Download Thunderstorms Three stages of Thunderstorm Development

Thunderstorm and Tornado Lecture
Thunderstorms
Thunderstorms 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 atmosphere
More frequently in moist environments
Thunderstorms 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 Stage
Starts with a warm
plume of rising air.
Condensational
heating causes air to
remain warmer than
the surrounding
environment.
Water droplets are
carried above
freezing level.
Mature Stage
The 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
Dissipating Stage
• Hail can form in clouds with
– High supercooled liquid water
content
– Very strong updrafts
The downdraft takes over the entire
cloud and cuts off the
supply of warm, humid air from below.
• Hailstones associated with
deep and intense
cumulonimbus
Precipitation decreases/the cloud
evaporates.
– Typically make 2-3 trips up
through cloud
– Layering tells about hailstone
history
Such single cell storms are short lived,
and cause relatively little damage.
The Coffeyville hailstone (5.5”, 1.7 lbs)
Note the density layering
What leads to more severe thunderstorms?
Low-level winds bring
warm, moist air from
the southeast
Downdrafts often lead to development
of more storms (multicell storms).
Severe thunderstorm schematic - above
Severe Thunderstorm/Tornado frequency
Why do we have so many tornadoes in the US?
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
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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 leftmover
Vertical stretching
and 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 supercell
thunderstorm
• 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 Scale
F-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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