Precipitation

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Precipitation

• EAS 211
• Spring 2005
• 04/01/05

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Precipitation Processes

• Condensation
• Responsible for initial stages of cloud droplet
  growth.
• Air rises and cools to become saturated
• Some H2Ovapor then condenses on cloud
  condensation nuclei.
• This can produce rapid growth of very small water
  droplets (lt20µm) which are too small to fall as
  precipitation.

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There are two major processes that form
precipitation in clouds.

• Collision-Coalescence
• Bergeron Process

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Collision-coalescence

• Warm cloud process (Tgt-15ºC)
• Large droplets may form on large CCN or by random
  collisions w/droplets these begin to fall.
• Droplets collide with smaller, slower moving
  droplets essentially absorbing those in their
  path.
• The larger the droplet, the more likely it is to
  collide and stick to other droplets
• Coalescencethe merger of cloud droplets by
  collisions
• NOTE This is most common growth process for
  clouds and precipitation in the Tropics.
• Factors governing raindrop production
• Increase liquid water content
• Range of droplet sizes
• Cloud thickness
• Cloud updrafts
• Electrical charge of droplets

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Formation of ice crystals through nuclei

• There are two ways ice nuclei can be used to form
  an ice crystal.
• Homogeneous freezing
• Ice nuclei

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Homogeneous freezing

• No nucleus is present, so enough molecules with
  in the water droplet join together to from a tiny
  ice embryo.
• At higher temps, thermal agitations will break up
  the embryos.
• So only larger cloud droplets can freeze in this
  way at Temp gt -40 Celsius
• Colder than this, however, and any ice embryo
  will grow larger.

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Ice nuclei

• Are present in the atmosphere, but in much
  smaller quantities than cloud condensation nuclei
• There are 3 types of ice nuclei
• Deposition nucleiwater vapor changes directly to
  ice upon the nucleus
• Freezing nucleinuclei that cause freezing after
  immersion in a super-cooled drop
• Contact nucleinuclei that can cause super-cooled
  water to freeze upon contact with the nucleus.

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HOWEVER

• Both of these processes are not dominant and do
  not produce many ice crystals. Another process
  is at work
• Bergeron Process

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Bergeron

• Cold cloud process in which at least part of the
  cloud is composed of ice crystals usually
  involves a co-existence of super-cooled (water
  present in small quantities below freezing) and
  frozen droplets.
• Given a cloud containing super-cooled water and
  ice crystals, this cloud will be saturated. So
  the number of molecules entering and leaving the
  droplet and ice crystal must be equivalent.
• But since the es (saturation vapor pressure) is
  greater than the es over ice at a given temp,
  water molecules will escape the droplet surface
  more easily than the ice sfc.
• The difference in vapor pressure will cause water
  vapor to move from the droplet to the ice
  crystal.
• To remain in equilibrium, the droplet must
  replenish the water vapor molecules surrounding
  it, so it will evaporate some more of its water
• So we can see that ice crystals grow larger at
  the expense of the surrounding water droplets.
• This is enough to cause light precip then
  further growth results from riming (accretion)
  and aggregation.

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• Rimingwhen ice crystals fall through a cloud and
  collide with super-cooled droplets, the droplets
  freeze onto them. This causes a rapid growth of
  ice crystals, which further increases fall speed
  and further increases riming. Cyclic
• Aggregationthe joining of ice crystals to form a
  single, larger one.
• Occurs most easily when the ice crystals have a
  thin coating of liquid water to make them more
  adhesive (such as when the temp is just below
  freezing)

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Bergeron Process
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Precipitation Types

• Rainfalling drops of liquid waterdiameter of
  drops is greater than or equal to 0.5 mm (average
  1-6 mm)
• Drizzlefalling drops of liquid waterdiameter of
  drops is less than or equal to 0.5 mm
• Virgaprecip which evaporates before reaching the
  ground.
• Snowprecipitation of ice crystals commonly
  shaped as dendrites or plates with six sides.
  They can be as small as 50 microns or as large as
  5 mm.
• Wet snowtemp. just below freezing dense, wet
  snowpack, large flakes.
• Dry snowtemp well below freezing small, powdery
  flakes.

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Precip. Types continued

• GraupelIce crystals that form by riming to
  produce a spongy texture (diameter about 5 mm)
  can fall to the ground as precip or remain in
  cloud to serve as nuclei for hail development.
• If it falls to the ground it is called snow
  pellets or during the summer, soft hail
• Common in higher elevations.
• Hailice pellets formed in roughly concentric
  layers
• Mostly ice with small amounts of air mixed in
• Size of hail depends on strength of updraft,
  range from pea-size to softball size.
• The larger the hailstone, the greater the
  terminal velocity
• Terminal Velocityfinal speed of object falling
  through the atmosphere, when friction of the air
  balances gravity

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More precip.

• Sleet
• Raindrops which freeze in air while falling to
  the surface.
• Precip usually falls initially as snow, passes
  through a warm layer (inversion) (gt0 ºC) and
  melts to rain closer to the sfc, temp lt 0ºC so
  the rain freezes.
• Semi-transparent pellets smaller than 5 mm in
  diameter
• Commonly associated with warm fronts
• Ice pelletssame as sleet but smaller diameter
  (lt0.5 mm)
• Freezing rain
• Super-cooled rain drops which freeze when coming
  into contact with the surface (Tlt0ºC) to form a
  continuous layer of ice on the sfc.
• Process is similar to that for sleet, where
  snowflakes melt in a warm layer, but the sfc is
  shallower in this case so the rain drops do not
  freeze in the air.
• Mistsuspended microscopic water droplets in the
  air
• Drifting snowsnow moved by light winds to low
  heights above ground level.
• Blowing snowsnow moved by stronger winds, raised
  to moderate or high heights, that impairs
  visibility.

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Formation of Hail
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Formation of Sleet
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Measuring Precipitation

• There are several ways in which to measure
  precipitation
• Raingauge
• Diameter is 8 inches
• Precip funnels into a tube with 1/10 the sfc area
  of the collector, so the depth of the accumulated
  water undergoes a ten-fold increase.
• This amplification allows us to measure the
  amount of precip precisely (to the nearest
  hundredth) by inserting a calibrated measuring
  stick into the water, and measuring the depth of
  the wet portion

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• Tipping bucket
• An automated collector which provides a record of
  the timing and intensity of the precip
• When precip accumulates, it funnels into one of
  two pivoting buckets
• When the upright bucket collects 0.01 of water,
  it tips over, emptying contents and causing the
  other bucket to become upright
• As the bucket tips, it sends a signal to the
  gauges computer telling it to record 0.01 of
  rainfall.
• Weighing bucket
• Similar to the tipping bucket, new accumulations
  of rain are constantly recorded
• Weighing mechanism translates weight of
  accumulated water into a depth and records it
  automatically.

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Problems with raingauges

• It is a point source
• Wind can deflect precip away
• Wind can blow precip into
• Water can splash out of or into
• Water may evaporate before it is measured.

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Other precip measurements

• Radar
• Radar sends microwave signals which are scattered
  by precipitation particles.
• The amount backscattered to the radar depends on
  precipitation intensity
• Computer algorithms translate intensity to
  precipitation rate (mm/hr)
• Not very accurate, more useful when calibrated
  with rain gauge measurements
• Snow measurements
• Can use a raingauge that is heated (to melt snow
  for liquid-equivalent) but snow does not always
  fall vertically, and heating can lead to
  evaporation, so this can be reliable
• Best way is to use a ruler to measure snow depth
  and melt it to get the water equivalent.
• Usually, 10 inches of snow will equal 1 inch of
  water, however, wet snow may have a ratio of
  3-41 while very dry snow may have a ratio of
  20-301