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Thunderstorm and Tornado Lecture ThunderstormsThunderstorms occur where you have strong lifting of air

parcels:• Near fronts or in advance of large scale upper air troughs• In an unstable environment

– Summer time surface heating destabilizes the atmosphereMore frequently in moist environmentsThunderstorms are a special case of the cumulonimbus cloud

– only becomes a thunderstorm in the presence of lightning

Generally require instability in the atmosphere

Instability re-visited Three stages of Thunderstorm Development

• Cumulus Stage• Mature Stage• Dissipating Stage

Cumulus StageStarts with a warm plume of rising air.

Condensational heating causes air to remain warmer than the surrounding environment.

Water droplets are carried abovefreezing level.

Mature StageThe top of the cloud approaches the tropopause and forms an anvil top.

A downdraft is initiated by frictional drag of increasingly large raindrops.

Entrainment brings in dry air: leads to evaporation of falling particles.

Evaporative cooling leads to negative buoyancy, stronger downdraft.

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Hail• Hail can form in clouds with

– High supercooled liquid water content

– Very strong updrafts

• Hailstones associated with deep and intense cumulonimbus– Typically make 2-3 trips up

through cloud– Layering tells about hailstone

history

The Coffeyville hailstone (5.5”, 1.7 lbs)Note the density layering

Dissipating Stage

The downdraft takes over the entire cloud and cuts off thesupply of warm, humid air from below.

Precipitation decreases/the cloud evaporates.

Such single cell storms are short lived, and cause relatively little damage.

Downdrafts often lead to development of more storms (multicell storms).

What leads to more severe thunderstorms? Severe thunderstorm schematic - above

Low-level winds bring warm, moist air from the southeast

Upper-level winds bring colder, dryer air from the polar regions

Net effect – warm moist air underneath colder, dryer air. This is a good start.

Even more important –wind profile as you go up with height is ‘pushing’ precipitation away from the warm moist inflow – this helps maintain the storm

Severe Thunderstorm/Tornado frequencyWhy do we have so many tornadoes in the US?

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Thunderstorm motion

What causes the storm to rotate?

• Varying theories, but the predominate theory is one of vertical stretching of a surface vortex tube

• A vortex tube is simply a ‘tube’ of fluid that is being rotated horizontally by the low-level wind shear

• The convection this picks this tube up, and stretches it into the vertical – thus causing rotation around the vertical axis

A source of the spin As the vortex tube is stretched, it becomes vertical, creating rotation around a vertical axis

The precipitation may at some point bisect the vortex tube, creating two oppositely rotating updrafts

This is seen as the thunderstorm splitting, creating two new thunderstorms; one we call the right-mover, the other, the left-mover

Vertical stretchingand conservation of angular momentum will intensify the rotation of the dominant storm, which can create a mesocyclone and perhaps even a tornado

Tornado Conditions

• Strong winds aloft (> 25,000 ft)– Often associated with leading edge of trough

• Cool, dry air at mid levels (~500-mb)– Wind typically out of the south west

• Warm humid air at low levels (~700-mb)– Strong gusty winds typically out of the south to south,

south-west• Rotation of the parent storm due to wind shear for

supercell tornadoes

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When do Tornadoes occur?• Tornadoes can occur anytime when you have severe

thunderstorms (including hurricanes).• Most large-scale outbreaks of tornadoes occur in the

spring , but can also occur throughout the summer thunderstorm season

Simulation of a tornadic supercellthunderstorm

• Storm occurred on the 20th of May, 1977 near Del City, Oklahoma

• What you’re about to see is a computer-generated numerical model of this thunderstorm, generated by the environmental conditions that existed on the 20th of May

• Names have been changed to protect the innocent

Gratuitous Tornado Photos

• Sequence of tornado formation from mesocyclone, taken on 30 May 1996 by Greg Thompson (former CSU grad student, now scientist at NCAR)

• Storm occurred near Elba, CO• Don’t try this at home, kids

Fujita Tornado Intensity ScaleF-0 (40-72 mph): Breaks branches off trees; damages sign boards. F-1 (73-112 mph): Peels surface off roofs; mobile homes pushed off

foundations or overturned; moving autos pushed off the roads.F-2 (113-157 mph): Roofs torn off frame houses; mobile homes demolished; large

trees snapped or uprooted.F-3 (158-206 mph): Roof and some walls torn off well-constructed houses; trains

overturned; most trees in forest uprooted. F-4 (207-260 mph): Well-constructed houses leveled; structures with weak foundations

blown off some distance; cars thrown and large missiles generated. F-5 (261-318 mph): Strong frame houses lifted off foundations and carried considerable

distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; steel-reinforced concrete structures badly damaged.

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