Genesis Criteria for Diabatic Rossby Vortices: A Model Study

A suite of idealized mesoscale model simulations are conducted to examine the dynamic pathway to the genesis of short-scale, moist baroclinic disturbances. It is shown that in an initially subsaturated environment, two distinct stages of development are necessary before a preexisting surface-concentrated, warm-core vortex can begin to amplify. The first stage, termed environmental preconditioning, involves the moistening of the lower atmosphere via the transport of relatively high equivalent potential temperature air into the immediate environment of the translating vortex. The second stage results from continuous cloud diabatic processes and it involves the emergence of a low-level positive potential vorticity (PV) anomaly with some evidence of a further, more diffuse, negative PV anomaly at higher elevations. The PV structure is characteristic of a diabatic Rossby vortex (DRV). The disturbance does not begin to amplify until the magnitude and coherence of the low-level PV structure allows for the sufficient production of eddy available potential energy through diabatic processes to overcome frictional dissipation, and this necessitates forced convection for a finite period of time. A comparison of the simulated disturbance structure with observed DRVs lends credence to the idealized model results. Furthermore, the simulations facilitate the identification of an amplitude threshold for DRV genesis that is defined as a function of both environmental parameters (baroclinicity and moisture content) and the strength of an initial disturbance. In particular, if, given a background environment, the amplitude of an initial disturbance is not sufficiently large, frictional processes will inhibit the genesis of a growing disturbance within a realistic time frame.