A supercell is not a big thunderstorm. It is a thunderstorm that has solved the problem in lesson 2, and rotation is how it solves it.

In this lesson you will

  • Define a supercell: a storm with a persistent, deep rotating updraft.
  • Explain the origin of midlevel rotation: shear creates horizontal vorticity, the updraft tilts it, streamwise vorticity yields a rotating updraft.
  • Explain storm splitting and why clockwise-curved hodographs favour the right-mover.
  • Describe the dynamic pressure effect: rotation lowers pressure aloft and enhances the updraft beyond buoyancy alone.
  • Label the conceptual model: updraft, FFD, RFD, gust fronts, occlusion, flanking line, rain-free base and precipitation distribution.

The definition, and what it excludes

A supercell is a thunderstorm with a persistent, deep, rotating updraft, called a mesocyclone. Every word in that is doing work. Not a rotating storm, a rotating updraft. Not briefly, persistently, on the order of half an hour and up. Not rotation in a shallow layer, rotation through a deep one.

Size is not in the definition and neither is intensity. A modest storm with a persistent rotating updraft is a supercell; a huge, violent, non-rotating storm is not. Lesson 7 covers miniature supercells that are supercells in every dynamical sense at a fraction of the scale.

The Field Guide gives two traits as the ones that distinguish a supercell in the field: persistent rotation at the rain-free base, and the rear-flank downdraft. Both come back in modules 3 and 4 as things you look at.

Where the rotation comes from

Not from the earth's rotation, which is far too weak at this scale, and not from the storm spinning something up on its own. It is already in the environment before the storm exists, lying on its side.

  1. Shear makes horizontal spin. Wind increasing with height means air at one level is moving faster than the air just below it. Put an imaginary paddlewheel in that flow, side-on, and it turns. The atmosphere is full of horizontal vorticity wherever there is shear, doing nothing visible.
  2. The updraft tilts it upright. A developing updraft lifts those horizontal vortex tubes and turns them toward the vertical, and horizontal spin becomes vertical spin at mid-levels.
  3. Streamwise vorticity makes the updraft itself rotate. If the horizontal vorticity points along the storm-relative inflow rather than across it, the spin is carried directly into the updraft and the updraft as a whole rotates. Davies-Jones 1984 is where that argument is made properly, and it is the reason a curved hodograph matters and not just a long one.

That whole sequence is about mid-level rotation, which is what makes the storm a supercell. Low-level rotation, and what turns it into a tornado, is a separate problem with a separate mechanism, and it is module 4's.

Rotation makes the updraft stronger

Here is the part that turns a curiosity into a storm mode. A rotating column of air has low pressure at its center, the same way any vortex does. Rotation aloft therefore lowers the pressure aloft, which creates an upward-directed pressure gradient force underneath it, which pulls air up from below independently of buoyancy.

That is why a supercell can be far stronger than lesson 1's CAPE arithmetic suggests, and why supercells occur in high-shear, low-CAPE environments where parcel theory says there is barely enough energy for a thunderstorm at all. The dynamic contribution is not a correction term. On some cool-season days in a moist climate it is most of the updraft.

Teaching point
Rotation is not a symptom of a strong updraft. It is one of the causes of it. That feedback, updraft to tilting to rotation to lower pressure to a stronger updraft, is what lets a supercell run for six hours while the ordinary cells around it live for thirty minutes.

Splitting, and why right is favoured

On a straight hodograph the physics is symmetric. Precipitation falls into the middle of the updraft and splits it in two, and the dynamic pressure effect works equally on both halves. You get a cyclonically rotating right-mover and an anticyclonically rotating left-mover, diverging as mirror images.

Add clockwise curvature to the hodograph and the symmetry breaks. The dynamic forcing now reinforces the right-mover and opposes the left-mover, so the right-mover dominates and the left one weakens or never gets going. Both movers deviate from the mean wind rather than following it, which is why a supercell so often appears to be going the wrong way compared with the other storms on the radar. Bunkers et al. 2000 turned this into the operational method for predicting right-mover motion; the concept is what this course needs, and the hodograph arithmetic is not on the exam.

The parts of the storm

The conceptual model below is Lemon and Doswell's, and it is the same structure the Field Guide's simplified supercell diagram is drawn from, so the vocabulary lines up with what you were shown in class. Directions assume a storm moving roughly northeast in the northern hemisphere.

Updraft
The rotating core, on the rear right flank of the storm. Under it sits the rain-free base, and this is where the tornado, if there is one, will be.
Rain-free base
The flat, dark, precipitation-free cloud base beneath the updraft. The place to look for persistent rotation, and the reason a supercell often looks least threatening from the direction that matters.
FFD
Forward-flank downdraft. The main precipitation region, ahead of and to the left of the updraft. Heavy rain and most of the hail fall here.
RFD
Rear-flank downdraft. A dry, often warm downdraft wrapping around the back of the updraft. It is one of the two defining supercell features, it can produce outflow winds over 100 mph on its own, and it is central to tornadogenesis in module 4.
Forward-flank gust front
The boundary between the FFD outflow and the inflow, running roughly along the direction of storm motion.
Rear-flank gust front
The leading edge of the RFD outflow, curling around the updraft from behind. The clear slot you may have heard about is this feature seen from outside.
Occlusion
What happens when the rear-flank gust front wraps the whole way around the updraft and cuts it off from its inflow. The mesocyclone occludes, the storm often produces a new one to the southeast, and the cycle repeats.
Flanking line
The line of towering cumulus trailing back southwest from the updraft, stepping up in height toward it. New updrafts feeding the storm, and a good visual cue that the storm is still organized.
Where are you standing
The Field Guide frames its diagram as a position question, and it is the right way to learn it. In the forward flank you get heavy rain and hail and see nothing. To the southeast, in the inflow, you have warm wind at your back blowing toward the storm and the best view of the rain-free base there is. Behind the storm you are in the RFD outflow, dry and gusty, looking at the back of it. Your report is only interpretable if the office knows which of those three places you were in, which is lesson 1 of module 1 arriving from a different direction.

Hail, in one paragraph

A strong updraft supports large hailstones against gravity, and a tilted, rotating updraft gives them a long residence time in the growth region rather than dropping them straight through it. Those two together are why supercells produce the very large hail and other storm modes mostly do not. Sizing, reporting and the reference-object problem are module 5.

What this looks like on radar

Three names, so the words are not new when you meet them properly. The hook echo is precipitation being wrapped around the back of the mesocyclone by its own rotation. The inflow notch is the concavity where inflow is eroding the reflectivity on the storm's right rear. The BWER, or bounded weak echo region, is the updraft itself: a column so strong that precipitation has not had time to form inside it, appearing as a hole in the reflectivity with an echo overhang above.

That is the whole radar content of this module. Module 9 does radar interrogation properly, including the artifacts that fool experienced people. Here, radar is used for one thing only: identifying storm mode.

Why this module spends 35 minutes on one storm type

Because of the share of the damage. In Brotzge et al.'s 2003 to 2004 sample, supercells produced 97% of tornado fatalities, 96% of injuries and 92% of tornado damage. Lesson 8 will show that QLCSs produce a substantial share of tornadoes by count. They do not produce that share of the harm.

Check yourself

Why can a supercell updraft be stronger than the CAPE alone would allow?

  1. Rotation lowers the pressure aloft, and the resulting upward pressure gradient force adds to buoyancy
  2. The mesocyclone reduces entrainment to zero, so the parcel keeps all of its buoyancy
  3. Rotation adds centrifugal force, which pushes air upward through the core
  4. It cannot. CAPE sets a hard ceiling that no storm exceeds
Hint

What is true of the pressure at the center of any rotating column of air?

Answer

Rotation lowers the pressure aloft, and the resulting upward pressure gradient force adds to buoyancy

A rotating column has low pressure at its center, so rotation aloft creates an upward-directed pressure gradient force beneath it that lifts air independently of how buoyant it is. That dynamic contribution is why supercells exist at all in the low-CAPE, high-shear environments lesson 7 covers, where parcel theory alone would predict almost nothing.

Sources

  1. NWS Weather Spotter's Field Guide (YPA-201154, June 2011), section 2, the supercell and the supercell diagram
  2. Lemon & Doswell 1979, Mon. Wea. Rev. 107(9), 1184–1197: severe thunderstorm evolution and mesocyclone structure as related to tornadogenesis
  3. Rotunno & Klemp 1982, Mon. Wea. Rev. 110, 136–151: the influence of the shear-induced pressure gradient on thunderstorm motion
  4. Davies-Jones 1984, J. Atmos. Sci. 41(20), 2991–3006: streamwise vorticity, the origin of updraft rotation in supercell storms
  5. Bunkers et al. 2000, Wea. Forecasting 15, 61–79: predicting supercell motion using a new hodograph technique
  6. Brotzge et al. 2013, Wea. Forecasting 28(5), 1261–1276: tornado warning performance by storm type
  7. Reference only, not redistributable: Markowski & Richardson 2010, Mesoscale Meteorology in Midlatitudes, Wiley