Idealized simulations of nocturnal severe wind-producing convective systems

Organized convection has long been recognized to have a nocturnal maximum over the central United States. Many nighttime convective systems produce severe surface winds, even though they often appear to be elevated above and decoupled from a stable nocturnal boundary layer. 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 severe wind production of elevated 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).

Previously suggested mechanisms for severe wind production by elevated convection include the following. Convective systems that are elevated may nevertheless produce strong downdrafts that penetrate the inversion and produce severe winds at the surface. As well, stable surface air that is present near a convective system may be dynamically lifted and cooled at the system's leading edge, after which it descends rapidly to the surface owing to its negative buoyancy. Finally, a mature system may produce a sufficiently perturbed pressure field that the sub-inversion air, even if decoupled from the convective overturning, can yet be accelerated to severe speeds.

In the light of a recent observational study, however, an additional mechanism is also envisioned. Observations suggest that, once a convective system has matured and produced a surface cold pool, the introduction of CIN to the sounding via nocturnal chilling does not necessarily disrupt the convective system, nor does it prevent the system from producing severe surface winds, provided that at least some boundary layer CAPE remains. The nocturnal parcels need not have large CAPE values, but it is hypothesized that they must be capable of taking part in the deep convection, with whatever vigor, such that the storms continue to overturn the lower part of the troposhere. In this scenario, even though the nocturnal convection appears to be feeding on elevated air with higher θ_e and greater potential buoyancy, the key to its severe wind production is the lifting of boundary layer air by an outflow boundary. The idealized numerical simulations in progress will be used to evaluate the preceding hypotheses and elucidate the key parcel trajectories and governing dynamics for severe wind production in a nocturnal environment.