Thunderstorms:

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Thunderstorms ordinary or single cell
storms, multicell storms, supercell storms
Typical cumulonimbus single cell thunderstorm
produces heavy shower, possibly with hail and
lightning
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Reading

• Ahrens, Chapter 14 Thunderstorms and Tornadoes
• This lecture next (Lightning, tornadoes) will
  cover the topic.

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What meteorological conditions precede a
thunderstorm?

• A conditionally unstable atmosphere
• Substantial boundary layer moisture
• A trigger to release the instability
• On a skew T-log p plotCAPEConvective
  Available Potential Energy energy that can be
  released
• CINConvective INhibition energy barrier
  that has to be overcome

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Real example tephigram large amount of CAPE
thunderstorm v.likely
CAPE is given by thearea between SALR and
environmentallapse rate
CAPE
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An important forecaster tool for predicting
thunderstorms Maps of CAPE (contours) and
vertical velocity ()
Fri Nov 7 12Z2008
http//expert.woeurope.eu/cape_frame.htm
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Sunday 1200 (8 Nov 2009)
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Monday 31 Oct 2011 (03z)
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Ordinary or single cell thunderstorms

• Relatively small
• Isolated
• Typically just produce a single heavy shower,
  then dissipate.
• Very little vertical wind shear (come back to
  this later)

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Stage 1 Cumulus
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Cumulus stage (continued)

• Buoyant updraught
• Vertical velocity increases with height, to 10
  ms-1 at top
• Surrounding air mixed in (entrainment)
• Inside cloud, raindrops and supercooled drops
  grow, releasing latent heat
• At edges, drops evaporate into entrained air
  moistens the surrounding air.
• As the environment moistens, successive
  updraughts sustain clouds to higher and higher
  levels
• No rainfall at this stage

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Stage 2 Mature
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Mature stage (continued)

• Top of cloud extends to near tropopause levels
  (gt10 km), well above 100 freezing level
• Growth of drops ice continues until updraught
  can no longer support them start to fall
• Entrainment of surrounding drier air tends to
  evaporate drops, cooling air
• Both these processes lead to development of a
  downdraught
• Updraughtdowndraughtcell single cell
  thunderstorm
• Most intense stage heavy rain, thunder,
  lightning
• Anvil starts to form at top

-40C
10 km
5 km
0C
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Stage 3 Dissipating
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Dissipating stage (continued)

• Downdraught grows until it cuts off flow of air
  to the updraught the storm has its fuel
  supply stopped
• Rainfall declines and the lower part of the cloud
  evaporates
• Rainfall stops all that is left is the anvil
• All 3 stages pass in typically about 1 hour - a
  rapid, heavy shower

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Summary single cell storm
Mature
Dissipating
Cumulus
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Vertical wind shear

• Why might this be important?

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Approaching mature stage
Dissipating stage
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Multi-cell thunderstorms

• This type of thunderstorm is where once one cell
  subsides, another grows in its place, adjacent to
  the last cell
• The downdraught causes a gust front when it
  meets the surface. This may push up surrounding
  moist air and trigger a new cell to develop.
• The presence of vertical wind shear can help
  thunderstorm development and persistence by
  separating the updraught from the downdraught

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Vertical Wind Shear
Shear tilts the storm, helping it propagate,
increases its lifetime and severity Promotes
formation of new cells i.e. a multicell storm
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Shear and rotation
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Horizontal shear combined withan updraught can
lead to a stormacquiring vorticity about a
verticalaxis
Vorticityassociatedwithhorizontalshear
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Generating a supercell storm
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Supercell, Kansas, rotating updraught
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Supercell thunderstorms

• Rotating updraught
• Rotation causes the storm to be more robust
  longer-lived, and therefore more dangerous
• Forms an area of low pressure at centre of
  rotation, called a mesolow
• Updraught centred on the low pressure
• Circulation around the low is in cyclostrophic
  balance