Response of simulated squall lines to low-level cooling

Organized convection has long been recognized to have a nocturnal maximum over the central United States. However, the role of the stable boundary layer and its effects on the governing dynamics of convective systems' inflow and updraft trajectories have been treated somewhat speculatively to this point. The present study uses idealized numerical simulations to investigate the mechanisms for the maintenance, propagation, and evolution of nocturnal-like convective systems. As a litmus test for the basic governing dynamics, the experiments use horizontally homogeneous initial conditions (i.e. they include neither fronts nor low-level jet streams).

The simulated storms are allowed to mature as surface-based convective systems before the boundary layer is cooled. In this case it is then surprisingly difficult to cut the mature convective systems off from their source of near-surface inflow parcels. Even when 10K of low-level cooling has been applied, the pre-existing system cold pool is sufficient to lift boundary layer parcels to their LFCs. The present results suggest that many of the nocturnal convective systems that were previously thought to be “elevated” may actually be “surface-based”. With additional cooling, the simulated systems do indeed become elevated. First, the CAPE of the near-surface air goes to zero; and second, as the cold pool's temperature deficit vanishes, the lifting mechanism evolves toward a gravity wave or bore atop the nocturnal inversion. Provided that air above the inversion has CAPE, the system then survives and begins to move at the characteristic speed of the wave/bore. Interestingly, as the pre-convective environment is cooled and approaches the temperature of the convective outflow, but before the system becomes elevated, yet another distinct behavior emerges. The comparatively weaker cold pool entails slower system motion, but also more intense lifting (apparently because it is more nearly balanced by the lower tropospheric shear, e.g. Rotunno et al. 1988). This would explain the frequent observation of intensifying convective systems in the evening hours without the need for a nocturnal low-level jet.

The governing dynamics of the simulated systems, as well as the behaviors of low-level tracers and parcel trajectories will be discussed in the extended abstract and formal presentation.