Transcript 11_ThunderstormsTornadoes

NAS 125: Meteorology
Thunderstorms and Tornadoes
Tornado outbreak, part 1
• On 3 May 1999, more than 70 tornadoes struck an
area stretching from northern Texas to south-central
Kansas.
– Twenty-six tornadoes struck in or near Oklahoma City.
• The most intense tornado, F5 on the Fujita scale, tore
through Moore and Bridge Creek, claiming 28 lives.
– An F4 tornado killed six at Haysville, Kansas, a few hours
later.
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Tornado outbreak, part 2
• Rawinsonde data from Norman, Okla., indicated a
shallow layer of warm, humid air near the surface
with dewpoints near 21 °C.
• The warm, humid layer was capped by a temperature
inversion at about 1,500 m with much drier air above.
– The inversion layer kept convection currents in check, but
as the temperature and humidity contrast grew during the
day, deep convection began, leading to the developmen tof
severe thunderstorms.
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Tornado outbreak, part 3
• The rawinsonde indicated a vertical wind shear, with
southerly winds at the surface to westerly or westsouthwest winds aloft.
– The wind shear contributed to the development of severe
weather.
• The arrival of a jet streak in the afternoon lifted the
capping inversion, allowing the development of
massive supercell thunderstorms.
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Thunderstorms, part 1
• Thunderstorms are violent convective storms
accompanied by thunder and lightning; they are
usually localized and short-lived.
– Vertical air motion, considerable humidity, and instability
combine to create towering cumulonimbus clouds, so
thunderstorms are always associated with this combination.
– They frequently occur in conjunction with other kinds of
storms (hurricanes, tornadoes, fronts [especially cold
fronts]) in midlatitude cyclones, and orographic lifting.
– They are associated with other mechanisms that can trigger
unstable uplift.
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Thunderstorms, part 2
• Thunderstorms (continued):
– Thunderstorms are virtually unknown poleward of 60˚ of
latitude.
– Thunderstorms often occur in clusters.
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Life cycle, part 1
• Thunderstorms have a life cycle that consists of there
stages: cumulus stage, mature stage, and dissipating
stage.
• In the cumulus stage, updrafts prevail and clouds
grow.
– Cumulus clouds quickly surge to altitudes of more than
8,000 m.
– Cumulus clouds also merge horizontally, such that the
lateral size may reach 15 km.
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Life cycle, part 2
• Cumulus stage (continued):
– Two types of convection drive storm development: free
confection and forced convection.
• Free convection is caused by intense heating of the Earth’s surface,
but is not usually adequate to generate a thunderstorm.
• Forced convection is driven by frontal or orographic lifting or by
converging surface winds; most thunderstorms result from forced
convection.
– Latent heat is released by condensation of water; this extra
heat adds to the buoyancy of saturated air parcels, which
rise and cool at the moist adiabatic rate.
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Life cycle, part 3
• Cumulus stage (continued):
– Two types of cumuliform clouds form:
• Clouds with significant vertical development that resemble a huge
cauliflower are called cumulus congestus.
• Cumulus congestus clouds that continue to develop vertically such
that precipitation, lightning, and thunder are produced, are called
thunderstorm clouds – cumulonimbus clouds.
– No precipitation falls during the cumulus stage because
updrafts are strong enough to keep water droplets and ice
crystals suspended in the upper reaches of the cloud.
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Life cycle, part 4
• The mature stage begins when precipitation reaches
the ground.
– It is the most active and largest stage, with cloud tops
reaching 18,000 m; they often have an anvil shape where
high-altitude winds distort cloud shape.
– The weight of water droplets and ice crystals becomes too
great for them to remain suspended by the force of the
updrafts.
– As the water droplets and ice crystals fall, they drag
adjacent air with them to form a downdraft, which
descends along the updrafts building the storm.
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Life cycle, part 5
• Mature stage (continued):
– Unsaturated air at the edge of the cloud is dragged into the
cloud in a process called entrainment.
• Entrainment causes some of the water mixes with the saturated air,
causing water droplets and ice crystals to vaporize, which in turn
weakens the updrafts and strengthens the downdrafts.
– The downdraft exits the base of the cloud, hits the ground
and spreads out across the surface in a strong, cool wind
resembling a mini cold front called a gust front.
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Life cycle, part 6
• The mature stage begins when precipitation reaches
the ground.
– Unusual clouds are associated with the gust front.
• Roll clouds are tube-shaped clouds, not connected to the
cumulonimbus cloud, that form behind the gust front; they appear
to roll horizontally.
• Shelf clouds – or arcus clouds – are low, elongated, wedge-shaped
clouds that are attached to the cumulonimbus cloud; they form at
the edge of the gust front.
• In the dissipating state, downdrafts dominate, and
turbulence ceases.
• Adiabatic warming in the downdrafts contributes to the drying up
of moisture that fuels the storm.
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Classification, part 1
• Single-celled thunderstorms are weak, isolated
systems that form anywhere in warm, humid air
masses.
– They usually form long some boundary within the air mass.
• Multi-cellular thunderstorms consist of more than one
storm cell, each of which may be at different stages of
the thunderstorm life cycle.
– A squall line is an elongated cluster of thunderstorm cells.
• They typically form along frontal boundaries, thus individual
storms are called frontal thunderstorms.
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Classification, part 2
• A mesoscale convective complex (MCC) is a nearly
circular cluster of interacting thunderstorm cells.
– MCCs may be 1000 times larger than individual storms.
– MCCs occur mostly in the warm season, and are not
associated with a front.
• Supercell thunderstorms are long, lived, large, intense
storms, composed of a single cell, with an updraft
that may exceed 240 km per hour.
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Distribution, part 1
• Thunderstorm formations require three conditions:
– Humid air in the low- to mid-troposphere;
– Atmospheric instability;
– A source of uplift.
• Most form in maritime tropical (mT) air masses.
• Most form when mT air is lifted:
– Along fronts;
– Up mountain slopes;
– Via horizontal convergence of surface winds.
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Distribution, part 2
• Thunderstorm day: A statistic used to express
thunderstorm frequency; defined as a day in which
thunder is heard.
– Thunderstorm days/month
– Thunderstorm days/year
• Globally, thunderstorm days are highest:
– In humid tropics;
– Along ITCZ.
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Distribution, part 3
• In North America, thunderstorm days are highest:
– In Central Florida;
– Along Colorado Front Range.
• Thunderstorms are common along coastal areas.
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Severe thunderstorms, part 1
• Severe thunderstorms are (by convention)
accompanied by locally damaging winds, frequent
lightning, or large hail.
– According to the National Weather Service, Severe
thunderstorms must meet one or more of the following
criteria:
• Surface winds stronger than 93 km/hr (58 mph);
• Tornadoes or funnel clouds;
• Hailstones 1.9 cm (0.75 in) or more in diameter.
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Severe thunderstorms, part 2
• The taller the storm, the greater the potential for it
becoming severe.
– Vertical wind shear (change in horizontal wind speed or
direction) is key to the development of severe
thunderstorms.
• Most severe storms develop over the Great Plains.
– They are usually associated with synoptic-scale cyclones.
– They usually form ahead of a squall line within the warm
sector, ahead of and parallel to a fast-moving cold front.
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Severe thunderstorms, part 3
• Feature of typical Southern Plains storm outbreaks:
– Mature cyclone centered over Western Kansas;
– Well defined cold front trailing into West Texas;
– Interaction between the westerly polar front jet aloft and a
southerly low-level jet blowing up from Gulf of Mexico
create vertical wind shear and instability; effect is enhanced
if subtropical jet is near
– Jet streak induced horizontal divergence and convergence
aloft, which in turn triggers cyclogenesis under the leftfront quadrant of the streak and subsidence under rightfront quadrant;
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Severe thunderstorms, part 4
• Southern Plains storm outbreaks (continued):
– Compressional warming in area of subsidence leads to
formation of a capping inversion;
– Persistent capping inversion intensifies differences between
air masses;
– Updrafts eventually break through capping inversion,
allowing explosive convection;
– Updrafts may penetrate the stratosphere, thus creating a
severe thunderstorm.
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Severe thunderstorms, part 5
• Southern Plains storm outbreaks (continued):
– Dry lines typically form between eastern edge of cold front
and western edge of mT air;
– Mammatus clouds, formed from blobs of cold, cloudy air,
often appear at the base of mature cumulonimbus clouds.
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Thunderstorm hazards
•
•
•
•
Lightning
Downbursts
Flash floods
Hail
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Lightning, part 1
• There are more than 8.5 million lightning bolts daily
in world.
• Lightning most frequently occurs as exchanges
between adjacent clouds or between the upper and
lower portions of the same cloud; it also occurs as an
electrical connection of ionized air from cloud to
ground.
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Lightning, part 2
• The sequence that leads to lightning discharge is
known, but the mechanism for electrification is not.
– Large cumulonimbus cloud experiences a separation of
electrical charges:
• Positively charged particles are mostly high in cloud, while
negatively charged particles tend to concentrate in base.
– A growing negative charge in base attracts a growing
positive charge on Earth’s surface immediately below
cloud.
• An insulating barrier lies between cloud base and surface.
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Lightning, part 3
• Sequence (continued):
– Growing negative charge (continued):
• The contrast between cloud base and surface builds to tens of
million volts and overcomes the insulating barrier.
• A finger of negative current flicks down from cloud and meets a
positively charge darting upward from the ground, causing
lightning.
• The cause is unknown; theories differ.
– Most popular theory is that updrafts carry positively
charged particles to the top of the clouds, while falling ice
pellets gather negative charges and transport them
downward.
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Lightning, part 4
• Cause (continued):
– Scientists are investigating if cosmic rays from deep space
may be involved.
• Thunder is an instantaneous expansion of air caused
by the abrupt heating that lightning bolt produces.
This expansion creates a shock wave that becomes a
sound wave.
– One can time the distance that lightning is away because of
the different rates thunder and lightning travel at (speed of
sound vs. speed of light).
• A three-second interval equals about a kilometer.
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Downbursts
• Downbursts are exceptionally strong downdrafts that
diverge horizontally after they reach the ground.
• Macrobursts cause destruction over distances of more
than 4 km with winds that may exceed 210 km/hr.
– They last up to 30 minutes.
• Microbursts are smaller and of shorter duration, but
wind speeds may reach 270 km/hr.
– Microbursts are hazardous to aircraft.
• Derechos are straight-line downburst winds that may
span hundreds of kilometers.
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Flash floods, part 1
• Flash floods are short-term, localized, difficult to
predict rises in stream level, usually forming in
response to high precipitation amounts in watershed
areas.
– Flash floods typically rise and fall within 6 hours of rain
event.
– Often form under stationary or slow-moving
thunderstorms.
– Typically form under conditions of weak vertical wind
shear.
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Flash floods, part 2
• Big Thompson Canyon flood, 31 July 1976:
– Canyon located in Colorado Front Range below Rocky
Mountain National Park.
– Persistent thunderstorm over headwaters of Big Thompson
River dropped 25 to 30 cm of rain; about 20 cm fell in 2
hours.
– Floodwaters poured into narrow canyon, raising the river as
much as 6 m.
– 139 killed; 418 houses and 197 motor vehicles destroyed;
$35.5 million in property damage.
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Hail, part 1
• Hail is precipitation in the form of balls or chunks of
ice more than 5 mm in diameter.
• Hail almost always falls from tall cumulonimbus
clouds with strong updrafts and abundant supercooled
water droplets.
• Largest hailstone recorded near Coffeyville, Kansas,
on 3 September 1970: 766 g, 44.5 cm.
• The form when ice pellets fall, get picked in updrafts,
and grown as additional layers of water freeze on the
surface.
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Hail, part 2
• Hail falls to the ground when updrafts are no longer
able to keep it suspended.
• Hail may cover ground in long, narrow stripes called
hailstreaks.
– Hailstreaks may reach widths of 2 km and lengths of 10
km.
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Tornadoes, part 1
• Tornadoes are localized, typically cyclonic, lowpressure cell surrounded by a whirling cylinder of
wind spinning so violently that partial vacuum
develops within the funnel.
– They have the most extreme pressure gradients known (as
much as 100-millibar difference between tornado center
and air immediately outside funnel).
– Extreme pressure differences produce winds of
extraordinary speed.
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Tornadoes, part 2
• Tornadoes (continued):
– How fast are the winds? No one knows, because tornadoes
blow to bits the anemometer (and instrument for measuring
wind speed). Maximum estimates range from 320 to 800
kilometers.
– The exact mechanism of formation is unknown.
– They usually develop in warm, moist, unstable air
associated with midlatitude cyclones.
– They most often develop along a squall line that preceded a
rapidly advance cold front, or along the cold front.
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Tornadoes, part 3
• Tornadoes (continued):
– The spring and early summer are favorable for
development because there are considerable air-mass
contrasts present and greater instability in the lower
troposphere at those times.
– They most frequently occur in midafternoon, at the time of
maximum heating.
– Less than 1 percent of thunderstorms produce tornadoes.
– Waterspouts occur over ocean and lakes; they have less
pressure gradient, gentler winds, and reduced destructive
capability.
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Tornadoes, part 4
• Tornadoes (continued):
– More than 90 percent of all reported tornadoes occur in the
United States. This reflects optimum environmental
conditions:
• The relatively flat terrain of the central and southeastern U.S.
provides uninhibited interaction of Canadian cP and Gulf mT air
masses.
• Tornado alley runs from east Texas and Texas Panhandle north into
southeastern South Dakota.
• The United States can expect between 700 and 1100 tornadoes,
with the record being 1389 in 1998.
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Tornadoes, part 5
• The deadliest tornado was the Tri-State tornado of 18
March 1925, which claimed 695 lives.
– The tornado lasted 3.5 hours, moved at a speed of 118
km/hr, and wreaked havoc along a path 350 km in length.
• Tornado hazards:
–
–
–
–
Extremely high winds
Strong updrafts
Subsidiary vortices
Abrupt drops in air pressure
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Tornadoes, part 6
• Tornado intensity is measured using the Fujita scale.
– The Fujita scale is a six-point scale rating tornado strength
and damage to structures.
– Weak tornadoes
• F0: estimated wind speed 65-118 km/hr; 40-73 mph
• F1: estimated wind speed 119-181 km/hr; 74-112 mph
– Strong tornadoes
• F2: estimated wind speed 182-253 km/hr; 113-157 mph
• F3: estimated wind speed 254-332 km/hr; 158-206 mph
– Violent tornadoes
• F4: estimated wind speed 333-419 km/hr; 207-260 mph
• F5: estimated wind speed 420-513 km/hr; 261-318 mph
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Tornadoes, part 7
• Tornado intensity (continued):
– Tornado intensity can fluctuate over the course of its life
cycle.
• The Wichita-Andover (Kansas) tornado of 26 April 1991 began as
an F1, worked up to an F5 (as it struck a mobile home park in
Andover, Kansas, where it destroyed 241 mobile homes), then
decreased to an F1 before it ended.
• Most tornadoes are spawned from supercell
thunderstorms.
– Supercell thunderstorms have updrafts with speeds
exceeding 240 km/hr.
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Tornadoes, part 8
• Tornadoes and supercell thunderstorms (continued):
– Tornadoes develop in the interaction between supercell
updrafts and horizontal winds.
• Horizontal winds exhibit strong vertical shear in terms of both
speed and direction.
– Wind strengthens and turns clockwise with altitude.
• Horizontal cylinder of rolling air is tilted upright by the updraft;
and vertical shear within cloud intensified rotation – typically in the
counterclockwise direction.
– The spinning thunderstorm cloud is called a mesocyclone.
• Wall clouds typically accompany mesocyclones, forming in the
zone of the strongest uplift.
• Wall clouds do not normally form tornadoes.
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Tornadoes, part 9
• Tornadoes and supercell thunderstorms (continued):
– Mesocyclones (continued):
• Tornadic wall clouds are generally long-lived, with strong surface
winds blowing in toward the center.
• Mesocyclone circulation is most intense at about 6,100 km.
• Updrafts strengthen as more air converges toward the base of the
supercell.
• The mesocyclone narrows (with an accompanying increase in wind
speed) and builds toward the ground.
• As interior air pressures drop, water vapor condenses to form a
funnel cloud.
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Tornadoes, part 9
• Tornadoes and supercell thunderstorms (continued):
– Mesocyclones (continued):
• A strong downdraft develops near the back of the supercell, and the
funnel follows the downdraft to the ground.
• Squall lines and mesoscale clusters also spawn very
productive thunderstorms.
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Monitoring tornadic storms, part 1
• In the early 1980s, storm chasers tried to place
instruments in the path of tornadoes, without much
success.
– Remote platforms, such as portable Doppler radar, are
much more effective tools.
• Radar signs
– Hook echoes, which are detected by reflectivity radars,
often indicate possible tornadoes.
– A tornado vortex signature on Doppler radar signfies the
presence of a possible tornado.
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