The simulation of high-precipitation supercells on preexisting boundaries in multicellular environments

The documented observations of supercells along preexisting boundaries in multicellular environments represent either a fundamental limitation of known diagnostic parameters to discriminate convective mode, irrespective of the presence of localized heterogeneities, or the modulating effect of preexisting boundaries. It is our belief that such observations demonstrate the modulating role of boundaries. The primary objective of this proposed work is to identify the specific mechanisms by which boundaries yield this modulation.

Observations and climatological surveys have also noted that many boundary-anchored supercells possess a high-precipitation (HP) morphology. Since the most probable hazards associated with a supercell often depend on its morphology, an explanation for this tendency needs to be identified. Therefore, the second objective of this proposed work is to examine the preference for an HP morphology in supercells on boundaries in multicellular environments.

Previous studies have addressed the role of preexisting boundaries on tornadogenesis and low-level mesocyclogenesis but a direct examination of the role of preexisting boundaries in the development of high-precipitation supercells in multicellular environments has not been undertaken. To explore this phenomenon we will conduct a set of cloud-scale, numerical experiments with the Illinois Collaborative Model for Multiscale Atmospheric Simulations (ICOMMAS). Experiments have been designed to test the following complimentary hypotheses. First, we propose that the augmentation of the low-level shear within the airmass along and just behind a preexisting boundary, associated primarily with the boundary’s solenoidal circulation, can foster supercellular updraft maintenance/propagation and rotation in environments that would otherwise be unfavorable for supercells. Furthermore, we propose that, while this enhancement of low-level shear along and behind a boundary will yield a vertical profile of wind with sufficient shear for supercells, in a multicellular environment characterized by “modest” deep-layer shear the mid/upper-level winds will remain relatively weak. Supercellular environments with relatively weak mid/upper-level winds have been shown to favor an HP morphology. Results from preliminary simulations will be presented at the conference.