Mesoscale Meteorology METR 4433
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Tornado Dynamics Readings Klemp 1987 Dynamics of Tornadic Thunderstorms handout Bluestein Vol II Section 348 Rotunno R 1986 Tornadoes and tornadogenesis In P Ray Editor Mesoscale Meteorology and Forecasting AMS Boston 414436 DavieJones Robert R J Trapp and H B Bluestein 2001 Tornadoes and Tornadic Storms In C A Doswell Editor Severe Convective Storms AMS 167222 Houze Sections 868 De nition tornado n A rotating column of air usually accompanied by a funnelshaped downward extension of a cumulonimbus cloud and having winds whirling destructiver at speeds of up to 300 miles per hour American Heritage Dictionary of the English Language This de nition re ects a fundamental property of the tornado it occurs in association with a thunderstorm Tornado Climatology in the US Tornadoes in the United States are reported most frequently in a band stretching from West and North Texas through Oklahoma central and eastern Kansas and into eastern Nebraska This region is referred to as tornado alley Figure 3 68 Frequency distribution of tornadoes 193071976 in the Uniled States Tornadoes per 2quot latitudelongitude overlapping quadrilateral normalized to nrrl2 area per year from Doswell 1985 adapted from Kelly el alr 1978 Courtesy of the American Meteorological Society A v z Ei f i a 39 A 1 i K a t 4 39 21W 5 Based on dma from 1921 m 1995 me man number or days per century or 2 m in Hue United Slams 4 imcnsin or greater mmndo occilrmncl wihin 40 m Concnnnon 2 nl 2000 Centaur imervul is 5 days vuh minimum level equal u 5 3 The elevated terrain to the west and southwest of the Great Plains and the Gulf of Mexico play important roles in producing soundings having high amounts of CAPE The sloping terrain is partially responsible for the lee trough and lowlevel jet the latter which advects the Gulf marine layer northward while warm dry air caps the moist layer The ubiquitous dryline and fronts approaching from the north and northwest are frequent locations of storm formation Most tornadoes occur during the late afternoon and evening hours in the late spring This suggests that solar heating plays an important role in producing a potentially unstable environment for tornadic storm formation during the afternoon Major outbreaks occur primarily during the spring when strong disturbances in the middle and upper troposphere promote regions of strong lifting and an environment of strong vertical shear and high values of CAPE Tornadic activity begins along the Gulf States in late Winter and migrates northward so that summer activity is highest in the Northern Plains Environments for tornado outbreaks in the United States have been produced as far east as the East Coast Tornado outbreaks in late spring and early summer are often associated with short waves moving through northwesterly ow aloft as a ridge is built up to the west and a trough is ensconced to the east In spring outbreaks usually occur in southwesterly ow aloft downstream from major troughs Relation oftornadoes t0 supercell storms Although not all supercell storms produce tornadoes most of the intense tornadoes are generated by them In a radar study of Oklahoma storms during 19711975 for example Burgess 1976 found that 62 of the 37 storms that exhibited strong stormscale rotation developed tomadoes While none occurred in storms which did not rotate OVERSHOOTING TOP gt mm NR B AL 4 quotquot iv 39 quot 2l3 KM at quoti gtCltummrus COLD AIR FLANKING LINE OWERS I Hm KM 5 39 mwnanFLANK 39 E umquot quotIquot nus r snonr PRECIPITATION CURTAIN I HoaK EEHO PRECIPITAYIONFNEE CLOUD BASE REARFUNK Gust FHONT STORM MOTION Thu cuslwzlnlxllm um Fm 51 Tornado m Oanuhnwk Nunh Dahlm un 2 July 1978 L mun 1mng m m nonh ml nhumgmplw v v v A um1um puwu on r m by m oquot purl un gnmmd rumicmmu Ilsc t f InilL L um Cup iqu phmn Transition of storms into their tornado phase When a storm does move into its tornadic phase significant alteration of the stormscale structure occurs that disrupts the nearly steady configuration These changes include a rapid increase in lowlevel rotation a decrease in updraft intensity a smallscale downdraft forming behind the updraft and a ow at low levels in which coldout ow and warmin ow air spiral around the center of circulation As the downdraft labeled RFD adjacent to the updraft intensifies downdraft out ow progresses cyclonically around the center of rotation marked by the northern encircled T which is the likely location for tornado formation As this out ow pushes into the path of the oncoming moist in ow a new updraft and center of rotation may also develop tornadic intensity denoted by the southern encircled T in Figure 11 At the same time the spreading downdraft out ow cuts off the supply of warm moist air to the original circulation center called occlusion causing the original updraft to weaken Although a supercell storm may persist in a nearly steady configuration for up to several hours the transition to the tornadic phase illustrated in the gure may take place in less than about 10 min STORM MOTION Figur 11 Schematic plan View of a tornadi thundenton39n nu the lurface The thick line encompasses the ra ar echo The barbed line denotes the boundary between the warm in ow and cold out ow and illuatratea the occluding gust front Lowlevel position of tlu updraft is nely atippled while the forward ank FFD and rear ank RFD downdraft are comzly atipplzdl Stormrelative surface ow is shown along with the liker location of tornadoen encircled T s Frum Lemon 3L Doawzll 1979 as adapted by DaviesJone 1985 UNION CITY TORNADO Ws cmo 24 MAY 973 FLOW l550 55 CS39I N IL 0 5m Figure 183 Schematic plan View of a tornadic thunderstorm from the surface observations of Golden and Purcell 1978a showing the location of the surface gust front major low level cloud boundaries and precipitation types and intensities Note the ankingline cloud base relative to surface gust front stream ow and tornado Figure 12 illustrates schematically the ow structure Within a numerically simulated supercell evolving in a unidirectional Wind shear at a time When the lowlevel rotation is intensifying rapidly but prior to the formation of the occluded gust front shown in Figure ll II 4 ll Figure 12 Threedimensional schematic view at a numerically simulated supercell thun dentorm at a stage when the lawlevel rotation is intens39 ying The storm is evolving in westerly environmental wind shear and is viewed from the southeast The cylindrical arrows depict the ow in and around the storm The thin lines show the lowlevel vor tex lines with the sense of rotation indicated by the circular ribbon mews The heavy barbed line marks the boundary of the cold air beneath the stormt Figure 12 10 Barnes 1978 and Lemon and Doswell 1979 have suggested that this transition is initiated by the rear ank downdraft see Figure 11 which forms at midlevels descends to the surface and then intensifies the lowlevel rotation by producing strong shear Barnes or temperature gradients Lemon amp Doswell between this downdraft and the updraft The numerical storm simulations of Klemp amp Rotunno 1983 Rotunno amp Klemp 1985 and observational studies Brandes 1984ab indicate a reverse sequence of events the lowlevel rotation intensifies followed by formation of the rear ank downdraft Klemp amp Rotunno 1983 proposed that the rear ank downdraft that promotes this occlusion is in fact dynamically induced as strong lowlevel rotation lowers the pressure locally and draws down air from above By decomposing the pressure field at a time when the simulated rear ank downdraft was intensifying they demonstrated that the uid shear term was responsible for Virtually the entire adverse vertical pressure gradient near the ground Since the intensifying rotation is largest near the ground a downward directed pressure gradient results which in turn promotes the downdraft The retarding in uence of rotation has been called the vortex valve effect39 Lemon et a1 1975 suggested that as the rotation increases within the storm this effect may be responsible for its collapse In the numerical simulation just described the maximum lowlevel vertical vorticity remained less than one half the maximum at midlevels for over an hour and then in less than 10 min it intensified to double the midlevel maximum Figure 13 Expuded threedimensional perspective viewed from the uoutheut of the lowlevel ow a at the time depicted in Figure 12 and b about 10 min later nfter t has intensi ed Futures are drawn as deacribed in Figure 12 the r ear unk downdra except that the vector direction of vortex lines are indicated by mow along the lines The shaded mow in 0 represents the rotationally induced vertical pressure gradient Figure 1 3 and the striped mow in 6 denote the rear unk downdraft 1 2 Factors responsible for rapid ampli cation of the lowlevel rotation Analyses of the storm simulations demonstrate clearly that the intensification is stimulated by the baroclinic generation of strong horizontal vorticity along the lowlevel boundary of the cold air pool forming beneath the storm Klemp and Rotunno 1983 Rotunno and Klemp 1985 Remember d a B dt 0x This horizontal vorticity is then tilted into the vertical and strongly stretched as the in ow enters the lowlevel updraft To see how this situation arises notice that in the evolving storm precipitation is swept around to the northern side of the cyclonically rotating storm As it falls to the north and northeast of the updraft evaporation cools the low level air With time this cold pool of air advances progressively into the path of the lowlevel in ow to the storm A signi cant portion of the in ow can be approaching along the boundary ofthis cold air pool The horizontal temperature gradients thus baroclinically generate horizontal vorticity that is nearly parallel to the in owing streamlines This process generates horizontal vorticity that is several times the magnitude of the mean shear vorticity and that is more favorably oriented to be tilted into vertical cyclonic vorticity This same mechanism may also be responsible for tornadoes that form occasionally in nonsupercellular storms If a storm encounters a preeXisting cold front or an out ow boundary from another storm strong horizontal vorticity baroclinically generated along that boundary may be swept into the storm and amplified The lowlevel vortex lines depicted in Figures 12 and 13 further illustrate the baroclinic vorticity generation mechanism Since the environmental shear is westerly the horizontal vortex lines embedded in the shear are oriented south north with the sense of rotation as indicated in the undisturbed region southeast of the storm in Figure 12 As these vortex lines penetrate the lowlevel pool of cold air they turn rapidly toward the center of convergence and are swept into the updraft At this stage the lowlevel updraft is located along the boundary between the warm and cold air and it intertwines the warm and cold flow in the rising air As the rear ank downdraft intensifies this baroclinic generation supports the rapid intensification of rotation in the secondary updraft forming farther to the east along the gust front in Figure 13b Theory of rotation near ground DavisJ ones points out that the tilting of horizontal vorticity into the vertical and the subsequent intensification of rotation due to stretching cannot eXplain the intensification of rotation near ground because the tilted vorteX tubes cannot intercept the ground and the vertical stretching is strongest above the ground see DavisJ ones et al 2001 DavisJones points to the importance of the presence of rainy downdraft Three roles are identified of the downdraft l A negatively buoyancy downdraft can impact the ground with considerable force and spring out rapidly 2 A downdraft may transport highmomentum air down to the surface In the presence of midlevel mesocyclone this would transport the angular momentum associated with the midlevel mesocyclone downward and inward This provides vertical vorticity to the near ground levels to be concentrated and intensified Cool downdraft enhances baroclinic boundaries therefore cause more generation of horizontal voriticity that can be tilted into vertical direction U Pages 184186 of DavisJones et al 2001 provides a very good synthesis of our current understanding of the typical processes involved with the intensification of lowlevel rotation within a supercell l4 After the original updraft is cut off from the warm in ow it begins to dissipate while the new updraft continues to strengthen The strong rotation may then cause a new downdraft that spreads out at the surface cuts off the in ow to this updraft and promotes yet another convergence center farther east In Figure 13 Visualize the eastem circulation center in 13b becoming the center shown in 13b and then repeating the cycle Such cyclical redevelopments or tornadogenesis accompanied by successive tornadoes are not uncommon in tornadic storms Burgess et al 1982 See next example Figure 345 Example of cyclical tornado production Rope stage of the second tornado in a series and its wall cloud left and mature stage of the next tornado right and its wall cloud Looking to the northwest from a position 122 km south of Canadian Texas 2236 UTC May 7 1986 photograph by H Bluestein Courtesy of the American Meteorological Society MESOVORTEX CORE EVOLUTION 60 Na 2 Figure 344 Conceptual model of mesocyclone core evolution Lowlevel wind discontinuities thick lines and tornado tracks shaded Inset shows the tracks of the tornado family and the small square is the region expanded in the figure from Burgess et alt 1982 Courtesy of the American Meteorological Society In summary the typical sequence of events of tornadogenesis see good summary in Houze section 86 Stormscale circulation and development creates the forward ank downdraft and gust front The gust front creates intense horizontal vorticity that can be significant stronger the environmental horizontal vorticity this vorticity gets advected into the center of storm and updraft and is tilted into vertical and stretched to intensify the vertical rotation This process is believed to be responsible for the intensification of rotation at the low levels but may not be at the ground the lower end of the mesocyclone while the tilting of environmental vorticity is primarily responsible for the midlevel rotationcyclone The downdraft can also transport significant amount of vertical vorticity in the midlevel mesocyclone down to the surface When this vorticity gets concentrated towards the lowlevel circulation center it intensifies due to angular momentum conservation This is believed to be at least one of the sources of ground level rotation in tornado The increased lowlevel rotation creates pressure minimum at the low levels and downward PGF which promotes the RFD and weakens the original downdraft The advection of cold air from RFD by the rotating circulation at the low levels cuts off warm air supply to the original updraft and causes occlusion of FF and RF gust fronts and the demise of tornado at the occlusion the tip of gust front wedge and New tornadoes often form again at the occlusion point However there are cases where tornado forms and remains inside the cold air behind the gust front Such tornadoes are less connected toaffected by the occlusion process In some cases during Vortex field experiment surface mobile network failed to find significant surface temperature gradient The tornadogenesis problem remains unsolved at least not fully Typical Tornado Life Cycles from Observational Perspective Read Houze section 88 The tornado frequently rst becomes visible as a dust whirl 0n the ground under the wall cloud The condensation funnel soon appears aloft Late in life it may bend at the ground while it stretches in length and narrows in width This final decaying stage in the tornado s life history is called the rope stage owing to the ropelike appearance of the condensation funnel The gust front associated with the RFD may tilt the tornado so that it becomes nearly horizontal The tornado life cycle of 1030 min is common but not exclusive For example tornadoes sometimes occur in the absence of any condensation funnel always look like a rope or never go through a rope stage Some long damage paths are associated with tornadoes that last much longer than 30 mm Most tornadoes in supercells are cyclonic Anticyclonic tornadoes are rare They have been documented however along the RFD gust front away from the mesocyclone 20 Therefore the life cycle can be divided into 0 Organizing stage Visible funnel touching ground intermittently o mature stage tornado reaches its full strength and damage more severe o shrinking the funnel decreases to a thin column 0 decay stage funnel becomes fragments contorted and still destructive 21 m 4 4 I 7 Figure 346 A series of photographs depicting the lite cycle of a tornado a a dust whirl appears near the ground underneath the wall Cloud b a ondensation funnel builds downward from Cloud base c the tornado becomes tilted and stretches out just before it dissipates from Bluestein 1983 This tornado occurred near Cordell Oklahoma on May 221981 photographs copyright H Bluestein Cl courtesy ot the American Meteorological Society 22 UNION cmr OKLAHOMA 24 MAY I973 TORNADO 41 41 AN Dunli n 15 min Wm Pam Langlh 17 km quot 3c Mu wmm 05 km W cloud Figure and associated dams Multivo rtex tornadoes Although tornadoes frequently consist of a single vortex they occasionally exist as two or more smaller quotsuction V0 rticesquot that rotate around the center of the wall cloud Laboratory model and numerical model evidence suggests that multiple vortices are associated with high quotswirl ratioquot that is relatively large azimuthal flow compared to radial flow Suction vortices often have a lifetime of only one revolution around the wall cloud new ones form and dissipate in the identical location relative to the mesocyclone Sometimes multiplevortex tornadoes evolve into singlevortex tornadoes and Vice versa Some suction vortices extend all the way to cloud base others are visible only near the ground underneath a broader rotating cloud base The multiplevortex phenomenon was first postulated by T Fujita on the basis of cycloidal damage swaths The quotsuction vortices create greater maximum wind speed inside the tornadoes vortex because the effect due to the smaller and larger vortices are additive They are observed to be particularly dangerous and damaging 24 mum mu n o lam u v RoiunnI I n Ilaund lamen cm a b Figure 348 a Model of tornado with multiple vortices from Fujita 1981 proposed by Fujita in 1971 b Photograph of a multiplewortex tornado view to the south on Ma 2 1979 in northcentral Oklahoma near Orienta photograph taken by H Bluestein for NSSL Also see the multiplevortex tornado in Vol I Fig 16 a courtesy of the American Meteorological Societyl Nonsupercell tornadoes and Gustnadoes Read Houze section 87 Tornadoes don39t occur exclusively in supercells Nonsupercell tornadoes can occur along a lowlevel convergence line underneath growing cumulus clouds Preexisting vortices along the line get stretched in the vertical by updra of the growing cumulus cloud and intensify to form usually relative Weak tornadoes la lt11 N g 3L 3 3quot V Zc l iii 22 Q39 Figure 349 Schematic model of the life cycle of the nonsupercell tornador The black line is the radar detectable convergence boundary Lowlevel vortices are labeled with letters At the left clouds begin to form over the convergence zone along which there are preexisting vortices In the middle strong updrafts develop beneath the growing cumulus congestus clouds At the right a strong updraft becomes superimposed on one of the preexisting vortices and a tornado forms The tornado dissipates when precipitation falls out of the updraft and the cell collapses from Wakimoto and Wilson 1989 Courtesy of the American Meteorol ogical Society Lee and Wilhemson 1997 present a very nice numerical study on nonsupercell tornadogenesis The following gure is taken from the paper 1440 and 1360 no 4 Threerdimensmual model renderings afth ewimg amenable ofleadmg edge Vm uces 5mm and out ow boundary for 1260 d Luau i571 7 man g kgquot is surfaeerbased room suutheasl Reference Lee B D and R B Wilhelmson 1997 The Numerical Simulation of Nonsupercell Tornadogenesis Part 11 Evolution of a Family of Tornadoes along a Weak Out ow Boundary J Atmos Sell 54 23872415 28 Lowlevel convergence helps concentrate the vertical vorticity The vorteX intensification can be further aided by rotation in the convective updraft Gustnadoes are short lives vortices occurring along preexisting gust fronts coming into eXistence mainly due to horizontal shear instability Waterspouts Vortices forming over water often along out ow boundaries of nearby shower The processes involved are similar to the landbased nonsupercell tornadoes forming along a gust front 29 Flow structure inside tornadoes The following schematic shows ow regions inside a tornado REGION DI REGION I u REGION 11 III REGION II Figure 833 Schematic of new region In a tornado Adapted from Monun I97 and from Davieslunch l9 ll Regionl is in cyclonstrophic balance o la is called the outer ow region 7 region of converging ow Where angular momentum is conserved la is the core 7 in a state of solid body rotation 7 the angular velocity is constant and the core is stable against radial displacement The boundary between Ia and 1b is Where max tangential velocity lies 0 Flow along the core axis can be either up or down and there is little mixing With outside air 30 H is region of turbulent boundary layer where cyclonstrophic is upset by friction Strong radial in ow exists in 11 towards 111 because of net inward force centrifugal force is weaker due to frictional reduction in wind speed 111 is called the corner region where in ow turns into core region lb Region IV is the region of buoyant updraft Wind Speed inside tornadoes Horizontal wind speeds in tornadoes can be as high as 100150 ms There is some theoretical evidence that even stronger upward vertical velocities can occur near center of a tornado near the ground The maximum possible tornadic Wind speed is a function not only of the hydrostaticpressure de cit owing to latentheat release in the updraft which is a function of the CAPE but also to the dynamicpressure de cit which is a function of the character of the wind field itself especially in the surface boundary layer 31 FScale Number F0 F1 F2 F3 F4 F5 Intensity Phrase Gale tornado Moderate tornado S i gnific ant tornado S evere tornado Devastating tornado Incredible tornado Inconceivable tornado Wind Speed 4072 mph 73 l 12 mph 1 13 157 mph 158 206 mph 207 260 mph 261 318 mph 319 379 mph The Fujita Scale of Tornado Intensity Type of Damage Done Some damage to chimneys breaks branches off trees pushes over shallowrooted trees damages sign boards The lower limit is the beginning of hurricane wind speed peels surface off roofs mobile homes 7 pushed off foundations or overturned moving autos pushed off the roads attached garages may be destroyed Considerable damage Roofs torn off frame houses mobile homes demolished boxcars pushed over large trees snapped or uprooted light object missiles generated Roof and some walls torn off well constructed houses trains overturned most trees in fores uprooted Wellconstructed houses leveled structures with weak foundations blown off some distance cars thrown and large missiles generated Strong frame houses lifted off foundations and carried considerable distances to disintegrate automobile sized missiles y through the air in excess of 100 meters trees debarked steel re inforced concrete structures badly damaged These winds are very unlikely The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds Missiles such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 damage If this level is ever achieved evidence for it might only be found in some manner of ground swirl pattern for it may never be identifiable through engineering studies 32 33