3/3/2010 ATS 351 Lab 7 Precipitation

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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

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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

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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

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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

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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

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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

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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

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Lake effect snow

Lake effect snow

• Heating

– Water warmer

than land in

fall and early

winter

– Unstable

environment

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Lake effect snow

• Air rises,

quickly

reaching

saturation due

to addition of

moisture from

lake

(evaporation)‏

Lake effect snow

Lake effect snow

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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)‏

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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

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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.