Three-Dimensional Evolution of Simulated Long-Lived Squall Lines

Abstract

Simulations of squall lines, using nonhydrostatic convection-resolving models, have been limited to two dimensions or three dimensions with the assumption of along-line periodicity. The authors present 3D nonhydrostatic convection-resolving simulations, produced using an adaptive grid model, where the lines are finite in length and the restriction to along-line periodicity is removed. The base state for the simulations is characterized by weak, shallow shear and high convective available potential energy (CAPE), an environment in which longlived midlatitude mesoscale convective systems (MCSs) are observed. The simulated systems bear strong resemblance to many observed systems, suggesting that large-scale forcing, absent in the horizontally homogeneous environment, is not needed to produce many of the distinguishing features of midlatitude MCSs. In simulations without Coriolis forcing, the presence of line ends leads to mature symmetric systems characterized by a central region of strong convection, trailing flanks of weaker convection, and a strong, centrally focused rear inflow. Simulations that include Coriolis forcing lead to asymmetric systems with significant system growth and migration to the right (south) of the original system centerline. In both cases the evolution of the leading-line convection is primarily controlled by the surface cold pool expansion, with Coriolis forcing promoting rightward system propagation. In the Coriolis simulation, a midlevel mesoscale convective vortex (MCV) forms in the north, to the rear of the convection, while the outflow region aloft is strongly anticyclonic. The northern location of the MCV is coincident with and influenced by a northward bias in the positive buoyancy anomaly aloft. Midlevel vertical vorticity generation by tilting of horizontal vorticity, both ambient and baroclinically generated, is observed in both the Coriolis and no-Coriolis simulations. On larger scales, the convergence of Coriolis rotation generates significant vorticity and is crucial to the formation of the MCV.