Transcript
3/3/2010
1
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
3/3/2010
2
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
3/3/2010
3
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
3/3/2010
4
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’sexpense
– 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
3/3/2010
5
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
3/3/2010
6
43oF 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
3/3/2010
7
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
3/3/2010
9
Lake effect snow
Lake effect snow
• Heating
– Water warmer
than land in
fall and early
winter
– Unstable
environment
3/3/2010
10
Lake effect snow
• Air rises,
quickly
reaching
saturation due
to addition of
moisture from
lake
(evaporation)
Lake effect snow
Lake effect snow
3/3/2010
11
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)
3/3/2010
12
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
top related