Balanced turbulence on the tropopause

It is well known that tropopause disturbances are important for weather-producing events such as extratropical cyclones. Observations show that these disturbances are often vortical, with pronounced structural and population asymmetries between cyclones and anticyclones. Cyclones are typically characterized by compact structure and stronger pressure, wind and temperature perturbations when compared to broader and weaker anticyclones. Neither the origin of these vortices or the reasons for the preferred asymmetries are well understood.

We explore these problems using a novel small Rossby-number approximation to the primitive equations applied to a simple model of the tropopause in continuously stratified fluid. This model resolves the dynamics that give rise to vortical asymmetries, while retaining both the conceptual simplicity of quasigeostrophic dynamics and the computational economy of two-dimensional flows. Solutions for random initial conditions (i.e., freely decaying turbulence) exhibit structural asymmetries typical of observations. Moreover, we find that cyclones have a preferred length scale whereas anticyclones do not. These results differ significantly from previous studies of cyclone--anticyclone asymmetry for the shallow-water primitive equations and the periodic balance equations.