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

– 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

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 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.