This study concerns mechanisms responsible for the maintenance and discrete propagation of nocturnal squall lines. "Discrete propagation" refers to the development of new and independent convection in the upstream environment lying ahead of an already established storm. This new convection may subsequently merge with the pre-existing storm or may instead supplant it. In either case, this mode of propagation is distinguished from "continuous propagation", a situation in which new cells are formed in the immediate vicinity of a gust front. The most commonly cited mechanism for triggering this discrete propagation involves gravity waves (e.g., Carbone et al. 1990; Crook et al. 1990; Cram et al. 1992; Stensrud and Fritsch 1993, etc.)

The mechanisms allowing for discrete propagation may help an organized storm persist for considerably longer periods of time and/or result in regeneration of convection after a period of decay and quiescence, particularly when a storm encounters an unfavorable environment. This is seen in a numerical simulation of discretely propagating nocturnal convection made using a cloud model with realistic surface physics and atmospheric radiative forcing. Throughout its mature phase, and extending into the evening hours, the storm is found to possess a midtropospheric cool and moist tongue of air extending ahead of it. The development and maintenance of this feature, which may be viewed as "preconditioning" of the upstream environment, was discussed recently by Fovell (2002). Unsurprisingly, the storm declined in intensity after sunset as the lower troposphere cooled and CAPE decreased.

Yet, this particular storm did not perish in part because new convection began appearing in the cool, moist tongue extending ahead of the storm. As the night progressed, these new cells merged with the pre-existing line, resulting in a renewed burst of convective activity. Several times during the simulation, one of these new cells developed into deep convection prior to merging with the line. In these instances, the newly established convection became the new squall line, resulting in what would appear as a discrete jump in the position of the convection. We will show how and why this phenomenon took place, and make comparisons with observations.