Precipitation

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Precipitation

• Chapter 7

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http//www.gewex.org/gpcp.html
Global Precipitation Climatology Project (GPCP)
Most rain falls near the equator. Notice how dry
it is on the west coast of the US and South
America. The Eastern US is relatively wet.
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A fallen drop stops speeding up when gravity is
balanced by air resistance. The speed that a
drop must fall so air resistance is large enough
to balance gravity is the terminal velocity.
Large drop have much larger terminal velocities
than small drops. In fact the reason cloud drops
dont fall from the sky is that their terminal
velocities are so slow (like 1 inch per minute)
that any small updraft can easily keep them
suspended.
Air resistance
Gravity
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Table 7-1, p. 166
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Conditionally unstable atmosphere. The atmosphere
is conditionally unstable when unsaturated,
stable air is lifted to a level where it becomes
saturated and warmer than the air surrounding it.
If the atmosphere remains unstable, vertical
developing cumulus clouds can build to great
heights. The rain drop falls out of the cloud
when its terminal velocity exceeds the mean
updraft speed.
Stepped Art
Fig. 7-5, p. 169
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Super cooled liquid water drops are droplets that
are present at temperature between -40 oC and 0
oC. The freeze on contact with either another
particle, tree branch, airplanewing, and make
rime ice.
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Fig. 5, p. 183
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Precipitation Processes

• Topic Freezing of Cloud Droplets
• Spontaneous or homogeneous freezing
• Ice embryo

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Precipitation Types

• Rain falling drop of liquid water
• Drizzle less than 0.5 mm
• Virga
• Cloudburst
• Snow frozen water falling from sky (crystal or
  flake)
• Most precipitation starts as snow
• Freezing level, snow cloud appearance, fall
  streaks, drifting snow, blizzard
• A blanket of snow is a good insulator

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Precipitation Types

• Topic Tear Drops
• Raindrops not tear shaped.
• Shape is size dependent
• Less than 2 mm sphere
• Greater than 2 mm flattened sphere

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Small drops less than 2 mm
Larger drops
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Precipitation Types

• Topics Sounds and snow
• A blanket of snow will act like an acoustic tile
  and absorb sound waves.
• Topics Snow with Temperature above Freezing
• Unsaturated wet bulb temperature below or equal
  to 0C, rain cooled by evaporation forms snow
  despite environmental temperature above freezing.

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Warm air typical has more moisture than cold air
giving larger snow flakes.
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Precipitation Types

• Sleet air below freezing, then travels through a
  layer of air above freezing, begins to melt and
  then falls through a layer of air below freezing
  just above the ground surface.
• Freezing Rain ground surface is freezing as rain
  hits the surface it freezes.

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Precipitation Types

• Observation Aircraft Icing
• Aviation hazard is created by the increase in
  weight as ice forms on the body of the airplane.
• Spray plane with anti-freeze.

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Precipitation Types

• Snow Grains solid equivalent of drizzle, no
  bounce or shatter
• Snow Pellets larger than grains, bounce, break,
  crunch underfoot
• Graupel ice particle accumulation with rime
• Hail graupel act as embryo in intense
  thunderstorm, grow through aggregation as pushed
  up by updraft.

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Stepped Art
Fig. 7-29, p. 185
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Measuring Precipitation

• Instruments
• Rain gauge standard, tipping bucket, weighing
• Snow average depth at 3 locations, 101 water
  equivalent
• Doppler Radar
• Transmitter generates energy toward target,
  returned energy measured and displayed
• Brightness of echo amount/intensity of rain
• Doppler measures speed of horizontal rain

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Stepped Art
Fig. 7-33, p. 188
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Somewhat like police radar, Doppler radar can
also determine the storm velocity away from or
towards the radar station by measuring the
frequency difference between the transmitted and
reflected signal. Here m/s stands for meters per
second. 60 m/s 130 mph The figure shows a
tornado. The purple region is moving rapidly
towards the station and then 200 meters to the
right the pink is moving rapidly away from the
station.
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Measuring Precipitation

• Measuring from space
• Specific satellites designed to assess clouds,
  atmospheric moisture, and rain
• TRMM
• CloudSat

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