Modeling Boundary Crossing Supercells

Supercell storms cause major destruction and loss of life every year.  However, only some supercells cause significant societal impacts and, of those that do, are only destructive during certain segments in their lifecycle.  Why does a supercell intensify and why do some produce more prolific damage than others?  Particular environmental conditions are strongly correlated with peak supercell and tornado strength (e.g., Thompson et al. 2003; 2007).  However, supercells, especially those long-lived (Bunkers et al. 2006) do not experience homogeneous environments.  Rather, they encounter changes in their surrounding environment spatially and temporally.  While some of the changes can be gradual and small (Richardson et al. 2008), supercells that cross boundaries can experience a rapid change (e.g., Markowski et al. 1998; Gilmore and Wicker 2002).

While tornadoes can form when supercells interact with warm front boundaries, they can also form when the supercells interacts with pre-existing outflow boundaries from previous convection (e.g., Maddox et al. 1980).  Fierro et al. 2006 used a cloud model to study how a supercell intensified in updraft speed, rotation, and electrification as it crossed a pre-existing outflow boundary into an environment of larger instability, larger storm relative environmental helicity, and lower cloudbase. Fierro et al’s, idealized outflow boundary was based upon two soundings sampled either side of an outflow boundary in west Texas (Rasmussen et al. 1998). Fierro’s coarse resolution, however, was not adequate for resolving tornadic circulations.

This study focuses on the behavior of a storm’s mesocyclone and the vorticity production that may lead to tornadogenesis after a boundary crossing.  Using a three-dimensional cloud model, CM1 (Bryan and Fritsch 2002), two suites of simulations are performed.  One supercell suite of simulations largely replicates Fierro et al’s earlier work with 1 km horizontal grid spacing while the tornado suite uses 100 m isotropic grid spacing.  

Time-height plots are used to analyze how certain variables, such as vertical velocity, change with height as the supercell intensifies.  Horizontal cross sections of updraft, radar reflectivity, and vertical vorticity reveal storm structure as typically viewed from Doppler radar.  Additionally both Eulerian and Lagrangian analyses are performed. The Eulerian circulation analysis is performed following Trapp and Weisman (2003), to study how the mesocyclone circulation changes over time, along with vorticity sources that act to increase the circulation.  Lagrangian trajectory analysis is used to analyze how the vorticity of air parcels change as they enter the mesocyclone and/or tornado circulation.  Potentially important vorticity processes that air parcels may experience as they enter the storm circulation are horizontal and vertical stretching and the tilting of horizontal into vertical vorticity (e.g., Naylor and Gilmore 2014).  A supercell storm’s ingesting of baroclinic vorticity and remnant vertical vorticity along the pre-existing boundary are also potentially important in the study herein.