Thunderstorms

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Thunderstorms

• Thunderstorms are mesoscale systems
• Large Scale -- Extratropical cyclones
• Mesoscale -- Thunderstorms
• Small Scale -- Tornadoes
• Thunderstorms are convective systems
• Develop in an unstable atmosphere
• Rising motion may be brought about by
• Free convection
• Forced lifting
• Upper level lifting

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Thunderstorms

• We shall now look at the life-cycle of a typical
  single cell thunderstorm.

Cumulus Stage
If conditions are favorable (conditional or
absolute instability), cumulus clouds can grow
both vertically and laterally.
Initially Free Convection
The sun warms the ground which warms a parcel of
air above the surface.
Warm
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Thunderstorms

• These thermals rise and cool at the dry adiabatic
  lapse rate.
• Mixing occurs with the environment to slowly heat
  the lower layers of the atmosphere.
• This mixing can change a stable layer near the
  ground to a dry adiabatic layer.

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Thunderstorms
Mixing Ratio Dry Adiabat Sounding
Convective Temperature
p
p
T
T
Sunrise
Afternoon
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Thunderstorms

• The more humid the surface air is initially, the
  less lifting is required to reach the CCL.
• Once the CCL is reached, the vapor condenses
• Latent heat is released
• Further cooling is at the moist (wet) adiabatic
  rate.

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Thunderstorms
As the afternoon wears on, the surface is
continued to be heated by the sun. Convective
bubbles (parcels) continue to rise from the
surface of the earth. These convective
bubbles bring more moisture from the surface
allowing the cumulus clouds to grow.
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Thunderstorms

• The rising stream of moist air below the cloud
  and feeding into it is called the updraft.
• The updraft is sufficiently strong to keep water
  droplets and ice crystals aloft.
• There is no precipitation in the cumulus stage.

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Air Mass Thunderstorms

• First studied just after World War II
• Many commercial and military aircraft accidents
• Newly developed radar was exploited for weather
  studies
• The Thunderstorm Project
• Resulted in first life cycle of a thunderstorm
• Air mass thunderstorms are also referred to as
  Garden Variety!

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Air Mass Thunderstorms

• We shall examine three phases of an air mass
  thunderstorm
• Cumulus Phase (Growth Phase)
• Mature Phase
• Dissipating Phase
• Life cycle can last from 45 minutes to over an
  hour

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Air Mass Thunderstorms
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Cumulus Phase

• Development of towering cumulus
• Region of low level convergence
• Warm moist air
• Nearby cumulus may merge to form a much larger
  cloud
• Dominated by updraft
• Mixing and entrainment occur in the updraft

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Entrainment

• Entrainment is the process by which saturated air
  from the growing cumulus cloud mixes with the
  surrounding cooler and drier (unsaturated) air.
• Entrainment causes evaporation of the exterior of
  the cloud and tends to reduce the upward buoyancy
  there.

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Air Mass Thunderstorms
Mature Stage

• The mature stage begins when precipitation
  occurs. Cu transitions into a Cb.
• Evaporation caused by entrainment causes regions
  of cooler air that begin to descend.
• This is the beginning of the downdraft.

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Mature Phase

• Precipitation, formed by the Bergeron cold rain
  process, begins to reach the ground.
• The precipitation drags some of the surrounding
  air down creating the downdraft.

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Downdraft

• The downdraft is the descending column of air in
  a thunderstorm.
• Created and maintained by three processes
• Evaporational cooling of entrained air
• Downward drag caused by falling precipitation
• Evaporational cooling of the air below the cloud
  base

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Downdraft

• When the downdraft reaches the ground, it spreads
  out in all directions.
• The leading edge of this cold, often gusty wind
  is called the outflow boundary or gust front.
• The outflow boundary behaves like a cold front
• Strong wind shift (speed and direction)
• Much colder air behind the gust front
• Acts as a location for additional lift for future
  storm development.

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Downdraft and Gust Front

• When the downdraft hits the ground, it spreads
  out.
• The leading edge of this spreading cold air is
  called the gust front.

Gust Front
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Mature Phase

• The mature phase represents the peak intensity of
  the storm.
• Updrafts and downdrafts are about equal in
  strength.
• Precipitation is typically heavy and may contain
  small hail
• Gusty winds result from the downdraft spreading
  out on the ground.
• The anvil, or cloud top, begins to turn to ice,
  or glaciate.

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Thunderstorms

• The thunderstorm reaches its maximum intensity
  during the mature phase.
• Elements of the mature thunderstorm
• Lightning
• Thunder
• Heavy Rain
• Small Hail
• Gusty Winds

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Thunderstorms
Dissipating Stage

• The downdraft and precipitation fall down through
  the updraft cutting off the updraft.
• The gust front surges out in front of the storm
  cutting off the inflow of warm, moist air.
• The storm begins to die of starvation.

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Dissipating Phase

• Eventually the downdraft overwhelms the updraft
  and convection collapses.
• Precipitation becomes lighter and diminishes.
• Cloud begins to evaporate from the bottom up
  often leaving behind an orphan anvil.
• Cirrus Spissatus cumulonimbogenitus

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Thunderstorms

• The previous discussion was a description of an
  ideal single cell thunderstorm.
• These are the garden variety thunderstorms seen
  in the summer.

• Warm Humid Air Mass
• Widely Scattered
• Usually Weak
• Usually Short Lived

• Produce a good portion of summertime rainfall.
• Produces temporary cooling
• Slight potential for producing
  severe weather.

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Thunderstorms

• Criteria for Severe Weather
• Winds ? 50 kts
• Hail Diameters ? ¾ inch
• Funnel Cloud
• Tornado

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Thunderstorms

• Multicell Thunderstorms
• A storm that has a succession of cells in various
  stages of their life cycle.
• Most thunderstorms are multicellular.

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1
1
2
2
2
3
3
3
Each cell moves The group moves
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4
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Thunderstorms

• Multicell Thunderstorms
• Each cell goes through the complete life cycle of
  a single cell.
• Each cell is often unique and can be identified
  on radar.
• Although each cell grows and dies, the group of
  storms can last for over an hour.
• Has the potential for producing severe weather.

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Multicellular Thunderstorms

• A cluster of short lived single cells.
• Cold outflow from each cell combine to form a
  much larger and stronger gust front.
• Convergence along the gust front tends to trigger
  new updraft development. This is the strongest
  in the direction of storm motion.
• New cell growth often appear disorganized to the
  naked eye.

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Multicellular Thunderstorms

• Individual cells typically move with the mean
  (average) wind flow.
• Multicell storms may last a long time. They
  constantly renew themselves with new cell growth.

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Importance of Wind Shear

• Single cell
• Storm is vertically stacked
• Outflow boundary may outrun the motion of the
  storm cell
• New storms that develop may be too far from the
  original to be a part of it
• Multicell
• Storm motion keeps up with outflow
• New development forms adjacent to the older cells
  and connects with the old cell

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Multicellular Thunderstorms

• In multicell storms, the development of new cells
  is very close to the older ones.
• The cells tend to merge.

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Multicellular Thunderstorms
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Multicellular Thunderstorms

• As each cell matures, the updraft holds the
  precipitation aloft. Eventually the
  precipitation unloads and a gush of heavy rain
  occurs at the ground.

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Multicellular Thunderstorms

• At low levels
• The outflow produces the surface convergence to
  form new updrafts
• New updrafts often form at the right or
  right-rear flank of the storm
• At high levels
• As the cell matures, the updraft may pass the
  equilibrium level
• This forms a penetrating updraft of an
  overshooting top

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Multicellular Thunderstorms
EL
Overshooting Top
LCL
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Multicellular Thunderstorms

• As the next cell grows and develops, the top of
  the older cells tends to collapse.
• This enhances the upper-level divergence which
  helps to expand the anvil
• The collapsing air mixes with the downdraft air
  and enhances the downdraft.

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Multicellular Thunderstorms

• Some multicellular storms show little or no
  motion
• Very localized rainfall that can cause
  significant flooding
• Rapid cell development on the rear of the storm
• New cells move over the same land region as the
  previous cells
• The individual cells move with the mean wind but
  the storm is stationary

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Thunderstorms

• Supercell Thunderstorms
• Relatively long lived intense thunderstorm.
• Consists of one large cell usually with
  exceptionally strong updrafts.
• The structure of the storm is such that the
  updraft and downdraft are separated from each
  other.
• Often the entire storm can begin to rotate.
• May produce large tornadoes.

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Thunderstorms
Supercell Thunderstorms
The updraft and downdraft are tilted and are
thereby separated from each other.
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Thunderstorms

• Synoptic conditions favorable for severe weather
  in Oklahoma
• Mature cyclone that has formed in the lee of the
  Rocky Mountains
• Well defined surface boundary
• Cold Front
• Dryline
• Outflow Boundary
• Warm, moist low level jet

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Thunderstorms

• Cold, Dry air at mid-levels
• Strong trough just to the west at 500 mb
• Strong upper level jet possibly with embedded jet
  streaks
• Temperature inversion aloft to hold back
  convection until later in the afternoon

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Dryline

• A boundary between moist air and much drier air.

Warm, Dry Air (cT)
Warm, Moist Air (mT)
Dryline at the surface. Convergence zone
Gulf of Mexico
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Severe Thunderstorms

• Severe Thunderstorm or Tornado Watch
• Conditions are favorable for the development of
  severe weather or tornadoes.
• Does not mean that severe weather or tornadoes
  will occur at your location.
• Severe weather or tornadoes are likely in and
  around the watch area.

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Severe Thunderstorms

• Severe Thunderstorm or Tornado Warning
• Severe weather or tornadoes are occurring or have
  been indicated by Doppler weather radar.
• Take immediate action to protect life and
  property.

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Classic Severe Weather Conditions

• Idealized Classic conditions for severe weather.

• Low level moisture
• Instability -- Cold air aloft associated with the
  upper level trough
• Inversion (Cap) -- Warm air near 800 mb
• Strong Wind Shear
• Lifting Mechanism -- Cap Breaker