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Collision and Coalescence

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					  ATS 351
   Lab 7
Precipitation


March 7, 2006
   Droplet Growth by Collision and
            Coalescence
• Growth by condensation alone takes too long
• Occurs in clouds with tops warmer than 5°F (-15°C)‫‏‬
• Greater the speed of the falling droplet, the more air
  molecules the drop encounters
• Important factors for droplet growth
   –   High liquid water content within the cloud
   –   Strong and consistent updrafts
   –   Large range of cloud droplet sizes
   –   Thick cloud
Collision and Coalescence
    Droplet Growth by the Bergeron
               process
•   Cold clouds
•   Homogeneous nucleation of ice
•   Vapor deposition
•   Accretion
•   Aggregation
 Homogeneous nucleation of ice
• Freezing of pure water
   – Enough molecules in the droplet must join together in a
     rigid pattern to form an ice embryo
   – Smaller the amount of pure water, the lower the
     temperature at which water freezes
• Supercooled droplets
   – Water droplets existing at temperatures below freezing
• Homogeneous nucleation (freezing) occurs at
  temperatures of –40°C
• Vapor deposition
   – From vapor to solid
   – Not likely in our atmosphere
                          Ice nuclei
• Ice crystals form in subfreezing air on particles called ice nuclei
• Ice nuclei are rare; only one out of 10 million aerosols is an effective
  ice nuclei
• Fewer sources than CCN
   – Desert and arid regions: silicate particle (dominant)‫‏‬
   – Clay particles: for temperatures between –10 and –20°C
   – Volcanic emissions
   – Combustion products
   – bacteria
   – IN may be de-activated when exposed to atmospheres with high
      concentrations of Aitken nuclei produced by industrial processes
   – Oceans are NOT good sources of IN
                  IN requirements
• Insolubility
   – If soluble, cannot maintain molecular structure requirement for ice
• Size
   – Must be comparable, or larger than, that of a critical ice embryo
      (typically 0.1 microns)‫‏‬
• Chemical bond
   – Must have similar hydrogen bonds to that of ice available at its
      surface
• Crystallographic
   – Similar lattice structure to that of ice (hexagonal)‫‏‬
• Active Site
   – Pits and steps in their surfaces
     Heterogeneous nucleation
• Vapor deposition
  – Direct transfer of water vapor to nucleus
• Condensation-freezing Condensation of vapor
  onto surface, followed by freezing
• Immersion
  – Ice nucleus immersed within a drop
• Contact
  – Collision with supercooled droplets, freezing upon
    impact
          Growth mechanisms
• Vapor deposition
  – Saturation vapor pressure over water greater than over
    ice
  – Supercooled liquid droplets more readily evaporate and
    contribute to the vapor pressure than sublimation from
    ice
  – When ice and liquid coexist in cloud, water vapor
    evaporates from drop and flows toward ice to maintain
    equilibrium
  – Ice‫‏‬crystals‫‏‬continuously‫‏‬grow‫‏‬at‫‏‬the‫‏‬water‫‏‬droplet’s‫‏‬
    expense
  – The process of precipitation formation in cold clouds by
    ice crystal diffusional growth at the expense of liquid
    water droplets is known as Bergeron process
         Growth mechanisms
• Diffusional growth alone not sufficient for
  precipitation formation
• Accretion
  – Ice crystals collide with supercooled droplets,
    which freeze upon impact
  – Forms graupel
  – May fracture or split as falls, producing more
    ice crystals
         Growth mechanisms
• Aggregation
  – Collision of ice crystals with each other and
    sticking together
  – Clumping of ice crystals referred to as a
    snowflake
           Precipitation Types- Ice Habits



Environmental             Crystal Habit
Temperature (°C)‫‏‬


0 to -4                   thin plates

-4 to -6                  needles

-6 to -10                 columns

-10 to -12                plates

-12 to -16                dendrites, plates

-16 to -22                plates

-22 to -40                hollow needles
                    Snow
• Snowflakes can generally fall 300m
  (1000ft) below the freezing level before
  completely melting
• Dry vs. wet
  – Moist air slightly above freezing, snowflakes
    slightly melt forming thin film of water along
    edges; snowflakes stick together
  – Extremely cold air with a low moisture content,
    small, powdery flakes fall
           43 oF   and Snow?
• Snow occurs when air temperature above
  freezing if very dry air
• Evaporative cooling can allow a rainy day
  to change to snowfall
• Need a wet-bulb temperature at freezing or
  below
                   Graupel
• Ice crystals falls through cloud,
  accumulating supercooled water droplets
  that freeze upon impact
  – Creates many tiny air spaces
  – These air bubbles act to keep the density low
    and scatter light, making the particle opaque
• When ice particle accumulates heavy
  coating‫‏‬of‫‏‬rime,‫‏‬it’s‫‏‬called‫‏‬graupel
                        Hail
• Hailstones form when either graupel particles or
  large frozen drops grow by collecting copious
  amounts of supercooled water
• Graupel and hail stones carried upward in cloud
  by strong updrafts and fall back downward on
  outer edge of cloud where updraft is weaker
• Hail continues to grow and carried into updraft
  until so large that it eventually falls out bottom of
  cloud
               Hail growth
• As hailstone collects supercooled drops
  which freeze on surface, latent heat
  released, warming surface of stone
• At low growth rates, this heat dissipates into
  surrounding air, keeping surface of stone
  well below freezing and all accreted water
  is frozen
• Referred to as dry growth of hailstone
               Hail growth
• If hailstone collects supercooled drops
  beyond a critical rate or if the cloud water
  content is greater than a certain value, latent
  heat release will warm surface to 0°C
• Prevents all accreted water from freezing
• Surface of hailstone covered by layer of
  liquid water
• Referred to as wet growth of hailstone
                    Hail layers
• Alternating dark and light layers
• Wet growth
   – solubility of air increases with decreasing temperature
     so little air dissolved in ice during wet growth
   – Ice appears clear
• Dry growth
   – Hailstone temperature close to environmental
     temperature so at cold temperatures, large amount of air
     dissolved
   – Ice appears opaque
Lake effect snow
Lake effect snow
            • Heating
              – Water warmer
                than land in
                fall and early
                winter
              – Unstable
                environment
Lake effect snow
            • Air rises,
              quickly
              reaching
              saturation due
              to addition of
              moisture from
              lake
              (evaporation)‫‏‬
Lake effect snow
Lake effect snow
              Lake effect snow
• Wind fetch
   – Length of trajectory of wind across lake
   – Greater the distance the wind blows over warm water,
     the greater the convection
• Frictional difference
   – When wind moves from over water to land, friction
     slows it down, resulting in surface convergence and
     lifting
• Large-scale forcing
   – Enhancement of lake-effect snow
Case study (Dec 1998)‫‏‬
Case study (Dec 1998)‫‏‬
Case study (Dec 1998)‫‏‬
Case study (Feb 2007)‫‏‬
    Global Distribution of Precipitation
• Annual precipitation on
  earth is equal to the
  annual evaporation.
• The general circulation
  of the atmosphere gives
  clues as to where maxima
  and minima in
  precipitation can be
  found.
  – Precipiation minima are
    found in regions of
    widespread subsidence
  – Precipitation maxima are
    found in regions of
    widespread upward
    vertical motion
             Rain Shadow
• A rain shadow is an arid region on the lee
  side of a mountain range
• Caused by the adiabatic cooling and
  warming of air parcels as they travel over
  the topography
• Why the western slopes in CO receive more
  snowfall than the eastern slopes.

				
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