Convective evolution in along–line shear

Convective lines in environments with line-parallel shear seem to have received relatively little attention in the literature. This may be because such systems cannot be reproduced in 2D and periodic-3D numerical simulations, which have heretofore been widely used in studies of linear convective systems. The present study considers some of the preferred structures that occur in environments with line–parallel vertical wind shear. Such systems are frequently implicated in flash flooding because they entail both the along–line movement of hydrometeors and backbuilding convective development.

Although the studied environments are initially supportive of supercells, the merging of outflows soon renders a predominant linear forcing. Thereafter, along–line flow within the system's cold pool entails backbuilding on both the mesoscale and the convective scale. As well, along–line flow in the upper troposphere within the system entails along–line hydrometeor transports, especially in the leading and trailing anvils. These behaviors lead to the archetypal structure of a convective line with parallel precipitation. Along–line hydrometeor advection means that much of the system's precipitation falls very near its outflow boundary, and that the convective cells can seed other updrafts farther down the line. As a result, convective systems in line–parallel shear can intensify their cold pools quite rapidly. As well, in time the convective line begins to possess diminished upper tropospheric along–line flow within its axis. These factors may hasten transition toward a predominantly rearward–sloped updraft and the production of trailing precipitation. Even in the absence of Coriolis accelerations, this kind of convective evolution in environments with line-parallel shear will lead to highly asymmetric structures, such as are commonly observed in midlatitudes.