Large-scale flow responses to mountain gravity waves breaking in the mid-troposphere

The balanced synoptic scale flow response to the absorption of short mountain gravity waves (GWs) at 3D directionnal critical levels was studied by Martin and Lott (2005). The aim was notably to characterize the impact in the context of lee cyclogenesis.

In the present study, we tend to go further on this problem, using nonlinear 2D anelastic simulations of the Eady problem, without tropopause. In these simulations, an isolated mountain range composed of several small-scale ridges generates short mountain GWs that are absorpted at critical levels and lead to breaking. We analyze the large-scale flow (LSF) response to this forcing.

The LSF response consists of two contributions that we separate: a balanced part fully described by the PV distribution, and an unbalanced part constituted of inertia-gravity waves (IGWs). Where the mountain GWs are absorpted and break, they generate a potential vorticity (PV) dipole steered by the shear. This results in a forced growing baroclinic pattern, which is the balanced part of the motion. The unbalanced response can consists of i) secondary IGWs generated by the momentum deposition in the critical zone (Vadas et al, 2003), and ii) non-geostrophic unstable modes of the Eady problem that couple spatially balanced Eady edge waves with IGWs (Plougonven et al 2005).

We quantify the relative importance of those parts in the energy budget, and analyse the mechanisms that generates the IGWs, notably when the regime of the critical levels interaction becomes nonlinear. In this situation, nonlinear feedbacks between the LSF response and the GWs forcing can prevent the IGWs part to increase with the orographic forcing. We also discuss the significance of each contribution to lee cyclogenesis.