3-DIMENSIONAL EVOLUTION OF SIMULATED LONG-LIVED SQUALL LINES

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 nonhy drostatic 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 simulatio ns is characterized by weak, shallow shear and high convective availab le potential energy (CAPE), an environment in which long-lived midlati tude mesoscale convective systems (MCSs) are observed. The simulated s ystems bear strong resemblance to many observed systems, suggesting th at large-scale forcing, absent in the horizontally homogeneous environ ment, is not needed to produce many of the distinguishing features of midlatitude MCSs. In simulations without Coriolis forcing, the presenc e of line ends leads to mature symmetric systems characterized by a ce ntral region of strong convection, trailing flanks of weaker convectio n, and a strong, centrally focused rear inflow. Simulations that inclu de Coriolis forcing lead to asymmetric systems with significant system growth and migration to the right (south) of the original system cent erline. 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 simul ation, a midlevel mesoscale convective vortex (MCV) forms in the north , to the rear of the convection, while the outflow region aloft is str ongly anticyclonic. The northern location of the MCV is coincident wit h and influenced by a northward bias in the positive buoyancy anomaly aloft. Midlevel vertical vorticity generation by tilting of horizontal vorticity, both ambient and baro-clinically generated, is observed in both the Coriolis and no-Coriolis simulations. On larger scales, the convergence of Coriolis rotation generates significant vorticity and i s crucial to the formation of the MCV.