Thunderstorms

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Thunderstorms and Severe Weather (Chapt 15)
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The Big Picture

• Weve emphasized horizontal transport of energy
  to balance the planetary energy budget
• Hadley Cell
• Subtropical divergence
• Midlatitude cyclones and conveyor belts
• What about vertical motion?
• Up-warm, down cold
• Up moist, down-dry
• Severe weather is all about vertical motion, and
  represents local release of energy that
  contributes to planetary energy balance

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Atmospheric Thermal Structure

• Heated from below by latent and sensible heat
  fluxes, in stratosphere by ozone absorption
• Thunderstorms (esp in tropics) provide the energy
  to mix the troposphere, determine height of
  tropopause

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How is Energy Transportedto its escape zones?

• Both atmospheric and ocean transport are crucial
• Buoyancy-driven convection drives vertical
  transport
• Latent heat is at least as important as sensible
  heat

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Thunderstorms Occurwhere moist air is lifted in
an unstable environment

• Near fronts
• In advance of large scale upper air troughs
• In warm air sector of mid latitude cyclones
• Sometimes isolated without any synoptic scale
  support
• In association with surface convergence
• In an unstable environment
• Where surface heating or cooling aloft
  destabilizes the atmosphere
• More frequently in moist environments

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Three Stages of Thunderstorm Development

• Cumulus Stage
• Mature Stage
• Dissipating Stage

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Cumulus Stage
Starts with a warm plume of rising air.
  The updraft velocity increases with
height. Entrainment pulls outside air into
the cloud.   Supercooled water droplets are
carried far above freezing level.
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Mature Stage
The heaviest rains occur.   The downdraft is
initiated by frictional drag of the raindrops.
Evaporative cooling leads to negative buoyancy.
  The top of the cloud approaches tropopause and
forms anvil top.
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Dissipating Stage
The downdraft takes over entire cloud. The
storm deprives itself of supersaturated updraft
air. Precipitation decreases. The cloud
evaporates.
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Thunderstorm Sounding
Temperature and dew point have typical vertical
profile in the warm sector before a thunderstorm
occurs, including the shallow inversion at 800 mb
that acts like a cap on the moist air below. The
cold dry air above warm humid air produces
convective instability
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Thunderstorm Climatology
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Hail Climatology
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Severe Thunderstorms
Figure 15.5
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Severe Thunderstorm Structure
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Gust Front Roll Clouds
Turbulence in the fast moving gust front will
spawn eddies and possibly roll clouds beneath the
shelf cloud. These clouds spin about a
horizontal axis near the ground.
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Flying into a Microburst
Figure 15.10
A pilot flying into a microburst must anticipate
sudden and strong changes in wind direction and
speed. Initially a headwind is encountered that
lifts the plane, followed by a strong downdraft,
and when leaving the storm a tailwind causes a
loss of altitude.