Thunderstorms
Review of last lecture
1. Two types of lightning (cloud-to-cloud 80%, cloud-toground 20%)
2. 4 steps of lightning development.
3. How fast does thunder travel?
4. Climate impacts of lightning: nitrogen cycle, ozone,
wildfire
5. Lightning safety
Convective systems
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Tornadoes: about 100-600 m, last 1 minute to 1 hour
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Thunderstorms: about 10 Km, last 10 minutes to a
couple of hours. 3 types: ordinary, multicell, supercell
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Mesoscale convective systems (MCSs): A cloud
system that occurs in connection with an ensemble
of thunderstorms and produces a contiguous
precipitation area on the order of 100 Km or more in
at least one direction, and often last for several hours
to a couple of days.
Thunderstorms
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A storm containing lightning & thunder
Convective; form when warm, humid air
rises in conditionally unstable environment
The warmer the rising air parcel is relative
to environment, the more buoyant force is
driving it upwards (stronger convection)
Trigger to start uplift: warming sfc, terrain
(orography), converging sfc winds, frontal
zones, divergence aloft (or combination)
Global distribution of thunderstorms
Thunderstorms I. Ordinary Storms
Three stages have been identified in ordinary thunderstorms:
a) DEVELOPING: unstable atmosphere, vertical updrafts keep precipitation
suspended
b) MATURE: entrainment of dry air that causes cooler air from evaporation, triggering
downdrafts and falling precipitation and gust fronts
c) DISSIPATING: weakening updrafts and loss of the fuel source after 15-30 minutes.
An ordinary thunderstorm
Thunderstorms II. Multicell Storm
Cool downdrafts leaving a
mature and dissipating storm
may offer relief from summer
heat, but they may also force
surrounding, low-level moist air
upward.
Hence, dying storms often
trigger new storms, and the
successive stages may be
viewed in the sky.
A Multicell Thunderstorm
Video: Development of a supercell
thunderstorm
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http://www.youtube.com/watch?v=36vGiE
5JQzs (90 min -> 2 min lapse)
Thunderstorm III. Supercell Storm
Storms producing a minimum of
a) 3/4 inch hail and/or
b) wind gusts of 50 knots and/or
c) tornado winds, classify as severe.
Formation of supercell thunderstorms
1. Before thunderstorms develop, a
change in wind direction and an increase
in wind speed with increasing height
creates an invisible, horizontal spinning
effect in the lower atmosphere.
2. Spinning horizontal vortex tubes created
by surface wind shear may be tilted and
forced in a vertical path by updrafts. This
rising, spinning, and often stretching
rotating air may then turn into a
mesocyclone.
3. Most strong and violent tornadoes form
within this area of strong rotation.
Vertical structure of a supercell thunderstorm
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In ordinary storms, the downdraft and falling precipitation cut
off the updraft. But in supercell storms, winds aloft push the
rain away and the updraft is not weakened and the storm
can continue maturing and maintain its structure for hours.
Cloud structure of a supercell thunderstorm
Horizontal structure of a supercell
thunderstorm
Radar echo of a supercell
Satellite image of a supercell
Different types of supercell thunderstorms
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Low precipitation
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High precipitation
Effects of supercell
thunderstorms
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Large hails
Damaging winds
Flooding
Dangerous cloud-to-ground lightning
Deadly tornadoes
Video: Multiple tornadoes from
one supercell
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http://www.youtube.com/watch?v=FPfOyjomig
Summary
1. The general size and lifetime of mesoscale
convective systems, thunderstorms and tornadoes
2. 3 types of thunderstorms.
3. 3 stages of the ordinary thunderstorms. Downdraft
and falling precipitation cut off the updraft.
4. Formation of multi-cell thunderstorms. Downdrafts
initiate new thunderstorms in nearby regions.
1. 3 stages of the supercell thunderstorms. Winds aloft
push downdraft/precipitation away and the updraft is not
weakened.