Abstract
The squall line's impact on its upstream environment is examined using traditional cloud and simplified (parameterized moisture) models. The study was motivated by the need to explain significant differences between the two dynamical frameworks. In both, the first environmental response to the convection consists of a rapidly propagating gravity wave characterized by deep tropospheric subsidence. This gravity wave accelerates the low-level storm inflow in its wake, with largest effect seen near the surface.
In the more sophisticated model, however, this wave is eventually followed by a shorter vertical wavelength feature, one possessing lower tropospheric ascent. This second gravity wave, absent from the simplified model runs, substantially alters the upstream environment yet again. The gentle low-level uplift establishes the ``cool/moist tongue'' of air that has been found to stretch well ahead of the storm in simulations. Between the second wave and the main storm updraft, the accelerated inflow is shifted from the ground to the middle troposphere where it helps to push dry air into the convecting region.
This subsequent environmental adjustment responds to the establishment of a small yet persistent area of weak cooling during the early mature phase. Located in the middle troposphere at the upstream edge of the main cloud mass, this cooling is the combined effect of cloud water evaporation and the ascent of subsaturated air. Though the cooling that excites it is of relatively small magnitude ($<$ 2 K), this wave's effect is dramatic and significant. A crude fix to the moisture parameterization served to establish a qualitatively similar tongue-like feature in the simplified model simulations, bringing the results from the two dynamical frameworks more into line.