Tornadogenesis in a High-Resolution Simulation of the 8 May 2003 Oklahoma City Supercell

Abstract

A 50-m-grid-spacing Advanced Regional Prediction System (ARPS) simulation of the 8 May 2003 Oklahoma City tornadic supercell is examined. A 40-min forecast run on the 50-m grid produces two F3-intensity tornadoes that track within 10 km of the location of the observed long-track F4-intensity tornado.The development of both simulated tornadoes is analyzed to determine the processes responsible for tornadogenesis. Trajectory-based analyses of vorticity components and their time evolution reveal that tilting of low-level frictionally generated horizontal vorticity plays a dominant role in the development of vertical vorticity near the ground. This result represents the first time that such a mechanism has been shown to be important for generating near-surface vertical vorticity leading to tornadogenesis.A sensitivity simulation run with surface drag turned off was found to be considerably different from the simulation with drag included. A tornado still developed in the no-drag simulation, but it was much shorter lived and took a substantially different track than the observed tornadoes as well as the simulated tornadoes in the drag simulation. Tilting of baroclinic vorticity in an outflow surge may have played a role in tornadogenesis in the no-drag simulation.

VORTEX2 Observations of a Low-Level Mesocyclone with Multiple Internal Rear-Flank Downdraft Momentum Surges in the 18 May 2010 Dumas, Texas, Supercell*

“…A final observation on the evolution of surges B-D is that a near-surface vortex developed and persisted to the north and trailing the apex of each surge, reaching a maximum intensity as it interacted with the leading edge of surge D (Figs. 7, 8), which is similar to the evolution of horizontal momentum surges in prior dual-Doppler studies Wurman et al 2010;Marquis et al 2012;Kosiba et al 2013) and numerical simulations (Dahl et al 2014;Schenkman et al 2014).…”

“…8,9) are consistent with the tilting and subsequent stretching of vorticity originating within the RFD surges by upward acceleration. Though calculation of a vorticity budget for air parcels within the RFD surges entering the lowlevel mesocyclone is not possible in the present study, the presence of a vorticity dipole straddling the leading RFD surge is similar to previous studies in which counterrotating vortices were attributed to upwardarching, baroclinically generated vortex lines Markowski et al 2008Markowski et al , 2011Markowski et al , 2012aMarquis et al 2012;Kosiba et al 2013;Markowski and Richardson 2014) or to downward-sagging, frictionally generated vortex lines (Schenkman et al 2014). Evolution of the low-level mesocyclone can be discussed in terms of an inferred dynamic vertical perturbation pressure gradient force, which will be proportional to the magnitude of the azimuthal wind shear if an assumption of pure rotation is made (Rotunno and Klemp 1982).…”

“…This feature was first discussed in detail and labeled as an ''RFD surge'' or ''second surge'' by Finley and Lee (2004) and Lee et al (2004), though it was previously briefly discussed and has subsequently been identified using many different monikers, including ''outflow surge'' (Dowell and Bluestein 2002b), ''surging RFD'' (Markowski et al 2002), ''jet'' , ''double gust front'' (Adlerman 2003;Wurman et al 2007), ''embedded surge'' (Hirth et al 2008), ''secondary RFD surge/gust front'' Mashiko et al 2009;Wurman et al 2010;Skinner et al 2011;Marquis et al 2012;Kosiba et al 2013;Bluestein et al 2014), ''internal RFD surge'' (Finley and Lee 2008;Romine et al 2008;Karstens et al 2010;Lee et al 2011;Schenkman et al 2014), and ''rear-flank downdraft internal surge'' (RFDIS) (Lee et al 2012;Karstens et al 2013). It appears that, despite the variation in nomenclature, these terms refer to the same phenomenon.…”

Imported and Storm-Generated Near-Ground Vertical Vorticity in a Simulated Supercell*

“…Our results are based on a free-slip simulation; that is, surface friction was neglected, thereby ignoring a potentially important source of near-surface horizontal vorticity (Schenkman et al 2014). However, the basic mechanisms described herein would not be rendered invalid if an additional source of near-ground horizontal vorticity was added.…”

Aboveground Thermodynamic Observations in Convective Storms from Balloonborne Probes Acting as Pseudo-Lagrangian Drifters

“…This has been a hot topic in the severe storms community lately (Schenkman et al 2012(Schenkman et al , 2014Xu et al 2015;Roberts et al 2016) and unfortunately one that is exceptionally difficult to study both observationally and with simulations. Observational studies of the effects of surface friction are problematic because one cannot know how an observed storm might have behaved differently if in a frictionless setting.…”