1. Introduction and considerations
The first proposed tornado taxonomy was presented by Agee and Jones (2009, hereafter AJ) consisting of three types and 15 species, ranging from the type I (potentially strong and violent) tornadoes produced by the classic supercell, to the more benign type III convective and shear-driven vortices such as landspouts and gustnadoes. This original taxonomy was presented to (i) help organize and sort out the variety of tornado occurrences, with different roles played by varying strengths and patterns of buoyancy/CAPE and shear/helicity, and (ii) to accommodate the change in nomenclature made by the American Meteorological Society (AMS) in the Glossary of Meteorology from its original 1959 definition to the revised definition in 2000 (Huschke 1959; Glickman 2000). These comments are being provided now because the AMS has revised the definition again in October 2013 (see http://glossary.ametsoc.org/wiki/Tornado), which has direct impact on the Agee–Jones taxonomy. The succession of three tornado definitions are (i) 1959—“a violently rotating column of air, pendant from a cumulonimbus cloud”; (ii) 2000—“a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud”; and (iii) 2013—“a rotating column of air, in contact with the surface, pendant from a cumuliform cloud, and often visible as a funnel cloud and/or circulating debris/dust at the ground.” In view of the latest definition, a few changes are warranted in the AJ taxonomy. Considering the roles played by buoyancy and shear on a variety of spatial and temporal scales (from miso to meso to synoptic), coupled with the requirement in the latest definition that a tornado must be pendant from a cumuliform cloud, it is necessary to reexamine the AJ taxonomy.
a. Changes in the taxonomy
There are some minor and/or significant changes in each of the three types of tornado classification due to a combination of the following: the new tornado definition, recent research investigations, comments by Markowski and Dotzek (2010, hereafter MD), and e-mails received by the author. Purely shear-driven vortices (although indirectly associated with cumuliform convective clouds) must be dropped from the original AJ taxonomy. This includes the gustnadoes (type IIId), as well as hurricane eyewall shear vortices (type IIIe).
Contrary to the wishes of many in the severe storms community, the 2000 Glossary defined gustnadoes as tornadoes (which AJ had no choice in the matter in 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 “must be pendant from a cumuliform cloud” implicates the presence and role of convective buoyancy in vortex formation (thus eliminating shear vortices as noted above) but continuing to allow tornadoes in the type III class, namely landspouts, waterspouts (with landfall), and even a few cold-air funnels when in contact with the ground. Simply stated, the combined roles of shear and buoyancy, as well as the associated dynamical and kinematic processes of tilting–convergence–stretching, must act together in the presence of a cumuliform cloud updraft embedded in a wind shear environment to form a vortex that is a candidate for becoming a tornado. It is further noted that the anticyclonic secondary vortex (type IIIf) has been relocated in the revised taxonomy to type I (and labeled as Id). This relocation is consistent with the recommendation made by MD, as well as by Agee and Jones (2010, hereafter AJ2). Changes in type II species are minor, but the nomenclature of rear inflow jeats (RIJs) has been changed to inflow jets (IJs) since inflow features that occur in quasi-linear convective system (QLCS) events can be either from the front or the rear. Accordingly, an updated taxonomy is presented in Fig. 1, as well as a newly revised table of taxonomy species criteria (Table 1). The comment and reply articles by MD and AJ2, as well as the reviews received for this publication, require additional comments regarding tornadic supercell thunderstorms. Admittedly, there are some mixed views concerning the placement (or not) of supercells in lines (i.e., in the type II classification). Although QLCS may contain storm cells with some characteristics of the supercells, they do not meet the definition of discrete entities as defined in the type I classification. Tornadic supercells can be in a line but separated (and not in a solid QLCS) and thus consistent with the classification criteria.
Revised tornado taxonomy (after Agee and Jones 2009).
Citation: Weather and Forecasting 29, 5; 10.1175/WAF-D-14-00058.1
Criteria for applying tornado taxonomy.
b. Multiple vortices and tornado definition
The occurrence of multiple vortex tornadoes has long been recognized, as seen in the early observations of the 3 April 1974 tornado outbreak (Agee et al. 1975). A single tornadic thunderstorm is also capable of supporting two or more minitornado cyclones (Agee et al. 1976) capable of producing individual tornadoes, resulting in a parallel mode tornado family [also see Fujita (1974)]. Over the decades there have been many observations and investigations of vortices associated with tornado events, but nothing comparable to those reported on by Wurman and Kosiba (2013, hereafter WK). The complexity of their Doppler observations of a multitude of vortices on several different scales has resulted in their proposal for a new tornado definition and, thus, requires some consideration in this contribution. The author views that tornadoes (particularly strong and violent tornadoes) can (and should) display multiple vortex features with a variety of sizes. Large two-cell vortices (such as wedge tornados) can be viewed as a coalescence or bundling of vortex tubes of different sizes. Such are sometimes visible to even the naked eye and at an impressive level, as is evident in the movies of the Tuscaloosa, Alabama, tornado of 27 April 2011. However, the unprecedented findings by WK bring into focus the complexity of tornado formation and structure, with its plethora of vortices. Many, if not most, of these cases are shear-driven vortices that are also capable of coalescing into a spectrum of vortex sizes. In spite of this complexity and the importance of their findings, the author does not see a basis for changing the taxonomy presented or the AMS definition of a tornado.
2. Summary and conclusions
In summary, the author is pleased with the latest AMS definition of a tornado and equally pleased to eliminate two tornado species from the original AJ taxonomy. Also, this revision has provided an opportunity to make additional minor changes in the taxonomy (as suggested by others in the research community). Further, a brief discussion of the potential impact of the WK Doppler investigation of tornado-associated vortices on the AMS definition has been provided. Equally important is consideration of the study by Smith et al. (2012), which defines convective modes for significant severe thunderstorms and tornadoes, based on 78.5% of all such CONUS reports from 2003 to 2011. Their three categories were QLCS, supercells, and disorganized, along with a number of subcategories such as bow echo, discrete cell, cell in cluster, cell in a line, marginal supercell, and linear hybrid. Clearly, these convective categories bear a strong similarity to the tornado taxonomy classifications (and should), but they are not the same.
Although it has taken several decades, the newest tornado definition seems solid and is not likely to change again. It is not viewed as being compromised by new discoveries such as those by WK (although change is always possible when warranted).
REFERENCES
Agee, E., and Jones E. , 2009: Proposed conceptual taxonomy for proper identification and classification of tornado events. Wea. Forecasting, 24, 609–617, doi:10.1175/2008WAF2222163.1.
Agee, E., and Jones E. , 2010: Reply. Wea. Forecasting, 25, 341–342, doi:10.1175/2009WAF2222353.1.
Agee, E., Church C. , Morris C. , and Snow J. , 1975: Some synoptic aspects and dynamic features of vortices associated with the tornado outbreak of 3 April 1974. Mon. Wea. Rev., 103, 318–333, doi:10.1175/1520-0493(1975)103<0318:SSAADF>2.0.CO;2.
Agee, E., Snow J. T. , and Clare P. R. , 1976: Multiple vortex features in the tornado cyclone and the occurrence of tornado families. Mon. Wea. Rev., 104, 552–563, doi:10.1175/1520-0493(1976)104<0552:MVFITT>2.0.CO;2.
Fujita, T. T., 1974: Jumbo tornado outbreak of 3 April 1974. Weatherwise, 27, 116–126, doi:10.1080/00431672.1974.9931693.
Glickman, T., Ed., 2000: Tornado. Glossary of Meteorology.2nd ed. Amer. Meteor. Soc., 585.
Huschke, R. E., Ed., 1959: Tornado. Glossary of Meteorology. Amer. Meteor. Soc., 781.
Markowski, P., and Dotzek N. , 2010: Comments on “Proposed conceptual taxonomy for proper identification and classification of tornado events.” Wea. Forecasting, 25, 338–340, doi:10.1175/2009WAF2222343.1.
Smith, B. T., Thompson R. L. , Grams J. S. , Broyles C. , and Brooks H. E. , 2012: Convective modes for significant severe thunderstorms in the contiguous United States. Part I: Storm classification and climatology. Wea. Forecasting, 27, 1114–1135, doi:10.1175/WAF-D-11-00115.1.
Wurman, J., and Kosiba K. , 2013: Finescale radar observations of tornado and mesocyclone structures. Wea. Forecasting, 28, 1157–1174, doi:10.1175/WAF-D-12-00127.1.
