Supercell Thunderstorm Structure and Evolution, Slides of Thermodynamics

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Supercell Thunderstorm

Structure and Evolution

• Most uncommon, but most dangerous storm type
• Produces almost all instances of very large hail and violent (EF4-EF5) tornadoes
• Highly organized due to strong environmental vertical wind shear and dynamic process in storm
• Buoyancy important, but less so than pulse/MCS storms since supercells dynamically controlled
• Long lifecycle; 1-4 hours is quite common; “super” cell was coined based on duration
• Contains a sustained rotating updraft (mesocyclone)

Supercellular Convection

• Mesocyclone
• Hook Echo
• Weak Echo Region (WER)
• Bounded Weak Echo Region (BWER)
• Creation of rotation in updraft
• Dynamic process
  • Front Flank Downdraft (FFD)
  • Rear Flank Downdraft (RFD)
  • Splitting storm; right and left movers
  • Wall cloud; tail cloud; mammatus cloud
  • V-Notch

Important Concepts of a Supercell

500 mb heights (black) and isotachs (green; image) Sfc/850 mb winds; sfc LIs (black); 500 mb isotachs (image)

Typical supercell pattern:

Upper trough/low to NW with jet exit region (even right exit region) over area. LLJ increases in response

pulling axis of instability and strong shear northward. Storms often form in gradient zone of instability

Supercell Environment

Classic Supercell Structure

• Rotating updraft in a supercell; strongest in mid-levels
• Rotation develops as environmental horizontal vorticity tilts and accelerates into vertical. Storm dynamic process important
• Parent circulation that can lead to a tornado (necessary but not sufficient for a tornado; must have strong low-level rotation too)
• Can last for hours. Associated with EF0-EF5 tornadoes (almost all violent tornadoes associated with mesocyclones)

Supercell Characteristics: Mesocyclone

Low-level (top) and mid-level (bottom) cyclonically- rotating updraft (radar to south)

• Weak echo region (WER) is a low-level area of weak/low reflectivity on radar as strong updraft suspends and prevents precipitation from falling in this area
• Coincident/near inflow and updraft zone in and near hook echo
• Bounded weak echo region (BWER) is a mid-level weak/low reflectivity (cavity) aloft as intense updraft suspends and prevents precipitation from forming and falling in this area. Sometimes called an echo free vault
• Area of low reflectivity is “bounded” on all sides by higher reflectivity and signifies significant overhang/tilt of reflectivity
• BWER location is coincident with core of meso- cyclone (rotating updraft)

Supercell Characteristics: WER/BWER

Mid-level BWER

Low-level WER

WER and BWER coincident with mesocyclone

• Forms from water loading (dragging cool air downward), melting, and evaporation which creates a cold pool and outflow boundary northeast of hook echo and tornado location
• Outflow boundary is on leading and southern side of FFD. Storm-relative inflow advances into boundary from the south or east as boundary becomes quasi-stationary
• Horizontal vorticity occurs along boundary as inflow air rides over top of outflow from north. Vorticity can be pulled west along boundary, converge into hook area, and be tilted into updraft (U) to augment low- and mid-level mesocyclone. However, RFD seems to be most associated with tornadogenesis

Supercell Characteristics:

Front Flank Downdraft

• Complex thermodynamically and dynamically induced region of subsiding air that develops on rear side of main updraft of supercell
• Formation can occur due to thermodynamics – evaporative cooling and increase in negative buoyancy, or dynamics – blocking flow and vertical pressure gradient forces that lead to descent
• Rotating updraft acts as an obstruction (barrier) to mid-upper level flow. As high pressure builds on upwind end of storm, air begins to sink forming RFD on back side of supercell. Drier air entrained from behind storm can increase negative buoyancy. When RFD hits ground, it spreads out along RFD gust front and can cause significant straight-line wind damage
• RFD can arrive at surface warmer than surroundings due to adiabatic warming in absence of precipitation and evaporative cooling, or arrive cooler due to evaporative cooling. Properties of RFD have an impact on whether a tornado will form or not
• RFD gust front is leading edge of downdraft and arcs back into updraft/hook area where it can enhance low-level convergence and low-level spin up of vorticity to enhance tornadogenesis
• Just south of tornado location, significant straight-line wind damage can occur along RFD gust front

Supercell Characteristics: Rear Flank Downdraft

• Wall cloud : A lowering, rotating cloud base where humid rain-cooled air from downdraft is drawn into updraft. An indication of low-level mesocyclone from which tornado drops
• Tail cloud : Tail of wall cloud that points toward FFD. Feeder air coming into low-level mesocyclone
• Mammatus cloud : Forms on underside of anvil. Composed of pockets of cooled air as particles fall and evaporate. Does not produce severe weather, but may be associated with a strong/severe storm
• V-Notch : Slot of lower reflectivity values that can appear on downwind edge of reflectivity field surrounded by higher values in strong supercells. Due to blocking flow aloft around updraft core

Supercell Characteristics: Types of Clouds/V-Notch

Development and Enhancement of Updraft Rotation

• Convergence : Consider a sink. Initially there is some weak rotation, but as water converges toward drain, speed of rotation increases. This is conservation of angular momentum – radius of rotation decreases so rotational velocity must increase. Convergence into updraft can result in rotation development or increase in existing rotation (ice skater effect)
• Low-level boundary : Air travelling along a frontal zone or outflow boundary will develop rotation about a horizontal axis (horizontal vorticity) as warm air rides over top and cool air flows underneath horizontal boundary
• Convergence and low-level boundary are smaller- scale sources of horizontal vorticity than that created by environmental wind shear. These smaller sources augment the necessary larger scale shear in enhancing and focusing horizontal vorticity generation

• Tilting : In order to create vertical rotation (vorticity) from horizontal rotation (vorticity), must tilt horizontal rotation into vertical. In thunderstorms, this tilting is achieved by the updraft

Development and Enhancement of Updraft Rotation

Dynamical Process in Supercells

• Environmental shear leads to a rotating updraft as horizontal vorticity is titled and stretched into updraft. This causes development of a dynamic process in storm which affects evolution, strength, longevity, and motion of supercell
• Rotation about a vertical axis must be balanced by a pressure gradient force pointed toward center of rotation, i.e., low pressure is dynamically produced at center of rotation with PGF directed toward it. Low is strongest in mid-levels of storm where rotation/updraft is strongest
• Low pressure leads to an even stronger updraft in mid-levels as supercell can actually “suck up” air into low. This in turn causes even more rotation (due to vertical stretching) as updraft speed increases with height, which in turn can feed back and cause an even stronger mid-level low. Results in enhanced steady-state updraft. Deeper shear usually means more efficient dynamic process
• Dynamic forces are as or more important than buoyancy forces in supporting updraft strength and rotation. Dynamic process can cause splitting supercells and a right movement of storm compared to mean wind. Process is also important to tornado development

Hodograph Type Vs. Supercell Characteristics

Straight hodograph favors storm split with equally strong left and right movers. Right (left) mover has an cyclonically (anticylonically) rotating updraft. The right (left) mover moves to right (left) of the mean wind at a speed slower (faster) than the mean wind.

S-R inflow flank on north side of storm

S-R inflow flank on south side of storm