BALANCED DYNAMICS OF MESOSCALE VORTICES PRODUCED IN SIMULATED CONVECTIVE SYSTEMS

Long-lived, mesoscale convective systems are known to occasionally pro duce mesoscale convective vortices (MCVs) in the lower to middle tropo sphere with horizontal scales averaging 100-200 km. The formation of M CVs is investigated using fully three-dimensional cloud model simulati ons of idealized, mesoscale convective systems (MCSs), initialized wit h a finite length line of unstable perturbations. In agreement with ob servations, the authors find that environmental conditions favoring MC V formation exhibit weak vertical shear confined to roughly the lowest 3 km, provided the Coriolis parameter (f) is chosen appropriate for m idlatitudes. With f = 0, counterrotating vortices form on the line end s, positive to the north and negative to the south with westerly envir onmental shear. The MCV and end vortices are synonymous with anomalies of potential vorticity (PV). Using PV inversion techniques, the autho rs show that the vortices are nearly balanced, even with f = 0. Howeve r, the formation of mesoscale vortices depends upon the unbalanced, sl oping, front-to-rear and rear inflow circulations of the mature squall line. End vortices form partly from the tilting of ambient shear but more from the tilting of the perturbation horizontal vorticity inheren t in the squall line circulation. With the addition of earth's rotatio n, an asymmetric structure results with the cyclonic vortex dominant o n the northern end of the line. The key to this MCV formation is organ ized convergence above the surface cold pool and associated mesoscale ascent and latent heating. A simulated MCV can even form in an environ ment with no ambient shear. Using a balanced model, the authors perfor m extended time integrations and show that the MCV produced in a shear ed environment remains largely intact because the shear is confined to low levels and is relatively weak. In addition, the interaction of th e vortex with the shear produces sufficient, mesoscale vertical motion on the downshear side of the vortex to trigger convection in typical, observed thermodynamic environments. Results suggest that balanced dy namical arguments may elucidate the long-term behavior of mesoscale vo rtices. However, because the balance equations neglect the irrotationa l velocity contribution to the horizontal vorticity, the formation of the mesoscale updraft that leads to an MCV and the generation of verti cal vorticity through vortex tilting are both treated improperly. Thus , the authors believe that existing balanced models will have serious difficulty simulating MCS evolution and mesoscale vortex formation unl ess mesoscale environmental forcing determines the behavior of the con vective system.