Synoptic responses to mountain gravity waves encountering directional critical levels

Abstract

A heuristic model is used to study the synoptic response to mountain Gravity Waves (GWs) absorbed at directional critical levels. The model is a Semi-Geostrophic version of the Eady model for baroclinic instability adapted by Smith (1986) to study lee cyclogenesis. The GWs exert a force on the large-scale flow where they encounter directional critical levels (Shutts 2003). This force is taken into account in our model, and produces Potential Vorticity (PV) anomalies in the mid-troposphere. For the case of an idealized mountain range in which the orographic variance is well separated between the small-scales (that produce the GravityWaves) and large-scales (that produce Synoptic Eady Waves), the PV produced by the GWs force has a surface impact that is significant compared to the surface response due to the large scales of the mountain. For a cold front, the GWs force produces a trough over the mountain and a larger amplitude ridge immediately downstream. It opposes somehow to the large scale Eady wave response to the mountain, which is anticyclonic aloft and cyclonic downstream.