A Theory for Strong, Long-Lived Squall Lines

We study herein the mechanics of long-lived, line-oriented, precipitating cumulus convection (squall lines) using two- and three-dimensional numerical models of moist convection. These models, used in juxtaposition, enable us to address the important theoretical issue of whether a squall line is a system of special, long-lived cells, or whether it is a long-lived system of ordinary, short-lived cells. Our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but, on occasion, a squall line may be composed of essentially steady, supercell thunderstorms. The numerical experiments presented herein show that either type of squall line may develop from an initial line-like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line. With shallow shear, oriented perpendicular to the line, a long-lived line evolves containing individually short-lived cells. Our analysis of this type of simulated squall line suggests that the interaction of a storm cell's cold surface. outflow with the low-level shear produces much-deeper and less-inhibited lifting than is possible without the low-level shear, making it easier for new cells to form and grow as old cells decay. Through interecomparsion of two- and three-dimensional squall-line simulations, we conclude that the essential physics of this type of squall line is contained in the two- dimensional framework. We argue that these results describe the physics of both midlatitude and tropical squall lines. Under conditions of deep strong shear at an angle to the supposed line, a line of supercells develops in which their respective three-dimensional circulations do not interfere with one another.