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Vorticity Evolution Leading to Tornadogenesis and Tornadogenesis Failure in Simulated Supercells

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Vorticity Evolution Leading to Tornadogenesis and Tornadogenesis Failure in Simulated Supercells MARCH 2014 N A Y L O R A N D G I L M O R E 1201 Vorticity Evolution Leading to Tornadogenesis and Tornadogenesis Failure in Simulated Supercells JASON NAYLOR AND MATTHEW S. GILMORE Department of Atmospheric Sciences, University of North Dakota, Grand Forks, North Dakota (Manuscript received 19 July 2013, in final form 30 September 2013) ABSTRACT A three-dimensional idealized cloud model was used to study the storm-scale differences between simu- lated supercells that produce tornado-like vortices and those that do not. Each simulation was initialized with a different Rapid Update Cycle, version 2 (RUC-2), sounding that was associated with tornadic and non- tornadic supercells in nature. The focus is an analysis of vorticity along backward-integrated trajectories leading up to tornadogenesis (19 simulations) and tornadogenesis failure (14 simulations). In so doing, the differences between the nontornadic and tornadic cases can be explored in relation to their associated en- vironmental sounding. Backward-integrated trajectories seeded in the near-surface circulation indicate that the largest differences in vertical vorticity production between the tornadic and nontornadic simulations occur in parcels that de- scend to the surface from aloft (i.e., descending). Thus, the results from this study support the hypothesis that descending air in the rear of the storm is crucial to tornadogenesis. In the tornadic simulations, the descending parcels experience more negative vertical vorticity production during descent and larger tilting of horizontal vorticity into positive vertical vorticity after reaching the surface, owing to stronger horizontal gradients of vertical velocity. The larger vertical velocities experienced by the trajectories just prior to tornadogenesis in the tornadic simulations are associated with environmental soundings of larger CAPE, smaller convective inhibition (CIN), and larger 0–1-km storm-relative environmental helicity. Furthermore, in contrast with what might be expected from previous works, trajectories entering the incipient tornadic circulations are more negatively buoyant than those entering the nontornadic circulations. 1. Introduction d tilting of horizontal vorticity that was baroclinically generated (i.e., vorticity generated by the storm’s own Although tornadoes have been studied extensively horizontal density gradients), over the last 50 years, many unanswered questions re- d tilting of horizontal vorticity associated with the main regarding the storm-scale processes responsible vertical wind shear of the environment (also known for their development. Numerical modeling studies have as barotropic vorticity), or repeatedly shown that supercells do not develop low- d transport of vertical vorticity to the surface. level rotation until downdrafts reach the surface (e.g., Klemp and Rotunno 1983; Davies-Jones and Brooks More than one of these processes contributes to the 1993; Walko 1993; Trapp and Fiedler 1995; Wicker and rotation within the low-level mesocyclone (Klemp and Wilhelmson 1995, hereafter WW95; Adlerman et al. Rotunno 1983; Davies-Jones and Brooks 1993; A99; 1999, hereafter A99). While these studies agree that Markowski et al. 2008); however, the relative impor- downdrafts are critically important, they do not agree tance of these processes to tornadogenesis appears to on the exact mechanisms that produce low-level vor- vary among cases. First, modeling studies by Davies- ticity. These works have concluded that downdrafts can Jones and Brooks (1993) and Grasso and Cotton (1995) produce positive (cyclonic) near-surface vertical vor- found that the largest source of vertical vorticity in the ticity via the low-level mesocyclone is baroclinically generated and tilted into the vertical in air that descends cyclonically Corresponding author address: Jason Naylor, NorthWest Research around the updraft. Second, Markowski et al. (2003) and Associates, 3380 Mitchell Lane, Boulder, CO 80301. Davies-Jones (2008) demonstrated that tornadogenesis E-mail: [email protected] could occur only through the transport of vertical vorticity DOI: 10.1175/JAS-D-13-0219.1 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 10/06/21 04:52 AM UTC 1202 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 71 to the surface via downdrafts and curtains of rain that wrap tornadic and nontornadic supercells. Trapp (1999) com- cyclonically around the low-level mesocyclone. Third, pared six supercells (three nontornadic and three torna- the Brandes (1984) observational study and WW95 nu- dic) observed during the Verification of the Origins of merical study concluded that the primary source of cyclonic Rotation in Tornadoes Experiment (VORTEX) and vertical vorticity was via tilting and stretching of horizontal found that while the nontornadic supercells experi- vorticity originally generated along the forward-flank gust enced less stretching of vertical vorticity and less low- front, and that descending parcels either did not strongly level convergence, the supercells were similar in other contribute to low-level vertical vorticity or, as in WW95, respects—including the presence of low-level mesocy- contributed adversely to vorticity of the opposite sign. clones and rear-flank gust fronts. Using a similar da- Much of the research involving nontornadic storms taset from VORTEX, Markowski et al. (2008) found has only focused on understanding differences in the that both tornadic and nontornadic supercells exhibit near-storm environments (NSEs) of tornadic and non- vortex line ‘‘arches’’ that straddle the hook echo— tornadic storms (e.g., Darkow 1969; Maddox 1976; Davies suggesting that near-surface rotation development in and Johns 1993; Brooks et al. 1994; Rasmussen and both types of supercells was aided by baroclinic vorticity Blanchard 1998; Thompson et al. 2003, 2012; Togstad generation in the rear-flank downdraft. Wakimoto and et al. 2011). For example, it is now known that tornadic Cai (2000) found that while a nontornadic supercell had supercells occur more often in NSEs with large values of ‘‘more extensive’’ precipitation (as indicated by radar storm-relative environmental helicity, large CAPE, low reflectivity) in the rear flank, stronger inflow, and stron- LCL heights (Rasmussen and Blanchard 1998; Thompson ger updrafts along the rear flank compared to a tornadic et al. 2003, 2012), and small convective inhibition (CIN) supercell, the nontornadic supercell had an order-of- (Thompson et al. 2012). However, there are still many magnitude less horizontal vorticity in the NSE despite questions regarding the storm-scale differences between both storms having an occlusion downdraft and horseshoe- tornadic and nontornadic supercells and how these dif- shaped updraft–downdraft signatures. Ziegler et al. (2001) ferences may be influenced by the NSE. concluded that a tornadic supercell had strong, low- One storm-scale feature that may help discriminate level stretching of cyclonic vertical vorticity associated between tornadic and nontornadic storms is the buoy- with a preexisting boundary layer vortex, while a nearby ancy of the low-level storm outflow. Observational studies nontornadic storm was characterized by negative stretch- have also shown that the evaporatively chilled storm ing. While the aforementioned studies show that tornadic outflow in significantly tornadic supercells often has and nontornadic storms share many structural similari- smaller negative buoyancy (not as cold/dense) relative ties, differences in vorticity production may explain why to the prestorm NSE compared to nontornadic super- some storms were tornadic and others were not. How- cells (e.g., Markowski et al. 2002; Shabbott and Markowski ever, the small number of cases and different analysis 2006; Grzych et al. 2007). Simulations by Markowski strategies makes it difficult to generalize differences in et al. (2003) using a model with a 2D axisymmetric co- vorticity production. ordinate system show that downdrafts with more nega- The main goal of the current study is to advance the tively buoyant air cannot be lifted by the updraft, thus current understanding of tornadogenesis by simulating disrupting near-surface convergence and stretching of numerous tornadic1 and nontornadic storms to deter- vertical vorticity. Markowski et al. (2011) computed mine the source(s) of vorticity-rich air at low levels, trajectories using dual-Doppler wind retrievals in three identify the processes that result in tornadogenesis and nontornadic supercells and found that the air entering tornadogenesis failure, and relate these to the NSE. the near-surface circulation only ascends a short dis- Idealized simulations were initialized with proximity tance before abruptly descending again, implying either soundings representative of the NSEs of tornadic and 1) the parcels in the nontornadic cases are too negatively nontornadic supercells. It is believed that the study buoyant to be lifted by the updraft or 2) the low-level herein contains the largest number of tornado-resolving vertical pressure gradient force is insufficient to lift the parcels. The findings from these studies suggest that barotropic vorticity is important to tornadogenesis and 1 Herein, when terms such as ‘‘tornado,’’ ‘‘tornadic,’’ ‘‘non- if the downdraft is too ‘‘cold,’’ this might inhibit torna- tornadic,’’ ‘‘tornadogenesis,’’ or ‘‘tornadogenesis failure’’ are used dogenesis despite stronger implied baroclinic production. to reference phenomena occurring within the hook echoes of However, none of these studies
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