Post on 16-Dec-2015
Precipitation Types
Global and Regional Mean Precipitation
1. Types of Precipitation:
2. Measurement:
FIT, Feb 15, 2012 1
Global Mean Precipitation
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Satellite measurement of precipitationWhat happens in Hawaii?
Hawaii Mean Precipitation
3Precipitation measurement by Rain Gauge
Mean = 70 in Without land = 25 in
Honolulu Mean Precipitation
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Surface Analysis (Feb 11, 2012, 18 UTC)
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Hawaii (Feb 12, 2012, 0245 UTC)
6 Radar measurement of rain
Precipitation Types
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Precipitation Types
Drizzle: diameter < 0.5 mmRain: diameter >= 0.5 mm Where are the raindrops larger, Miami or Seattle?
Raindrops almost always < 6 mm because• collision among raindrops• too large raindrops tend to break up
Start as rain, but may fall as drizzle.
Does rain always reach the surface?
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Does rain always reach the surface?
Falling precipitation that evaporate before reaching surface --- Virga
• Low humidity - - evaporation - - drops become smaller - - rate of fall decreases
• Strong updraft Sudden rain shower and cloudburst (Cumuliform) Rain and no-rain in places next to each other (Cumulonimbus) Continuous rain at small vertical currents (layered cloud or Nimbostratus)
How does “Virga” look like and where does it happen?
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Fig. 7-14, p. 174
Virga (Rain not reaching the surface)
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Fig. 2, p. 175
Shape of Raindrops
Spherical < 2 mm ; Parachute > 2 mm(surface area / volume) is minimum for 2.
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Table 7-2, p. 175
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Intensity of Rainfall
Precipitation Types
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•
Fig. 7-15, p. 176
14Falling ice-crystals and snowflakes (Fallstreaks) from cirrus clouds.
Snow
Fig. 7-16, p. 176
Fig. 7-17, p. 178
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Common forms of ice crystal
DendriteWhy is dendrite most common?
Table 7-3, p. 176
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Why is dendrite most common?
Growth = f(temperature, sat. vap. Pr. difference)Maximum growth rate is at -12 to -16 C, when saturation vapor pressure difference between water and ice is maximum.
Table 7-4, p. 178
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Intensity of Snowfall
Is visibility the best way to measure snowfall? Can wind lead to visibility problem?
Fig. 4, p. 179
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Some more terms related to snowfall
Flurries: light and intermittent from developing cumulus cloud
Snow squall: brief but more intense than flurries, usually from cumuliform clouds
Drifting snow: surface movement
Blowing snow: in air movement
Ground blizzard: after falling snow has ended
Blanket of snow: snow covering the landscape
Fig. 4, p. 179
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Snow as Sound Absorbant
Fig. 7-18, p. 179
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Annual Average Snowfall over US
Why is Hawaii not included?
Snow in Hawaii !!
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Mauna Kea
Precipitation Types
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Fig. 7-19, p. 180
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Sleet and Freezing Rain
Partially melted snowflake or cold raindrop - - freezesDeep freezing layer - - Sleet <= 5 mmShallow freezing layer - - freezing rain >=0.5 mm
Fig. 7-20, p. 180
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Formation of Rime
Supercooled droplets in contact with object at below freezing temperature - - white milky granular ice
Fig. 7-21, p. 180
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Destruction due to Freezing Rain
Syracuse, New York, January 1998
Fig. 5, p. 181
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Aircraft de-icing
Fig. 7-22, p. 181
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Average number freezing rain/drizzle days
Fig. 7-23, p. 182
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Vertical Temperature Profile and Precipitation Type
Precipitation Types
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•
Fig. 4, p. 179
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Snow Grains and Snow Pellets
Snow Grains Snow Pellets
Diameter < 1 mm < 5 mm
Shower no yes
Brittleness/bouncy no yes
Origin Stratus cloud Cumulus congestus
Opaque yes yes
Fig. 7-24, p. 182
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Formation of Graupel / Snow Pellets
Ice particle + cloud droplets -- rimed ice crystal -- graupel (dift shape than rime) -- snow pelletDuring summer: graupel - - large raindropVigorously convective cloud: graupel - - hailstones
Fig. 7-27, p. 183
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Formation of Hail/Hailstone
Fig. 7-25, p. 182Fig. 7-26, p. 183
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Hail and Hailstone
Dia = 17.8 cm (7 in) ;Circumference = 47.6 cm (18.7 in); weight > 1.75 lb
Table 7-5, p. 185
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Summary of Precipitation Types
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Measurement of Precipitation
In-situ measurement: Rain gauge; accurate but spotty.
Remote measurement: Radar, Satellite; not so accurate but large coverage.
Combination of the above.
Fig. 7-29, p. 184
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Components of the standard rain gauge
Area of collector = 10 x area of measuring tube
Fig. 7-30, p. 185
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Tipping bucket rain gauge
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Radar
Transmitted energy
backscatter
Fig. 7-31a, p. 187
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Reflectivity Precipitation
Oklahama, April 24, 1999
Fig. 7-31a, p. 187
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Reflectivity Precipitation
Precipitation due to Fay (2008)
THE END
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Fig. 7-23a, p. 182
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Fig. 7-23b, p. 182
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Fig. 7-23c, p. 182
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Fig. 7-23d, p. 182
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Fig. 7-28a, p. 184 Fig. 7-28b, p. 184
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Fig. 7-32, p. 189
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Fig. 7-32, p. 189
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