Kinematic and Thermodynamic Study of a Shallow Hailstorm Sampled by the McGill Bistatic Multiple-Doppler Radar Network

Abstract

In this paper, the authors examine the kinematic and thermodynamic characteristics of a shallow hailstorm sampled by the McGill bistatic multiple-Doppler radar network on 26 May 1997. This storm consists of two main shallow convective cells (depth less than 5 km) aligned along a SW?NE convective line propagating to the southeast. The authors also analyze the interactions between the two cells during the life cycle of the convective line. In particular it is shown that dynamic interactions play a major role in the intensification of the second cell. This storm is found to evolve in a manner that shares some characteristics with both multicell and supercell storms. A rotating updraft associated with a mesocyclone develops in the mature stage of the storm, which is characteristic of a supercell. However, the lack of a ?vault? structure on the precipitation field, the relatively fast evolution of the cells, and other characteristics detailed henceforth seem to indicate that this storm only shares a few of the typical characteristics of supercells. Some morphological and thermodynamic similarities are found between this storm and recent numerical simulations of shallow supercell storms. While the first cell starts dissipating, a cold downward rear inflow is developing, which resembles the ?rear-flank? downdraft documented in several numerical and observational studies of tornadic storms. This downdraft acts to intensify the updraft associated with the second cell and produces a precipitation overhang within which hail eventually forms. When this pocket of hail falls to the ground a bit later, it accelerates the low-level rear inflow that progressively cuts off the inflow ahead of the storm, leading to the progressive dissipation of the second cell. The physical processes involved in the evolution of rotation at low levels to midlevels within this storm are evaluated using the vorticity equation. It is shown that the time tendency of the positive and negative vertical vorticity anomalies associated with the two cells are mainly driven by tilting of horizontal vorticity. Strong negative vertical vorticity associated with the intensification of the rear-flank downdraft in the later stage of the storm is also produced by tilting, which is consistent with previous studies of tornadic storms.