A new theory for downslope windstorms and trapped mountain waves

Mountain gravity wave theories often neglect the observationnal evidence that the large scale winds can become very small near the earth surface. This common shortcoming is related to the fact that mountain waves have a critical level near the surface in this case, making the problem extremely involved. We briefly expose here how Lott (2016) circumvents this difficulty and derive a theory where linear mountain gravity waves are forced by a nonlinear surface boundary condition, where the background wind is null at the surface and increases smoothly to reach a constant value aloft, and where the buoyancy frequency is constant. In this configuration, the critical level just below the surface is dynamically controlled by the surface and minimum Richardson number J . When the flow is very stable (J & 1), this critical level dynamics easily produces large downslope winds and Foehn. The downslope winds are more intense when the stability increases and much less pronounced when it decreases (when J goes below 1). On the opposite, the trapped lee waves are very small when the flow is very stable, start to appear when J ≈ 1 and are more intense when the stability decreases further. For the trapped waves, this result is explained by the fact that the critical level absorbs the gravity waves downstream of the ridge when J > 0.25, while absorption decreases when J approaches 0.25. Two new examples for J = 2 and J = 0.3 are given here to illustrate these results. Download Lott (2016) at: http://www.lmd.jussieu.fr/~flott/articles/LOTT JAS15.pdf