A Numerical Investigation of a Supercell Tornado: Genesis and Vorticity Budget

Abstract

The mechanism of supercell tornadogenesis and its vorticity budget are investigated by means of a high-resolution (horizontally uniform grid size of 70 m) numerical simulation of the Del City storm, which occurred in Oklahoma, USA, on May 20, 1977. After 50 min of the simulation, a meso-low, which is generated by nonlinear interaction between the storm updraft and vertical wind shear associated with both environmental and storm-induced horizontal flow, develops at around 1.8 km above ground level (AGL). The meso-low acts to strengthen the underlying updraft, and generates a low-level mesocyclone via the tilting of horizontal vorticity associated with the environmental wind and that generated by baroclinic processes. In turn, a pressure depression associated with this low-level mesocyclone generates an updraft exceeding 43 m s -1 at 1.5 km AGL. In addition, small-scale vortices (pretornadic vortices) develop along a gust front. When the low-level updraft strengthened, one of these pretornadic vortices located immediately beneath the updraft shows a rapid growth into a major tornado. As the tornado develops, a downward pressure gradient force associated with an intense rotation of the tornado strengthens a tornado-scale downdraft on its north side. The developed downdraft compresses the vertical vortex of the tornado, eventually resulting in its dissipation. We also performed a vorticity budget analysis along a typical air-parcel trajectory. Air parcels in the mature tornado vortex originate mainly from the northwest in a layer between 10 and 500 m AGL. Vertical vorticity within the mature tornado is initially produced during descent via tilting of the horizontal vorticity, which is enhanced by stretching of the vortex tube.