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A Study of Synoptic-Scale Tornado Regimes

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A Study of Synoptic-Scale Tornado Regimes Garner, J. M., 2013: A study of synoptic-scale tornado regimes. Electronic J. Severe Storms Meteor., 8 (3), 1–25. A Study of Synoptic-Scale Tornado Regimes JONATHAN M. GARNER NOAA/NWS/Storm Prediction Center, Norman, OK (Submitted 21 November 2012; in final form 06 August 2013) ABSTRACT The significant tornado parameter (STP) has been used by severe-thunderstorm forecasters since 2003 to identify environments favoring development of strong to violent tornadoes. The STP and its individual components of mixed-layer (ML) CAPE, 0–6-km bulk wind difference (BWD), 0–1-km storm-relative helicity (SRH), and ML lifted condensation level (LCL) have been calculated here using archived surface objective analysis data, and then examined during the period 2003−2010 over the central and eastern United States. These components then were compared and contrasted in order to distinguish between environmental characteristics analyzed for three different synoptic-cyclone regimes that produced significantly tornadic supercells: cold fronts, warm fronts, and drylines. Results show that MLCAPE contributes strongly to the dryline significant-tornado environment, while it was less pronounced in cold- frontal significant-tornado regimes. The 0–6-km BWD was found to contribute equally to all three significant tornado regimes, while 0–1-km SRH more strongly contributed to the cold-frontal significant- tornado environment than for the warm-frontal and dryline regimes. –––––––––––––––––––––––– 1. Background and motivation As detailed in Hobbs et al. (1996), synoptic- scale cyclones that foster tornado development Parameter-based and pattern-recognition evolve with time as they emerge over the central forecast techniques have been essential and eastern contiguous United States (hereafter, components of anticipating tornadoes in the CONUS). Tornadic thunderstorms over the United States during the past half century Great Plains may emanate from a dryline (Schaefer 1986; Doswell et al. 1993). Initial initially. For example, the dryline environment efforts to classify environments known to have described by Bluestein et al. (1988) associated produced tornadoes include Beebe (1956) and with tornadoes on 7 May 1986 was characterized Miller (1972), among others, and focused mainly by a hot, deeply mixed afternoon boundary layer on composite means of surface and upper-level west of the dryline, and a low-level moist layer data for tornado days. These studies highlighted east of the dryline surmounted by steep midlevel synoptic-scale cyclones characterized by a warm, lapse rates. Resulting CAPE values ranged from moist low-level air mass extending poleward 3000–4000 J kg–1. Proximity hodographs beneath a cold, dry midlevel air mass. Changes showed clockwise turning of wind vectors with in wind speed and direction with height were height, and speed shear over the lowest 6 km also observed, and attributed to the presence of a AGL (hereafter all height references are AGL) low-level southerly jet stream beneath the exit around 22 m s–1. region of strong mid and upper-level jet streaks. These proximate jets were shown to enhance As the synoptic-scale cyclone matures and vertical wind shear as well as to aid in moves east of the Great Plains, a cold front may destabilization and the release of convective overtake the dryline. A preliminary study by instability (Beebe and Bates 1955; Uccellini and Guyer et al. (2006) found that 55% of cool- Johnson 1979). season significant tornadoes (rated at least F2) __________________________ from 1984–2004 in the Gulf Coast states Corresponding author address: Jonathan M. occurred in the warm sector along or ahead of a Garner, Storm Prediction Center, Norman, OK, cold front attendant to a synoptic-scale low- 73072, E-mail: [email protected] pressure system. Environments associated with 1 GARNER 06 August 2013 these events featured a moist surface air mass significant tornado parameter (STP; T03; (mean dewpoint of 18°C) but weak 700–500-mb Thompson et al. 2004) combines normalized lapse rates (6.0–6.8°C km–1), which resulted in values of CAPE, deep-layer bulk wind difference mixed-layer (ML) CAPE values generally (BWD), storm-relative helicity (SRH), and LCL ranging from 900–1700 J kg–1. Due to the height into a composite index. Each parameter southward migration of the polar jet during the was selected from a sample of tornadic and cool season, combined with strong flow in the 1– nontornadic Rapid Update Cycle (RUC; 2-km layer, vertical speed shear was pronounced. Benjamin et al. 2004a,b) model proximity soundings. Thresholds used to normalize each In contrast to tornadic storms moving into the parameter were derived from statistically warm sector away from drylines and cold fronts, significant differences that distinguish between tornadic storms occurring on boundaries are significantly tornadic and weak or nontornadic located along the edge of the warm sector within supercell environments. baroclinic zones. Boundaries that favor tornadic supercell development range from synoptic-scale The motivation for this paper is to evaluate warm fronts and stationary fronts, to outflow the most important tornado forecast parameters boundaries produced by earlier thunderstorms. for three prominent synoptic-scale regimes Most research studying tornadic storms along associated with significant tornadoes: cold baroclinic boundaries shows that the fronts, warm fronts, and drylines. Because the augmentation of vorticity in a baroclinic zone STP shows favorable skill in discriminating beyond what occurs in the ambient environment between significant and weak tornado increases the potential for significant tornadoes environments, the relative contribution of its as thunderstorms interact with it (e.g., Maddox et components (MLCAPE, 0–6-km BWD, and 0–1- al. 1980; Markowski et al. 1998; Rasmussen et km SRH) have been used to distinguish the three al. 2000). environments. In addition, MLLCL height, ML convective inhibition (CIN), and convective Current understanding of the parameter space mode also were evaluated in this study. Section favorable for significant tornadoes primarily 2 will present the methods used to collect and focuses on environments of supercells (Lemon classify events, as well as the data source and and Doswell 1979; Doswell et al. 1993) that method of computation for the STP. Results will acquire low-level updraft rotation and produce be given in section 3, and a summary and specific thermal characteristics in rear-flank discussion will conclude the paper. downdrafts (RFDs). While deep-layer shear is used to forecast supercells (Weisman and Klemp 2. Methodology 1982, Thompson et al. 2003; hereafter T03), the shear profile over the lowest 1 km appears to be For the 2003–2010 period, all recorded more relevant to low-level mesocyclone tornadoes (12 552) and their respective attributes development and increased tornado potential (such as F/EF-scale rating, tornado path length, (Rasmussen 2003; T03). Proximity sounding fatalities, and injuries) were evaluated for studies also have found that significant tornadoes possible inclusion in this study using the online are more likely when the height of the lifted program Severe Plot [Hart and Janish 2012; condensation level (LCL) is <2000 m, which (http://www.spc.nssl.noaa.gov/climo/online/sp3/ favors a greater potential for buoyant RFD’s plot.php)] and the online version of NOAA (Rasmussen and Blanchard 1998; Markowski Storm Data [NCDC 2012; 2002; Markowski et al. 2002; T03). (http://www.ncdc.noaa.gov/stormevents)]. Though the primary focus of this study is on the Combinations of CAPE and vertical wind environments of significant tornadoes, weak shear also are used in composite indices. tornadoes were included for comparison Though these composite indices have been purposes. shown to discriminate more strongly between tornadic and nontornadic environments when The Storm Prediction Center severe compared to their individual components thunderstorm events website (Rasmussen and Blanchard 1998), it is important (http://www.spc.noaa.gov/exper/archive/events/) to understand that no composite index will then was used to determine whether these perform favorably under every meteorological tornadoes occurred with a synoptic cyclone. A situation (Doswell and Schultz 2006). The synoptic cyclone is defined for this study as an 2 GARNER 06 August 2013 area of surface low pressure with winds flowing also were used for subjective pressure analysis, counterclockwise in the northern hemisphere, gradients in temperature and dew point (as in and not associated with a tropical cyclone. The Garner 2012), as well as streamlines during the cyclone had to display closed isobars around the hour of tornado occurrence, which allowed center of surface low pressure, as well as a surface frontal features associated with the frontal-wave structure that emanated from the cyclone to be determined. In addition, archived surface low. Typical cyclones generally visible and infrared satellite imagery were used resembled the idealized extratropical cyclone to aid further in determining frontal location. described by Heuboer et al. (1996), though the These procedures allowed the identification of size of the cyclones analyzed in the current study one or more of the following boundaries: the varied from an area of several states to almost dryline, the warm front, and the cold front (Figs. the entire CONUS. Archived surface 1 and 2). meteorological aviation report (METAR) plots Figure 1: Example of a typical surface dryline
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