A numerical investigation of a supercell tornado: Genesis and vorticity budget

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 occurredin oklahoma,usa,on may 20,1977. after 50 min of the simulation,a meso-low,which is generated by nonlinearinteraction between the storm updraft and vertical wind shear associated with both environmental andstorm-induced horizontal flow,develops at around 1.8 km above ground level (agl). the meso-low acts tostrengthen the underlying updraft,and generates a low-level mesocyclone via the tilting of horizontal vorticityassociated with the environmental wind and that generated by baroclinic processes. in turn,a pressure depressionassociated 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 thetornado strengthens a tornado-scale downdraft on its north side. the developed downdraft compresses the verticalvortex 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 maturetornado vortex originate mainly from the northwest in a layer between 10 and 500 m agl. vertical vorticitywithin the mature tornado is initially produced during descent via tilting of the horizontal vorticity,which isenhanced by stretching of the vortex tube. © 2010,meteorological society of japan.