The Oklahoma squall line of 19 May 1977. Part II: Mechanisms for maintenance of the region of strong convection

The observed meso-β structure on this day is believed to be made possible by a deep low-level layer of weak vertical wind shear and high water-vapor content. At other locations along the line, the numerical simulation indicates an unsteady behavior in the maintenance of squall line convection by gust frontal convergence. Perturbations in the vertical motion field are periodically initiated by either (i) enhanced convergence at the gust front resulting from diverging downdrafts at locations farther to the west, or (ii) Kelvin-Helmholtz instability produced at the gust front head.

These perturbations move westward relative to the gust front above the low-level cold air and periodically invigorate the main region of updrafts located a few tens of kilometers west of the gust front. Low-level updrafts, forced by diverging surface outflow from weak downdrafts, occasionally interact with the translating perturbations to increase their amplitude. The existence of the westward-moving perturbations is tentatively substantiated by the presence of similar structures in the analyzed Doppler wind fields.

Greater time resolution in Doppler data, in combination with more comprehensive surface and upper air data ahead of squall lines of this type, would aid in confirming the reported structures.