METEOROLOGY

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METEOROLOGY

• GEL-1370

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Chapter Ten

• Thunderstorms andTornadoes

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Goal for this Chapter

• We are going to learn answers to the following
  questions
• What atmospheric conditions produce
  thunderstorms?
• How severe thunderstorms are produced?
• Why severe thunderstorms are not common in polar
  latitude?
• How lightning are produced?
• How thunders are produced?
• What are tornadoes and how they are produced?
• What is Fujita scale?
• Major characteristics of a tornadoe?
• Why highest frequency of thunderstorms occur in
  US?

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Thunderstorms

• Thunderstorm A storm that contains lightning and
  thunder
• Birth occurs when warm humid air rises in a
  conditionally unstable environment
• What can trigger the birth of thunderstorm
  unequal heating of the surface, terrain, lifting
  of warm air along a frontal zone
• Ordinary thunderstorms (or air-mass
  thunderstorms) Develop in warm, humid air masses
  away from weather fronts usually short-lived and
  rarely produce strong winds or large hail
• Severe thunderstorms may produce high winds,
  flash floods, changing hail tornadoes

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Thunderstorms

• Stages of ordinary thunderstorms
• Cumulus Stage Humid air rises, cools,
  condenses in to cumulus clouds
• Transformation of water-vapor into liquid or
  solid cloud particles releases large quantities
  of latent heat this keeps the air inside the
  cloud warmer than the surrounding air
• During cumulus stage, insufficient time for
  precipitation to form, and the updrafts keep
  water droplets and ice crystals suspended within
  the cloud no lightning or thunder during this
  stage
• As the cloud builds well above the freezing
  level, cloud particles grow larger and heavier
  drops begin to fall drier air around the cloud
  is being drawn into it entrainment of drier air
  leads to evaporation of raindrops air becomes
  colder heavier air begins to descend as a
  downdraft

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Thunderstorms contd.

• Appearance of the downdraft marks the beginning
  of the mature thunderstorms downdraft updraft
  within the mature thunderstorm constitute a
  cell
• In most storms, there are several cells, each of
  which may last for an hour or so
• Updrafts downdrafts reach their greatest
  strength in the middle of the cloud, creating
  severe turbulence
• Overshooting Intrusion of the updraft above the
  cloud top in to the stable atmosphere
• Dissipating stage When updrafts weaken
  downdrafts tend to dominate throughout much of
  the cloud
• Three stages Cumulus stage, maturing
  thunderstorm stage, dissipating stage

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Thunderstorms contd.

• A single ordinary thunderstorm may go through its
  three stages in an hour or less
• The cold downdraft may force warm, moist surface
  air upward this air may condense and can
  gradually build into a new thunderstorm
  multicell thunderstorms
• Most ordinary thunderstorms are multicell storms
• Severe Thunderstorms Capable of producing large
  hail, strong, gusty surface winds, flash floods,
  and tornadoes
• Can form from moist air when it is forced to rise
  into a conditionally unstable atmosphere severe
  thunderstorms also form in areas with a strong
  vertical wind sheer

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Air motions associated with thunderstorms
severity depends on the intensity of the storms
circulation pattern
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Ordinary thunderstorm in its mature stage
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A multicell thunderstorm in the middle is in its
mature stage to its right of the cell, a
thunderstorm is its cumulus stage
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A simplified model describing air motions other
features associated with a severe thunderstorm
severity depends on the intensity of the storms
circulation pattern
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Severe Thunderstorms contd.

• The storm in the previous figure, moves from left
  to right the upper-level winds cause the system
  to tilt so that the updrafts move up and over the
  downdrafts
• The updrafts in a severe thunderstorm may be so
  strong that the cloud top is able to intrude well
  into the stable atmosphere top of the cloud may
  even extend to more than 18 km above the surface
• Gust Front The boundary separating the cold
  downdraft from the warm surface air
• Along the leading edge of the gust front, the air
  is turbulent strong winds here can pick-up loose
  dust and soil and lift them into a huge tumbling
  cloud

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Gust Front Microburst

• Downbursts A severe localized downdraft that can
  be experienced that fall slowly and reduce
  visibility more than light rain
• Microburst A downburst with winds extending only
  4-kms or less
• Supercell and Squall-line thunderstorms
• Supercell Storm An enormous severe thunderstorm
  whose updrafts (can exceed 90 knots) and
  downdrafts are nearly in balance, allowing it to
  maintain itself for several hours. It can produce
  large tornadoes hail (gt grapefruit size) most
  supercell storms move to the right of the
  steering winds aloft
• Squall-line storms form as a line of
  thunderstorms along a cold front or out ahead of
  it

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The lower half of a severe squall-line type
thunderstorms and some of the features associated
with it
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Dust clouds rising in response to the outburst
winds of a microburst north of Denver, CO
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Doppler radar display showing a line of
thunderstorms bent in the shape of a bow (Red,
orange, and yellow)
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Supercell near Spearman, TX has a tornado
extending downward from its base
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Some of the features of a classic supercell
thunderstorm, viewed from southeast
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Diagram of the thunderstorm from above, looking
down on the storm shaded red updraft shaded
gray downdraft
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Severe Thunderstorms contd.

• Dry Line (dew-point fronts) A zone of
  instability along which thunderstorms form dew
  point temp may drop along this boundary by as
  much as 9C/km
• Mesoscale Convective Complexes A large organized
  convective weather system comprised of a number
  of individual thunderstorms size of an MCC
  1000 times larger than individual thunderstorm

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Surface conditions that can produce a dryline
with severe thunderstorms A developing
mid-latitude cyclone with a cold front, warm
front, and three distinct air masses (cP, cT mT)
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IR image showing a Mesoscale Convective Complex
extending from central Kansas across western
Missouri
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Floods Flash Floods

• Flash floods Floods that rise rapidly with
  little or advance warning results when
  thunderstorms stall or move slowly, causing heavy
  rainfall over a relatively small area
• Causes for Flash Floods
• Thunderstorms stall or move slowly
• Thunderstorms move very quickly but keep passing
  over the same area (phenomenon called training)
• Heavy rain and melting of snow taking place in
  spring
• Torrential rains from tropical storms
• Summer of 1993 rain in the upper Midwest caused
  the worst flood 6.5 billion crop lost 43 human
  lives 45,000 homes were lost evacuation of
  74,000 people

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Distribution of Thunderstorms

• gt40,000 thunderstorms/day (14 millions/yr) in the
  world
• 14 million/year
• Conducive conditions for thunderstorm formation
  Combination of warmth and moisture
• Where thunderstorms are prevalent i)
  Southeastern states along the Gulf Coast with a
  maximum in Florida (mainly during summer) ii)
  Central Rockies iii) Over water along the
  intertropical convergence zone where the
  low-level convergence of air helps to initiate
  uplift
• Where thunderstorms are rare i) Dry regions such
  as polar regions and the desert areas of the
  subtropical highs ii) Pacific coastal and
  interior valleys

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Average number of days each year on which
thunderstorms are observed in US mountainous
west has sparse data
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Average number of days each year hail observed
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Thunderstorms and Lightning

• Lightning A giant spark discharging electricity
  that occurs in mature thunderstorms can take
  place within a cloud, from one cloud to another,
  cloud to surrounding air or cloud to ground
  (20) 80 within the clouds
• Lightning stroke can heat the air surrounding it
  to 30,000C which in turn causes the air to
  expand, thus initiating a shock wave that becomes
  a booming sound wave-thunder
• Light travels faster than sound (345 m/s _at_25 C)
• Time difference between the light and sound can
  be utilized to determine how far away the stroke
  took place

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Lightning Thunder contd.

• Close distance lightning Clap sound or crack
  followed immediately by a loud bang
• Farther away rumbling sound due to sound
  emanating from different areas of the stroke
• Lightning, but no thunder Thunder waves were
  refracted and the sound waves got attenuated,
  making the thunder inaudible
• Sonic boom Produced when an aircraft exceeds the
  speed of sound at the altitude at which it is
  flying
• Condition for lightning to occur Separate
  regions containing opposite electrical charges
  must exist within the cumulonimbus cloud

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Electrification of clouds

• Several theories to explain the formation of
  lightning
• When hail fall through supercooled droplets, the
  droplets freeze and release latent heat this
  heat warms the hailstone contact of warmer
  hailstone and colder ice crystal leads to a net
  transfer of positive ions from the warmer object
  to the colder object --- hailstone is negatively
  charged and ice crystals ively charged
• Positively charged ice particles carried to the
  upper part of the cloud by updrafts larger
  haldstones with ive charge fall toward the
  bottom of the cloud
• Cold, upper part becomes ively charged middle
  of the cloud becomes ively charged

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Electrification of the Clouds contd.

• Another school of thought Regions of separate
  charge exist within tiny cloud droplets and
  larger precipitation particles during the
  formation of precipitation --- Negative charge in
  the upper part of these particles ive charge
  in the lower part of the particles --- when
  falling precipitation collides with smaller
  particles, larger precipitation particles become
  negatively charged and the smaller particles
  positively charged --- updraft sweeps smaller
  sized particles leading to net ive charge

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Generalized charge distribution in a mature
thunderstorm
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The Lightning Source

• Negative charge at the bottom of the cloud causes
  a region of the ground beneath to become ively
  charged as the thunderstorms move, the positive
  charge moves along with it the positive charge
  is most dense on protruding objects charge
  separation causes electric field existence
  electric potential difference between cloud and
  the ground --- when electric potential builds up,
  current flow results and lightning occurs
• Cloud-to-ground lightning begins when the
  localized electric potential gradient gt3 million
  volt/m --- leads to the discharge of electrons
  toward the cloud base and then to the ground

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Lightning source contd.

• Stepped Leader An initial discharge of electrons
  that proceeds intermittently toward the ground in
  a series of steps in a cloud-to-ground lightning
  stroke
• Return Stroke The luminous lightning stroke that
  propagates upward from the earth to the base of a
  cloud
• Dart Leader Discharge of electrons that proceeds
  intermittently toward the ground along the same
  ionized channel taken by the initial lightning
  stroke
• Different types of Lightning Forked lightning
  (crooked or forked in shape), ribbon lightning
  (ribbon hanging in the cloud), bead lightning
  (series of beads tied to a string), ball
  lightning (sphere appears to float in the air)
  sheet lightning (cloud appears like a white sheet)

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Development of stepped leader when ive charge
near the bottom of the cloud becomes large enough
to overcome airs resistance, flow of electrons
rushes to the earth
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As the electrons approach the ground, a region of
charge moves up into the air through any
conducting object, such as trees, buildings
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When the downward flow of electrons meets the
upward surge of ive charge, a strong electric
current a bright return stroke carries ve
charge upward into the cloud
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Lightning rod extends above the building when
lightning strikes, it follows an insulated
conducting wire into the ground
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Four marks on the road surface represent areas
where lightning, after striking a car entered the
roadway 3 tires were flattened
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Lightning Detection Suppression

• Heat Lightning Distant lightning from
  thunderstorms that is seen, but not heard
• As the electric potential near the ground
  increases, a current of ive charge moves up
  pointed objects, such as antennas
• Lightning rods (made of metal with a pointed tip)
  are placed that extend well beyond the height of
  the structure
• Lightning Direction-finder It detects the
  direction of lightning by measuring the
  radiowaves produced by lightning

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Damages by lightning in US

• 10,000 fires/yr in US are started by lightning
• 50 million worth of timber is destroyed per yr
• Can we reduce the cloud-to-ground lightning??
  Seeding cumulonimbus clouds with hair-thin pieces
  of Al wire (10-cm long) --- metal will produce
  many tiny sparks and prevent the electrical
  potential in the cloud from building to a point
  where lightning occurs

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Tornadoes

• Tornadoes A product of thunderstorms rapidly
  rotating winds that blow around a small area of
  intense low pressure
• Tornadoes circulation is present on the ground
  either as a funnel-shaped cloud or as a swirling
  cloud of dust debris majority rotate
  counterclockwise
• Other shapes
• Twisting ropelike funnels
• Cylindrical-shaped funnels
• Massive black funnels
• Funnels that resemble an elephants trunk hanging
  from a large cumulonimbus cloud

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Tornadoes Features and stages

• Diameter (most) 100-600 m (few meters 1,600 m
  rare)
• Most last only a few minutes average path
  length of 7 km (largest one 470 km lasted for
  7 hrs) in Illinois and Indiana in 1917
• Stages of a Tornadoes (most common)
• Dust-Whirl stage Dust swirling upward from the
  surface damage is light
• Organizing Stage Tornado increases in intensity
  with an overall downward extent of the funnel
• Mature Stage funnel reaches its greatest width
  is almost vertical damage is most severe
• Shrinking stage Overall decrease in the funnels
  width increase in the funnels tilt still
  capable of intense

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Tornadoes Features and stages

• Sometimes violent damage
• Decay Stage The final stage, usually finds the
  tornado stretched into the shape of a rope
• Minor tornadoes may evolve only through certain
  stages
• Damages
• 100 people/year killed (11/10/02 37 people
  died on a single day)
• 45 mortalities in mobile homes
• March 18, 1925 tornadoes 695 people died, 7
  tornadoes traveled a total of 703 km across
  portions of Missouri, Illinois and Indiana

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Tornado outbreak

• Tornado Outbreak A series of tornadoes that
  forms within a particular region, often
  associated with widespread damage and
  destruction a region may include several states
• April 3, 1974 16 hour period, 148 tornadoes cut
  through parts of 13 states, 307 people killed,
  gt3700 people injured, damage gt600 million
• Occurrence Most numbers in US average
  gt1,000/yr 1,424 during 1998
• Tornado alley Tornado belt, Central Plains,
  stretches from central Texas to Nebraska

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A mature tornado with winds gt150 knots rips
through southern illinois
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Tornado incidence by State uppernumber by each
state (25 yrs) lower average annual
number/100,000 square miles darker greater
frequency
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Tornadoes and their impact

• Lifting railroad coach with 117 passengers and
  dumping it 25 m away
• Schoolhouse was demolished and 85 students inside
  were carried over 100 m without one of them being
  killed
• Most tornadoes have winds of less than 125 knots
• Pressure in the center of a tornado may be more
  than 100 mb lower than the surrounding there is
  a momentary drop in outside pressure when tornado
  is above a structure
• When confronted with a tornado, take shelter
  immediately (basement, stay away from windows,
  small bathroom, closet, interior hallway)

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Fujita scale for damaging wind
Scale Category Mi./hr knots Expected damage
F0 F1 Weak 40-72 73-112 35-62 63-97 Light tree branches broken sign boards damaged Moderate trees snapped windows broken
F2 F3 Strong 113-157 158-206 98-136 137-209 Considerable large trees uprooted, weak struc. Destroyed Severe trees leveled, cars over-turned, walls removed from bldg.
F4 F5 Violent 207-260 261-318 180-226 227-276 Devastating frame houses dstroyed Incredible structure the size of autos moved over 100 m

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Fujita Scale contd.

• Fujita Scale Theodore Fujita in late 1960s ---
  classifying tornadoes according to their
  rotational wind speed based on the damage done by
  the storm
• Majority of tornadoes are F0 and F1 (weak ones)
  and only a few are above the F3(violent) with
  1 F5/yr
• Tornado Formation Tornadoes tend to form with
  intense thunderstorms and a conditionally
  unstable atmosphere is essential for their
  development
• Most strong and violent tornadoes develop near
  the right rear sector of a severe thunderstorm
• In order for a tornado to spawn a tornado, the
  updraft must rotate

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Total wind speed of a tornado is greater on one
side than on the other
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A powerful multi-vortex tornado with three
suction vortices
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Conditions leading to the formation of severe
thunderstorms that can spawn tornadoes red boxed
area tornadoes are likely to form
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Where tornadoes are common

• Greatest tornado activity shifts northward from
  winter to summer
• Winter contrast between warm and cold air masses
  are the greatest over the southern Gulf states
  tornadoes are most likely to form in this region
• Spring humid Gulf air surges northward, jet
  stream also moves northward tornadoes more
  prevalent from the southern Atlantic states
  westward into the southern Great Plains
• Summer contrast between air masses lessens the
  jet stream is normally near the Canadian border
  tornado activity tends to be concentrated over
  the northern plains

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Features associated with tornado-bearing
thunderstorm thunderstorm moves northeast
tornadoes form in the southwest part
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Mesocyclone

• Mesocyclone A vertical column of cyclonically
  rotating air within a severe thunderstorm
• Severe thunderstorms form in a region of strong
  vertical wind sheer most strong and violent
  tornadoes form within the mesocyclone
• Existence of the swirling winds of the
  mesocyclone inside tornado-producing
  thunderstorms were observed 1970s (first time)
  using Doppler Radar
• 30 of all mesocyclones produce tornadoes 95
  produce severe weather
• Time between mesocyclone identification tornado
  touching the ground is 20 minutes

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Tornadoes contd.

• Gustnadoes Tornadoes that form along the gust
  front
• Wall cloud An area of rotating clouds that
  extends beneath a severe thunderstorm and from
  which a funnel cloud may appear
• Tornado Watch Issued by Storm Prediction Center
  in Norman, Oklahoma
• Doppler radar can detect areas of precipitation
  measure rainfall intensity
• Tornado Vertex signature (TVS) An image of a
  tornado on the Doppler radar screen that shows up
  as a small region of rapidly changing wind
  directions inside a mesocyclone

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A computer model illustrating motions inside a
severe tornado-generating thunderstorm
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Waterspouts

• Doppler Lidar uses a light beam (instead of
  microwaves) to measure the change in frequency of
  falling precipitation, cloud particles, and dust
• Waterspout A rotating column of air over a large
  body of water tend to move slowly than
  tornadoes last for only 10-15 minutes

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Doppler radar display of large supercell
thunderstorm that is spawning an F4 tornado
(circled are) near Lula, OK
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Average annual number of tornadoes tornado
deaths by decade

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Decade Tornadoes/year Deaths/year
1950-1959 480 148
1960-1969 681 94
1970-1979 858 100
1980-1989 819 52
1990-1999 1,220 56
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Summary Chapter - 10

• Stages of a thunderstorm and a tornado
• Air-mass thunderstorm, multicell supercell
  thunderstorm
• Gust front, causes for downdraft, microburst
• Squall line, dry line
• Suitable time for the formation of thunderstorm
• Lightning and thunder formation and features
• Funneling cloud, mesocyclone, wall cloud
• Fujita scale
• Direction of movement of tornadoes, conditions
  for its formation, waterspout