Convective initiation ahead of squall lines: observations and simulations

On 21 June 2003, over 30 new convective cells appeared ahead of an eastward-advancing nocturnal squall line. At first glance, the initiation locations of these new cells appeared to be relatively unordered. However, more careful analysis reveals that cells preferentially appeared on one of several parallel lines having a certain spacing and oriented at an acute angle to the squall line. This behavior suggests that organized structures existed in the nocturnal boundary layer that served to make localized areas of the storm's upstream environment more relatively favorable for convective initiation.

We have been studying this case by analyzing observations and by conducting idealized numerical experiments. Our working hypothesis is the organized structures represent the remnants of moisture bands established by horizontal convective rolls that existed on the previous afternoon. The active roll circulations, which are stationary relative to the ground, decay after sunset as the surface layer stabilizes, but are presumed to leave behind quasi-linear, parallel zones of enhanced moisture where the roll updrafts had existed. These zones represent favored areas for convective initiation. Since the cells crop up well ahead of the main storm's gust front, an action-at-a-distance mechanism is needed to trigger initiation, and a reasonable candidate is vertically trapped gravity waves excited by transience in the main storm's convective region. The viability of this mechanism is demonstrated and evaluated using idealized numerical simulations.