Sensitivity of main downdraft features to vertical wind shear and ice-phase in a microburst-producing thunderstorm

Abstract

This paper presents results from numerical simulations of a severe storm producing a microburst, using a convective numerical model. The numerical experiments are done in order to understand the role of the environmental vertical wind profile and/or the ice phase in organizing cloud dynamics and the strength of the microburst. Three particular downdraft types (the mid level, the penetrative and the precipitation related) and their relationship with cloud microphysics and the environmental wind profile are analyzed. To accomplish this objective, a well documented storm developed on 20 July 1986 over northern Alabama (USA), is used as case study. Similar environmental conditions to those that characterize this event have been observed prior to thunderstorms that produce severe low-level winds over northeastern Argentina. Model results show a good representation of the observed main storm features during the different cloud stages for the control experiment. Numerical simulations confirm the role of the vertical wind shear, in spite of its weak magnitude, in the genesis and support of the mid-level downdraft. The inclusion of the ice phase provides additional buoyancy that promotes a stronger updraft and vertical growth and reduces the loading contribution related to supercooled raindrops to the mid-level downdraft. Melting provides an additional cooling that reinforces both the low-level precipitation related downdraft and the divergent outflow velocity differential in the microburst.