The dynamics of gap flow over idealized topography

Abstract

A flow past an idealized topography with a narrow gap is examined using a numerical model. The obstacle is a finite ridge in the N-S direction with a narrow gap in the W-E direction and the fluid has a simple vertical structure with a constant atmospheric stability and wind.In the first part, flows perpendicular to the obstacle, with no rotation and no surface friction, are compared to cases without a gap in the ridge. The only varying parameter is a nondimensional mountain height epsilon = Nh/U. Upstream and down stream effects of the gap on the flow are assessed. Mass flux budgets of a low-level flow for both types of topography indicate a shift in the flow regime from "flow over" to "flow around" at epsilon ∼ 1, whereas the gap flow remains relatively constant. Using a smaller volume centered over the gap, a similar analysis combined with a momentum budget computation suggests two mechanisms responsible for the gap flow enhancement. The first one is characterized by a mountain lee-wave (epsilon ∼ 1), and the second one by an upstream flow blocking (epsilon ≥ ∼5).In the second part, the effects of rotation and surface friction are included, and there are four different initial wind directions relative to the obstacle (N, NW, W SW). Numerical simulations are performed for three values of epsilon = 1.4, 2.5 and 5.0. In the northerly case, there is no gap flow without the surface friction, despite the synoptic scale pressure gradient oriented parallel to the gap, independent of epsilon. In the southwesterly case, the gap flow is present for all values of epsilon, despite a component of the synoptic scale pressure gradient being oriented in the opposite direction. Only when the gap is elongated, the low-level flow in the channel away from the entrance and exit decelerates and reverses. In the westerly and northwesterly cases, signature of both gap flow enhancing mechanisms are observed. In flows from these two directions, the strongest downslope at the surface winds are observed in the presence of the mountain wave. As the upstream blocking becomes more important, the highest surface winds are observed in the gap.In the third part, the idea of a constant height gap flow is revisited. Observational evidence as well as results from numerical simulations suggest that in order to enhance the mass flux through the gap a subsidence is required.