Twin peaks: stable flow past two identical mountains

Over the past decade considerable progress has been made in understanding air flow past high, mesoscale mountains. Numerical modelling work has tended to either focus on reproducing complex, real case studies or on understanding simple flows past isolated idealised orography. In this work we aim to bridge the gap between these two approaches by investigating idealised stable flow past two identical, idealised mountain ridges, where the second mountain lies in the wake of the first and is thus sheltered from the mean flow. Of particular interest to us is the drag exerted by the second mountain and how it changes as the second mountain is moved away from the first. We are interested in this problem because we would like to know whether sheltering effects should be included in orographic drag parametrizations for NWP and climate models.

We find that sheltering effects can be significant. When the mountains are close together very little drag is exerted by the second mountain because it is sheltered by the first. In this scenario current parametrizations, which do not account for sheltering effects, would overestimate the drag on the two mountains by a factor of two. As the second mountain is moved progressively further from the first, the drag on the second mountain converges towards the drag on the first. When the mountains are sufficiently far apart, the second mountain lies outside of the wake of the first and the current parametrizations would correctly predict the drag.

Our experiments show that the drag variation is not only controlled by the distance between the two mountain crests but also by factors which govern the length of the wake behind the first mountain. These factors include the incident wind speed and the surface roughness. Full analysis of these experiments will be presented and the sensitivity of the drag to various parameters discussed, together with implications for orographic drag parametrizations.