THUNDERSTORMS

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THUNDERSTORMS
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• Types of Thunderstorms
• Airmass or Ordinary Cell Thunderstorms
• Supercell / Severe Thunderstorms

• Limited wind shear
• Often form along shallow
• boundaries of converging
• surface winds

• Precipitation does not fall
• into the updraft
• Cluster of cells at various
• developmental stages due
• to cold outflow undercutting
• updraft

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• ORDINARY CELL THUNDERSTORMS
• CUMULUS STAGE
• Sun heats the land
• Warm, humid air rises
• Condensation point is
• reached, producing a
• cumulus cloud
• Grows quickly (minutes)
• because of the release of
• latent heat
• Updrafts suspend droplets
• Towering cumulus or

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• MATURE STAGE
• Droplets large enough
• to overcome resistance
• of updrafts (rain/hail)
• Entrainment
• Drier air is drawn in
• Air descends in
• downdraft, due to
• evaporative cooling
• and falling rain/hail
• Anvil head when stable
• layer reached (cloud
• follows horizontal wind)
• Strongest stage, with

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Mature, ordinary cell thunderstorm with anvil head
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Downdrafts and Gust Fronts
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Microbursts create aviation hazards
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• 3. DISSIPATING STAGE
• Updrafts weaken as gust front moves away from the
  storm
• Downdrafts cut off the
• storms fuel supply
• Anvil head sometimes
• remains afterward
• Ordinary cell
• thunderstorms may pass through all three stages
  in only 60 minutes

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Review of Stages Developing (cumulus), mature
and dissipating
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• Thunderstorms
• Typical conditions
• Conditional instability
• Trigger Mechanism
• (eg. front, sea-breeze front, mountains,
• localized zones of excess surface heating,
• shallow boundaries of converging surface
• winds)

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Conditional Instability
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Thunderstorm Development

• 1. Heating within boundary layer
• Air trapped here due to stable layer aloft
• increasing heat/moisture within boundary layer
• (BL).
• External trigger mechanism forces air parcels
• to rise to the lifted condensation level (LCL)
• Clouds form and temperature follows MALR
• 3. Parcel may reach level of free convection
• (LFC). Parcel accelerates under own buoyancy.
• Warmer than surroundings - explosive updrafts
• 4. Saturated parcel continues to rise until
• stable layer is reached

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CAPE Convective available potential energy (J/kg)
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CAPE (J/kg) 0 Stable lt1000 Marginally Unstable
1000-2500 Moderately Unstable 2500-3000 Very
Unstable gt3500 Extremely Unstable
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• The Severe Storm Environment
• High surface dew point
• Cold air aloft (increases conditional
  instability)
• Shallow, statically-stable layer capping the
• boundary layer
• 4. Strong winds aloft (aids tornado development)
• 5. Wind shear in low levels (allows for
• long-lasting storms)
• Dry air at mid-levels (increases downdraft
• velocities)

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A squall line (MCS)
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Radar image of squall line
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Wind shear and vertical motions in a squall line
thunderstorm
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Mesoscale convective complex (MCC)
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Outflow Boundaries
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Thunderstorm movement in an MCC
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See http//rsd.gsfc.nasa.gov/rsd/movies/preview.h
tml
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• Tornado Development
• Pre-storm conditions
• Horizontal shaft of rotating air at altitude of
• wind shift (generally S winds near surface
• and W winds aloft)
• 2. If capping is breached and violent
• convection occurs, the rotating column is
• tilted toward the vertical

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• Supercell Thunderstorms
• Defined by mid-level rotation (mesocyclone)
• Highest vorticity near updraft core
• Supercells form under the following conditions
• High CAPE, capping layer, cold air aloft, large
• wind shear

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• Tornadogenesis
• Mesocyclone 5-20 km wide develops
• Vortex stretching Lower portion of
• mesocyclone narrows in strong updrafts
• Wind speed increases here due to conservation
• of angular momentum
• Narrow funnel develops visible due to adiabatic
• cooling associated with pressure droppage

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2 hours after the Lethbridge tornado
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Tornado producing supercell
insert fig 11-29
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Global tornado frequency
insert fig 11-32
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insert table 11-2
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• Waterspouts
• Similar to tornadoes
• Develop over warm waters
• Smaller and weaker than tornadoes

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Distribution of lightning strikes
insert fig 11-23
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• Lightning
• Source of lightning the cumulonimbus cloud
• Collisions between supercooled cloud particles
  and
• graupel (or hail) cause clouds to become charged
• Most of the base of the cumulonimbus cloud
• becomes negatively charged the rest becomes
• positively charged (positive electric dipole)
• Net transfer of positive ions from warmer object
  to
• colder object (hailstone gets negatively charged
  
• fall toward bottom - ice crystals get charge)
• Many theories exist open area of research

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Development of lightning
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Flashes per square kilometre per year
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Four types of cloud- ground lightning
Most common
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• Intracloud Discharges
• Cloud to Ground Discharges
• - death and destruction of property
• - disruption of power and communication
• - ignition of forest fires
• - Lightning is an excellent source of soil
• nitrogen!

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Cloud-ground lightning 90 induced by negatively
charged leaders 10 induced by positively charged
leaders Sometimes, there are ground to cloud
leaders Negative cloud-ground lightning Leaders
branch toward the ground at about 200 km/s, with
a current of 100-1000 Amperes The return stroke
produces the bright flash
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• Potential difference between lower portion of
• negatively-charged leader and ground
• 10,000,000 V
• As the leader nears the ground, the electric
• potential breaks the threshold breakdown
• strength of air
• An upward-moving discharge is emitted from
• the Earth to meet with the leader

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The return stroke lasts about 100
microseconds, and carries a charge of 30
kiloAmperes, producing the main flash The
temperature along the channel heats to 30,000
K, creating an expanding high pressure channel,
producing shockwaves
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Blue jet
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Multiple suction vortices greatly increase damage
insert fig 11-37