Download Precipitation I

Cerro Torre
Precipitation
C. David Whiteman
Atmos 3200/Geog 3280
Mountain Weather and Climate
© D. Rabiger
Types of precipitation- Hydrometeors
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Drizzle - fine drops < 0.5 mm close together,
appear to float, but fall to ground (therefore
different from fog). Freezing drizzle, if freezing on
impact.
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Rain - drops > 0.5 mm, widely separated. Freezing
rain if freezes on impact.
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Snow - Branched ice crystals in form of 6-pointed
stars.
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Ice crystals - Unbranched needles, columns or
plates, sometimes called diamond dust.
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Hail - Small balls of ice, falling separately or frozen
together in lumps.
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Snow grains - Very small white, opaque grains of ice
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Ice pellets - Transparent or translucent pellets of
ice, round or irregular (rarely conical) ≤ 5 mm
diameter.
Small hail and/or snow pellets - white, opaque grains
of ice 2-5 mm in diameter, sometimes round,
sometimes conical. Snow pellets also called
graupel.
Justin Cox
Plates
Stellar crystals
Columns
Needles
Spatial dendrites
Capped columns
Irregular particles
Graupel
Sleet
Hail
Depositional Growth
Resulting solid precipitation types - ICSI
Riming
Refreezing of melted snow
“Big-time” riming
Precipitation intensity
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How measured? (see Table 8.2)
The precipitation intensity of rain is determined by the
rate of fall, x
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Light (x ≤ 0.1 inches/hr)
Moderate (0.11 ≤ x ≤ 0.30 inches/hr)
Heavy (x > 0.30 inches/hr)
or by wetting or spray of rain on surfaces.
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The intensity of snow or drizzle occurring alone is
determined by the obstruction to visibility, v
Light (v > 1/2 mile)
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Moderate (1/4 < v ≤ 1/2 mile)
Heavy (v ≤1/4 mile)
How is precipitation measured?
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Rain gauges
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Funnel type gauge with calibrated measuring stick
Recording rain gauges
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Weighing rain gauge
Tipping bucket rain gauge
Snow measurements
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Use rain gauge that is protected from wind by slatted wind
screen
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Average the snow accumulations at several sites (to
compensate for drifting)
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Water equivalent can be determined by melting snow
Specific gravity - a measure of the water content of snow;
water depth equivalent/depth of snow. Range 0.01-0.40. New
snow is typically 0.04-0.10. High snow densities associated
with warm temperatures, small crystals and high winds.
Aerosols are important in precipitation formation
– Cloud condensation nuclei (CCN) –
Tiny particles (aerosols) that assist
in the early stages of cloud droplet
formation and growth
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There are about 10 times more
CCN in continental than
maritime airmasses
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Thus, maritime clouds typically
have fewer but bigger cloud
droplets
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Maritime clouds favor riming
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Bigger droplets
Warmer temperatures (more
cloud water, less ice)
Jay Shafer/Mt. Washington
Why your anemometer
says the air is calm
How do most clouds form?
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Cloud formation:
Lift unsaturated air (DALR) until condensation occurs on
small aerosol particles (cloud condensation nuclei,
CCN, typically 1000s of CCN per cc). Water condenses
preferentially on the largest and most hygroscopic
particles. Condensation releases heat. Clouds have
hundreds of droplets per cc, typically 20 micrometers in
diameter). Additional lifting occurs at MALR. Lifting
above freezing level can occur without the cloud
droplets freezing (supercooling). The small cloud
droplets follow air motions within the cloud, rather than
falling. Typical raindrop is 2 mm in diameter (1,000,000
times larger in volume than cloud droplets). How can
the droplets combine to form rain droplets? There are
two main precipitation formation processes.
How does precipitation form?
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Warm rain process:
In warm clouds (clouds above about -15°C), cloud droplets of different
size collide and coalesce into larger drops. Process is enhanced when
droplets are of different size (and, hence, fall speed) so that the larger,
faster falling drops collide with the smaller, slower falling droplets). The
broad droplet size distribution usually arises because of the initial
presence of aerosol particles of different size. This broad size distribution
is typical of marine aerosols, so this process is common over oceans and
coastal areas.
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Cold rain or ice crystal or Bergeron-Findeisen Process:
Requires the presence of both supercooled water droplets and ice crystals
in a cloud at temperatures below freezing. This process produces most of
the precipitation in temperate continental regions. How are crystals
introduced into a sub-freezing water cloud? They can fall from above,
some of the supercooled droplets can freeze, or ice particles can form on
small aerosols called ice nuclei. [Nucleation] Ice nuclei are much less
common than CCN. Ex: 1 ice nucleus per 1 M droplets at -10°C.
Vapor deposition - Bergeron/Findeisen process
Whiteman (2000)
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Saturation vapor pressure for ice is lower than that for water
Air is near saturation for water, but is supersaturated for ice
Ice crystals/snowflakes grow by vapor deposition
Cloud droplets lose mass to evaporation
Cold rain process
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Larger numbers of IN become activated as air
temperature decreases. Nonetheless, relative scarcity of
IN explains why cloud droplets do not freeze at T < 0°C.
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When cloud droplets and ice crystals coexist, water
vapor diffuses from liquid droplets to ice particles
because the saturation vapor pressure over water is
greater than that over ice. The ice particles grow at the
expense of the water droplets and then fall, oftentimes
gaining additional mass through riming (accretion).
When the ice particles fall through the freezing level
they may melt and reach the ground as rain. The ice
particles, during their fall, may collide and stick to one
another (aggregation).
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At higher elevations, the particles will often reach the
ground as graupel, snow, ice crystals or sleet.
Cold rain process, continued
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Additional lifting can bring the rising air to
lower temperatures where more IN will be
activated and more ice particles form.
The cloud can become glaciated, with
additional heat release (latent heat of
freezing).
Accretion/Riming
Graupel (UCLA)
Hexagonal
Lump
Cone
Magono and Lee (1966)
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Growth of a hydrometeor by collision with supercooled cloud
drops that freeze on contact
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Graupel – Heavily rimed snow particles
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3 types: cone, hexagonal, lump
Aggregation
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Ice particles colliding and adhering with each other
Can occur if their fall speeds are different
Adhering is a function of crystal type and temperature
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Dendrites tend to adhere because they become entwined
Plates and columns tend to rebound
Crystal surfaces become stickier above –5°C
Glaciation
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Glaciation – Conversion from liquid to
mixed-phase (water and ice) cloud
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Mainly ice
Gotta happen for snow
Jay Shafer
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But! Water doesn’t freeze at 32°F/0°C
– Supercooled cloud droplets – exist at
temperatures below 0°C
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They need an ice nucleus to freeze
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Clouds with “warm” cloud tops
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Number of ice nuclei is low when you
are just below freezing
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May have a difficult time glaciating
Need cold cloud tops, or “ice
multiplication” for cloud to glaciate
Mainly water
Summary - Formation of precipitation
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Mechanisms for hydrometeor growth
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Warm rain process
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Condensation
Collision-coalescence
Cold rain process
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Vapor deposition (Bergeron-Findeisen process)
Riming/accretion
Aggregation
Whiteman (2000)
Orographic lifting
Whiteman (2000)
Example - Precipitation in Washington
Green areas are topography; labeled lines
are mean annual precipitation in inches.
West-east cross section through
Washington showing how precipitation
varies with elevation.
Whiteman (2000)
Rising motions from low pressures in mountains
Rising motions can
be produced by the
upslope flow on
mountainsides
caused by flow
around traveling low
pressure centers.
Which direction is
the flow (CCW or
CW)?
Lifting mechanisms
Whiteman (2000)
Cumulonimbus life cycle
Cumulus stage
Mature stage
Decaying stage
Whiteman (2000)
Graupel and rain
Rain shafts versus virga.
Graupel shower over Trail Ridge Road,
RMNP, Colorado
Rainshaft and virga over Mesa Verde, Colorado
Stages in the formation of graupel
© C. D. Whiteman
Gust fronts and downbursts
Whiteman (2000)
Mean lightning flash density in Arizona
Southwest Monsoon
Lopez et al. (1997)
Lightning
Whiteman (2000)
© C. D. Whiteman
Peterson (1962)
Summary
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Precipitation is not produced solely by condensation
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Collision-coalescence is needed for cloud droplets to grow into
rain if cloud >0°C
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In mixed phase clouds
A cloud condensation nucleus is needed to initiate cloud
droplet growth
Mix of ice crystals and supercooled liquid water
Ice crystals form when cloud droplets are activated by an ice nuclei or through
ice multiplication
Ice crystals grow “at expense” of cloud drops (Bergeron-Findeisen)
Accretion can increase the density of falling snow and SWE at ground
Aggregation can further increase hydrometeor size
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Most mid-latitude, continental rain is produced by mixed-phase
clouds and involves the cold rain (ice crystal) process