Evolution of Quasi-Two-Dimensional Squall Lines. Part I: Kinematic and Reflectivity Structure

Abstract

Doppler radar observations that establish common patterns in the evolution of the reflectivity and flow structures of squall lines are described. A number of squall lines have been analyzed with unprecedented time resolution in order to identity these patterns. All of the squall lines appeared to be approximately two-dimensional and featured a solid leading edge at some time during their life cycle, instead of being composed of discrete cells separated by echo-free regions. A large variety of intensities and evolution lime scales was observed. It is shown that squall lines of this type evolve through identifiable stages of reflectivity structure. This evolution appears to be strongly related to changes that occur in the kinematic structure. As a typical system evolves, a rearward-sloping zone of horizontal vorticity, predominantly associated with vertical shear, develops on the scale of the system, presumably driven by the horizontal buoyancy gradients across the system. The vorticity that is generated allows further generation to take place by causing the superposition of a saturated, precipitating anvil cloud aloft over potentially cooler air below in the trailing region. The rearward-sloping vorticity zone gradually tilts toward the horizontal. The rate at which this zone tilts seems to be the primary difference between the systems studied. To a first approximation, the inflow streamlines parallel the sloping vorticity zone, so as it approaches a horizontal slope, vertical motion becomes smaller. Eventually, convective-scale ascent ceases, giving the impression that the gust front has surged out ahead of the precipitation.