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Chapter 8 - Precipitation

Rain Drops, Cloud Droplets, and CCN• Recall the relative sizes of rain drops, cloud drops, and CCN:

– raindrops - 2000 μ m = 2 mm• fall at a speed of 4-5 ms-1

– cloud drops - 20 μ m = 0.02 mm• remain suspended in the air

– CCN - 0.2 μm = 0.0002 mm• remain suspended in the air

• To get a droplet (20 μ m) to grow to raindrop size (2000 μ m) it must increase in size by a factor of 100 (two orders of magnitude): – 2000 μ m/20 μ m = 100 – this occurs in about 30 minutes in a

thunderstorm!

• How does this happen?– condensation– collision/coalescence– ice-crystal process

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Cloud Droplet Growth by Condensation• Consider pure water in equilibrium with air above it

– then the RH = 100% – evaporation = condensation – vapor pressure (e) = saturation vapor pressure (es)

• if evaporation > condensation, water is ________ • if evaporation < condensation, water is __________• Now, a droplet surface is not flat as shown to the right ->

instead, it has curvature.....

Flat versus Curved Water Surfaces• More energy is required to maintain the "curvature" of the

drop – The water molecules on the surface of the drop have more energy – Evaporation rate off curved surface > evaporation rate off of flat

surface• Since air above both surfaces is saturated

– evaporation rate = condensation rate– condensation rate onto droplet > condensation rate onto flat water

surface– esdrop > esflat

• Therefore:– if RHflat = 100%, then RHdrop

> 100% – the air surrounding the drop

must be supersaturated!! • This is called the

curvature effect– the curvature effect is larger

for smaller drops

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Solution Droplets• Note that the previous discussion

is valid for a pure water drop• if a droplet is comprised of a

solution – it can be in equilibrium with the

environment at a much lower RH – this explains the formation of haze

• This process of condensation will grow drops , but not to precipitation sizes ~2 mm

• Q: So, if a droplet grows to some size by condensation, how can it continue to grow to precipitation size?

Droplet fall speed is called its terminal velocity

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Collision/Coalescence• This is a dominant process for

precipitation formation in warm clouds (tops warmer than about -15°C)

• Some cloud droplets will grow large enough and will start to fall in the cloud -->>

• Since the bigger drops fall faster than the smaller drops, they will "collect" the smaller drops - the bigger drop grows

• Need droplets of different sizes for this process to really work

• Q: what determines the droplets fall speed relative to the ground?? – A: droplet size and updraft

strength

Collision/Coalescence - cloud droplet growth by collision

Life cycle of a droplet1. The drop initially forms in

the updraft of the cloud near cloud base

2. It grows in size by condensation and collisions

3. then the drop will begin to fall when Vt > w

4. Continue growth by coalescence

• Factors promoting growth by collision/coalescence– Different drop sizes -->– thicker clouds– stronger updrafts– consider a shallow stratus

deck....

• Vg = w + Vt– Vg = ground relative fall speed of the

drop – w = updraft velocity – Vt = drop's terminal velocity

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Precipitation Growth in Cold Clouds -Warm versus Cold Clouds

• Our previous discussion regarding droplet growth by condensation and collisions is valid for warm clouds:– warm clouds - have tops warmer than about 0°C– comprised entirely of water

• Cold Clouds– Cold clouds are defined as those clouds with tops colder than

0°C– can be comprised of:

• water • super-cooled water - liquid droplets observed at temps less than

0°C • ice

– Notice that super cooled water is found at altitudes where:• -40°C < Temp < 0°C

– only ice is found at altitudes above -40°C– Q: So how does frozen precipitation form in cold clouds?

The distribution of ice and water in a cumulonimbus cloud.

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Ice crystal formation by Homogeneous Freezing

• Pure water drops do NOT freeze at 0°C– it needs to be colder

• Bigger water drops will freeze at warmer temperatures than smaller drops

• Smaller water drops require colder temperatures to freeze

• Hence, you will find more smaller drops than larger drops higher in the cloud

• Any liquid water droplet, no matter how small, must freeze spontaneously below –40°C

Ice formation through Ice nuclei

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Ice Crystal Growth through Deposition

• After ice crystals are initialized through homogeneous freezing or ice nuclei, ice crystal will growth initially through vapor deposition.

• The deposition is normally slow, but in the presence of supercooled water, the growth can be fast.

Saturation vapor pressure over water and ice

• Recall that es(water) > es(ice)

• Where is this difference a maximum?

• es(water) - es (ice) is a maximum at -15°C

• Given a mixed cloud containing water and ice, which type of particle (ice v.s. water) will grow more quickly? – ice particles will grow more

readily than water particles

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The ice-crystal (Bergeron) processIce crystals grow

larger at the expense of the surrounding water droplets.

The fate of falling large ice crystals

Collect super cooled water droplets

Break up Collect small ice crystals

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Accretion/RimingIce collect water

• Occurs when super cooled drops freeze onto ice particle

• The resultant particle is often referred to as graupel

Aggregation

• Occurs when two ice particles stick together, forming one larger particle

• All of the aforementioned processes are occurring in a cold cloud to form ice particles– homogeneous freezing – deposition – contact freezing – accretion – aggregation

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Natural seeding by cirrus clouds may form bands of precipitation downwind of a mountain chain.

Ice crystals falling from a dense cirriform cloud into a lower nimbostratus cloud

How ice crystals grow and produce precipitation in clouds with a low liquid-water content and a high liquid-water content.

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Precipitation Types- Rain• Rain - drop diameter > 0.5 mm• drizzle - drop diameter < 0.5 mm (largely

produced by stratus)• virga - rain leaving cloud base and evaporates

before hitting the ground– often visible as evaporating streaks of precipitation

Precipitation Types- Rain

Rain Events:• Showers - localized, sometime heavy rain

events – usually associated with Cb– sometimes called a "cloud burst"

• continuous rain - from nimbostratus....

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Rain Events• Showers - localized, sometime heavy rain events

– usually associated with Cb– sometimes called a "cloud burst"

• Continuous rain - from nimbostratus....

Precipitation Types - Snow• Snow - often visible as fall streaks associated

with high cirrus

• Snow Events:• Flurries - weak, intermittent - produced from

developing Cu• Snow squalls - brief, heavy snow fall -

produced from Cu• Steady Snow - continuous for hours - produced

from Nb• Blizzard - low temperatures, strong winds,

blowing snow...

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Average annual snowfall over the United States. (NOAA)

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The dangling white streamers of ice crystals beneath these cirrus clouds are known as fallstreaks. The bending of the streaks is due to the changing wind speed with height

Precipitation Types- Ice

Habits

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Computer color-enhanced image of dendrite snowflakes.

Other types of frozen precipitation

• The vertical variation of temperature near the ground can have a dramatic influence on the type of precipitation that is observed at the ground:– Sleet– Freezing rain

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Vertical Temperature Profile for Sleet• If a deep freezing layer exists at low levels, sleet

may form• sleet - tiny ice pellets that are transparent

Vertical Temperature Profile for Freezing Rain

• If a shallow freezing layer exists at low levels, freezing rain, or glaze may form upon striking a cold object.

• This is the type of temperature structure that is necessary for an ice storm

• Note: at low levels on will experience freezing rain, at higher levels, it will be rain

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Rime Ice• Rime ice - super cooled drops in fog/cloud

that freeze onto objects on ground• Big problem on Mt. Washington

A heavy coating of freezing rain (glaze) covers Syracuse, New York, during January, 1998, causing tree limbs to break and power lines to sag.

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An aircraft undergoing de-icing during inclement winter weather.

Preventing aircraft icing when aircraft fly through an area of freezing rain or through a region of large supercooled droplets.

Average annual number of days with freezing rain and freezing drizzle over the United States. (NOAA)

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Vertical temperature profiles (solid red line) associated with different forms of precipitation.

Snow Grains and Snow Pellets• snow grains—(Also called

granular snow.) Precipitation in the form of very small, white opaque particles of ice; the solid equivalent of drizzle. They resemble snow pellets in external appearance, but are more flattened and elongated, and generally have diameters of less than 1 mm; they neither shatter nor bounce when they hit a hard surface.

• snow pellets—(Also called soft hail, graupel, tapioca snow.) Precipitation consisting of white, opaque, approximately round (sometimes conical) ice particles having a snowlike structure, and about 2–5 mm in diameter. Snow pellets are crisp and easily

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The formation of snow pelletsIn the cold air of a convective cloud, with a high liquid-water content, ice particles collide with supercooled cloud droplets, freezing them into clumps of icy matter called graupel. Upon reaching the relatively cold surface, the graupel is classified as snow pellets.

Hail• Usually formed in strong storms by accretion of

super-cooled water onto graupel, or other frozen particles

7 in.

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Hailstone Formation

1. Hailstones begin as embryos (usually ice particles) that remain suspended in the cloud by violent updrafts.

2. When the updrafts are tilted, the ice particles are swept horizontally through the cloud, producing the optimal trajectory for hailstone growth. Along their path, the ice particles collide with supercooled liquid droplets, which freeze on contact.

3. The ice particles eventually grow large enough and heavy enough to fall toward the ground as hailstones.

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A large hailstone first cut then photographed under regular light (top) and polarized light (bottom). This procedure reveals its layered structure.

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Measuring Precipitation

• Rain Gauges:

• tipping bucket rain gauge– bucket tips for

every 0.01" of precipitation

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Tipping bucket rain gauge• Each time the bucket fills with one-hundredth of an

inch of rain, it tips, sending an electric signal to the remote recorder.

Measuring Precipitation

• Radar estimates of rain -->>– radars transmit

electromagnetic waves that bounce off of precipitation particles

– more radiation is scattered if:

• have many particles• particles are large

• R= aZb relationship– Z: radar reflectivity– R: Rain rate

Z

R