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Chapter 7 – Precipitation Processes

Understanding Weather and ClimateAguado and Burt

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Precipitation

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Why Cloud Droplets Don’t Fall

• Gravity vs. Updraft– Terminal Velocity – The final speed obtained

by an object falling through the atmosphere. A balance between gravity and the frictional forces (drag) on the object.

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How Large is Large Enough?

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Growth of Cloud Droplets

• Condensation• Warm Clouds

– Collision and Coalescence• Cool and Cold Clouds

– Bergeron Process– Riming and Aggregation

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Condensation

• Can lead to quick growth for small water droplets.

• Once radii of 20 microns is passed, the efficiency of condensation is reduced.

• By itself condensation would produce few raindrops.

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Collision

• Collisions between drops– Dependent on absolute

size of the collector drop and relative size of the droplets below.

– Most efficient when droplets are slightly smaller than the collector drop.

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Coalescence

• When droplets stick together after colliding.• Coalescence efficiencies are usually

assumed to be near 100%

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Bergeron Process

• Depends on the co-existence of supercooled water and ice in the same cloud.

• Saturation vapor pressure over ice is less than that over water at the same temperature.

• Leads to a continuous transfer, in which supercooled droplets surrender water vapor which is subsequently deposited onto the ice crystals.

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Bergeron Process

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Riming

• Analogous to droplet collision• Riming is the collision of ice crystals and

supercooled water droplets.• Causes rapid grow of ice crystals and

acceleration of their fall speeds.

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Aggregation

• The joining together of two ice crystals to form a single larger crystal.

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Types of Precipitation

• Snow• Rain• Graupel• Hail• Sleet• Freezing Rain

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Snow

• Results from the growth of ice crystals through deposition, riming and aggregation.

• A snowflake’s structure depends on the temperature and moisture conditions that exist when the crystal is formed.– Warmer conditions provide for riming and a

wet snow pack– Colder conditions provide for less adhesion

(riming) and a “powder”

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US Snowfall

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Raindrop Shape

• Not tear drop shaped!

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US Precipitation

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Graupel

• When riming coats an ice crystal and its 6-sided sharp edges are lost into a milky-white spongy texture.

• Graupel can fall to the ground or remain in the cloud providing a nucleus for hail.

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Hail

• Ice pellets forms in roughly concentric layers.

• Formed by a repetitive sequence, where ice falls into a region with liquid droplets and provides a liquid coating, upon lifting the ice crystal higher the coating freezes and the process restarts. (Figure 6-13 in text book)

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Hail

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Sleet

• Raindrops freeze in the air while falling to the surface.

• Must have a layer in the lowest portion of the atmosphere with a temperature below 0°C.

• Requires an inversion aloft.

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Sleet

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Freezing Rain

• Light rain or drizzle of supercooled water droplets fall through air at or slightly below 0°C. It freezes when it hits the ground.

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Measuring Rainfall

• Raingauge – A instrument which collects the rain and enables its measurement– Tipping Rain Gauges – Weighing-Bucket Gauges

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Measurement Errors

• Rain gauges only represent a point measurement.

• Errors:– Wind generated turbulence can deflect

precipitation from entering the gauge– Splashing– Snow drifts

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Snow Measurement

• Measure depth• Water equivalency – depth of water which

would result if all of the snow was melted.• Conversion factors – 10:1• Snow pillows – big mattresses filled with

antifreeze liquid and connected to a pressure regulator.

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Measuring Precipitation by Radar

• Intensity of backscattered radiation to the radar indicates the size of the particles (droplets, snow flakes, gruapel or hailstones)

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Radar Estimates