Radar-detected Mesocyclone Tilt in Tornadic and Nontornadic Supercells

While supercell thunderstorms are the storms with the greatest potential of producing tornadoes, the majority of supercells do not produce tornadoes.  Recent work has demonstrated that low-level (LL) vertical wind shear and lifting condensation level (LCL) height in the storm inflow region are the most promising discriminators between tornadic and nontornadic supercells.  However, it is not yet understood why these parameters are the most favorable.  It is anticipated that as the horizontal distance between the LL and mid-level (ML) mesocyclones, the mesocyclone tilt, decreases, the likelihood and intensity of a tornado both increase.  It is expected that there is an orientation of vertical shear that results in a smaller mesocyclone tilt.  It is expected that lower LCL height, caused by higher boundary layer relative humidity, also results in a smaller mesocyclone tilt.  This study builds a climatology of radar data to distinguish between tornadic and nontornadic supercells.  Level-II and -III Weather Surveillance Radar-1988 Doppler data were collected for a subset of isolated supercells in the contiguous United States from 2009 to 2014.  

From this initial climatology, LL and ML azimuthal wind shear maxima are located, representing the LL and ML mesocyclones, and the horizontal distance between each maximum is calculated during the evolution of each supercell.  Results connecting the radar-derived mesocyclone tilt to aspects of the near-storm environment, including low-level shear magnitude and orientation and boundary layer relative humidity, will be discussed.  Characteristics of the near-storm environment are obtained from proximity soundings derived from the Rapid Update Cycle and Rapid Refresh model analyses.  Statistical analyses of the climatology and of individual case studies will be presented.