Convective Modes for Significant Severe Thunderstorms in the Contiguous United States. Part III: Tropical Cyclone Tornadoes

Abstract

A gridded, hourly, three-dimensional environmental mesoanalysis database at the Storm Prediction Center (SPC), based on objectively analyzed surface observations blended with the Rapid Update Cycle (RUC) model-analysis fields and described in Parts I and II of this series, is applied to a 2003-11 subset of the SPC tropical cyclone (TC) tornado records. Distributions of environmental convective parameters, derived from SPC hourly mesoanalysis fields that have been related to supercells and tornadoes in the midlatitudes, are evaluated for their pertinence to TC tornado occurrence. The main factor differentiating TC from non-TC tornado environments is much greater deep-tropospheric moisture, associated with reduced lapse rates, lower CAPE, and smaller and more compressed distributions of parameters derived from CAPE and vertical shear. For weak and strong TC tornado categories (EFO-EFl and EF2-EF3 on the enhanced Fujita scale, respectively), little distinction is evident across most parameters. Radar reflectivity and velocity data also are examined for the same subset of TC tornadoes, in order to determine parent convective modes (e.g., discrete, linear, clustered, supercellular vs nonsupercellular), and the association of those modes with several mesoanalysis parameters. Supercellular TC tornadoes are accompanied by somewhat greater vertical shear than those occurring from other modes. Tornadoes accompanying nonsupercellular radar echoes tend to occur closer to the TC center, where CAPE and shear tend to weaken relative to the outer TC envelope, though there is considerable overlap of their respective radial distributions and environmental parameter spaces.

A Thermodynamic, kinematic and microphysical analysis of a jet and gigantic jet‐producing Florida thunderstorm

“…However, only two of these are associated with negative jets. The differences in precipitable water are consistent with published values for supercell versus land-falling tropical cyclone environments [Edwards et al, 2012]. The mean MUCAPE (2963 J/kg) for the events is on the low end of significant when compared to supercell climatology-lying just above the significant (2860 J/kg) threshold for tornadic storms (Storm Prediction Center, http://www.spc.noaa.gov/sfctest/help/sfcoa.html) but has a large standard deviation (1277 J/kg).…”

Relationship of Low-Level Instability and Tornado Damage Rating Based on Observed Soundings

“…Therefore, 1973 was chosen as the start date for this study to limit the influence of over-reported significant and violent tornadoes. The term "EF" is used for the damage ratings for all tornadoes, including those rated prior to the enhanced Fujita scale's implementation in 2007 (Edwards et al 2012). The National Centers for Environmental Information publication, Storm Data, was examined from 1973-2015, and a database of significant and violent tornadoes was obtained for review.…”

“…Thompson et al (2003) and Grams et al (2012) found that composite parameters such as STP do best in distinguishing between significant tornado environments and non-tornadic environments. The authors opted to use the latest update for the STP (Thompson et al 2012), which uses effective shear (EBWD) and effective storm-relative helicity (ESRH) and incorporates mixed-layer convective inhibition (MLCIN). The formula for the STP is as follows: STP = (MLCAPE/1500 J kg The EBWD term is capped at a value of 1.5 and the STP is 0 if the value is less than 12 m s -1 .…”

“…Studies by Thompson et al (2003Thompson et al ( , 2012, Togstad et al (2011), Grams et al (2012), and others have evaluated parameters to discriminate atmospheric conditions associated with higher probabilities of significant tornadoes (a magnitude of EF2 and higher) versus non-significant tornadoes. Less research has been conducted regarding parameters that distinguish environmental conditions specifically favorable for violent tornadoes (EF4 and EF5).…”

Tornadoes in the Central United States and the “Clash of Air Masses”

“…Such upslope severe weather regimes typically are found on the cool side of (not along) a synoptic-scale front or outflow boundary produced by an antecedent mesoscale convective system (e.g., Doswell 1980). Third, supercells may even form along rainbands in hurricanes (e.g., McCaul 1987;Baker et al 2009;Molinari and Vollaro 2010;Green et al 2011;Edwards et al 2012). Thus, there are diverse situations in which strong tornadoes could form with no strong temperature gradient present.…”