non-blocked flow) to 'flow around' (blocked flow) is generally identified by flow stagnation upstream of the mountain and increased deflection of flow around the edges of the mountain. Numerical calculations of simple flow past a smooth mountain ridge presented here reveal another fundamental property of this change of flow regimes. In the presence of the Coriolis force, the drag exerts a cyclonic couple on non-blocked flows, but an anticyclonic couple on blocked flows. The delaying effect of the Coriolis force on the transition to the blocked flow regime is confirmed and the strength of the couple in blocked flows is also found to increase when the Rossby number U/fL decreases. A change in the flow regime from blocked to non-blocked flow when the Rossby number decreases gives a new high-drag regime at high non-dimensional mountain height (Nh/U) and increases substantially the temporal fluctuations of the drag. The sensitivity of the couple to the Rossby number is confirmed in realistic simulations of blocked flow from PYREX. Data from Iceland defines two regimes: at low Rossby number and high Nh/U, the drag on the right hand side is favoured, while at high Rossby numbers and low Nh/U the drag on the left side is larger.