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Rajul E. Pandya and Dale R. Durran

Abstract

The dynamical processes that determine the kinematic and thermodynamic structure of the mesoscale region around 2D squall lines are examined using a series of numerical simulations. The features that develop in a realistic reference simulation of a squall line with trailing stratiform precipitation are compared to the features generated by a steady thermal forcing in a “dry” 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; it includes an ascending front-to-rear flow, a midlevel rear inflow, a mesoscale up- and downdraft, an upper-level rear-to-front flow ahead of the thermal forcing, and an upper-level cold anomaly to the rear of the thermal forcing. It is also shown that a steady thermal forcing with a magnitude characteristic of real squall lines can produce a cellular vertical velocity field as the result of the nonlinear governing dynamics. An additional dry simulation using a more horizontally compact thermal forcing demonstrates that the time-mean thermal forcing from the convective leading line alone can generate a mesoscale circulation that resembles the circulation in the reference simulation and around real squall lines.

The ability of this steady thermal forcing to generate the mesoscale circulation accompanying squall lines suggests that this circulation is the result of gravity waves forced primarily by the low-frequency components of the latent heating and cooling in the leading line. The gravity waves in the dry and reference simulation produce a perturbed flow that advects diabatically lifted air from the leading line outward. In the reference simulation, this leads to the development of leading and trailing anvils, while in the dry simulation this produces a pattern of vertically displaced air that is similar to the distribution of cloud in the reference simulation. Additional numerical simulations, in which either the thermal forcing or 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.

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Rajul E. Pandya, Dale R. Durran, and Morris L. Weisman

Abstract

Midlatitude squall lines are typically trailed by a large region of stratiform cloudiness and precipitation with significant mesoscale flow features, including an ascending front to rear flow; a descending rear inflow jet; line-end vortices; and, at later times, mesoscale convective vortices. The present study suggests that the mesoscale circulation in the trailing stratiform region is primarily determined by the time-mean pattern of heating and cooling in the leading convective line. Analysis of the line-normal circulation shows that it develops as thermally generated gravity waves spread away from the leading line. Midlevel line-end vortices are the result of diabatically driven tilting of horizontal vorticity generated by the time-mean thermal forcing. In the presence of the Coriolis force, a symmetric thermal forcing generates an asymmetric stratiform circulation and a pattern of vertical displacement that resembles the comma-shaped stratiform anvil observed in real systems; this suggests that asymmetries in the cloud and circulation behind midlatitude squall lines are not necessarily the result of asymmetries in the convective leading line.

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