Tropical squall lines of the Arizona monsoon.

Abstract

Squall lines possessing nearly all the characteristics of tropical squall lines occasionally develop during the summer monsoon over southern Arizona and northwestern Mexico. Initial thunderstorm formation is over the mountains along the Continental Divide in the late afternoon. Satellite imagery, cloud-to-ground lightning strike data, and surface observations indicate the squall lines move from east to west or northeast to southwest by discrete propagation faster than all the winds below 20 kPa so that most of the anvil clouds lag behind. The synoptic-scale circulation is anomalous with a strong ridge located over the western United States and a deep trough located over the eastern United States. West to northwest winds are found in the boundary layer over southern Arizona and northwest Mexico while a deep layer of east winds are observed above. As a result, most of the environmental wind shear is confined to the lowest 2.5 km above the ground. The low-level wind shear seems to be required for the westward propagation of thunderstorms and the formation of the squall lines. Extremely dry midtropospheric air develops in the easterly flow through some combination of advection and subsidence and also appears to be an important factor in the development of the squall lines. A two-dimensional, nonhydrostatic, numerical model was able to simulate many of the features observed in these squall lines. Solar heating of the elevated terrain in the model caused the initial thunderstorm to develop over the Continental Divide. Continued development of new thunderstorms to the west of the Divide produced a squall line that travelled westward by translation of cells and discrete propagation, wherein new cells would develop 10-25 km ahead of the old ones, at a speed greater than all the winds below 30 kPa. Upward motion produced by westward propagating gravity waves and by the strong low-level convergence found just ahead of the gust front appeared to cause several episodes of discrete propagation. The creation of horizontal potential temperature gradients and the vertical and horizontal advection of preexisting vorticity gradients combined to produce the vorticity field associated with the rear inflow jet that developed beneath the simulated squall line.