The supercell: why it rotates and how it is built

The definition, and what it excludes

A supercellA thunderstorm with a single rotating updraft that can persist for hours. The storm type that produces most strong tornadoes, very large hail and the most damaging winds. is a thunderstorm with a persistent, deep, rotating updraftThe column of rising air inside a storm. Everything a thunderstorm does, from hail to tornadoes, is downstream of how strong and how organised this is., called a mesocycloneThe rotating updraft inside a supercell, typically a few miles across. Radar can see it aloft; whether anything is rotating at the ground is a separate question.. 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 baseThe smooth, flat, dark cloud base on the rear flank of a storm, where warm air is flowing in and rising. It is named for the air going up, not for being dry: hail and large drops may still fall from it., and the rear-flank downdraftRear-Flank DowndraftA dry downdraft wrapping around the back of a supercell updraft. One of the two features that distinguish a supercell in the field, and it can produce damaging outflow winds entirely on its own.. 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. ShearChange of wind with height, measured as the difference between the wind at the top and bottom of a layer. It is not needed to make a thunderstorm and it decides almost everything about what kind you get. 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 vorticitySpin in the air, about any axis. Wind shear fills the atmosphere with spin about a horizontal axis, doing nothing visible until an updraft stands it upright. 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 vorticityHorizontal spin that points along the direction air is flowing into the storm, rather than across it. It is what gets carried straight into the updraft and makes the updraft itself rotate. 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 hodographA plot of the wind at each height, joined up, so the shape of the wind profile is visible at a glance. A straight one means storms that split evenly; a clockwise curve favours the right-moving storm. 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 centre, 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 CAPEConvective Available Potential EnergyHow much energy is available to an updraft, in joules per kilogram. Bigger numbers mean a stronger updraft is possible, and a number on its own says nothing about what kind of storm you get. arithmetic suggests, and why supercells occur in high-shear, low-CAPE environments where parcelAn imaginary blob of air, followed upward to see whether it stays warmer than its surroundings. Almost every number on this page comes from comparing one of these against the air around it. 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.

SplittingA storm dividing into two, one turning clockwise and moving right of the mean wind, the other turning anticlockwise and moving left. A clockwise-curving wind profile favours the right-mover and starves the left one., 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 downdraftForward-Flank DowndraftThe main precipitation area of a supercell, ahead of and to the left of the updraft. Heavy rain and most of the hail fall here.. 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 downdraftAir descending through and out of a storm, driven by the weight of the precipitation and by rain evaporating into drier air below. It is what produces the gust front and the damaging winds. 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 slotA brightening slot of sinking, drying air cutting into the cloud base around the back of a wall cloud. It marks the rear-flank downdraft wrapping in, and it often appears shortly before a tornado. you may have heard about is this feature seen from outside.

Occlusion

What happens when the rear-flank gust frontThe leading edge of cool air rushing out of a storm, ahead of the rain. It is what makes the wind pick up sharply just before a storm arrives. 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 cumulusA tall, hard-edged, cauliflower-textured convective tower with no anvil. Capable of heavy showers, and not yet a thunderstorm. 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 organised.

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 echoA hook-shaped appendage on the reflectivity image, wrapped around the back of a supercell. One of the oldest radar clues that a storm is rotating. is precipitation being wrapped around the back of the mesocyclone by its own rotation. The inflow notchA concave bite out of the reflectivity on the inflow side of a storm, where air being drawn in is eroding the precipitation. A radar clue that the storm has a strong, organised updraft. is the concavity where inflow is eroding the reflectivityHow much of the radar pulse bounced back from whatever the beam was passing through on that tilt. It is the familiar green-to-red precipitation picture, and it describes what is up in the beam, not what is reaching the ground. on the storm's right rear. The BWERBounded Weak Echo RegionA hole in the radar reflectivity inside a storm, with precipitation above it. It is the updraft itself, rising so fast that nothing has had time to form in it yet., 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 artefacts 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.