2.2 TORNADOGENESIS IN SUPERCELL STORMS –WHAT WE KNOW AND WHAT WE DON’T KNOW

Robert Davies-Jones*National Severe Storms Laboratory, NOAA

Norman, Oklahoma

1. COMMON OBSERVATIONS

The advent of Doppler radar, 3Dnumerical simulations and scientificstorm chasing have led to hugeadvances in our knowledge ofsupercells during the last 35 years.However, complete understanding oftornadogenesis in supercell storms stilleludes researchers. To put mydiscussions of current knowledge andmysteries in context, I followRasmussen and Straka’s (1997)example by first describing features oftornadic supercells that are observedwith quite remarkable repetition fromstorm to storm and then reviewingviable theories that are congruent withthese observations. Some of thefollowing is based on their paper.

Background on tornadic storms iscontained in the most recent reviewarticle (Davies-Jones et al. 2001) andreferences cited therein.

1.1 STORM AND ENVIRONMENT

From detailed examination of radardata, which had rather coarseresolution by today’s standards,Browning (1964) concluded that mosttornadoes form within large, steady,and virulent thunderstorms. Theseappeared to be unicellular, so hecalled them supercells.

The introduction of pulsed Dopplerradars into severe-storm research in1971 quickly led to new results.Signatures of mesocyclones andtornadic vortices were evident in theDoppler velocity fields of a singleradar. The mesocyclone typicallyformed aloft first and then near theground prior to any tornadogenesis.____________*Corresponding author address:Robert Davies-Jones, National SevereStorms Laboratory, NOAA, 1313Halley Circle, Norman, Oklahoma73069-8493;e-mail: Bob.Davies-Jones@noaa.gov

Fawbush and Miller (1954)identified environmental conditionsthat are conducive to large, long-livedtornadoes. The sounding generallyhas large convective availablepotential energy (CAPE) and strongshear associated with winds veeringand increasing with height. Althoughboth shear and CAPE are importantparameters, theoret ic ians werebewildered by cases when tornadoesformed in environments either withhigh CAPE and little shear or with lowCAPE and high shear. The formerevents occurred either in hurricanes orin mid-latitudes during the coolseason, and the latter ones in mid-latitudes during the warm season. Wenow know that the low-CAPE, high-shear tornadoes form in mini-supercellswith low tops and the high-CAPE, lowshear tornadoes develop in supercellsthat are not very steady.

1.2 MESOCYCLONE ALOFT

The initial mesocyclone is a rotatingupdraft with maximum vertical vorticityexceeding 10-2 s-1 and a largecorrelation between vertical velocityand vertical vorticity. It forms aloft fromtilting of low-level storm-relativestreamwise vorticity associated withvertical shear in the large-scale ormesoscale environment. The resultingvertical vorticity is subsequentlystretched and advected vertically inthe updraft. The storm may amplify thelow-level streamwise vorticity in itsinflow by stretching it horizontally.Helical environments (i.e., ones withlarge shear vectors that veer withheight in the lowest few kilometers),either on a large scale or aroundbaroclinic boundaries are especiallyconducive to tornadoes. Very largehelicity in the lowest 1 km is particularlyfavorable for tornadoes. This isprobably because the mesocyclonealoft has a base that is not far off theground.

1.3 NEAR-GROUND MESOCYCLONE

In 1949 Brooks (1949) discoveredfrom tornado passages nearmicrobarographs that tornadoesformed within larger cyclones, which henamed tornado cyclones and whichnowadays are termed low-levelmesocyclones irrespective of whetheror not they produce tornadoes. Incomparison to mid-level meso-cyclogenesis, rotation near the grounddevelops later in the supercell’s lifetimeand by a different process. It seemsto await the formation of downdraftswithin the mesocyclone.

F u j i t a ( 1 9 5 9 ) collectedphotographs and movies of the cloudbase and sides of a 1957 tornadicstorm near Fargo, North Dakota, andfound that the entire updraft wasrotating cyclonically (Fig. 1). This wasthe first visual confirmation of amesocylone.

1.4 REAR FLANK-DOWNDRAFT

The data from two radars withdifferent viewing angles allowed theconstruction of 3D wind fields insidesevere storms. It soon becameevident that downdrafts played a rolein tornadogenesis. Dual-Doppleranalyses and observations by stormchasers revealed that tornadoesformed not in the early lifetimes of asupercell when it consisted almostentirely of a rotating updraft but lateron after downdrafts reached theground. Low-precipitation (LP)supercells seldom produce tornadoes,probably because they never developa significant downdraft.

A supercell is generally consideredto have two main downdrafts (Fig. 2)even though they may not be a gapbetween them. The forward flankdowndraft (FFD) forms first and iscollocated with precipitation on the leftfront side of the updraft in the northernhemisphere. [For southern-hemispheresupercells interchange left and rightthroughout this paper.] The rear-flankdowndraft (RFD) develops at the rearof the rotating cloud tower, which mayhave a quite circular base initially. TheRFD may arise from evaporative

cool ing or from a downwardnonhydrostatic vertical pressure-gradient force (NHVPGF). With time itpropagates around the rotatingupdraft, and its outflow boundary isthe RFD gust front. The mesocycloneat low levels now is divided into updraftand low cloud base on its left side,and downdraft (the left side of theRFD) on its right. (Lemon and Doswell1979). The right side of the RFD isanticyclonic and so lies outside themesocyclone. Convergence at theRFD gust front causes the updraft toextend along it and thus becomehorseshoe shaped. At low levels thesupercell now resembles an extra-tropical cyclone with the outflowboundaries from the FFD and the RFDin the warm-frontal and cold-frontalpositions, respectively. The RFD isoften visible as a clear slot, a narrowdeep slot of cloud-free air that wrapsaround the region where the tornadodevelops about 5-10 min prior to itsformation. Rapidly sinking andevaporating cloud fragments are oftenseen near its edges. The advancingRFD occludes the mesocyclone withthe tornado typically forming near thepoint of occlusion. At this stage a newmesocyclone may be forming alongthe bulge in the RFD to the right of theold one (Fig. 2).

1.5 HOOK ECHO

With the advent of weather radarcame the recognition that tornadicstorms close to the radar often had anecho with a hook-shaped appendageon the right rear side (Stout and Huff1953, Markowski 2002). The hook isoften attributed to precipitation beingdrawn into a cyclonically rotating raincurtain by the mesocyclone (Browning1964). If this were true, the hookwould elongate gradually. However, itoften seems to form all at once,suggesting that it may be associatedwith the sudden development ofprecipitation at the rear of a newupdraft. The hook is often narrow,consistent with chasers’ observationsof thin curved rain curtains withhydrometeors that are advectedhorizontally by the mesocyclonic windsas they fall. At the ground the curtains

typically consist of large drops.Sur face measurements revealdivergent flow towards and away fromthe mesocyclone axis and locallyhigher pressure in the region of thehook. The tip of the hook may flareout both cyclonically to the right andanticyclonically to the left.

1.6 TORNADO CYCLONE

Tornadoes have circulations that arean order of magnitude smaller than thecirculation of a typical maturemesocyclone. Fortunately nature isonly able to contract a part of a strongmesocyclone into a tornado. This partis thought to be a vortex with a typicalcore radius of 1 km within themesocyclone, and is named a tornadocyclone [not to be confused withBrook’s (1949) tornado cyclone inSection 1]. The tornado cyclone hasbeen observed occasionally in high-resolution Doppler radar observations.In one case (Burgess et al. 2005) itdeveloped though a deep columnwithin an intensifying mesocyclone. Itsmaximum winds were aloft and atornado built upward from within it.The co-existence of a tornado andtornado cyclone is also evident inDoppler on Wheels observations oftornadoes with secondary windmaxima at distances of 500-1000 mfrom the axis (Wurman and Gill 2000).Thus, the entire tornado cyclone maynot contract into a tornado.

1.7 OCCLUSION DOWNDRAFT

The occlusion downdraft is a small-scale downdraft that forms after thedevelopment of intense rotation andattendant low pressure next to theground (Rotunno and Klemp 1985).Thus, it is a response to the near-ground rotation rather than aninstigator of it. It is driven by strongdownward NHVPGF associated with adeep low at the surface. Visually, it ispractically indistinguishable from theRFD because it is located near thefront edge of the RFD as it wrapsaround the developing tornado.

1.8 WALL CLOUD

In his study of the Fargo storm,Fujita (1959) realized that an abruptlowering of the cumulonimbus cloudbase was a significant feature andnamed it the wall cloud. It marked thelower portion of a strong rotatingupdraft (Fig. 1). Supercell tornadoesare generally suspended from wallclouds until they are overtaken bydivergent air late in their life times.Rotating wall clouds containing strongupward motions often precedetornadoes by tens of minutes (Fujita1959). Based on numericalsimulations, Rotunno and Klemp(1985) attribute their formation to rain-cooled, nearly saturated air thatdescends, and flows along the groundinto the updraft. The lowering is dueto this air having a lower liftedcondensation level than air in thestorm’s inflow.

1.9 TORNADO POSITION IN STORM

Neil Ward chased one of thetornadic supercells studied byBrowning (Browning and Donaldson1963). He observed that thetornadoes formed in the storm’s mainupdraft and near its gust front.Chasers observed that tornadoesusually form in a region of the stormthat is a few kilometers away fromprecipitation and the nearest cloud-to-ground lightning (Davies-Jones andGolden 1975; Fig. 3). Thus, theoriesthat rely on electrical heating fromrepeated lightning strikes (Vonnegut1960) or on a precipitation-loadeddowndraft along the axis of rotation(Eskridge and Das 1976) have noobservational support.

Doppler-radar analyses andchasers’ observations revealed thattornadoes generally form close to thecirculation center of the mesocycloneand near the interface of updraft anddowndraft. Roughly speaking, theyare also near the center of curvatureof the wrapping rain curtain where theinward outflow from the associateddowndraft converges.

Observations from chase teamsshowed that the tornado below cloudbase was coincident in time and spacewith the tornadic-vortex signature(TVS). For the first time meteorologists

could observe tornadoes (or moreprecisely their signatures) above cloudbase (Brown et al. 1978). Largeviolent tornadoes were associated withTVSs that extended to near thetropopause. Note that, if the radar isnot close enough to resolve the parenttornado cyclone, the TVS may be dueat least partly to the tornado cyclone.

In numerical simulations ofsupercells, vertical motions intensifyand pressure falls at 1-3 km aboveground just prior to tornadogenesis(Wicker and Wilhelmson 1995). Atthese levels the poorly resolvedtornado forms in large gradients ofvertical velocity where the tilting term islarge.

1.10 ANTICYCLONIC VORTICITY

At low levels, anticyclonic vorticityis often located near tornadoes. Ananticyclonic flare is occasionallyobserved at the tip of the hook and isvisible in the field as anticyclonicrotation in the base of the flankingcloud line to the right of the clear slot(Fig. 2). Sometimes an anticyclonictornado forms at this location, usuallywhen there is a stronger cyclonictornado located on the other (left) sideof the hook (Knupp and Brown 1980).In other cases, the region ofanticyclonic vorticity advances arounda strong cyclonic tornado as the RFDwraps around the tornado.

In his photogrammetric analysis ofthe 1957 Dallas tornado, Hoecker(1960) found a region of anticyclonicvorticity just outside the core at aroughly 100 m from the axis.

2. WHAT WE KNOW

Tornadogenesis divides naturallyinto three stages. The first step in thevorticity concentration is the formationof a rotating updraft.

2.1 STAGE 1 OF TORNADOGENESIS

Since some tornadoes occur inlow-shear environments, meteor-ologists debated, prior to 1977,whether the concentrated vorticity insupercell tornadoes was simply a resultof stretching of pre-existing planetary

vertical vorticity by a large persistentand strong updraft or whether itoriginated ultimately from tilting ofhorizontal vorticity.

Stretching of planetary vorticity isat odds with the observations becauseit would produce rotation near theground f i rs t where horizontalconvergence is largest. In contrast,Browning and Landry (1963)hypothesized that updrafts insupercells rotated cyclonically by tiltingupdraft-relative streamwise vorticitypresent in their inflow. Thismechanism would produce amesocyclone aloft, as observedinitially. Lilly (1982) and Davies-Jones(1984) developed mathematicaltheories of this process.

The first successful 3D numericalsimulations of supercells settled thisdebate (Klemp and Wilhelmson 1978).The model storms resembledsupercells even with the Earth’srotation switched off. In simulationswith unidirectional (straight) shear,storms split as often observed intosevere right- (SR) and left-moving (SL)supercells. The updraft in the SR (SL)storm rotated cyclonically (anti-cyclonically). Inclusion of Coriolisforces simply made the right mover thestronger storm, but only slightlybecause the Rossby number forsupercells has an order of magnitudeof 100. Veering of the shear vectorwith height enhanced the right-movingupdraft and inhibited the left-movingone to a much greater degree.

Since updraft-relative streamwisevorticity is equal to the strength timesthe rate of veering with height of theupdraft-relative winds, updraft rotationdepends on updraft motion. Updraftspropagate towards (away from) theside where NHVPGF is upward(downward). Rotunno and Klemp(1985) found that in nearly straightshear, updraft propagation dependson the nonlinear part of the NHVPGFarising indirectly from the updraft-shearinteraction. The updraft forms amidlevel vortex pair by pulling up loopsof environmental vortex tubes. Thelow pressure in these vortices isassociated with upward NHVPF belowthem. Midway between the vorticesthere is high pressure due to water

loading and deformation withdownward NHVPGF below this high.This configuration of NHVPGF causesthe initial updraft to split into acyclonical ly rotat ing SR andanticyclonically rotating SL updrafts.

When the hodograph is highlycurved, Davies-Jones (2002) foundthat the propagation depends mainlyon the linear part of the NHVPGFinduced directly by the shear-updraftinteraction (as first proposed byRotunno and Klemp 1982 for nearlystraight shear).

Updra f ts acqu i re cyc lon iccirculation by propagating into cyclonicand out of anticyclonic regions. Forexample, the SR updraft maintains itsrotation by propagating towards theoriginal cyclonic vortex, which staysahead of it because the updraft iscont inual ly t i l t ing environmentalvorticity upward at its leading edge.Davies-Jones (2004) found that thegrowth of circulation around the edgeof an updraft at a given level is equalto the line integral around the edge ofvertical vorticity times either the localpropagation of the edge normal toitself or, equivalently, the localNHVPGF divided by the local gradientof vertical velocity. Thus, the firsts tage o f tornadogenesis, thedevelopment of a mesocyclone aloft, iswell understood.

2.2 STAGE 2 OF TORNADOGENESIS

Tilting by an updraft of horizontalvorticity fails, however, to producerotation very close to flat groundbecause the vertical vorticity isgenerated as the air is rising (Davies-Jones 1982). This rule could beviolated if streamwise vortex lines weretilted upward abruptly by a gust frontand stretched by an overhead updraftas proposed for waterspouts bySimpson et al. (1986). This is aneffect that is absent in simulations withlimited horizontal resolution. However,parcels approaching a gust frontgenerally start rising before they reachit and, since the vortex lines tendsomewhat to be frozen into the flow,they would also begins lifting ahead ofthe gust front. Tornadogenesis mustawait the development of downdrafts

that either tilt initially horizontal vortextubes as they advect them downwardor simply transport vertical vorticitydownward. This second stage oftornadogenesis, the development ofrotation very close to the ground, isthe one that requires much moreresearch.

2.3 STAGE 3 OF TORNADOGENESIS

The third stage, the formation ofthe tornado, appears to be simply theresult of amplification of verticalvorticity in air parcels that are beingstretched vertically by an updraft(Walko 1993). This view is supportedby Doppler radar observations ofstrong low-level convergence just priorto tornado formation. Frictionalinteraction between the low-levelmesocyclone and the ground may aidthis process by inducing radial inflowalong the ground (Rotunno 1986). At‘ground zero’, stretching of verticalvorticity is the dominant term in thevertical-vorticity equation once verticalvorticity is present very near theground. Ward’s (1972) laboratorytornado simulator exemplified thisstretching process in a wide updraftand reproduced several observedfeatures of tornadoes such ascharacteristic surface pressure profiles,vortex breakdown, and multiplevortices. Given sufficient time withoutdisruptions such as cold poolsspreading beneath the updraft, thisstage should mimic Ward’s laboratorymodel. At large swirl ratios, centrifugalforces prevent the convergence of airto near the axis of rotation, in whichcase either there is a single large weakvortex or multiple tornadoes formaround the periphery o f themesocyclone.

There is no need to find exoticenergy sources for tornadoes since wenow know that the “thermodynamicspeed limit” can be broken (Fiedler andRotunno 1986). The frictionalinteraction between the tornado andthe ground drives strong radial inflow.Parcels can penetrate closer to theaxis than the radius dictated bycyclostrophic balance above theboundary layer. Because their angularmomentum is nearly conserved, they

rotate very quickly. Their excessivekinetic energy is compensated for byloss of pressure energy. To conservemass, the boundary layer eruptsviolently upward into an intensevertical jet along the axis.

When the convergence increasesand the ambient angular momentum isconstant with height or when theconvergence is constant and theangular momentum increases withheight, the tornadic vortex forms aloftbecause high-angular-momentum airfirst arrives near the axis aloft. Thevortex then builds downward slowly(tens of minutes) to the surfacethrough a dynamic pipe effect (Smithand Leslie 1978; Trapp and Davies-Jones 1997). If there is insufficientangular momentum near the ground,the vortex remains as a funnel cloudand never becomes a tornado. If theconvergence and angular momentumare constant with height in the lowestfew kilometers, the vortex contractsuniformly and a tornado forms rapidlyin 5-10 min.

Incidentally, a popular suggestionfor tornado modification is to explode adevice in the core. A laboratoryexperiment by Chang (1976) showedthat this method can disrupt a vortextemporarily, but it immediately reformsas long as the larger-scale flow nearthe ground remains convergent andgyratory.

3. WHAT WE DON’T KNOW

As stated above, most of themysteries concerning tornadogenesisconcern the development of rotationvery close to the ground through someprocess that seems to be very differentfrom midlevel mesocyclogenesis. Thepossibility exists that the process maynot be the same one in all cases.Several processes have beensuggested and none have beencomplete ly ver i f ied by f ie ldobservations. Genesis of a stronglong-lived tornado requires that thenear-ground mesocyc lone beunderneath the mid-level mesocycloneso that there is a continuous widevortex column from the ground to nearthe tropopause. The tornado has to

terminate in a strong updraft toprevent it from filling from above.

3.1 BAROCLINIC MECHANISMS

High-resolution numerical cloudmodels now produce poorly resolvedtornadoes within simulated supercellsin non-rotating atmospheres. In thesesimulations, rotation near the ground isdue to tilting of horizontal vorticity thatis generated baroclinically in subsidingair that has spent considerable time(around 10 min or more) in a strongbuoyancy gradient (Rotunno andKlemp 1985). Davies-Jones andBrooks (1993) showed that the vorticityin this air changes from anticyclonic tocyclonic during its descent in thebaroclinic zone by the mechanismdescribed in Davies-Jones et al.(2001). Its cyclonic vorticity is thengreatly amplified as it passes into theupdraft. In the simulations, the near-surface vorticity maximum or tornado-like vortex is typically located ingradients of both temperature andequivalent potential temperature.

Unexpected ly , in -s i tu fieldobservations made during and afterVORTEX (Verification of the Origins ofRotation in Tornadoes EXperiment)have failed to detect rain-cooled air atthe surface near several strong andviolent tornadoes even though thetornadoes were close to a wind-shiftline. The discrepancy betweenobservations and simulations may bedue to the microphysics schemeproducing too strong a cold pool toosoon owing to excessive rainproduction too low in the cloud andthe evaporation rate being too fast.The observations do not preclude thepossibility that air, which enters thetornado at its base, may have passedslowly through a remote barocliniczone that may even be above theground.

3.2 P O S S I B L E R O L E O FMICROBURSTS

Do microbursts sometimes triggertornadoes or help maintain them?Radar observations and damagesurveys sometimes reveal microburstsprior to the touchdown of tornadoes or

on the right sides of tornado tracks. Inthe radar echoes of several tornadicstorms, ‘blobs’ of higher reflectivity,perhaps associated with microbursts,have been observed within the hooks.These descend to low elevations priorto tornado touchdowns on their leftforward sides. The blob in the wellobserved 2 June 1995 Dimmitt, Texastornadic storm had an anticyclonicvortex on its left side and a cyclonicone that appeared to evolve into thetornado on its right side. This vorticityconfiguration could arise fromdownward transport of high-momentumair (represented by the double arrow inFig. 2). Alternatively, it could originatefrom tilting by a downstream updraft ofbaroclinic vorticity. A downdraft that isheavy aloft owing to water loadingand/or evaporative cooling would beencircled by descending quasi-horizontal vortex rings with the vorticityvectors directed clockwise. These ringswould spread along the surface uponreaching it, Lifting of the leadingedges in the RFD gust frontconvergence zone would give rise tovertical vorticity of the observedconfiguration. The cyclonic vortex isgenerally the stronger vortex and theone that turns into a tornado becauseit can enter the main updraft. A majorquestion is whether enough circulationcould be generated for a majortornado. The other vortex couldbecome an anticyclonic tornado if it isstretched by an updraft in the flankingline.

3 . 3 FUJITA’S BAROTRONICMECHANISM

In cases without significantly coolnear the tornado, is there a remotebaroclinic zone aloft (possibly aloft) oris a barotropic tornadogenesismechanism operating? To show that abarotropic mechanism is possible, Ibuilt a ‘bare-bones’ axisymmetric modelo f a m e s o c y c l o n e withoutthermodynamics. The initial conditionis a Beltrami flow that consists of acyclonic updraft (or midlevelmesocyclone) surrounded by adowndraft. If left unperturbed, the flowdecays slowly without changingpattern. When hydrometeors are

introduced through the top boundaryabove the updraft, they fall around theperiphery of the updraft and dragangular momentum down to theground. Some of the angularmomentum that is transporteddownward advects inwards towardsthe axis as in Fujita’s (1975) “recyclingprocess”. Fujita inferred thismechan ism f rom eyewi tnessphotographs that showed the rotatingrain curtain tilting inwards towards atornado (like as in Fig. 2b). In thesimulation, a tornado forms andultimately decays owing to theabsence of a ‘buoyant cork’ aloft. Themechanism is unequivocally barotropicbecause differential drag forcesgenerate azimuthal vorticity, whichcannot be tilted in axisynmmetry flow.In this simulation, the rotating raincurtain instigates the tornado.

The axisymmetric model constrainsthe flow unrealistically by making partof the outflow from the downdraft tofocus on (i.e., converge towards) animposed axis. However, it has theadvantage over a fully 3D model ofbeing much easier to interpret.

3.4 W A L K O ’ S B A R O T R O N I CMECHANISM

Davies-Jones (1982) proposedthat a rear-flank downdraft, althoughdivergent, is vital to the lowering ofvertical vorticity and rotation to thesurface. In westerly shear, thedowndraft would draw down loops ofvorticity, producing a north-southoriented cyclonic-anticyclonic vortexpair near the surface. The mainupdraft lies ahead of and to the left ofthe RFD, in the right position for the airwith cyclonic vorticity to flow along theground and into it. This would result ina near-ground mesocyclone throughstretching. Note that the outflow fromthe downdraft enhances the low-levelconvergence in the updraft.

A numerical experiment by Walko(1993) validates this process for anidealized flow. In a westerly shearflow, Walko used a fixed heat sink andsource to produce a RFD southwest ofan updraft. A tornado-like vortexformed barotropically beneath theupdraft on the left edge of the cold

pool. Although baroclinicallygenerated vorticity was tilted, itcontr ibuted negat ively to thecirculation of the vortex and hence toits genesis.

3.5 PRE-EXISTING VERTICALVORTICITY

Low-level rotation could be theresult of a supercell updraftconcentrating pre-existing verticalvorticity on a front or shear line (Walko1993). Since the horizontal vorticity insheared supercell environments is anorder of magnitude larger than pre-existing vertical vorticity, tilting ofhorizontal vorticity would provide amore abundant source of verticalvorticity. But this mechanism cannotbe dismissed in the case of high-CAPE, low-shear tornadoes (seesection 3.10).

3.6 VORTEX SHEET INSTABIITY

Another barotropic mechanisminvolves shearing instability on thepseudo-cold front. When unstable,the quasi-vertical vortex sheet at thewind discontinuity rolls up intoindividual vortices, which can bestretched by overhead convection(Barcilon and Drazin 1972; Davies-Jones and Kessler 1974; Wakimotoand Wilson 1989; Lee and Wilhelmson1997). In some simulations of tornadicsupercell storms, two or three cyclonicvortices move up the gust front andflanking line to the main updraft wherethey merge into a tornado-like vortex(Adlerman and Droegemeier 2005).But, in other simulations of tornadicstorms, the vortices move down thegust front away from the main updraft,and so play no role in thetornadogenes is . Thus, thismechanism cannot be the only one.

3.7 TWO-CELL MESOCYCLONE

Agee at al. (1976) explained long-lived tornado families by postulatingthe existence of smaller-scale tornadocyclones that revolved around theparent mesocyclone. Tornadocyclones do exist but tornado familiesare due to a succession of

mesocyclones within a single supercell,(Burgess et al. 1982). Rotunno (1986)Brandes (1978) and Wakimoto and Liu(1997) proposed that the cylindricalvortex sheet between cells in a two-celled mesocyclone with a centralocclusion downdraft could becomeunstable. The mesocyclone wouldthen transform into two or moretornado cyclones, as in Ward’ssimulator at large swirl ratio. Thetornado cyclones could producetornadoes through frictional interactionwith the ground,

3.8 TORNADOGENESIS FAILURE

Why do some imminent tornadoesnever develop when strong warningsigns are present? Possibleexplanat ions are a s follows.Markowski et al.’s (2003) axisymmetricmodel with idealized thermodynamicsand precipitation shows that a tornadowill not form if the air at the ground istoo negatively buoyant and cannot belifted. This happens in the real worldwhen the outflow is cold and deep andthe storm-relative inflow is not strongenough to prevent the density currentfrom propagating ahead of theupdraft. Presumably a tornado alsowill not form (at the axis) if strongc e n t r i f u g a l f o r c e s p r e v e n tconvergence.

3.9 NEGATIVE EDDY VISCOSITY

Lilly (1969) attributed the existenceof anticyclonic vorticity just outside thecore of the Dallas tornado to inwardeddy angular momentum flux, anegative eddy viscosity phenomenonthat would contribute to tornadomaintenance. This process requireslarge asymmetries of the flow, possiblyin the form of spiral bands or inwardmoving eddies or secondary vortices.

3.10 WARM-SEASONTORNADOGENESIS

Storms in high-CAPE, low-shearenvironments occasionally producestrong and violent tornadoes. Theseenigmatic storms have rarely beenobserved during field experiments.Since there is little shear in the

environment, they must either availthemselves of pre-existing vorticityalong a frontal or outflow boundary ormanufacture horizontal vorticitybaroclinically and then tilt it towardsthe vertical. Since environmentalinflow winds are light, the cold poolshould spread out ahead of theupdraft and inhibit tornadogenesis.There are two factors that may mitigatethis effect (Davies-Jones et al. 2001).First, the updraft may be so strong thatit creates moderate inflow windsthrough suction. Second, theenvironmental CAPE is so high thatparcels at a considerable distancebehind the leading edge of the coldpool still have moderate CAPE andcan be lifted.

3.11 CYCLIC TORNADOGENESIS

Burgess et al.’s (1982) conceptualmodel of cyclic mesocyclone coreformation, based on Doppler radardata , essent ia l ly f i ts themesocyclogenesis in both simulatedand observed storms. Adlerman et al.(1999) found in simulations of cyclicmesocyclogenesis that the baroclinicmechanism was common to all cycles.The succeeding cycles occurred morerapidly than the first one because coldair from the previous cycle lends to abuoyancy orientation that is favorablefor horizontal baroclinic vorticitygenera t ion . Ad lerman andDroegemeier (2005) simulated cyclictornadogenesis. The model stormproduced six mesocyclones, one ofwhich produced two tornadoes andtwo others one each. Although someof the mesocyclones formed very neartheir predecessors and entrainedvorticity-rich air from them (aspostulated by Davies-Jones 1982),these were nontornadic.

Dowell and Bluestein (2002)analyzed data collected duringVORTEX of a cyclic tornadic supercelland, in contrast to the simulations,found no cold air behind the mostintense and longest-lived tornado.They concluded that ”cyclic tornadoformation may result if the horizontalmotion of the tornadoes repeatedlydoes not match the horizontal motion

of the main storm-scale updraft anddowndraft”.

High–precipitation (HP) supercellsare not prodigious tornado producers,perhaps because their mesocyclonesocclude too rapidly for tornadogenesisto occur.

4. SUMMARY

In this paper I argue that the bigunknown in supercell tornadogenesisis how rotation develops next to theground. There may be a variety ofmodes for this second stage oftornadogenesis. Several plausiblemechanisms are reviewed.

Acknowledgments. This work wassupported in part by NSF grant ATM-0340693. I am indebted to Drs. ErikRasmussen and Jerry Straka forallowing me to base some of thispaper on parts of their 1997unpublished article.

5. REFERENCES

Adlerman, E. J., K. K. Droegemeier,and R. P. Davies-Jones, 1999: Anumerical simulation of cyclicmesocyclogenesis. J. Atmos. Sci.,56, 2045-2069.

Adlerman, E. J., and K. K.Droegemeier, 2005: A numericals i m u l a t i o n o f c y c l i cmesocyclogenesis. (unpublished).

Agee, E. M., J. T. Snow, and P. R.Clare,1976: Multiple vortex featuresin the tornado cyclone and theoccurrence of tornado families.Mon. Wea. Rev., 104, 552-563.

Barcilon, A., and P. Drazin, 1972: Dustdevil formation. Geophys. FluidDyn., 4, 147-159.

Brandes, E. A., 1978: Mesocycloneevolution and tornadogenesis:Some observations. Mon. Wea.Rev., 105, 113-120.

Brooks, E. M.,1949: The tornadocyclone. Weatherwise, 2, 32-33.

Brown J. M., and K. R. Knupp, 1980:The Iowa cyclonic-anticyclonictornado pair and its parentthunderstorm. Mon. Wea. Rev.,108, 1626-1646.

Brown, R. A., L. R. Lemon, and D.W.

Burgess, 1978: Tornado detectionby pulsed Doppler radar. Mon.Wea. Rev., 106, 29-38.

Browning, K. A., 1964: Airflow andprecipitation trajectories within thesevere local storms that travel tothe right of the winds. J. Atmos.Sci., 21, 634-659.

Browning, K. A., and R. J. Donaldson,1963: Airflow and structure of atornadic storm. J. Atmos. Sci., 20,533-545.

Browning, K. A., and C. R. Landry1963: Airflow within a tornadicthunderstorm. Preprints, 1 0 t hWeather Radar Conference,Washington, DC, Amer. Meteor.Soc., 116-122.

Burgess, D. W., V. T. Wood, and R. A.Brown, 1982: Mesocycloneevolution statistics. Preprints, 10th

Conf. On Severe Local Storms,Omaha, NE, Amer. Meteor. Soc.,422-424.

Burgess, D. W., D. C. Dowell, L. J.Wicker, and A. Witt, 2005: Detailedcomparison of observed andmodeled tornadogenesis. Preprints,32nd Conf. On Radar Meteorology,Albuquerque, NM, Amer. Meteor.Soc., CD-ROM, 10R.4.

Chang, C. C., 1976: Preliminary worktoward “ki l l ing the tornado”laboratory par t ia l simulationexperiment. Proc. Symp. OnTornadoes: Assessment ofKnowledge and Implications forMan, Lubbock, TX, Texas TechUniversity, 493-499.

Davies-Jones, R., 1982: observationaland theoret ical aspects oft o r n a d o g e n e s i s . I n t e n s eAtmospheric Vortices, L. Bengtssonand J. Lighthill, Eds., Springer-Verlag, 175-189.

Davies-Jones, R., 1984: Streamwisevorticity: The origin of updraftrotation in supercell storms. J.Atmos. Sci., 41, 2991-3006.

Davies-Jones, R., 2002: Linear andnonlinear propagation of supercellstorms. J. Atmos. Sci., 5 9 , 3178-3205.

Davies-Jones, R., 2004: Growth ofc i rcu la t ion around supercellupdrafts. J. Atmos. Sci., 61, 2863-2876.

Davies-Jones, R., and E. Kessler,

1974; Tornadoes. Weather andClimate Modification, W. N. Hess,Ed., Wiley, 552-595.

Davies-Jones, R., and J. H. Golden,1975: On the relation of electricalactivity to tornadoes. J. Geophys.Res., 80, 1614-1616..

Davies-Jones, R., and H. Brooks,1993: Mesocyclogenesis from atheoretical perspective. T h eTornado: Its Structure, Dynamics,P r e d i c t i o n a n d H a z a r d s .Geophysical Monogr., No. 79,Amer. Geophys. Union, 105-114.

Davies-Jones, R., R. J. Trapp, and H.B. Bluestein, 2001: Tornadoes andtornadic storms. Severe ConvectiveStorms. Meteor. Monogr., No. 50,167-221

Dowell, D. C., and H. B. Bluestein,2002: The 8 June 1995 McLean,Texas, storm. Part II: Cyclic tornadoformation, maintenance, anddissipation. Mon. Wea. Rev., 130,2649-2670.

Eskridge, R. E. and P. Das, 1976:Ef fec ts o f precipitation-drivendowndraft on a rotating wind field:A possible trigger mechanism fortornadoes. J. Atmos. Sci., 33 , 70-84.

Fawbush, J., and R. C. Miller, 1954:The type of air masses in whichNorth American tornadoes form.Bull. Amer. Meteor. Soc., 35, 154-165.

Fiedler, B. H., and R. Rotunno, 1986:A theory for the maximumwindspeeds in tornado-like vortices.J. Atmos. Sci., 33, 2328-2340.

Fujita, T. T., 1959: Detailed analysis ofthe Fargo tornadoes of June 20,1957. U.S. Weather Bureau TechRep. 5, Severe local Storms project,University of Chicago, 29 pp. + figs.

Fujita, T. T., 1975; New evidence fromApr i l 3-4, 1974 tornadoes.Preprints, 9th Conf. On SevereLocal Storms, Norman, OK, Amer.Meteor. Soc., 248-255.

Fujita, T. T., G. S. Forbes, and T. A.Umenhofer, 1976:Close-up view of20 March 1976 tornadoes: Sinkingcloud tops to suction vortices.Weatherwise, 29, 116-131, 145.

Golden, J. H., 1974: The life cycle ofFlorida keys waterspouts. I. J. Appl.Meteor.,13, 676-692.

Hoecker, W. H., Jr., 1960: Wind speedand air flow patterns in the Dallastornado and some resultingimplications. Mon. Wea. Rev., 88,167-180.

Klemp, J. B., and R. B. Wilhelmson,1978: The simulation of three-dimensional convective stormdynamics. J. Atmos. Sci., 35, 1070-1096.

Lee, B. D., and R. B. Wilhelmson,1997: The numerical simulation ofnon-supercell tornadogenesis. PartI: Initiation and evolution of pre-tornadic misocyclone circulationsalong a dry outflow boundary. J.Atmos. Sci., 54, 32-60.

Lemon. L. R., and C. A. Doswell III,1979; Severe thunderstormevo lu t i on and mesocyclones t ruc tu re as re l a ted t otornadogenesis. Mon. Wea. Rev.,107, 1184-1197.

Lilly, D. K.,1969: Tornado dynamics.NCAR Manuscript No. 69-117(available from the National Centerfor Atmospheric Research, Boulder,CO.)

Lilly, D. K.,1982: The developmentand maintenance of rotation inconvective storms. I n t e n s eAtmospheric Vortices, L. Bengtssonand J. Lighthill, Eds., Springer-Verlag, 149-160.

Markowski, P. M., 2002: Hook echoesand rear-flank downdrafts: A review.Mon. Wea. Rev., 130, 852-876.

Markowski, P. M., J. M. Straka, and E.N. Rasmussen, 2002: Direct surfacethermodynamic observations withinthe rear-flank downdrafts ofn o n t o r n a d i c a n d tornadicsupercells. Mon. Wea. Rev ., 130,1692-1721.

Markowski, P. M., J. M. Straka, and E.N . R a s m u s s e n 2 0 0 3 :Tornadogenesis resulting from thetransport of circulation by adowndraft: Idealized numericalsimulations. J. Atmos. Sci., 60, 795-823.

Rasmussen, E. N., and J. M. Straka,1997: Tornadogenesis: A reviewand a new conceptual model.Unpublished manuscript.

Rotunno, R., 1986: Tornadoes andtornadogenesis. M e s o s c a l eMeteorology and Forecasting, P. S.

Ray, Ed., Amer. Meteor. Soc., 414-436.

Rotunno, R., and J. B. Klemp, 1982:The influence of the shear-inducedpressure gradient on thunderstormmotion. Mon. Wea. Rev., 110, 136-151.

Rotunno, R., and J. B. Klemp, 1985:On the rotation and propagation ofsimulated supercell thunderstorms.J. Atmos. Sci., 42, 271-292.

Simpson, J., B. R. Morton, M. C.McCumber, and R. S. Penc, 1986:Observations and mechanisms ofGATE waterspouts. J. Atmos. Sci.,43, 753-783.

Smith, R. K., and L. M. Leslie, 1978:Tornadogenesis. Quart. J. RoyalMeteor. Soc., 104, 189-199.

Stout, G. E., and F. A. Huff, 1953:R a d a r r e c o r d s Illinoistornadogenesis. Bull. Amer. Meteor.Soc., 34, 281-284.

Trapp, R. J., and R. Davies-Jones,1997: Tornadogenesis with andwithout a dynamic pipe effect. J.Atmos. Sci., 54, 113-133.

Vonnegut, B., 1960: Electrical theoryof tornadoes. J. Geophys. Res., 65,203-212.

Wakimoto, R. M., and J. W. Wilson,1989: Non-supercell tornadoes.Mon. Wea. Rev., 117, 1113-1140.

Wakimoto, R. M., and C.-H. Liu, 1998:The Garden City, Kansas stormduring VORTEX-95. Part II: Thewall cloud and tornado. Mon. Wea.Rev., 126, 393-408.

Walko, R. L., 1993: Tornado spin-upbeneath a convective cell: Requiredbasic structure of the near-fieldboundary layer winds. T h eTornado: Its Structure, Dynamics,P r e d i c t i o n a n d H a z a r d s .Geophysical Monogr., No. 79,Amer. Geophys. Union, 89-95

Ward, N. B., 1972: The exploration ofcertain features o f tornadodynamics using a laboratory model.J. Atmos. Sci., 29, 1194-1204.

Wicker, L. J., and R. B. Wilhelmson,1995: Simulation and analysis oftornado development and decaywithin a three-dimensional supercellthunderstorm. J. Atmos. Sci., 52 ,2135-2164.

Wurman, J., and S. Gill, 2000:Finescale radar observations of the

Dimmitt, Texas (2 June 1995)tornado. Mon. Wea. Rev., 128,1113-1140.

Fig. 1. Diagrams showing relationship of nearby rain shaft and cloud features to (a) tornado (basedon Fujita 1959 and from Fujita et al. 1976), and (b) waterspout (based on Golden 1974).

Fig. 2. Schematic plan view of an isolated SR supercell storm near the surface. Thick lineencompasses radar echo (note hook on southwest side). The wave-like gust front structure,resembling a synoptic-scale cyclone, is depicted by classic frontal symbols. Low-level positions ofthe updrafts are finely stippled. Coarse stippling marks location of forward-flank downdraft (FFD)and rear-flank downdraft (RFD). Favored locations for tornadoes are marked by encircled ‘T’s. Themajor cyclonic tornado is most probable near mesocyclone center. A minor tornado or a newpotentially major tornado may occur at the bulge in the RFD gust front. Southern T also marksposition where new mesocyclone core may form as old one occludes. ‘A’ is most probable locationof any anticyclonic tornado that forms. Thin arrows depict storm-relative streamlines (adapted fromLemon and Doswell 1979).

Fig. 3. Composite view of a typical tornado-producing cumulonimbus as seen from a south-easterlydirection. Horizontal scale is compressed. All the features shown cannot be seen simultaneouslyfrom a single point. Shelf cloud may not be present or may be south of wall cloud instead ofnortheast of it. With time a spiral precipitation curtain wraps cyclonically around the wall cloud fromthe northeast (diagram by C. Doswell III).