Triple-doppler analysis of a discretely propagating, long-lived, high plains squall line

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.