Mesoscale convective vortex formation in environments with differnt values of shear and CAPE

Idealized simulations have been conducted using the Penn State/NCAR MM5 model to investigate the dynamics of mesoscale convective vortex formation. The objectives of the simulations were threefold: (1) to to test the hypothesis that the formation and maintenance of midlatitude warm-season mesoscale convective vortices (MCVs) are largely influenced by balanced flow dynamics associated with a vortex that interacts with weak vertical shear; (2) to examine the mechanisms of potential vorticity (PV) generation in the simulated vortex and to elucidate the respective roles of convective and stratiform precipitation in the PV generation; and (3) to investigate the sensitivity of the MCV to different values of CAPE and shear in its background environment.

Results show the warm advection and lift associated with vortex balanced flow cause new convection to develop on the downshear side of the vortex where it reinforces the vortex against the destructive effects of the shear. Vorticity budget analyses, diabatic heating profiles, and PV animations show both the stratiform and convective processes to be important in the PV production in MCVs. The most rapid PV production is in the convective region, and individual PV maxima and minima are carried in the front to rear flow of the MCS from the convective to stratiform areas, where the PV centers merge to form the MCV.

In all simulated cases, important interactions occur between synoptic and mesoscale motions. Warm advection frontogenesis associated with a developing moist baroclinic wave is necessary to initiate the MCS in the idealized simulations. Once the MCS develops, the diabatic production of eddy available potential energy increases rapidly, and the baroclinic conversion of mean state available potential energy to eddy available potential energy (APE) undergoes a rapid increase a few hours later. In terms of PV evolution, a burst of PV production occurs with the initial development of the MCS. Then, as the cold pool expands and convection spreads outward along the leading edge of the cold pool, new PV is produced east of the original center and is advected cyclonically around this center, becoming part of a larger, positive PV anomaly. In this manner, the entire vortex grows upscale such that it becomes difficult to distinguish from the larger baroclinic system.

The initial results regarding sensitivity to CAPE and shear reveal that both increase the strength of the vortex but the vortex strength may be slightly more sensitive to increases in CAPE than to strengthening of the shear. The diabatic production of eddy APE is roughly proportional to CAPE. Comparisons of simulated MCVs with those observed in the Bow Echo And Mesoscale Convective Vortex Experiment (BAMEX) reveal similar environments in terms of shear and CAPE and similar vortex characteristics in terms of tangential flow and height and radius of maximum winds.