Triple-Doppler Analysis of a Discretely Propagating, Long-Lived, High Plains Squall Line

Abstract

A nonsevere squall line that developed on 21 June 1993 along the northern foothills of the Colorado Rocky Mountains is analyzed using a series of triple-Doppler analyses. This squall line developed in a relatively dry environment characterized by weak low-level but strong upper-level vertical shear of the horizontal winds. The drier thermodynamic profile, as characterized by a higher lifting condensation level, and weak low-level shear resulted in convection forming not along, but rather 7?10 km behind the leading edge of the shallow cold pool. The strong upper-level shear established a predominantly leading anvil. This led to a suppression zone immediately ahead of the leading line, which in turn resulted in a discrete mode of propagation of the squall line. Three different cycles were observed: each cycle had a distinct line of convective cells that initiate, intensify, and then decay. In each case the new cells developed 20?40 km out ahead of the decaying line of the previous cycle. Many studies have identified low-level shear as being critical to squall line development. Results from this study indicate that there may be a more extensive set of environmental conditions that will lead to long-lived midlatitude squall lines. In particular, the strong upper-level shear played an important role in the characteristics of this storm.