Balanced Dynamics of Mesoscale Vortices Produced in Simulated Convective Systems

Abstract

Long-lived, mesoscale convective systems are known to occasionally produce mesoscale convective vortices (MCVs) in the lower to middle troposphere with horizontal scales averaging 100?200 km. The formation of MCVs is investigated using fully three-dimensional cloud model simulations of idealized, mesoscale convective systems (MCSs), initialized with a finite length line of unstable perturbations. In agreement with observations, the authors find that environmental conditions favoring MCV formation exhibit weak vertical shear confined to roughly the lowest 3 km, provided the Coriolis parameter (f) is chosen appropriate for midlatitudes. With f = 0, counterrotating vortices form on the line ends, positive to the north and negative to the south with westerly environmental shear. The MCV and end vortices are synonymous with anomalies of potential vorticity (PV). Using PV inversion techniques, the authors show that the vortices are nearly balanced, even with f = 0. However, the formation of mesoscale vortices depends upon the unbalanced, sloping, 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 inherent in the squall line circulation. With the addition of earth's rotation, an asymmetric structure results with the cyclonic vortex dominant on the northern end of the line. The key to this MCV formation is organized convergence above the surface cold pool and associated mesoscale ascent and latent heating. A simulated MCV can even form in an environment with no ambient shear. Using a balanced model, the authors perform extended time integrations and show that the MCV produced in a sheared environment remains largely intact because the shear is confined to low levels and is relatively weak. In addition, the interaction of the 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 dynamical arguments may elucidate the long-term behavior of mesoscale vortices. However, because the balance equations neglect the irrotational velocity contribution to the horizontal vorticity, the formation of the mesoscale updraft that leads to an MCV and the generation of vertical 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 unless mesoscale environmental forcing determines the behavior of the convective system.