Download Severe Storms

Severe Storms
Readings: A&B Ch. 11 (308-326, 330-344)
Topics
1. Severe weather
d. Severe T-storms
i. Squall lines
ii.Mesoscale Convective
Complexes
iii.Supercells
a. Scale
b. Motion
2. Thunderstorms
a. Thunder & Lightning
b. Factors in T-storm
development
c. Air-mass T-storms
i. Cumulus stage
ii.Mature stage
iii.Dissipating stage
3. Tornadoes
a. Development
b. Location and Timing
c. Prediction
d. Fujita Intensity Scale
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Severe Storms
•
Thunderstorms, tornadoes, and hurricanes
ƒ Each have things in common and differences
Scale
Diameter
Mid-lat. Cyclone 1600 km
Hurricane
Tornado *
Thunderstorm
600 km
0.25 km
A few km
Rarely up to 50 km
Smaller & more
intense than a
mid-latitude
cyclone
* Tornadoes are too small to appear on a weather map
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Severe Storms
Motion
Mid-latitude
Cyclone
Hurricane
Tornado
Thunderstorm
Inward, spiral
Inward, spiral
Inward, spiral
Strong upward & downward motion
Very variable & gusty winds
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Thunderstorms
• Associated with cumulonimbus clouds
ƒ Generate heavy rainfall, thunder & lightning,
occasional hail
• Unlike hurricanes and tornadoes
ƒ Different air motion – updrafts, downdrafts,
variable winds
ƒ Can form:
• “On their own”: air mass thunderstorms
• In conjunction with cyclones (Low pressure)
ƒ Mid-latitude cyclones: t-storms frequently
form along the cold front
ƒ Hurricanes: generate widespread
thunderstorm activity
ƒ On rare occasions: tornado may descend from
thunderstorm (cumulonimbus tower)
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Thunder and Lightning
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Lightning:
ƒ 80% of lightning is cloud-to-cloud lightning
ƒ 20% of lightning is cloud-to-ground
•
During the formation of large cumulonimbus
cloud separation of charge occurs
ƒ Mechanism not known in detail
ƒ But, lightning only occurs in clouds which extend
above the freezing level and which form
precipitation
• Charge separation: related to Bergeron
process
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Thunder and Lightning
ƒ Part of the cloud has an excessive negative
charge and another part excessive positive
charge
ƒ Lightning: attempt to
equalize these
electrical differences
ƒ Air is a poor
conductor, therefore
the electrical
potential (charge
differences) must be
very high before
lightning will occur
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Thunder and Lightning
ƒ Stepped leader: advancing, branching shaft of
negative charges
ƒ Approaches the ground
ƒ When stepped leader
approached the
ground, a spark of
positive charges also
surges upward from
the ground
ƒ Stepped leader and
upward surge from
ground create a path
for flow of current
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Thunder and Lightning
ƒ Negative current flows downward from cloud
ƒ Return stroke: positive current flow upward from
ground
ƒ A single flash of lightning is a sequence of
strokes and return strokes
• Usually 2-3 strokes; sometimes up to 20
strokes
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Thunder and Lightning
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Thunder
ƒ Produced by lightning
ƒ Electrical discharge of lightning heats the air and
causes it to expand explosively
• Expansion produces sound waves → hear
thunder
ƒ Thunder travels at speed of sound (~300 m/s),
while the light seen from a flash of lightning
travels at the speed of light (3x108 m/s)
• Lag between them
• Count the seconds between lightning and
thunder; divide by 5, to determine how many
miles the storm is from you
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Thunderstorm Development
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Thunder and lightning are associated with
cumulonimbus clouds
ƒ Generate heavy rainfall, thunder, lightning,
occasional hail
•
Factors in development of cumulonimbus
cloud and thunderstorm:
ƒ Lifting: surface heating, presence of a front, or
other mechanism
ƒ Instability: tendency of warm air to lift
ƒ Moisture: release of latent heat provides energy
and buoyancy for additional lift
ƒ Wind shear: causes t-storm cells to tilt → more
severe storms
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Air Mass Thunderstorms
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Air mass (or isolated thunderstorms)
Occur within a warm humid (mT) air mass
Most frequent type of t-storms
Limited lifetime (~1-2 hours)
Very localized – limited in size
Most likely to occur:
• Spring & summer – when air is warmed
sufficiently from below
• Mid-afternoon when surface temperatures are
highest
• Sometimes after sunset as growth of immature
t-storm cells re-stimulated by cloud top cooling
→ Heavy rain, small hail, high winds, downbursts,
possibly weak tornadoes
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Air Mass Thunderstorm Development
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Three stages
ƒ Cumulus (developing) stage
ƒ Mature stage
ƒ Dissipating stage
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Cumulus (Developing) Stage
• Need continuous warm moist air
• Unstable air caused to rise by local heating or
other mechanism
• Develop cumulonimbus clouds 10-20 km tall
• Dominated by updrafts (up to speeds of 160
km/h)
• Once cloud passes
freezing level –
Bergeron process
ƒ Usually produces
precipitation within an
hour
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Mature Stage
•
Precipitation starts to fall
ƒ Falling precipitation causes drag on the air →
downdrafts
ƒ Some falling precipitation evaporates which is a
cooling process
•
Influx of cool dry air surrounding the cloud:
entrainment
ƒ Intensifies the downdraft (because the air is cool
and dense)
Downdraft – cooling process
• When downdraft leaves the bottom of cloud:
ƒ Precipitation falls to the surface
ƒ Cold air spreads laterally
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Mature Stage
→ T-storm cell composed of updrafts and
•
downdrafts, existing side-by-side
When cloud grows to top of unstable region
(often at the tropopause):
ƒ Updrafts spread laterally → Anvil top of cirrus
clouds
•
Most active stage of the
T-storm
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Gusty winds
Heavy precipitation
Sometimes hail
Intense thunder and
lightning
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Dissipating Stage
•
Cooling downdrafts cut off supply of warm
moist air which caused T-storm
• Downdrafts dominate
→ End of T/storm: without supply of moisture
the cloud soon evaporates
• Single cumulonimbus
cell has a life of 1-2 h
ƒ However, as storm
moves to fresh supplies
of warm water laden air,
new cells may be
generated
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Severe Thunderstorms
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Severe t-storms develop by a similar process
BUT, strong wind shear → tilt of updraft
ƒ Updraft and downdraft are not coincident
ƒ Downdraft does not cut off moisture supply to
updraft
ƒ Updrafts and downdrafts reinforce each other and
intensify the storm
→ Longer life: often 12 hours, sometimes several
days
→ Larger area
→ Severe storms have: stronger winds, heavier
rain, larger hail, more downbursts, more
likelihood of tornadoes
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Severe Thunderstorms
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Frequently form along or ahead of a cold
front in the wave cyclone
ƒ Cold air advances into a region of warm air
ƒ Warm air less dense - displaced upwards
•
Tend to appear in groups or clusters of
storm cells, 10-1000 km across
ƒ Squall line: linear band of t-storm cells
ƒ Mesoscale convective complex: oval or circular
cluster of t-storm cells
•
Can also occur with a single very large, very
intense updraft zone: supercell storm
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Severe Thunderstorms
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Squall line:
ƒ A group of thunderstorm cells, arranged in a
linear band
ƒ Approximately 500 km long, less than 100 km
wide
ƒ Typically last ~10 hours
ƒ Common in
southern U.S.
during spring /
summer
ƒ Usually form
immediately ahead
of a cold front
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Severe Thunderstorms
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Squall line:
ƒ Wind shear pushes updrafts ahead of downdrafts
ƒ Air spreads laterally ahead of downdraft: gust
front
• Causes uplift of warm air into cloud
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Severe Thunderstorms
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Mesoscale Convective Complex (MCC)
ƒ A group of thunderstorm cells, arranged in an
oval or circular cluster
ƒ Several 100 km across, typically last ~12 hours
ƒ Self-propagating group of storms:
• Downdraft from one cell converges with
surface flow
→ Updraft
→ Formation of a new cell
• New cell draws in moisture from downdraft of
old cell
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Severe Thunderstorms
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Mesoscale Convective Complex (MCC)
ƒ Frequently form “flanking line”, moving to NE
ƒ As storm at the north end of line dissipates, a new
one is formed at the south of the line
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Severe Thunderstorms
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Supercell storms
ƒ A single very large cell
• 20-50 km across
• Typical lifetime: 2-4 hours
ƒ Very intense updraft
ƒ More violent than MCC or squall line
ƒ Most likely setting for large tornadoes
ƒ Have large scale rotation, not observed in storm
cells in MCCs or squall lines
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Severe Thunderstorms
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Supercell storms
ƒ Large scale rotation → intense wind shear
→ Complex structure with updraft and downdraft
wrapping around each other
ƒ Downdrafts amplify updrafts
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Severe Thunderstorms
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Supercell storms
Track supercell storms on
radar, which shows intensity
of precipitation
Vault: inflow of warm air to
the updraft
ƒ Cloud droplets formed
ƒ Too small for rain
droplets
Vault may appear on radar
as hook echo
ƒ Hook echo indicates that
tornado formation is
imminent
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Severe Thunderstorms
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Doppler Radar
ƒ Shows wind toward (green) and away from (red)
the radar
ƒ Highlight region of intense rotation: mesocyclone
– often precedes tornado formation
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Tornadoes
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Tornadoes are:
ƒ Local violent storm of short duration and sporadic
occurrence
ƒ Very destructive – among nature’s most
destructive forces
ƒ Rotating column of air – vortex – extends
downward from a cumulonimbus cloud
ƒ Also referred to as a twister or a cyclone
ƒ In northern hemisphere, most tornadoes rotate
cyclonically (counter-clockwise), but a few spin in
the opposite direction (clockwise)
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Tornadoes
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Tornadoes can appear as a very thin ropeshaped column or as a funnel shape
•
Regardless of shape tornadoes are extremely
dangerous
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Tornadoes
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Strong winds within a tornado occur due to
extraordinarily large differences in pressure
over short distances
ƒ Pressures inside tornadoes have been estimated
to be as much as 10% (100 mb) lower than
immediately outside the storm
ƒ Pressure gradient force
→ Draws winds into the low pressure center
→ Generates winds of up to 480 km/h
ƒ Air near the ground rushes inward, spirals upward
around the core and merges with the airflow of
the parent thunderstorm deep in the
cumulonimbus cloud
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Tornadoes
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Tornado is visible:
ƒ Air sucked into the storm expands and cools
adiabatically
• May cool below dew point temperature
→ Condensation
→ Pale ominous cloud
ƒ Dust and debris
picked up by the
ground
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Profile of a Tornado
Width
Life
Length of path
Max. wind spd
Frequency of
Occurrence
•
Average
Weak
Violent
Tornado
Tornado
Tornado
150-600 m
100 m
1000 m
~5 minutes < 3 minutes > 3 hours
3-4 km
1 km
150 km
250-450 km/h < 150 km/h > 480 km/h
>50% of all <2% of all
tornadoes tornadoes
Tornadoes typically travel across the
ground:
ƒ At a speed of ~50 km/h
ƒ From SW to NE (because they occur ahead of a
cold front, where prevailing winds are from SW)
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Profile of a Tornado
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•
Most tornadoes rotate around a single
central vortex
Some stronger tornadoes have suction
vortices
ƒ Multiple smaller
intense whirls,
that rotate within
the main vortex
ƒ Suction vortices
have diameters of
~10m and rotate
very rapidly
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Development of a Tornado
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Form in association with severe t-storms:
< 1% of t-storms produce tornadoes, but
most severe storms must be monitored as
potential tornado producers
Can develop in any situation that produces
severe weather
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Along cold front or squall line
Mesoscale convective complex
Hurricane
Supercell t-storms
Not certain exactly what triggers formation
ƒ Product of the interaction between strong
updrafts in T/storm and winds in troposphere
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Development of a Tornado
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In supercell t-storms, a precondition to
tornado formation is a mesocyclone: vertical
cylinder of rotating air, ~3-10 km across, that
develops in updraft
ƒ Begins as slow, horizontal rotation of large
segment of cloud deep within cloud interior,
several km above surface
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Development of a Tornado
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Mesocyclone formation:
ƒ Under right conditions, strong updrafts in the tstorm tilt the horizontally rotating vortex to nearly
vertical, within the cloud interior
•
Mesocyclone often precedes tornado by ~30 minutes
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Development of a Tornado
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Mesocyclone intensifies:
ƒ At first, mesocyclone is wide, short, and rotating
slowly
ƒ Then, it narrows horizontally and stretches
vertically, causing winds to accelerate
ƒ Narrowing column of
rotating air stretches
downward, protrudes
below the cloud base
to produce a very
dark, slowly rotating
wall cloud
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Development of a Tornado
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Funnel cloud:
ƒ A slender rapidly spinning vortex, which emerges
from the base of the wall cloud
ƒ When a funnel cloud makes contacts with the
surface, then it is classified as a tornado
Formation of
mesocyclone does
not necessarily mean
tornado formation will
follow
ƒ Approximately 20%
of mesocyclones
produce tornadoes
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Location and Timing of Tornadoes
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Predominantly a N. American phenomenon
ƒ Many tornadoes touch down along wide strip
running SW to NE between the southern Plains
and the lower Great Lakes region: Tornado Alley
ƒ Florida: many
Tornado density
tornadoes
embedded in
passing
hurricanes &
tropical storms
ƒ On average, 770
tornadoes
reported
annually in U.S.
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Location and Timing of Tornadoes
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54% of all tornadoes
occur during spring
ƒ Most frequent in April
to June
ƒ May: ~5 per day in US
Air masses have
greatest contrast
ƒ cP – still cold and dry
ƒ mT – warm, humid,
and unstable
Greater contrast →
more intense storms
→ more likelihood of
tornadoes
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Tornado Prediction
•
Very difficult because:
ƒ Severe t-storms & tornadoes are short lived
ƒ Observation network sparse
• Surface station: ~160 km apart, hourly obs.
• Upper air stations: ~320 km apart, 12 hour obs.
ƒ Doppler radar
• Look for
characteristic
precipitation
pattern (hook
echo), and wind
patterns (rotation
of mesocyclone)
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Watches and Warning
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Severe storm watches and tornado watches
ƒ Issued by National Weather Service Storm
Prediction Center (Norman Oklahoma)
ƒ If a particular region of country is vulnerable to
impending storm activity, a watch is issued
Severe thunderstorm warning
ƒ Issued by local weather forecasting office
ƒ If a severe thunderstorm has developed
Tornado warning
ƒ Issued by local weather forecasting office
ƒ If an actual tornado has been observed (by trained
spotter)
ƒ Or: if mesocyclone and hook echo are detected by
Doppler radar
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Fujita Tornado Intensity Scale
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Tornado winds cannot be measured directly
Fujita Intensity Scale
ƒ Determined by assessing worst damage produced
by storm
ƒ 7 levels of intensity: F0 to F6
• F0 and F1: “weak storms” (69% of all
tornadoes) – moderate damage
• F2 and F3: “strong storms” (29% of all
tornadoes) – major structural damage
• F4 and F5: “violent storms” (<2% of all
tornadoes) – incredible devastation
• F6: never documented in nature
Most damage is caused by high winds; most
injuries and death result from flying debris
G109: Weather and Climate
13: Severe Storms