Transcript Class #23: Friday October 23

Class #26: Friday
October 30
Thunderstorms
Class #26: Friday, October 30, 2009
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Conditions for thunderstorm
formation (continued)
• For the more severe thunderstorms, vertical
shear of the horizontal wind
– The westerly (west to east) part of the wind
increasing as height increases
– Clockwise turning of the direction from which the
wind blows as height increases
• For example, southeasterlies at the surface, southerlies
at 850mb, southwesterlies at 700mb, and westerlies at
500mb
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Class #26: Friday, October 30, 2009
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An ordinary or air mass
thunderstorm
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Life cycle of a thunderstorm cell/air
mass thunderstorm
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Class #26: Friday, October 30, 2009
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Lifted Index
• Is a way to describe stability with one number
• Is not a perfect measure of stability
• Is useful for forecasting, but not the only
criterion forecasters use
• Is a difference in temperature at 500mb
• T(environment) – T(air parcel)
• Negative values are unstable
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Lifted Index (continued)
• Where do the temperatures come from
• For the environment, use a sounding
• For the parcel
– Start with surface conditions of temperature and
humidity
– Next, lift the parcel dry adiabatically (10°C/km)
until it is saturated
– Finally, raise the parcel moist (saturated)
adiabatically to 500mb
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Satellite-derived values of the
lifted index
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An environment that
favors the formation of
severe thunderstorms
At the highest levels there is a jet
stream with divergence aloft
At 500mb there is a trough to the west
At 700mb there is a warm dry air mass
Between 700mb and 850mb there is
convective or potential instability
Just above the surface there is a low
level jet of southerlies with warm
moist air
Overall, a clockwise turning wind in
the warm sector
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Drylines
• Are not true fronts because there is no
temperature contrast across the front.
• Resemble fronts because there is a boundary
between air masses, cT and mT.
• Resemble fronts because there is a wind shift,
pressure trough, convergence, and often
convective clouds along the dryline.
• Occur in West Texas in spring and early
summer, and severe weather can occur.
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Class #26: Friday, October 30, 2009
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Drylines (continued)
• Have a strong contrast in humidity and wind
direction across the front
• Can persist for several days.
• Can move westward at night (called the dewpoint front)
• Move from west to east during the day
• Are a powerful source of convergence
• The moister air is lighter, and rises
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Schematic view of a multicell
thunderstorm
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The shelf cloud signifies the arrival
of the gust front
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Composite radar imagery of a
squall line
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Satellite image of the southern
portion of the squall line
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Infrared image of a supercell
thunderstorm and squall line
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Photograph of an
overshooting
thunderstorm cloud
The updraft of the thunderstorm is so
strong that its top pokes into the
tropopause.
The overshooting cloud updraft is
decelerating and the updraft speed
decreasing.
Cloud top occurs when the updraft
slows to a stop.
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Visible satellite image of the same
storms
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Life cycle of the multicell
thunderstorm
A squall line would extend into the
slide
This storm is moving from right to left
on the slide
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