what is the name assigned to the rotating updraft of a supercell thunderstorm?

Structure and Dynamics of Supercell Thunderstorms

Supercell thunderstorms are perhaps the virtually fierce of all thunderstorm types, and are capable of producing damaging winds, large hail, and weak-to-violent tornadoes. They are nigh mutual during the spring beyond the central United States when moderate-to-potent atmospheric wind fields, vertical wind shear (change in current of air direction and/or speed with height), and instability are nowadays. The degree and vertical distribution of moisture, instability, lift, and wind fields have a profound influence on convective storm type, including supercells, multicells (including squall lines and bow echoes), ordinary/pulse storms, or a combination of storm types. In one case thunderstorms grade, minor/convective-scale interactions also influence tempest type and evolution. There are variations of supercells, including "classic," "miniature," "high precipitation (HP)," and "low atmospheric precipitation (LP)" storms. In general, however, the supercell form of storms is defined by a persistent rotating updraft (i.east., mesocyclone) which promotes tempest organization, maintenance, and severity. More than information concerning environmental weather condition and the structure of classic and HP supercells is given below. WSR-88D Doppler radar imagery showing the development of some supercell events beyond Kentucky and southward-key Indiana are available.

Dissimilar Thunderstorm Types

Ordinary: Brusque-lived (30-60 minutes) storm; generally is non-severe but pulse severe storm is possible; storm moves with hateful wind; little or no vertical wind shear/weak winds aloft in environment; chaotic hodograph (Fig. 1) ; typical in summertime; buoyancy process of import.

Multicell : Group of cells in different stages of evolution; tin can be severe or not-severe; frequently movement with the mean wind; show discreet propagation with new prison cell growth on the unstable inflow flank; weak-to-potent environmental air current shear/winds aloft; commonly a "straight-line" (unidirectional) hodograph indicating speed and/or directional shear conducive for MCSs, squall lines, and bow echoes (Fig. one) ; gust forepart process important (rest between convectively-induced depression-level cold pool strength and depth under the heavy rain and the ambience depression-level wind shear) to trigger new cells.

Supercell: Large severe storm occurring in a significant vertically-sheared surround; contains quasi-steady, strongly rotating updraft (mesocyclone); usually moves to the right (perhaps left) of the hateful current of air; can evolve from a not-supercell storm; moderate-to-strong vertical speed and directional wind shear in the 0-half dozen km layer; usually a "curved" hodograph in the lowest 0-3 km and a straight line in a higher place (Fig. i) ; dynamic process important resulting in a steady-state tempest (see beneath).

Fig. i: Hodograph showing vertical wind shear for ordinary, multicell, and supercell thunderstorms. Dots along hodograph line correspond end bespeak of arrowheads of vectors (not shown) originating from (0,0) point (x/y-axis intersection) that reveal wind speed and direction at the indicated height (in km). For example, on supercell hodograph, winds at ane km altitude are from the southeast, stronger from the south at 2 km, with winds increasing in speed (longer vectors from (0,0) signal to each dot) and veering to southwest at higher altitudes. The longer the hodograph, the greater the vertical wind shear. Not simply length, but shape of hodograph is of import. For example, direct-line hodograph for multicells and curved hodograph for supercells both betoken speed and directional shear. However, curved hodograph indicates presence of a low-level wind maximum (jet) which increases storm-relative flow into storm and potential for supercell development. Supercells can evolve from straight-line hodographs as well but are more common with curved hodographs. In contrast, only weak shear is shown for ordinary cells, although if high instability is present, then a severe pulse storm can occur, with hail and/or brief damaging winds.

Dynamics of Supercells; Environmental Characteristics

• Supercells are not divers by their depth or book. They tin can be large or small, high-topped or low-topped, and can occur anywhere, including the Ohio Valley. They are most common in the central Usa. While supercells are not as mutual equally other convective types, they often produce violent weather.
  
• The interaction between updrafts and the vertically-sheared environment strongly controls the degree of organization and severity of convection. Supercells and tornadoes are associated with moderate-to-stiff vertical wind shear (and helicity) and moderate-to-high Greatcoat (instability) (Fig. 2) . Rough total wind shear threshold for supercells is at least 40 kts (20 g/south) in the 0-6 km layer. To determine this threshold, await at the length of the hodograph (which includes speed and directional shear) in this layer, and "lay out" the hodograph along the x-axis to see if it exceeds xl kts. If so, supercells are quite possible; if not, supercells can still occur given some shear and high CAPE values.

Fig. 2: Scatter plot of potent and violent supercell tornadoes with respect to 0-2 km helicity (y-centrality) values in m2/s2 and CAPE (x-axis) in J/kg. Major tornado outbreaks typically associated with moderate-to-high Greatcoat (1500-3500 J/kg) AND helicity (150-450 m2/s2). Isolated to scattered tornadoes associated with low Cape and high helicity (upper left part of plot). Scattered tornadoes associated with high Greatcoat and depression helicity (lower correct part of plot).

• Strong 0-six km shear (long hodograph) causes loftier helicity/high potential for supercell and mesocyclone (rotating updraft) development, but Not necessarily tornadoes. Mesocyclone strength also is dependent on buoyancy. Tornado development is dependent on dynamical structure in the tempest. Generally, a supercell/mesocyclone occurring in an environment with significant low-level (0-ii km) curvature in the hodograph (indicating the presence of a low-level jet) is conducive to tornado development.
  
• Vertical wind shear causes the evolution of dynamic processes in the storm which affect the evolution, strength, longevity, and motion of the supercell. Explanation: one) Environmental shear results in a rotating updraft as horizontal vorticity is titled vertically into the updraft. 2) The diagnostic pressure level equation states that rotation about a vertical centrality (rotating updraft) must exist balanced past a pressure gradient forcefulness pointed toward the eye of rotation causing lowered pressure level in the middle-levels of a tempest where the rotation/updraft is strongest. 3) This vertical pressure perturbation leads to an even stronger updraft into the heart-levels, which in plough causes even more rotation (due to vertical stretching) as the updraft speed increases with tiptop, which in turn can feed back and cause an even stronger center-level pressure perturbation. The deeper the environmental wind shear, the more efficient the dynamic process should exist.
  
• This dynamic process results in an enhanced steady-state updraft; dynamic forces are as important or even more so than buoyancy forces in supporting updraft strength and rotation. The supercell actually tin "suck up" air and continue well into night despite the loss of heating, weaker instability, and dissipation of ordinary cells. The dynamic process also causes loftier (depression) force per unit area on the upshear/downdraft (downshear/updraft) side of the storm, which results in tempest tilt and a correct movement of the tempest compared to the hateful air current.
  
• Dynamic forces eventually can cause the primary updraft to split into 2 split updrafts, i.e., each supercell tin can develop both cyclonic (on the correct flank) and anticyclonic rotation (on the left flank) in the heart-levels. This can cause the storm to split into 2 separate cells, i moving right and the other left of the hateful wind. For a right (left) moving storm, the cyclonic rotation is within the updraft (downdraft) and the anticyclonic rotation is within the downdraft (updraft) with the tightest reflectivity slope on the south/east (north) side of the storm coincident with the updraft. A archetype example of a splitting tempest occurred on May 28, 1996 over south-fundamental Indiana. The correct mover evolved into a classic supercell that produced several tornadoes.
  
• Consider hodographs in evaluating the potential for tempest splitting and which cell volition dominate. A directly-line hodograph (unidirectional shear) is more conducive for storm splitting than a curved hodograph in the lowest few kilometers. Bold a split occurs, a hodograph with significant curvature (clockwise turning to the shear vectors) in the low-levels promotes a stiff correct and weak left moving supercell.
  
• The storm relative inflow direction and magnitude are very important. This determines which storm(s) will remain strong/astringent. For example, if 2 cells are aligned due north-s, both can remain potent despite footing-relative southerly arrival if the storm-relative arrival has an easterly component. Strong arrival speeds promote a stronger updraft forcefulness and more rotation. Strong middle-level storm-relative menstruum into the supercell also seems to correlate with a strong mesocyclone capable of tornadogenesis in the low-levels.

Tornado Mechanisms in Supercells

• Nearly all supercells produce some sort of astringent weather condition (large hail or dissentious winds) but but thirty percent or less produce tornadoes. Thus, one must attempt to differentiate a tornadic supercell from a non-tornadic one.
  
• In the environment, strong 0-6 km shear (long hodograph) and ample buoyancy is needed to generate a significant storm mesocyclone. Then, the supercell/mesocyclone occurring in an surround with significant depression-level (0-2 km) "curvature" in the hodograph seems to be conducive to tornado development.
  
• Yet, tornado development is dependent on the dynamical structure in the storm. There must be a strong updraft and source of vertical vorticity for strong mesocyclone and tornado evolution. Environmental horizontal vorticity acquired past ambient vertical wind shear is critical to form a rotating updraft (mesocyclone). The ecology vorticity may be crosswise or streamwise.
  
• Notwithstanding, tornado formation appears to be related to a storm scale process: the vertical tilting of baroclinically-induced horizontal vorticity . This process occurs along an outflow boundary associated with the forward flank downdraft (Fig. three) . Forth this boundary in or near the low-level claw region on radar, a small-scale-calibration circulation occurs every bit warm ecology air rises on the warm side of the boundary while cold air sinks and undercuts on the common cold side, which generates streamwise horizontal vorticity along the boundary (notation the sense of rotation in Fig. three ). This vorticity then is tilted and apace accelerated vertically into the storm updraft as the middle-level mesocyclone dynamically "sucks up" low-level air, resulting in a more prominent depression-level mesocyclone and likely tornadogenesis. This procedure sometimes tin exist seen visually as a tail cloud moving into the claw area from the east, and may be visible on the WSR-88D reflectivity/velocity as an outflow purlieus or fine line echo.
  
• The streamwise vorticity associated with this low-level process normally is Non axiomatic in the environment (i.e., identifiable in a sounding). It is generated through the storm's interaction with the environment. Thus, marginal ambience current of air shear may however back up supercells and fifty-fifty tornadoes given the presence of mesoscale/storm-scale interactions, which tin can profoundly increase the local wind shear, helicity, and therefore mesocyclone strength and tornado potential. Once shear is enhanced and maintained locally in the hook/weak repeat region, a series of mesocyclones and tornadoes are possible in the vorticity-rich local environment.

Fig. three: Thunderstorm-scale schematic of a supercell-ecology interaction, that tin consequence in the creation of vertical tilted baroclinically-induced horizontal vorticity. This can lead to enhancement of the low-level mesocyclone and possibly tornadogenesis. Text in schematic briefly describes this process.

Reflectivity Signatures Associated With Supercells

• For "classic" supercells, a depression-level pendant or hook often is nowadays on the correct rear side of the storm (Fig. 4) . Within the hook is a weak echo region (WER) signifying the location of a strong rotating updraft (mesocyclone). The hook is formed through the interaction of the frontward flank and rear flank downdrafts with the updraft area. The maximum reflectivity (heavy rain and large hail) cadre usually is located simply north and/or east of the WER. In the downwind (weaker) portion of the depression-level reflectivity design, a "V-notch" or "enhanced V" signature may be evident, indicating blocking flow aloft causing some environmental air to motility around the storm. An bodily supercell thunderstorm, as viewed by the KLVX WSR-88D Doppler radar over north-central Kentucky, is shown in Fig. 4a . A vertical cross-section of a typical classic supercell (along line C-D in Fig. 4 ) is shown in Fig. 5 .

Fig. 4: Plan view of a typical archetype supercell as viewed in radar reflectivity information. Bottom (top) picture represents low-level (upper-level) reflectivity. A weak echo region (WER) is noted in depression-levels, a bounded weak echo region aloft (BWER), with echo overhang higher up the BWER overtop the low-level WER (i.east., storm tilt). A large expanse of light atmospheric precipitation and deject extends well downwind in the upper anvil portion of the storm.

Fig. 4a: Low-level WSR-88D Doppler radar image of an bodily supercell thunderstorm over north-central Kentucky on May 28, 1996. Night carmine colour represents very heavy pelting and hail. A claw echo is seen on the southwest flank of the storm, coincident with a tornado on the footing at this time.

Fig. v: Vertical cantankerous-section of a typical classic supercell along line C-D in Fig. 4. The x-axis (y-axis) are horizontal (vertical) distance in km. Reflectivity values in dBZ are shown inside the storm. The low-level WER, elevated BWER, repeat overhang showing storm tilt, and downwind anvil debris clouds clearly are evident.

• To a higher place the WER, a Bounded Weak Echo Region (BWER) (i.e., donut hole) may exist present at college acme angles (Figs. 4 and 5) , indicating overhang in the storm and the location of a strongly rotating updraft. A persistent BWER is associated with a pregnant mesocyclone.

• High reflectivity often caps off the BWER above it. The superlative role of the storm (echo top) is shifted over the low-level reflectivity gradient or over the WER with possible significant anvil droppings extending downwind (Figs. 4 and 5) .
  
• Heavy Precipitation (HP) supercells : These showroom similar features as classic supercells. However, the low-levels oftentimes show a broad high reflectivity pendent or Front Flank Notch (FFN) (i.e., kidney bean shape) on the leading edge of the tempest, indicating the location of the WER and rotating updraft (Fig. 6) . Mesocyclones for HP storms may be embedded in heavy pelting. HP supercells are not every bit isolated as "classic" storms, and often may be embedded within squall lines and travel along boundaries. HP supercells occur in environments with rich low-level moisture and moderate-to-strong current of air shear, and are a threat for tornadoes, large hail, damaging winds, and flash flooding. An example of an HP storm embedded inside a squall line occurred over south-central Kentucky on May 18, 1995.

Fig. 6: Plan view of radar base reflectivity in the low-levels (bottom picture) and middle-levels (superlative picture) of a typical HP supercell. A WER is nowadays on the forward flank of the storm in depression-levels with repeat tilt aloft overtop the low-level WER. Highest reflectivity values in low-levels can resemble a kidney bean shape.

• Classic and HP supercells sometimes can evolve into a bow echo as the rear flank downdraft or a rear inflow jet causes the storm to accelerate outward, resulting in a bowing storm with damaging directly-line winds (Fig. seven) .

Fig. seven: Sequence of basic plan view reflectivity schematics showing how a supercell ("A") can transition into a bow echo storm ("D") due to development of a rear inflow jet and/or intense rear flank downdraft from the HP storm.

• Almost severe events occur near the updraft/downdraft interface on the correct rear (archetype) or front flank (HP) office of a storm. The strongest tornadoes often occur as the BWER begins to collapse.

Mesocyclone Signatures Associated With Supercells

Mesocyclone: A small-scale-calibration solid body rotation closely associated with a convective updraft. Truthful supercell mesocyclones (ones associated with tornadoes, e.g., Fig. 8 , must meet or exceed established thresholds for shear, vertical extent, and persistence. For supercells, the following gauge criteria seem to well for Kentucky:

• Shear: Distance between the maximum entering and maximum outbound less than equal to 5 nm. Rotational velocity Vr = [(max outbound velocity + max inbound velocity) ÷ 2]: Severe thunderstorm alert: greater than well-nigh twenty kts (15 kts) if the storm is less (greater) than 100 nm from the radar site. Tornado warning: greater than about 40 kts (30-35 kts) if the tempest is less (greater) than 100 nm away. These values are simply approximate, so detailed consideration of storm structure, trends, and trained spotter observations are very of import as well.
  
• Vertical extent: Shear extends at least viii,000-ten,000 ft in the vertical (only shear may NOT extend this loftier upwards for low-top storms or distant supercells that nevertheless can cause severe weather condition).
  
• Persistence: Coherent rotational signature persists at to the lowest degree ii volume scans.

Fig. 8: WSR-88D storm-relative reflectivity image of a tornado-producing mesocyclone nearly the town of Mt. Washington in north-central Kentucky (southeast of Louisville) on May 28, 1996. Red (dark-green) colors denote radial winds directed abroad from (toward) the radar located to the west (left) of the area shown. Thus, a tight, cyclonic (counterclockwise) circulation is shown most Mt. Washington. Simply northeast of the boondocks, the lighter shaded greenish colour represents storm-relative catamenia directed into the mesocyclone, which appears to aid in tornado evolution and maintenance. The mesocyclone is at the aforementioned time and position equally the claw echo in the reflectivity image in Fig. 4a above.

• Circumspection: Severe atmospheric condition and non-supercell tornadoes associated with squall lines and bow echoes may withal occur, despite these supercell criteria not existence met.

• Tornadoes are most likely during the menses of maximum mesocyclone cadre strength. Mesocyclones with the smallest diameters and highest rotational velocities (Vr) extending over a deep layer represent the greatest tornadic threat.
  
• Less than 30 of mesocyclones that see supercell criteria produce tornadoes, although about produce some sort of severe weather.
  
• Mature arcadian mesocyclone rotational structure in WSR-88D storm-relative velocity information: Depression-levels: Commonly run across cyclonic convergence (bold the storm is close enough to the RDA). Middle-levels: pure cyclonic rotation (maximum inbound/outbound are on neighboring radials at the same altitude from the radar site). Upper-levels: cyclonic divergence. Storm Top: pure departure (maximum inbound/outbound are forth same radial).
  
• Some mesocyclones produce a single rotational core; others produce a series of cores in a periodic fashion. The outset mesocyclone cadre has a relatively long organizing and mature stage. Yet, subsequent mesocyclones (if whatever) can develop and mature much faster in the vorticity-rich convective environs, resulting in a series of mesocyclones and a family of tornadoes.
  
• Multiple mesocyclones can evolve as the rear flank downdraft (mini-cold front) accelerates outward and catches up with the forwards flank downdraft (mini-warm/stationary front) resulting in a convective-scale triple signal occlusion at the mesocyclone/updraft center. Thus, the original mesocyclone weakens while a new 1 can spin up apace at the triple betoken (Fig. ix) .

Fig. 9: Conceptual model of mesocyclone core evolution. The "L" shows the mesocyclone location with convective-scale cold and warm/stationary fronts extending from the meso. The cold front is the leading edge of the rear flank downdraft, while the warm/stationary front represents the southern border of the forward flank downdraft from rain-cooled air n of the purlieus. The bold lines are tornado tracks. The insert shows tornado family tracks and the minor square in the insert is the region expanded in the schematic.

• The Tornado Vortex Signature (TVS) is a stiff, gate-to-gate (adjacent radials on the WSR-88D Doppler radar) shear associated with tornadic scale rotation that meets or exceeds established criteria for shear, vertical extent, and persistence. Identification of a low-level TVS suggests that a tornado may be occurring or may before long develop assuming a favorable reflectivity pattern. However, fifty-fifty without an identified TVS, mesocyclone identification and reflectivity and tempest-relative velocity structure is invaluable in assessing the need for a tornado warning.

Guidance for Warning Decisions for Supercells:

• Always consider every bit much data as possible, including one) pre-storm surround; 2) radar reflectivity structure and trends; 3) base and storm-relative velocity, including mesocyclone structure and trends; 4) other pertinent WSR-88D products; v) Four-Dimensional Storm Jail cell Investigator (FSI); 6) storm-calibration interactions (within storm environment) causing jail cell mergers, enhanced shear and rotation, etc.; and 7) sentry reports.
  
• Do not base of operations a warning decision solely upon mesocyclone force. Consider the information mentioned above. Withal, every bit a rule-of-thumb, if a supercell is identified, including one with just a "weak" mesocyclone, a severe thunderstorm warning should be issued; if a "moderate" or "potent" mesocyclone is indicated and is supported by favorable reflectivity structure and the presence of enhanced low-level storm-relative inflow, a tornado warning should be strongly considered.
  
• Know conceptual models of storm structure thoroughly. For example, even if velocity information are difficult to translate (e.g., range folding, improper dealiasing, weak mesocyclone at far ranges), just reflectivity structure or spotter reports suggest a severe or tornadic tempest, outcome the appropriate alarm at once.

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Source: https://www.weather.gov/lmk/supercell/dynamics

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