Thunderstorms
Straight Line Winds
This is the sort of wind that comes out of a thunderstorm.
They are produced by the downward momentum within the
downdraft region of a thunderstorm.
If the air is dry in the middle of the troposphere, the storm has
a fast rate of forward movement, and there are strong
updrafts and downdrafts, you have the right conditions for
strong Straight Line Winds
Dewpoint Depression
• Difference between temperature and dewpoint
at the surface and aloft
Dewpoint Depression
• This has the following practical results:
– When the lower level Tdd (temp – dew point) is small
and air is being lifted, clouds form low bases
– When the lower level Tdd is large and air is being
lifted, clouds form bases higher
– IOW, it took more lifting to saturate that air
adiabatically
– Can enhance downdrafting and hail formation by
enhancing evaporative cooling
Dewpoint Depression
• Suppose T = 15C and the dew point is 9C.
What is TDD?
• 15C-9C = 6.
• It does not equal 6C, it is not a temperature,
it’s a difference between temperatures.
Types of Thunderstorms
1) Air Mass Thunderstorms
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Most Common
Least destructive
Contained within a single air mass
Stages of an air mass thunderstorm
1) Cumulus Stage
• Air begins to rise
• Rising air cools adiabatically and forms fair
weather cumulus clouds
• These initial clouds last for minutes
• The initial clouds evaporate and increase the
amount of moisture in the air
• New clouds with vertical development form
• The new clouds extend to the upper
troposphere and ice crystals grow by the
Bergeron process
• The thick clouds cause the sky to darken
Stages of an air mass thunderstorm
1) Cumulus Stage
• Air begins to rise
• Rising air cools adiabatically and forms fair
weather cumulus clouds
• These initial clouds last for minutes
• The initial clouds evaporate and increase the
amount of moisture in the air
• New clouds with vertical development form
• The new clouds extend to the upper
troposphere and ice crystals grow by the
Bergeron process
• The thick clouds cause the sky to darken
Stages of an air mass thunderstorm
2) Mature Stage
• The storm is at its most intense
• Overshooting tops form
• Lightening and Thunder may be present
• Warm Updraft
• Cold downdraft
• Strong winds at surface via downdraft
Stages of an air mass thunderstorm
3) Dissipative Stage
• Falling rain or graupel forces air downward
causing down drafts
• As precipitation proceeds, downdrafts
increase at the base of the cloud.
• This cuts off the water supply
• Precipitation diminishes and the sky clears
Well Developed Cumulus Cloud
Severe Thunderstorms
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Most form along or ahead of cold fronts in Midlatitude cyclones
Wind speeds exceed 93 km/hr (60mile/hr)
Have hailstones larger then 2 cm in diameter
Spawn tornadoes
Last for 12 hours to several days
Account for ~60% of all annual rainfall in the
US and Canada
Typical occur in groups of individual storm cells
Severe Thunderstorms
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Mesoscale Convective Systems (MCS)
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Mesoscale Convective Clusters (MCC)
Squall Line
Supercell
Severe Thunderstorm Structure
http://www.theweatherprediction.com/severe/structure/
MCCs
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Occur as circular or oval clusters of storm
But all storms are part of the same system
Defined by satellite images
Responsible for most severe weather in
the US and Canada
Satellite Image
of
Mesoscale Convective Complex
Squall Lines
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Consist of a large number of individual storm
cells arranged in a linear band, ~500 km in
length
Tend to form 300 to 500 km ahead of cold fronts
Form mainly in spring and summer
Last 10 hours to days
Squall-line
Thunderstorms
Three types of Super Cells
Classic – Great Plains
High Precipitation – Eastern Great Plains and Eastward
Low Precipitation - leeward side of the Rockies
Supercell Thunderstorm
Cluster of Supercell Thunderstorms
Supercell Storms
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Very violent
Smaller in extent than MCCs or Squall lines (2050km in diameter)
Consist of a single, extremely powerful cell
instead of a number of cells
Undergo large scale rotation
Complex updrafts and downdrafts
How do we study storms?
Studying Storms
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Doppler Radar - makes use of the Doppler effect
to image storms, study their patterns, make
predictions and issue watches and warnings.
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Doppler Effect
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Sound travels in waves
If the object making the sound is traveling
toward the listener, the sound waves become
compressed (shift toward shorter wavelengths).
This results in a higher pitch.
If the object making the sound is moving away
from the listener, the sound waves are stretched
(shift toward longer wavelengths) and the pitch
is lower,
Studying Storms
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Doppler Radar takes advantage of the Doppler
Effect.
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A transmitter/receiver is used to emit a pulse
of electromagnetic radiation (l = several
centimeters)
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Can be used to observe the motion of cloud
constituents (raindrops and ice).
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Wind speed, direction and cloud structure
can be determined.
Studying Storms
Doppler Radar
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Hook - appearance of the hook usually indicates a
tornado will form.
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Vault - inflow of warm surface air enters the cell.
Vault results from small newly formed cloud drops
that are too small to be visible on radar.
Hook Echo
http://www.crh.noaa.gov/dtx/images/010521ref_expl.jpg
The mountain of Gold-Shwedagon
Pagoda
Pagoda
http://www.mir.com.my/leofoo/Burma2005/images/lake_kandagwi.jpg
Pagoda
http://www.mir.com.my/leofoo/Burma2005/images/lake_kandagwi.jpg
pagoda
http://www.risc.uni-linz.ac.at/people/sgerhold/images/myanmar/shwedagon.jpg
Bogalay Village
http://ratchasima.files.wordpress.com
Bogalay Village
http://ratchasima.files.wordpress.com
Cyclone Nargis,
May 2 2008
Cyclone Nargis,
May 2 2008
Hits the Irrawady delta
+22K known dead
~41K missing
10,000 dead in Bogalay
24 million people live in the disaster zone
Inaccessible with the damage to infrastructure
Military Junta had always justified its control as being needed to
respond to disasters and emergencies.
Military response to the cyclone has been much slower and
weaker than to the monk protests (the saffron revolution)
Reports as of a few years ago of diseases like Japanese
Encephalistis in the villages of the delta
Some US ambasadors reporting possiblity of the death toll being
100K