A Revised Tornado Definition and Changes in Tornado Taxonomy

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1256 WEATHER AND FORECASTING VOLUME 29

A Revised Tornado Deﬁnition and Changes in Tornado Taxonomy

ERNEST M. AGEE Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana

(Manuscript received 4 June 2014, in ﬁnal form 30 July 2014)

ABSTRACT

The tornado taxonomy presented by Agee and Jones is revised to account for the new definition of a tornado provided by the American Meteorological Society (AMS) in October 2013, resulting in the elimination of shear-driven vortices from the taxonomy, such as gustnadoes and vortices in the eyewall of hurricanes. Other relevant research findings since the initial issuance of the taxonomy are also considered and in- corporated, where appropriate, to help improve the classification system. Multiple misoscale shear-driven vortices in a single tornado event, when resulting from an inertial instability, are also viewed to not meet the definition of a tornado.

1. Introduction and considerations from a cumuliform cloud, and often visible as a funnel cloud and/or circulating debris/dust at the ground.’’ In The first proposed tornado taxonomy was presented view of the latest definition, a few changes are warranted by Agee and Jones (2009, hereafter AJ) consisting of in the AJ taxonomy. Considering the roles played by three types and 15 species, ranging from the type I buoyancy and shear on a variety of spatial and temporal (potentially strong and violent) tornadoes produced by scales (from miso to meso to synoptic), coupled with the the classic supercell, to the more benign type III con- requirement in the latest definition that a tornado must vective and shear-driven vortices such as landspouts and be pendant from a cumuliform cloud, it is necessary to gustnadoes. This original taxonomy was presented to reexamine the AJ taxonomy. (i) help organize and sort out the variety of tornado oc- currences, with different roles played by varying strengths a. Changes in the taxonomy and patterns of buoyancy/CAPE and shear/helicity, and There are some minor and/or significant changes in (ii) to accommodate the change in nomenclature made by each of the three types of tornado classification due to the American Meteorological Society (AMS) in the Glos- a combination of the following: the new tornado sary of Meteorology from its original 1959 definition to the definition, recent research investigations, comments revised definition in 2000 (Huschke 1959; Glickman 2000). by Markowski and Dotzek (2010, hereafter MD),and These comments are being provided now because the AMS e-mails received by the author. Purely shear-driven has revised the definition againinOctober2013(seehttp:// vortices (although indirectly associated with cumuli- glossary.ametsoc.org/wiki/Tornado), which has direct im- form convective clouds) must be dropped from the pact on the Agee–Jones taxonomy. The succession of original AJ taxonomy. This includes the gustnadoes three tornado definitions are (i) 1959—‘‘a violently rotating (type IIId), as well as hurricane eyewall shear vortices column of air, pendant from a cumulonimbus cloud’’; (ii) (type IIIe). 2000—‘‘a violently rotating column of air, in contact with Contrary to the wishes of many in the severe storms the ground, either pendant from a cumuliform cloud or community, the 2000 Glossary defined gustnadoes as underneath a cumuliform cloud’’; and (iii) 2013—‘‘a ro- tornadoes (which AJ had no choice in the matter in tating column of air, in contact with the surface, pendant presenting their taxonomy because of their adherence to the Glossary definition). Considering now in the new definition that the vortex in contact with the ground Corresponding author address: Ernest M. Agee, Dept. of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 ‘‘must be pendant from a cumuliform cloud’’ implicates Stadium Mall Dr., West Lafayette, IN 47907-2051. the presence and role of convective buoyancy in vortex E-mail: [email protected] formation (thus eliminating shear vortices as noted

DOI: 10.1175/WAF-D-14-00058.1

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FIG. 1. Revised tornado taxonomy (after Agee and Jones 2009). above) but continuing to allow tornadoes in the type III b. Multiple vortices and tornado definition class, namely landspouts, waterspouts (with landfall), and even a few cold-air funnels when in contact with the The occurrence of multiple vortex tornadoes has ground. Simply stated, the combined roles of shear and long been recognized, as seen in the early observa- buoyancy, as well as the associated dynamical and ki- tions of the 3 April 1974 tornado outbreak (Agee nematic processes of tilting–convergence–stretching, et al. 1975). A single tornadic thunderstorm is also must act together in the presence of a cumuliform cloud capable of supporting two or more minitornado cy- updraft embedded in a wind shear environment to form clones (Agee et al. 1976) capable of producing in- a vortex that is a candidate for becoming a tornado. It is dividual tornadoes, resulting in a parallel mode further noted that the anticyclonic secondary vortex tornado family [also see Fujita (1974)]. Over the de- (type IIIf) has been relocated in the revised taxonomy to cades there have been many observations and in- type I (and labeled as Id). This relocation is consistent vestigations of vortices associated with tornado with the recommendation made by MD, as well as by events, but nothing comparable to those reported on Agee and Jones (2010, hereafter AJ2). Changes in type by Wurman and Kosiba (2013, hereafter WK).The II species are minor, but the nomenclature of rear inflow complexity of their Doppler observations of a multi- jeats (RIJs) has been changed to inflow jets (IJs) since tude of vortices on several different scales has resulted inflow features that occur in quasi-linear convective in their proposal for a new tornado definition and, thus, system (QLCS) events can be either from the front or requires some consideration in this contribution. The the rear. Accordingly, an updated taxonomy is pre- author views that tornadoes (particularly strong and vio- sented in Fig. 1, as well as a newly revised table of tax- lent tornadoes) can (and should) display multiple vortex onomy species criteria (Table 1). The comment and features with a variety of sizes. Large two-cell vortices reply articles by MD and AJ2, as well as the reviews (such as wedge tornados) can be viewed as a coalescence received for this publication, require additional com- or bundling of vortex tubes of different sizes. Such are ments regarding tornadic supercell thunderstorms. Ad- sometimes visible to even the naked eye and at an im- mittedly, there are some mixed views concerning the pressive level, as is evident in the movies of the Tuscaloosa, placement (or not) of supercells in lines (i.e., in the type Alabama, tornado of 27 April 2011. However, the un- II classification). Although QLCS may contain storm precedented findings by WK bring into focus the com- cells with some characteristics of the supercells, they do plexity of tornado formation and structure, with its not meet the definition of discrete entities as defined in plethora of vortices. Many, if not most, of these cases are the type I classification. Tornadic supercells can be in shear-driven vortices that are also capable of coalescing a line but separated (and not in a solid QLCS) and thus into a spectrum of vortex sizes. In spite of this complexity consistent with the classification criteria. and the importance of their findings, the author does not

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TABLE 1. Criteria for applying tornado taxonomy.

Tornado type Characteristics for classification label Ia Discrete supercell with mesocyclone (typically a hook echo) with supportive values of CAPE and storm-relative helicity (SRH) with low-level directional shear Ib Discrete minisupercell with low top in a low-tropopause environment; typically minimal CAPE with large SRH; more common in early spring and late fall Ic Typically in the right-front quadrant (RFQ) of landfalling hurricanes; supportive values of CAPE, low-level shear and large ambient vertical vorticity Id Anticyclonic vortices that form in close proximity to much stronger cyclonic tornadoes and within the clockwise shear zone and region of anticyclonic downdraft tilting IIa Line echo wave pattern (LEWP)—a mesoscale wave pattern that adds to the local vorticity field and mesocyclone formation IIb Bow echo produced by a cold pool with enhancement of the solenoidal field and tilting with increased shear IIc Bookend vortex typically at the top or cyclonic end of the bow echo with associated mesocyclone IId IJs along sections of the QLCS that add to the local shear and vorticity field and the formation of mesovortices IIe Mesovortices that develop along a QLCS that are not associated with LEWPs, bows, or IJs IIf QLCS events are typical in the outer spiral bands of a hurricane and may produce tornadoes in the RFQ at landfall; supportive values of CAPE and ambient vertical vorticity IIIa Cumuliform cloud (sometimes not glaciated) with intense local updraft that converges and stretches vertical vorticity into a misocyclone in the PBL IIIb Similar to IIIa (but over water) and typically not glaciated; not to be confused with type I and type II tornadoes over water IIIc Convective instability due to cold air aloft and favorable shear for vortex development in a cooler environment (typically does not reach the ground) see a basis for changing the taxonomy presented or the REFERENCES AMS definition of a tornado. Agee, E., and E. Jones, 2009: Proposed conceptual taxonomy for proper identification and classification of tornado events. Wea. 2. Summary and conclusions Forecasting, 24, 609–617, doi:10.1175/2008WAF2222163.1. ——, and ——, 2010: Reply. Wea. Forecasting, 25, 341–342, In summary, the author is pleased with the latest AMS doi:10.1175/2009WAF2222353.1. definition of a tornado and equally pleased to eliminate ——, C. Church, C. Morris, and J. Snow, 1975: Some synop- two tornado species from the original AJ taxonomy. tic aspects and dynamic features of vortices associated with the tornado outbreak of 3 April 1974. Mon. Wea. Also, this revision has provided an opportunity to make Rev., 103, 318–333, doi:10.1175/1520-0493(1975)103,0318: additional minor changes in the taxonomy (as suggested SSAADF.2.0.CO;2. by others in the research community). Further, a brief ——, J. T. Snow, and P. R. Clare, 1976: Multiple vortex features in discussion of the potential impact of the WK Doppler the tornado cyclone and the occurrence of tornado families. Mon. investigation of tornado-associated vortices on the Wea. Rev., 104, 552–563, doi:10.1175/1520-0493(1976)104,0552: . AMS definition has been provided. Equally important is MVFITT 2.0.CO;2. Fujita, T. T., 1974: Jumbo tornado outbreak of 3 April 1974. consideration of the study by Smith et al. (2012),which Weatherwise, 27, 116–126, doi:10.1080/00431672.1974.9931693. defines convective modes for significant severe thunder- Glickman, T., Ed., 2000: Tornado. Glossary of Meteorology. 2nd storms and tornadoes, based on 78.5% of all such ed. Amer. Meteor. Soc., 585. CONUS reports from 2003 to 2011. Their three cate- Huschke, R. E., Ed., 1959: Tornado. Glossary of Meteorology. gories were QLCS, supercells, and disorganized, along Amer. Meteor. Soc., 781. Markowski, P., and N. Dotzek, 2010: Comments on ‘‘Proposed with a number of subcategories such as bow echo, discrete conceptual taxonomy for proper identification and classifica- cell, cell in cluster, cell in a line, marginal supercell, and tion of tornado events.’’ Wea. Forecasting, 25, 338–340, linear hybrid. Clearly, these convective categories bear doi:10.1175/2009WAF2222343.1. a strong similarity to the tornado taxonomy classifications Smith, B. T., R. L. Thompson, J. S. Grams, C. Broyles, and H. E. (and should), but they are not the same. Brooks, 2012: Convective modes for significant severe thun- Although it has taken several decades, the newest derstorms in the contiguous United States. Part I: Storm tornado definition seems solid and is not likely to change classification and climatology. Wea. Forecasting, 27, 1114– 1135, doi:10.1175/WAF-D-11-00115.1. again. It is not viewed as being compromised by new Wurman, J., and K. Kosiba, 2013: Finescale radar observations of discoveries such as those by WK (although change is al- tornado and mesocyclone structures. Wea. Forecasting, 28, ways possible when warranted). 1157–1174, doi:10.1175/WAF-D-12-00127.1.

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