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

Akira T. NODA, Hiroshi NIINO
2010 Journal of the Meteorological Society of Japan  
The mechanism of supercell tornadogenesis and its vorticity budget are investigated by means of a highresolution (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
more » ... (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 À1 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.
doi:10.2151/jmsj.2010-203 fatcat:fcnrmmoep5eiddsfasqvb7cbua