The role of nonlinear equilibration in limiting the up-down asymmetry of vertical motions in the extratropics

The effective static stability experienced by eddies in a moist atmosphere depends on the asymmetry between upward and downward vertical motions. Upward motions are generally more intense than downward motions in the extratropics, but previous work has shown that the magnitude of this asymmetry (as measured by an asymmetry parameter) does not vary as the climate is changed from cold to warm in simulations with a general circulation model (GCM). The invariance of the asymmetry parameter is surprising because the theory of moist baroclinic instability suggests that the asymmetry should increase strongly as the atmosphere approaches neutrality to moist convection in warm climates.

Here, we first use an idealized GCM with a range of horizontal and vertical resolutions to show that the invariance of the asymmetry parameter is not caused by an inability to resolve small-scale updrafts. We then show that the same GCM is capable of simulating highly-asymmetric moist baroclinic waves in the linear regime. Lastly, we present an example of how nonlinear equilibration leads to a reduction in the asymmetry parameter to values typical of those seen in GCM simulations and reanalysis.