Moist Baroclinic Instability over a Wide Range of Climates: From Periodic Waves to Diabatic Rossby Vortices

The distribution of vertical velocities in the extratropical troposphere becomes more skewed as the climate warms in simulations with general circulation models (GCMs). Previous studies of baroclinic instability in a moist atmosphere suggest that latent heating decreases the area-fraction of upward motion, and this may help to explain the greater skewness of the vertical velocity in a warmer and moister atmosphere. Here we examine moist baroclinic instability over a wide range of climates in an idealized GCM. We focus on the behavior of the vertical velocity, and we compare the behavior of the modes of moist baroclinic instability with fully nonlinear simulations in the same idealized GCM. Remarkably we find that the most unstable mode transitions from a periodic wave to an isolated vortex (known as a diabatic Rossby vortex) as the climate warms, with possible implications for cyclogenesis in very warm and moist climates. We also discuss the implications of the increases in skewness of the vertical velocity for the spatial extent of precipitation events.