Transcript Class #25: Friday, March 7

Class #24: Wednesday,
March 4
Clouds, fronts, precipitation
processes, upper-level waves,
and the extratropical cyclone
Class #24: Wednesday, March 4,
2009
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Lifting, fronts and cloud
formation
• At fronts, one, two, three or all four lifting
processes can be acting at the same time
• Frontal lifting forces the warmer air over
the colder air, and an upslope enhances
lifting
• Convergence occurs because the wind
direction changes at the front
• Convection can occur with surface heating
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2009
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The generic front: convergence
and frontal lifting
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2009
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Cold front: convergence, frontal
lifting, often convection
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2009
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Warm front: Convergence and
frontal lifting
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2009
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Cross section through a warm
front and cold front
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Cross sections at a later time:
convection in afternoon
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2009
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Review of the basic cloud types
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2009
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Frontal lifting and cloud types
• Frontal lifting is weaker at warm fronts
than cold fronts
• Convergence is weaker at warm fronts
than cold fronts
• Convection is rare at warm fronts,
common with cold fronts
• Layer clouds are common with fronts
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2009
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How clouds produce
precipitation
• Clouds produce precipitation with two
different mechanisms
• Both mechanisms can be active in the
same cloud
• First, the collision--coalescence process,
also called the warm rain process
• Second, the ice crystal process, also
called the Bergeron—Wegner process
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2009
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The collision—coalescence
process
• Cloud droplets are not all exactly the same
size
• Statistically speaking, there is a spectrum
of cloud droplet sizes
• Condensation alone is too slow to produce
precipitation-sized particles (it would take
days)
• Cloud droplets fall at different speeds
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2009
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Collision—coalescence
(continued)
• Terminal velocity in a cloud is the velocity
of a droplet relative to the surrounding air
– Dropping an object in a rising elevator, it will
fall to the floor of the elevator
– Cloud droplets can fall relative to the air
around them, even as they and the air rises
with respect to the ground
– Larger cloud droplets have a greater terminal
velocity than smaller cloud droplets
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2009
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Collision—coalescence
(continued)
• Larger drops have a greater terminal
velocity than smaller drops because they
are less buffeted by turbulent eddies.
• The larger drops, falling faster, collide with
some smaller drops.
• Some collisions result in sticking together
of the two drops, or coalescence.
• The result of coalescence is a larger drop
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2009
15
Collision and coalescence:
smallest drops can escape
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2009
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Warm rain
• Repeated collisions favor the largest
droplets, which continue to collide and
grow most quickly while they fall fastest.
• This process can produce raindrop-sized
drops in about 20 minutes, many times
faster than condensation.
• One typical raindrop contains about 1
million cloud droplets
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2009
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Warm rain isn’t the entire story
• The collision—coalescence process
explains how rain can form in clouds with
no ice, or in the lower (above-freezing)
portions of deeper/colder clouds
• Near mid-latitude fronts and in
extratropical cyclones, another process is
at work—the ice crystal process. It
depends on the presence of ice crystals
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2009
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Ice crystal formation
• The ice crystal process begins with the
formation of ice crystals
• At temperatures below -40ºC, ice crystals
can form spontaneously (deposition)
• At higher temperatures, small particles
called ice nuclei form surfaces for water
vapor to freeze.
• There are lots less ice nuclei than CCN
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2009
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Mixed clouds have water
droplets and ice crystals
• At temperatures just below freezing, few
substances can act as ice nuclei
• At lower temperatures (higher in the cloud)
more substances can act as ice nuclei
• Ice nuclei have molecular structures
similar to the ice crystal
• Condensation of supercooled water occurs
for T<0º without an ice nucleus
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2009
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Ice crystals can also act as ice
nuclei
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Ice crystal types depend on
temperature
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2009
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