Transcript Collision and Coalescence - CSU Radar Meteorology Group

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
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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
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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
– Ice crystals continuously grow at the water droplet’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
Temperature (°C)
Crystal Habit
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
o
43 F
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
coating of rime, it’s called graupel
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.