The influence of convectively generated thermal forcing on the mesoscale circulation around squall lines

Abstract

The dynamical processes that determine the kinematic and thermodynamic structure of the mesoscale region around squall lines are examined using a series of numerical simulations. The features that develop in a realistic two-dimensional reference simulation of a squall line with trailing stratiform precipitation are compared to the features generated by a steady thermal forcing in a "dry" two-dimensional simulation with no microphysical parameterization. The thermal forcing in the dry simulation is a scaled and smoothed time average of the latent heat released and absorbed in and near the leading convective line in the reference simulation. The mesoscale circulation in the dry simulation resembles the mesoscale circulation in the reference simulation and around real squall lines suggesting that the circulation around squall lines is the result of gravity waves forced primarily by the low-frequency components of the latent heating and cooling in the leading line.Other features of real squall lines squall lines and the reference simulation are reproduced in the dry simulation as well, including cellularity in the leading line and upper-level potential temperature perturbations in the trailing stratiform region. The gravity waves generated by the time-mean thermal forcing in the leading line are also shown to play a role in the distribution of stratiform cloudiness. Additional numerical simulations, in which either the thermal forcing or the large-scale environmental conditions were varied, reveal that the circulation generated by the thermal forcing shows a greater sensitivity to variations in the thermal forcing than to variations in the large-scale environment. Finally, it is demonstrated that the depth of the thermal forcing in the leading convective line, not the height of the tropopause, is the primary factor determining the height of the trailing anvil cloud.The comparison of dry and reference simulations is repeated using three-dimensional simulations. A three-dimensional thermal forcing, constructed from the pattern of latent heating and cooling in the leading convective line of a three-dimensional moist simulation, is shown to generate a circulation which resembles the circulation in the three-dimensional reference simulation and in real squall lines. This circulation included the line-normal components of the flow found in the two-dimensional simulations, as well as three-dimensional components of the flow such as the Mesoscale Convective Vortex. Additional simulations demonstrate that the Coriolis force can modify the circulation toward an asymmetric organization even when the thermal forcing and the mean-state environment exhibit no asymmetries.