Don Paul: What is a supercell, and how do they form? - The Buffalo News
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Don Paul: What is a supercell, and how do they form?

Supercells are long-lived, rotating thunderstorms. They can have life cycles up to and over 60 minutes, and they can reach up to 50,000 feet in the atmosphere; sometimes their tops extend even higher.

All supercells are dangerous thunderstorms, with frequent lightning and usually damaging hail. Nearly all especially violent and large, long-track tornadoes come from parent supercells, but only a minority of supercells produce tornadoes.

Here is a diagram of a typical supercell, though not all supercells have identical features:

(The National Severe Storms Laboratory)

If you've seen a huge thunderstorm in the distance with an anvil-shaped top that seems to stretch for miles to the east, you were probably looking at a supercell.

For supercells to develop, the lower atmosphere needs to be buoyant or, as I say on the air, "bubbly." Warm, humid air is buoyant by nature, but its upward motion can be capped by a dry, stable layer of air aloft. To enhance the upward motion, a greater contrast in moisture content and temperatures between the air aloft and an approaching airmass needs to be present.

In other words, the colder the air is aloft, the more buoyant the warm, humid air becomes relative to that colder air aloft, so the more rapidly it will rise.

(NOAA/National Weather Service)

Rather than go into all the dynamics of a capping layer aloft and what will cause a thunderstorm to form and break through that cap — that's an article all by itself — let's skip ahead to what makes a rotating storm more likely.

Wind shear must be present, and both types (speed and directional wind shear) must be present.

Speed shear is an increase in wind speed with increasing altitude. The greater the increase, the stronger the shear (see graphic at left).

(NOAA/National Weather Service)

Directional shear is a turning of the winds in a clockwise direction with an increase in altitude (graphic at left).

Directional shear can be a bit more difficult to understand. For example, if winds near the surface are blowing from the south or southeast and winds become more southwesterly or westerly with increased altitude, that turning with height in a clockwise direction enhances rotation in a rising column of air. The greater the change in direction, the stronger the directional shear.

Increased wind speed with height — speed shear — is very important as well, and the first 20,000 feet is the critical layer for wind shear to realize its full impact. This rotation produces what is called a mesocyclone, or a smaller diameter cyclone (all low pressure systems are cyclones, because their flow is cyclonic, or counterclockwise, in the northern hemisphere).

The two types of wind shear allow the mesocyclone to become deep and persistent and allows the storm to be tilted. The tilting is important because it produces a separate updraft and downdraft. The downdraft contains contains stable, rain-cooled air. Keeping the updraft with its warm, moist buoyant air separate from the downdraft is what allows the persistence to develop. Sinking rain-cooled air would contaminate and destroy the updraft if the tilting separation didn't develop.

Here is a nice time-lapse video of a supercell near Booker, Texas, from 2013:

Supercells are most common in the central United States, but we get them here as well, less frequently. Some supercells in the west produce lower amounts of precipitation. When we get them here, they are always heavy precipitation events. If nontornadic, they will still likely produce at least very strong straight line winds and typically contain large to very large damaging hail.

(National Weather Service)

At left is a topview of a classic heavy precipitation supercell on radar, with surface features superimposed.

In the high-moisture environment that is typical from the Gulf states all the way north to the Great Lakes and northeast, the features in the first diagram in this article are often difficult to see on the ground because the wall cloud and any possible tornado become rain-wrapped. Of course, that makes storm-chasing  more dangerous, particularly at night.

In cooler weather, miniature supercells become more common, with weaker buoyancy and weaker rotating updrafts. Supercells can evolve from miniature to full-blown supercells, and classification can be sometimes be difficult even for experienced meteorologists. The evolution to full-blown supercells is, to me, just as spectacular when viewed from our high-resolution GOES satellites:

The spectacle of a supercell has inspired many internet and social media hoaxes, such as this fake Superstorm Sandy photo that went viral at the time:

Fake Hurricane Sandy photos spread false rumours online: As Hurricane Sandy tore through New York City and the U.S....

Posted by CBC News on Tuesday, October 30, 2012

Which reminds me: In general, keep in mind many spectacular images you see on social media are photoshopped. This Donald J.'s scourge is Fake Weather.

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