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

Abstract

The change of flow regimes from “flow over” (non-blocked flow) to “flow around” (blockedflow) is generally identified by flow stagnation upstream of the mountain and increaseddeflection of flow around the edges of the mountain. Numerical calculations of simple flow pasta smooth mountain ridge presented here reveal another fundamental property of this changeof flow regimes. In the presence of the Coriolis force, the drag exerts a cyclonic couple on nonblockedflows, but an anticyclonic couple on blocked flows. The delaying effect of the Coriolisforce on the transition to the blocked flow regime is confirmed and the strength of the couplein blocked flows is also found to increase when the Rossby number U/f L decreases. A changein the flow regime from blocked to non-blocked flow when the Rossby number decreases givesa new high-drag regime at high non-dimensional mountain height (Nh/U) and increases substantiallythe temporal fluctuations of the drag. The sensitivity of the couple to the Rossby numberis confirmed in realistic simulations of blocked flow from PYREX. Data from Iceland definestwo 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.