Last update:01 October 2001
What's new: Accounted for the revised Glossarydefinitions of tornado and waterspout, plus a few minor changes.
The most widely-accepted definition of a tornado can be found in,among other sources, theGlossary of Meteorology (Huschke1959):
Tornado -- A violently rotatingcolumn of air, pendant from acumulonimbuscloud, and nearly always observable as a "funnel cloud" ortuba.
Curiously, this definition says nothing about that rotating columnbeing in contact with the surface. Perhaps the next edition of theGlossary will correct this oversight.
Update (01 Oct 2001): In fact, this definitionhasbeen corrected in the newGlossary (Glickman 2000):
Tornado -- 1. A violently rotatingcolumn of air, in contact with the ground, either pendant from acumuliform cloud or underneath a cumuliformcloud, and often (but not always) visible as afunnelcloud.
There is no definition of a tornado that has been extensivelypeer-reviewed.[4]
I want to take a moment to gripe about the wordtouchdownin association with tornadoes. I believe that "touchdown" isinappropriate to describe the actual process of tornadic windscommencing at the surface. There is nothing coming down, in the sensethat a solid tube would fall out of the sky. What actually goes onwhen a vortex is present in the atmosphere is that the vortex either(a) is already present at the surface, or (b) wraps around itself,like a smoke ring. If you find this confusing, please see myVorticityPrimer, where this concept is described in some detail. The lawsof Fluid dynamics tell us that (a) and (b) are theonly twooptions. As notedelswhere[informal form,here], theintense part of the vortexcan build downward, but this is notthe same as a tornado descending. What is actually happening is thatthe vortex at the surface increases its intensity (and decreasing itsscale at the same time) to tornadic proportions, eventually producingwinds capable of tornadic damage ... but the vortex itself is almostcertainly already in contact with the ground. Strictly speaking, "thevortex" should not be equated to "the tornado," since the vortex canbe present but with non-damaging windspeeds. Prior to thecommencement of damaging winds at the ground, the surface vortex isweak and spread out ... as it intensifies, the winds increase and thesize of the circulation contracts. The vortex also can intensifyupward (as we think happens in the tornadoes that are called"landspouts" - see below). Rather than "touchdown" I would prefer toconsider the observed process of the commencement of tornadic windsat the surface to be one of "spin-up" ... I hasten to add that "up"in this context doesnot imply ascent, but rather an increaseof spin intensity.
Perhaps some of you have seen some video of two funnel clouds thatare seen at cloud base ... with time, the vortices curl toward eachother and seem to "connect" ... what is happening there is a vortexthat wraps around itself, just like a smoke ring. Since air is afluid, it obeys the laws of fluid dynamics, which include this "law"of vortices (that they either form loops or they end only on a solidsurface). NSSL has a photo showing this phenomenon of a vortex ring(see photo #3 in theNSEA PhotoGallery). These have been given the name "bowtie funnels" by somestorm chasers.
During ourchasein 1989, my partner (Al Moller) and I happened to drive rightunder a strongly rotating towering cumulus. As it passed overhead,our surface winds changed from southeasterly, to easterly, tonortheasterly, to northerly, to northwesterly, to westerly, tosouthwesterly, all in within a minute or two! At no time were thewinds more than 10-15 knots. We experienced a surface vortex ofnear-tornadicsize but not of tornadicintensity! [anF-(minus)1 tornado?] Later on, this rotating cloud produced atornado.
The observed "descent" of cloud to form the funnel-shaped cloud ofa tornado is also not associated with true descent. As the vortexintensifies, its central pressure falls. When the pressure is reducedto that which permits condensation of water vapor into cloudmaterial, the funnel-shaped cloud appears. It often appears todescend for perhaps two distinct reasons: 1) Near cloud base, thepressure doesn't have to fall as far for the air in the vortex toreach condensation as it does further down, or 2) The circulationintensity is actually increasing downward. We do not know the actualdistribution of pressure in tornadoes, of course, and the tornadicpressure field may have many complexities. However, it is unlikelythat clouds from above are descending to create the funnel-shapedcloud ... generally,descent dissipates clouds. Of course, itcould be that descent in the core of a two-celled vortexmightproduce a funnel-shaped cloud simply by pulling down cloud matter ina way that evaporation of the cloud droplets is relatively slow alongthe axis and relatively faster along the margins, resulting in atapered funnel. Someday, perhaps we can get some answers ... .
The public at large has many misconceptions about tornadoes, anotable one being that unless the condensation cloud associated withthat rotating column of air is touching the surface, it is not atornado. This is manifestly untrue, as many storm chasers realizefully. Since it is thewind associated with the rotating aircolumn that does the damage, it is the moving air (wind) andnotthe cloud that constitutes the tornado. Many tornadoes have beenobserved (Fig. 1 is but one of countlessexamples) that do not have condensation funnels all the way to thesurface, but which clearly are in contact with the ground. It isquite possible for the circulation to be more or less completelyinvisible for at least some portion of the life cycle of the event.In the case of waterspouts (see below), this is frequently the case,but such events also occur in the dusty, dry mid-continental plains(Fig. 2). Chasers may refer to these eventswith such slang terms as "dust bowls" or "dirt-daubers."
When the visible funnel does not reach the surface, silly reportsof a "funnel cloud accompanied by wind damage" can occur, or suchabsurdities are reported as "The funnel only reached treetop level"or whatever. We probably will find it difficult ever to convince thepublic as a whole that a tornado iswind and not the funnelcloud. Hopefully, no self-respecting meteorologist would do anythingto perpetuate such misconceptions. Also, storm chasers who believethis could get into a lot of trouble, quickly! By the way, "tornadoon the ground" is redundant, since tobe a tornado, thedamaging winds have to be present at the surface at that time.
Many strong-to-violent tornadoes include a phase where the tornadoappears as a truncated cone funnel (not reaching the surface) withoccasional rope-like multiple vortices beneath it (e.g., the Xenia,Ohio tornado of 03 April 1974 or the Binger, Oklahoma tornado of 22May 1981,Fig. 3). Lay persons might haveconsiderable difficulty recognizing this for what it is, since itdoes not look like a "funnel-shaped cloud." Such an appearance,however, doesnot imply that one is in fact dealing with astrong-to-violent tornado ...a tornado's appearance is not areliable indicator of its intensity, chaser lore notwithstanding.
Among chasers, it is a common assumption that if a funnel cloudextends halfway or more to the ground from cloud base, there almostcertainly is a tornadic circulation at the surface. This may or maynot be true in any specific instance ... it probably is more rightthan wrong. For official purposes, however, such a "storm chaser'srule" is not a legitimate assumption; by definition, one must confirmthe existence of a damaging circulation at the surface before theevent can legitimately be called a tornado. If such a confirmationcannot be made, the event must be considered to be a funnel cloud ora "possible" tornado.
Recent research, some connected with theVORTEX program, hasmade it clear that vortical flows are often present at the surface,even without any visible funnel cloud. If a condensation funnel ispresent that does not "touch" the surface, some sort of "circulation"[10] is virtuallycertain to be present, but it may not be sufficient to raise debris.In the absence of debris, it is hard to know if the situation hasbecome tornadic or not. I'll return to this later.
Having mentioned waterspouts, this raises another topic. There isa special name for a tornado moving over the water: a waterspout. Whydo we not have special names for tornadoes moving over sand(sandspout?), or asphalt (tarmacnado?), or mobile homes(manufacturnado?), or eucalyptus trees (gumswirl?)? Is it awaterspout if the water is fresh water rather than sea water? Does itbecome a waterspout if it moves over a lake? What about a pond? Howabout encountering a swimming pool or perhaps a puddle? How big doesa body of water have to be to create a waterspout from a tornado?What about when crossing a river? A creek? A dry streambed? Wouldthis last example be a "dry waterspout"? I am engaging deliberatelyinreductio ad adsurdum here becauseI do not believe thereis any scientific distinction of consequence between a waterspout anda tornado!
In the newGlossary, in fact, thedefinition of awaterspout is now:
Waterspout -- 1. In general, atornado over a body of water. 2. In itsmost common form, a nonsupercell tornado over water.
For years, people believed that waterspouts were a distinctlydifferent phenomenon, uniquely associated with tropical andsubtropical convection that might not even qualify as cumulonimbusclouds. Of course, some "authorities" knew of the annoying problem ofsupercells over water; recognition of this produced the abominableterm: "tornadic waterspout." Of late, it has been observed thatphenomena quite comparable to waterspouts arise over the land,leading to another dubious term (that I have used!): "landspout" (byanalogy, a "waterspoutic tornado"?). In my opinion,all theseterms refer to the same phenomenon: an intense vortex associatedwith deep moist convection. Thus, I must quibble with the standarddefinition for its exclusion of convective vortices that happen withclouds not meeting the criteria to be cumulonimbi (e.g., thosewithout glaciation at the cloud top).
I am proposing the following definition:
Tornado --A vortex extendingupward from the surface at least as far as cloud base (with thatcloud base associated with deep moist convection), that is intenseenough at the surface to do damage should be considered atornado.
This is without regard to
My broadened definition is designed to ignore what I consider tobe incidental aspects of the situation. I believe that the physicalprocess giving rise to an intense vortex is not associated with anyof these coincidental issues and so the labeling of the real vorticesthat occur should not depend on them. It also excludes any phenomenanot associated with deep moist convection, such as dust devils or"mountainadoes," and avoids making artificial and scientificallyunjustified distinctions between "spouts" andtornadoes.[5]
I hasten to add that I donot believe that the physicalprocesses giving rise to tornadoes are all the same. It appears thattornadoes arise in many different ways, and perhaps different processcan be associated with the tornado at different times in its lifecycle. Moreover, not all tornadoes associated with a given moistconvective cloud arise via the same processes (see Doswell andBurgess 1993). Some of the relatively intense vortices associatedwith a convective storm probably shouldnot be consideredtornadoes; e.g., circulations not extending to the surface, and truegustnadoes (see below), assuming we can identify them as such. Thereis a fair amount of anecdotal evidence for non-tornadic intensevortices in association with convection (see Moller et al. 1974;Cooley 1978; Doswell 1985; Bluestein 1988; Doswell and Burgess 1993;Bluestein 1994), but not much hard information about the processesgiving birth to these vortices.
At present, we are more or less content to classify tornadoesaccording to whether or not they occur with supercells. In thefuture, it may become scientifically useful to sub-classify tornadoeseven further, as we learn more about how real events occur (asopposed to, say, events in our computer simulations!). If we mustclassify, then it seems to me that we should do so on the basis ofphysical processes and not be concerned with superficial aspects ofthe events. We are far enough along in our understanding of tornadoesthat we ought to be able to move at levels deeper than the surfacenow.
Now, I wish to move into much more speculative and unchartedterritory. For a tornado such as the famous Union City, Oklahomatornado of 24 May 1973 (hereinafter referred to as "UC"), the lifecycle of the event can be reasonably clear and understandable. Theearly stages with a rotating wall cloud give way to the developmentof a funnel cloud, then damage at the surface begins, followed by thedescent of the funnel cloud to the ground. The width of the funnelincreases to some maximum, and then begins to shrink, finallyreaching a rope-like dissipation. The documentation for the UC eventis extensive (see Brown 1976) and it might even be consideredprotoypical for many purposes. There is relatively little ambiguityin such events; there obviously is only the one tornadic vortex andits visual evolution can be inferred without much error from lookingafterward at the evidence along the path. The UC damage path,surveyed extensively immediately after the storm, matches the visualimages quite nicely. The visible funnel was more or less continuousuntil dissipation. The damage path coincides quite nicely with thevisible vortex evolution. During dissipation, the visible funneldisappeared, but a clear debris cloud was still present for a fewseconds longer at the surface.
All tornadoes are not this simple, however. The reality oftornadic events comprises a range of visible aspects that at timescan be confusing and defies simple classification schemes. Multiplevortices introduce some complications. The UC event did not exhibitany obvious multi-vortex phase, but many tornadoes do. To someextent, the multi-vortex phenomenon is relatively well understood;the formal scientific literature on the subject (e.g., Snow 1978;Rotunno 1982; Gall 1985; Lewellen 1993) is fairly extensive. Whereasvortices like the UC event are "stable" in the sense that theirvisual aspect changes only slowly with time, other vortices are notso stable. The classic video from a helicopter in Minneapolis on 18July 1986 illustrates the erratic, rapidly-changing visual structureof tornadoes having persistent multi-vortex behavior. Looked at fromthe perspective of a post-storm damage survey, however, the damagepath in such a case might give a much more continuous "picture" ofthe event. Whereas the visual images are changing rapidly, thedamaging winds at the surface can be more or less continuous, so ifwe did not have the video with which to compare, our damage surveymight conclude with little doubt that a single tornado was involved,even though eyewitness accounts might well seem to contradict thatrather vigorously. As scientists, we might smugly dismiss thoseeyewitness accounts with the observation that it probably was amulti-vortex event. For the Minneapolis event, all is well with sucha dismissal; when visual evidence is present, perhaps we can affordto be smug. In the absence of visual documentation, I am rather lessinclined to heap scorn on eyewitness accounts of odd behavior. Iwasn't there, so I can't be certain what the eyewitnesses saw![1]
After the 08 June 1974 tornado outbreak in Oklahoma and Kansas, Isaw some 8 mm film (this was prior to the video age!) footage of aremarkable tornadic evolution; I believe it was a tornado nearLuther, OK. I have been unable to relocate that footage,unfortunately. My memory of the film is that a quite distinct tornado(somewhat reminiscent of the UC funnel in shape, but with a lowercloud base) was moving along seemingly in a relatively stablefashion, when it seemed to dissipate suddenly and another similarfunnel appeared rapidly right next to where the original funnel hadbeen, all within about a second or two. The rapidity with which thisremarkable transformation occurred was amazing to me. It seemedrather obvious to me that this visual record would almost certainlybe the only clue that such an event occurred; now that record appearsto be gone or at least unlocatable. The proximity of the two funnelsvirtually guaranteed that the damage path, as surveyed, would beviewed as continuous. Thus, if there were some slight offset in thedamage path associated with the dissipation of one funnel and thedevelopment of the next, it almost certainly would not be evident tothose doing a damage survey. To my knowledge, the tornado isclassified as a single event. Perhaps this is a properclassification; perhaps this was simply a brief visual manifestationof a transient episode in the life of a single tornado. I raise thepoint simply because of the possible implications of that suddentransition.
Another, somewhat similar, episode has been documented by Davieset al. (1994) for the Hesston, Kansas tornado of 13 March 1990. Inthe Hesston storm, the visible funnel seemed to dissipate rapidly andre-form near the axis where it had been earlier but with asubstantially smaller diameter, all within a brief time span. Thequestion was asked, but not answered: Was this all a single tornadoor could it be viewed as the dissipation of one tornado, followed bythe nearly immediate development of another? Again, apparentcontinuity of the damage seems to leave this particular instancereasonably well-defined as a single event. Is it possible to conceiveof such an event with a somewhat longer time span between thedissipation of one funnel and the development of another? I certainlyhave no trouble imagining something of that sort happening, such thata superficial examination of the damage path still might not detectsuch a gap. Not all damage surveys involve a thorough analysis of thepath from start to end, both from the ground and from the air.Moreover, there might be several ways in which the illusion of acontinuous damage track would appear even with a reasonably thoroughsurvey. How big a gap in the path is enough to start considering theevents to be separate? This brings me to the following discussion:
By the way, in most of my experience, tornadoes do not skip.Rather, they may weaken or intensify and thereby create gaps in thedamage, but their continuity is relatively high. I have not heard alot of storm chasers discussing how the tornadoes they observe"bounce" and "skip" across country. Gaps in the damage path can arisebecause (a) there is nothing to damage, or (b) the circulation hasweakened to the extent that no evidence of the tornado's passage isleft. Should we consider the latter to signify that the tornado"skipped" or "lifted" during an otherwise continuous track? Even moreradically, would we say the cessation in damage constituted acessation of the tornado? In many such cases, it is clear from filmor video that a visible funnel and even a dust and debris whirl(albeit perhaps only small debris) is continuous even though "damage"is not occurring. In such a case, the damage path would giveerroneous input to the classification of the event, often leading tothe notion of "skipping" as a means of avoiding having each gap in anotherwise linear path represent a gap between individual tornadoes.
In footnote #7 (above), I proposed the following, slightlymodified definition of a tornado:
Tornado --A vortex extendingupward from the surface at least as far as cloud base (with thatcloud base associated with deep moist convection), that is intenseenough at the surface to do damage at one or more points along itspath, should be considered a tornado.
In some of these cases, it certainly can be argued that these aretransient, superficial fluctuations in what might well be interpretedproperly as a single event. A continuous, more or less linear damagepath would be a compelling argument in favor of such aninterpretation. However, the existence of multiple vortices, now acommonly-accepted variant of tornadic behavior, raises some troublingissues.
Furthermore, I have specificallyavoided defining thewindspeed at which "damage" begins (or ends!). The current windspeedsprovided in the Fujita scale [seehere(item #B.11) andhere]indicate that F0 damage begins at 40 mph. That is not even a windstrong enough to qualify for a "severe" thunderstorm by operationalstandards (58 mph)! However, a 40 mph wind certainly would raisesurface dust in, for example, many parts of West Texas! An observermust see "debris" at the surface if an event is to be considered atornado, but this requirement is pretty nebulous. The ability toraise debris is, in a very real sense, dependent on the underlyingsurface and the conditions preceding the event. It's harder to raisedust on a muddy field than a dry one. We are unlikely ever to be ableto obtain windspeed measurements associated with the surfacecirculations in real time (i.e., as an observer watches the event),so it seems fruitless to establish windspeed criteria in thedefinition. The only value it might have would be in the abstract, orfor the unforeseen future, where itmight someday be possibleto get detailed windspeed measurements. If I were pressed to providea threshold, I'd say that the minimum tornadic windspeed should be 58mph (50 kts or about 25 m s-1), which is the currentthreshold used for "straightline" winds in thunderstorms.
For clearly damaging tornadoes with prominent condensationfunnels, etc., there is little ambiguity. It's only on the margins ofthe definition that we have problems (although that seems to be acommon problem withany definition, as this essay, overall, issuggesting). The ambiguity of what winds are capable of producing"damage" clearly feeds into the problem of "skipping" tornadoes, ofcourse.
Let me digress for a moment, to establish some more background.
Atmospheric vortices form a spectrum, in my opinion, with no clearboundarieskinematically between events. That is, atmosphericflow can be organized into vortices of essentiallyany size.The fact that there arepreferred sizes often reflects theexistence of the dominance of some physical instability that has acharacteristic scale length (e.g., baroclinic instability). Thus,vortices can be classified according to thephysical processesthat give them birth, but there is no compelling reason forclassifying vortices solely on the basis of theirsize. Hence,the boundaries among events have an obvious tendency to be quite"fuzzy." We think we know, for instance, the characteristic scalesize of an extratropical cyclone. We also know that cyclonic stormsexist which we consider to be "distinct" in some way (typically sizeis a factor in making such distinctions) from extratropical cyclones;polar lows or tropical cyclones to name two. Nevertheless, there aretimes when polar lows or even tropical cyclones seem to have adecidedly baroclinic aspect and one can get into intense argumentsabout where any particular event fits in a taxonomy of cyclones. Itseems that even on synoptic scales, there is room for debate overclassifications.
If we put aparticularevent into a particular bin, we tend to apply a set of defaultassumptions about the character of such an event. This can be thesource for considerable confusion at times, as I have tried toindicate in acommentabout a particular classification scheme (Doswell 1991).Classification does not seem to be a very important aspect of scienceto some, but it affects the way we view the real world. In effect,putting an event into a "bin" seems to imply all sorts of thingsabout that event which may not be valid for the particular instance.
Having put up with this preamble, indulge me for a bit more. I amgoing to describe a pot pourri of events that (a) I have witnesseddirectly in storm chasing, or (b) have had described to me (perhapswith images) by fellow storm chasers, (c) I have seen in film andvideo of events, or (d) are reasonable extrapolations of thingsunobserved but whichcould be observed and yet not bedocumented as such.
a.Mesocyclones[2]. Acharacteristic of mesocyclones is that they represent a perturbationflow that is roughly the same size as the updraft region of acumulonimbus. Although cumulonimbus clouds do not cover a huge rangeof sizes, neither are they all the same size. Thus, mesocyclonesalmost necessarily have a range of sizes, mostly on the order of afew km in diameter; as opposed to a few tens of km in diameter or afew hundred m in diameter. Moreover, mesocyclones have a range ofintensities, with the intensity usually measured in terms of shear orvorticity. Arguably, another measure of intensity could be themaximum windspeed anywhere within the mesocyclonic circulation(the boundaries of which might be a bit tough to define, if pressed).Some mesocyclones, perhaps even some on the lower end of the sizerange, could easily be associated with windspeed maxima in a rangewhere damage would result if they interact with the surface. Such anevent, if it occurs in the real world, meets the definition of atornado I quoted earlier! That is, it is a violently rotating columnof air in association with a deep convective cloud and is in contactwith the surface of the earth. Such events could even have cloudmaterial right down to the surface in the right environment,manifesting themselves as a wide tornado.
b. Multiple-vortex mesocyclones.Gene Moore (areasonably well-known and highly respected storm chaser) once showedme photographs of an amazing event he witnessed in the TexasPanhandle. It involved a large rotating wall cloud, apparentlyseveral km in diameter. From time to time, a funnel cloud would form,producing what appeared to be a slender, ropy tornado around theperiphery of this wall cloud. The funnel cloud typically would extendto the surface briefly and then dissipate, to be followed a fewminutes thereafter by another such event elsewhere on the wallcloud's periphery. Apparently, this went on for some time. Is eachsuch event an individual tornado, or are they simply vorticesembedded within a single multi-vortex tornado with intensityfluctuations? If one were to look at the damage carefully, it is notentirely clear that a single damage path would be involved. Thedamage might appear to be quite chaotic, given the vagaries of theassociated events and the likelihood of a damaging event interactingwith something (trees and structures are pretty sparse in the Texaspanhandle) to mark the event. Short streaks of damage might havevarious orientations and could appear rather erratic. Without visualdocumentation, it might be possible to do a damage survey andconclude that what had happened was a series of microbursts. A ratherwell-respected survey team (which will remain anonymous) overflew thetrack of a documented tornado on 10 April 1979 near Seymour, Texasand concluded it was a downburst, so I have reason to believe that adamage survey is not necessarily definitive!
Given a mesocyclone, who is to say that they are all dynamicallystable, slowly-evolving phenomena? A mesocyclone might well manifestthe same multi-vortex complexity that virtually all vortices exhibit,resulting in a miniature tornado "outbreak" of a sort (as describedby Snow and Agee 1975). Suppose there was a fairly regular productionof such events associated with a moving storm, at intervals in spaceand time. Such a case would appear as a tornado "family" associatedwith a traveling supercell (the Fargo events of 20 June 1957 are theprototypical example, as documented by Fujita 1960). For such a casewe have universally chosen to accept them as separate tornadoes, butit might be argued that they really are simply manifestations of asingle event: a long-lived mesocyclone with embedded multiplevortices. How might one draw the line here? Is it so hard to imaginesituations where classification might prove troublesome? What about asmall, intense mesocyclone in a high-humidity environment? Such anevent might have a nearly continuous lowering and spotty damage, withsubvortices that could be considered tornadoes or tornado cyclones.How large (small) and intense (weak) can a tornado (tornado cyclone,mesocyclone) be? Where doyou propose to put the dividinglines between tornado-tornado cyclone-mesocyclone?
c. Large, multiple-vortex events. I have seenvideo (inTornado VideoClassics III) of another amazing event in the Texas Panhandle,near Lazbuddie on 10 May 1991. This event apparently involved arelatively large tornado (on the order of a UC funnel) at the centerof a ring of smaller tornadic vortices. The central funnel was morepersistent than the surrounding ones, with several funnel clouds incontact with the surface simultaneously from time to time. Is eachfunnel in such an event a single tornado, or is the whole collectionsimply a manifestation of a single tornadic event, with multiplevortices? Is it one tornado within a multivortex mesocyclone? Howdoes one classify such a mess? Presumably, these are rare events, butexactly how many tornadoes should be recorded for that event?
d. Tornado cyclones. During thePampa,Texas tornado I was privileged to witness on 08 June 1995, afascinating evolution occurred. The tornado I will call the "first"tornado began as a ragged, rotating lowering that produced severalbrief episodes of dust whirls and small debris at the surface,apparently out in open country southeast of Pampa. During that sametime, low-level, multiple-vortex funnels appeared and thendisappeared within a few seconds on several occasions. Followingthis, the tornado seemed to become very disorganized, with a new areaof rotation developing as a clear slot wrapped around a truncated,cone-shaped funnel near where the original rotation becamedisorganized. This second area evolved into a very "stable" tornado.As this was happening, another funnel was developing to the northeastof the event we were watching, which I will call the "third" tornado.The interaction of these two cyclones was odd in that the secondtornado (southwest of the developing third tornado the whole time)initially moved westward, then northward, and finally northeastward,whereas the third tornado (developing rapidly as the second one wasdissipating) apparently moved mainly northward. Was the first seriesof brief debris whirls and occasional multiple-vortex funnels(without a complete connection to cloud base) a separate event fromthe event that finally moved through western Pampa? Or was it all asingle tornado? Were there two mesocyclones or were we seeingseparate "tornado cyclones" within a single mesocyclone? Perhaps thedata available will permit answers .. but perhaps not. In most cases,of course, high-resolution data and videotapes of the event would notbe available. These details are simply lost for most events.
e. Other wierd things. It is plausible to believe thatgustnadoes can develop into tornadoes (see below); there are at leastsome indications [e.g., from Erik Rasmussen in some personalcommunications] that a true dust devil could, as well! Essentially,there are many ways to produce an intense vortex from a preexisting,nontornadic vortex. Since there's a lot we don't know or understand,if we look carefully, we may continue to find examples that don't fitour nice, cleanhypotheses.[8]
The events I have been describing above and others, some mentionedin the reviewed literature(Doswelland Burgess 1993, Davies et al. 1994, Forbes and Wakimoto 1983,etc.) and some not, all make me concerned for what we have recordedas tornadoes in the historical record. For example, insofar as thatrecord is concerned, the Tri-State tornado is a single tornado event.As already discussed elsewhere (Doswell and Burgess 1988), there isroom for debate about whether or not this actually was a singleevent. Continuity of the damage path is, as I have suggested, notentirely convincing that a single tornado was involved. Of course, Iam even questioning whether a tornado "family" of the sort thatmight be involved with the Tri-State event is really more thanone event. In other words, I am not taking a dogmatic position here... I am willing and eager to consider other views, but what concernsme is that (a) we have the debate, and (b) we reach some consensusabout how to record the real events.
The growth of scientific understanding about tornadoes does notmean that clarity of insight follows immediately. As noted, I feelsomewhat concerned about our concepts of tornadogenesis. A tornado(as I am fond of saying), irrespective of the details of thedefinition, is not anobject; it is aprocess. That is,the wind field that we define to be the tornado exists as a resultof, and evolves in response to, other processes. Observationally, itseems that vortices of this intensity are produced virtuallyexclusively in association with deep moist convection. There surelyare dust devils (dry convective vortices) that attain damagingproportions but they are uncommon and we know essentially nothingabout their distribution. Since the strongest dust devils probablyoccur in arid regions of extremely sparse population, our ignoranceof such events is large.
Within the range of vortices associated with deep moistconvection, I suspect that a lot of different processes can producevortices in convective clouds, very few of which affect the surface.There is some limited mention of such events in the literature(Moller et al. 1974; Cooley 1978; Doswell 1985 [p. 107]; Bluestein1988;Doswelland Burgess 1993; Bluestein 1994; etc.). For example, Cooley'spaper is the only mention of "cold air funnels" in the refereedjournals. At times, I have heard of funnels being tagged byforecasters as "cold air funnels" in situations quite radicallydifferent from the prototype described by Cooley and discussedbriefly in Doswell and Burgess (1993). An erroneous classification ofsuch vortices might be prompted by a perceived need to "explain"surprise observations of funnels, but it probably does more harm thangood to provide abad explanation. Unfortunately, science canoffer little more than speculation about the origins of thesenon-tornadic vortices (e.g., Bluestein 1994). It is conceivable thatsome tiny percentage of these minor events (examples shown inFig. 4 andFig. 5)could produce damaging winds at the surface, but I have no way ofknowing what that percentage might be, nor by what processes suchvortices might come to reach damaging intensity at the surface, tosay nothing of their origins.
Events that we might reasonably call tornadoes appear to beassociated with:
As noted in Davies-Jones and Brooks (1993) and discussed inanother essay of mine, the issue with a tornado is to get thehigh vorticity near to the surface. It appears that processesassociated with the mesocyclone and low-level boundaries are likelycandidates for obtaining tornadic vorticity at the surface. I notethat a supercell (in my opinion) need only have a mesocycloniccirculation meeting certain criteria (seeDoswell1996) to qualify as a supercell and that the mesocyclone need notbe present at low levels. (Other views exist; see, e.g., Droegemeieret al. 1996.) Low-level mesocyclones are apparently the result ofdifferent processes than those that produce mid-level mesocyclones(see Brooks et al. 1994). At this time, it is unclear what percentageof low-level mesocyclones produce tornadoes, but apparently fewerthan 50 percent of those storms having mesocyclones at any levelproduce tornadoes. []
In the case of "landspouts," it seems that pre-existing vorticesalong pre-existing low-level boundaries might "explain"tornadogenesis. With those tornadoes that seem to form aloft anddescend (the TVS - see Trapp and Davies-Jones 1996), the story may becomplex dynamically, and involve an interaction between the updraftand its own rain-cooled outflow. It also could be that real eventsinvolve a complex mixture of all three processes [(a)-(c), above] attimes. Whatever we find out about these processes in the real world,it almost certainly is going to be the case that there is no singlepath to a tornado, even when we restrict our attention to thoseevents that are clearly and unambiguously tornadoes (Includinglandspouts ...landspouts are tornadoes, by any reasonabledefinition of a tornado. Like the waterspout, I'd prefer not having aspecific term for this kind of event, since it only seems to createconfusion.). Not all significant tornadoes are the same, even thoughtheir flow fields might end up looking alike ... a vortex after all,has a certain characteristic kinematic structure irrespective of theprocesses that produced it.
There is another class of events that has caused a large amount ofheartburn:gustnadoes. Observations indicate clearly thatrelatively weak, short-lived vortices can form along the leading edgeof an outflow boundary. The mechanism(s) by which such vortices form?No one really knows. Hence, almost anything I can say about theseevents is pure speculation. We have no detailed Doppler radarobservations of them; we have no numerical simulations of them; wehave virtually no validated knowledge. All we have is anecdotalevidence from storm chasing and some analogies with things seen inlaboratory simulations (Idso 1975). The visual appearances of truegustnadoes (as opposed to tornadoes along a gust front, which aremanifestly different phenomena) indicate they are shallow (perhaps10-100 m deep) with no apparent connection to any process happeningat cloud base or above (Fig. 6). When theyarise, which I believe to be frequently, they occur in "swarms" suchthat there may be several in existence at the same time along thesame gust front, forming and dissipating within no more than a fewminutes and probably having only weak wind perturbations.Superimposed on a damaging gust front (i.e., a downburst), they mightrepresent local concentrations of damage. Superimposed on anon-damaging gust front, they might be manifest as isolated damageevents in an otherwise benign situation. My guess is that typically,they represent only a minor perturbation of essentially nosignificance, except in very rare examples.
I have some anecdotal evidence that a gustnado can evolve into atrue tornado [Dave Blanchard, personal communication], but such anevolution is almost certainly rare. Whereas some true tornadoes mightinitially resemble a gustnado at the start,I certainly would findit easy to deny gustnadoes (as I have defined them) the status oftrue tornadoes. Unfortunately, it may be hard to train folks tobe able to distinguish them from other vortices occurring inconjunction with deep, moist convection. I certainly have encountereda lot of different notions about gustnadoes, even amongmeteorologists, much less the lay public. There seems to be adisturbing trend to refer to all tornadoes occurring on a gust frontas "gustnadoes" whereas I have tried, apparently without success, toconfine the term to theshallow vortices on gust fronts thatseem not to extend as far as cloud base. Moreover, even with 20+years of chasing behind me, I still am encountering things I haven'tseen before. What about the person experiencing something like thisfor the first time? If that person is confused and has a hard timesorting out what he/she experienced, I think they can be forgiven.But we need not assume that the public is congenitally stupid,either. Some people can report quite accurately what they saw, butthey describe it in inappropriate terms (e.g., a tornado withmultiple vortices becomes several tornadoes merging into one)
There is a lot we don't know about what happens along seeminglyboring, 2-dimensional gust fronts! Perhaps gust fronts are prone todynamical instabilities on a variety of scales, some of which remainsmall (gustnadoes), some of which are large and persistent enough("misoscale" eddies) to become a bona fide tornado. There might be awhole spectrum of structures along gust fronts, many of which do notattain "tornadic" proportions (however we might choose to define atornado). If that is the case, then it might be quite difficult toanticipate when a gust front would produce damaging vortices, fromgustnadoes on up to and including what would unambiguously be calledtornadoes. The topic of what might happen along gust fronts waswritten about by Idso (1974; 1975) in a very speculative vein, butlittle of substance has been done with the topic.
This topic never seems to go away for long, so I suppose I shouldsay just alittle more about cold air funnels. There is notmuch about them in the literature ... the only published paper aboutthem of which I am aware is that by Cooley (1978) and it offersrelatively little insight. Doswell and Burgess (1993) have discussedthem, but not in depth, because little of depth can be said. Theyoccur in association with convective clouds (that may have little orno thunder) developing within cold pools aloft, in environments withrelatively little vertical wind shear. These should be distinguishedfrom tornadoes with low-topped supercells (that typically arise inenvironments that may be notably cold aloft, but with considerablevertical wind shear) and from "landspout"-type events (that haverelatively little vertical wind shear, but are not tied to cold poolsaloft). Cold air funnels arenot items for urgent studybecause of their lack of associated damage and casualties ... we haveother things to worry about that really do damage and createcasualties. In my experience, many events are labeled inappropriatelyas "cold air funnels" even when they are not associated directly withcold pools aloft, simply because the events are unexplained (as notedin section 7, above). If one of these should happen to do surfacedamage, then I see it as a tornado ... plain and simple.
If we admit that pre-existing boundaries often interact with theconvection and that the interaction might be very important intornadogenesis, the picture becomes quite complex, perhapsintimidatingly so. Sometimes the interaction seems to favortornadogenesis, while at other times, it seems to precludetornadogenesis. Not every boundary with which a storm might interactis the same. Nor does the interaction have to follow precisely thesame course every time even when the boundaries are nearly identical;perhaps there are many factors (age of the storm, its movementrelative to the boundary, its orientation relative to the boundary,etc.) that could alter the course of the interaction. In this arena,we are abysmally ignorant at the moment. We have only veryrudimentary knowledge of these interactions. It appears thatboundaries of all sorts exist in abundance in "clear" air and mayonly show up occasionally on satellites as cloud lines. Some of themappear on high-sensitivity radars (like the WSR-88D) but we know nextto nothing about them ... their origins, structure, and evolution areopen topics for speculation, especially operationally.
There is evidence to suggest that even supercells on "synopticallyevident" tornado outbreak days do not necessarily produce tornadoesvia some self-contained "cascade" process. Davies et al (1994)suggested that the Hesston tornado family may have involved aninteraction with a pre-existing boundary. Jim Purdom has been sayingsimilar things for years (e.g., Purdom and Sinclair 1988) about manystorms on outbreak days. At this point, the numerical stormsimulations (virtually all initiated with horizontally homogeneousinitial conditions) suggest that such interactions are not necessary.Nevertheless,real storms have a disturbing proclivity toproduce tornadoes in association with interactions involvingpre-existing horizontal inhomogeneities. It might be that we simplycannot understand tornadogenesis properly without includinginhomogeneous initial conditions in our numerical simulations; see myessay on post-VORTEX thoughts. Given the bewildering nature of suchinteractions with the variety of boundaries that seem to be showingup in our new, high-resolution observations, it is going to be achallenging time to follow the thread of tornadogenesis. It could besome time before we can establish common characteristics and begin tosee order in the apparent chaos. It seems to me that in order toaccomplish that future orderly synthesis, we are ill-served by ataxonomy (classification scheme) that fails to distinguish tornadoesby the relevant physical processes.
Unfortunately, since we do notknow (in a completelysatisfactory way, at least) what are the relevant physical processes,we are caught in a bit of a "Catch-22" conundrum: to understandtornadoes, we need a proper classification ... but to develop aproper classification, we need to understand tornadoes! I amconfident we eventually can work our way out of this seeming paradox,but it will involve small incremental steps, first learning somethingabout tornadoes and then applying that to the classification. Perhapsthe prior classifications cannot be salvaged without informationabout particular events that, if we ever had it, has been lost overtime. The tornado data base should be modified regularly to containthe information needed to classify the events as best we know how atany given time. Keeping continuity with the past is of little valueif by doing so we miss the chance to gain a better understanding.Better to change the record-keeping frequently as we develop betterunderstanding .. we always can "degrade" high-quality data to looklike low-quality data if consistency is the goal, but the reverseusually is not possible. My colleague,Erik Rasmussen hasproposed an interesting idea for a newdatabase for folks to consider.
The day of having a simple collection of "tornado events" ought tobe put behind us soon, if we are ever to gain a deeper understandingof the processes. We must expand our data base to include informationabout the storm that produced the event: was it a supercell (usingwhatever definition we can arrive at as a consensus)? Was it adeveloping cloud or was it a mature storm? Was it interacting with adetectable boundary? What is (are) the source(s) for informationabout the event? If assessments are quantitative, what data were usedand how were thresholds defined? If assessments are subjective, whatis their factual basis? And soon.[3] We also mustbegin to make some sort of distinctions among tornadoes, perhapsgropingly at first. Is the tornado "stable" in the visual sense Ihave described? Are there any gaps or irregularities in the damagepath and what is the nature of those irregularities? Was the tornadopendant from a wall cloud? What was the location of the eventrelative to the mesocyclone, if a mesocyclone was associated with thestorm (and what is the operative definition of a mesocyclone?)? Doesthe event fit any of the recognized categories of convective vortices(whatever those categories might be at the time)? Did it have anydistinctive characteristics (e.g., no visible funnel, multiplevortices, smooth and laminar, ragged and turbulent, very wide,dramatic changes in its characteristics during its life cycle, etc.)?
I realize fully that this is asking for alot ofinformation relative to where we are now. However, it seems to methat a "tornado database" that does not ask for a lot of informationabout tornadoes is not going to have much to offer, except perhaps toactuaries for the insurance companies, who aren't concerned withscientific subtleties.
Neil Stuart (presently at the Wakefield, VA office of the NationalWeather Service) has raised some interesting points via e-mailcorrespondence. In some cases, there is very strong horizontal windshear in storms (even tropical cyclones) ... the wind speedsassociated with such wind shear can attain damaging values and socreate damage paths. Obviously, this is true for thunderstorms, sincethey produce strong outflows (often called downbursts). Along theflanks of a downburst, strong horizontal gradients of the wind can befound, and such shears can be associated with substantial vorticity(both cyclonic and anticyclonic). In theory, such "sheets" ofhorizontal wind shear are unstable and should "roll up" (Fig. 7) intoa series of discrete vortices; in the process, the shear line'svorticity (originally spread out along the line) becomes concentratedinto a number of vortices.
Fig.7Thus, if strong vorticity spread out along a line is present atsome moment, it is likely that it will break down into a string ofseparate vorticity centers. This might have relevance for the originof gustnadoes, and has been proposed for tornadoes as well (althoughtornadoes are more complex because it seems not all tornadoes developin the same way). However, the notion that high vorticity tends toevolve into compact centers rather than being strung out along a lineis relevant.
As discussed in myvorticityprimer, the issue of whether or not a vortex (i.e., a compactregion of vorticity) produces a closed flow depends in part on thereference frame. If a moderately intense vorticity center is movingrelatively rapidly, the ground-relative flow may not be a closedvortex. In fact, many weak to moderate tornadoes that move rapidlyhave damaging winds only on one side of the vortex. This produces adamage path that contains little or no direct evidence of rotation... all the damage may appear to be coming from more or less the samedirection. This can lead some investigators to conclude that theevent was not a tornado, but rather a microburst. The giveaway,however, is the existence of along, narrow damage path. Asthe center of high vorticity moves along, it creates a narrow swathof damage.
If the damage is done by a moving center of high vorticity,resulting in a damage path that is narrow relative to its length, Iam inclined to call it a tornado (or a gustnado, depending on thecircumstances). I am not inclined to reject the tornado hypothesissimply because the damage is unidirectional, for reasons just given.That is, it is not obvious to me that the flowmust be closedin a ground-relative sense for the event to be considered a tornado.If there are Doppler radar data, thestorm-relative frameworkoften will show a fair amount of symmetry in a vortex flow, even ifthe ground-relative flow is highly asymmetric.
A downburst (or microburst) creates a difluent damage path,because the winds are forced to spread apart as they interact withthe solid ground ... the resulting divergence creates a fan-shapeddamage region that may start out concentrated but rapidly spreads outand weakens. This is quite different from the damage produced by aconcentrated vorticity center moving along.
On the other hand, people often are heard saying that such thingsas twisted trees, street signs, and other "indicators of rotation"imply that an event was a tornado. The scale of rotation in tornadoesis on the order of hundreds of meters, generally. A twisted tree canarise from purely straight-line winds if the tree's resistance to thewind is not symmetric, for example ... that is, the tree yields tothe wind in a asymmetric fashion. Those doing damage surveys need tobe aware of this all-too-common misunderstanding of the scale oftornadic vorticity; it usually is much larger than the structures andobjects that are damaged, soon the scale of the structures,the damaging winds are"straight."[9]
As a final footnote to all of this, it seems to me that VORTEX hasunderscored the relevance and value of observational programs. Wehave seen enough from the observations to cast real doubt on most ofwhat we thought we knew. There have been those who considered thetheory and numerical simulations to be so far advanced thatobservations could serve only to confirm the theoretical andnumerical findings. In my opinion, VORTEX has invalidated this view,as described in my other VORTEX-related essay; the observations haveshown the inadequacies and shortcomings of these approaches quiteclearly. Moreover, the new data sets are revealing a host of newphenomena about which we know virtually nothing. The relevance ofthese new phenomena to tornadogenesis has yet to be shown, but itshould be noted that we cannot rule them outa priori asimportant factors, either. It appears that observational meteorologyis in need of some support; shutting down our observational programsin favor of theory and numerical simulations would be a scientifictragedy (and travesty!!) for us all. Theory, modeling, andobservations each have their own roles to play in science, and wecannot do without any one of them. Observational meteorology has beengiven far too little attention and value over the past two decades.Decreasing budgets have resulted in a disproportionate share of theburden has falling on observational facilities. I believe that VORTEXcontains a message: the community should recognize that theory andmodeling are not by themselves sufficient to carry our science.
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