An observational and numerical study of a mini-supercell storm

The structure and evolution of a small, moderately intense supercell storm, observed over northern Alabama on 17 July 1992, is described through a combined observational analysis and numerical modeling study. Wind profiler measurements show that the storm formed within a favorable low-level wind shear regime confined only to northern Alabama over a limited time interval between conventional NWS rawinsonde times. While not severe, this cloud system was long-lived and exhibited characteristics similar to those of classic supercells, including a unicellular echo with multicellular substructures, an extended lifetime, a collar cloud and a flanking line of cumulus. Moreover, this supercell did not produce cloud-to-ground lightning over its lifetime, nor was thunder from intracloud lightning heard over a 20-min period as the storm moved by. Numerical simulations of the cloud system duplicate the general observational features, including horizontal and vertical dimensions, cyclic behavior and convective core and anvil characteristics. Simulated distributions of microphysical fields indicate that a juxtaposition of cloud ice, graupel and supercooled water did not occur in sufficient concentrations to support effective charge separation by the non-inductive charge mechanism, despite updraft magnitudes of 15–20 m s−1 within the −10 to −20°C levels of the cloud.