Notes

Outline

Chapter 7: Precipitation Processes

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Precipitations

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Terminal Velocity

Raindrops

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How Raindrop Grows?

Growth by Condensation

Growth in Warm Clouds

Collision Collector drops collide with smaller drops. Due to compressed air beneath falling drop, there is an inverse relationship between collector drop size and collision efficiency. Collisions typically occur between a collector and fairly large cloud drops. Smaller drops are pushed aside. Collision is more effective for the droplets that are not very much smaller than the collect droplet.

Coalescence When collisions occur, drops either bounce apart or coalesce into one larger drop.  Coalescence efficiency is very high indicating that most collisions result in coalescence.  Collision and coalescence together form the primary mechanism for precipitation in the tropics, where warm clouds dominate.

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Cool and Cold Clouds

An Example of  Cool and Cold Cloud

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Growth in Cool and Cold Clouds

Bergeron Process

Riming and Aggregation

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Forms of Precipitation

Snow Snowflakes have a wide assortment of shapes and sizes depending on moisture content and temperature of the air. Snowfall distribution in North America is related to north-south alignment of mountain ranges and the presence of the Great Lakes.

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"Rainshowers:" Rainshowers: are episodic precipitation events associated with convective activity and cumulus clouds Raindrop shape: As frictional drag increases, changes to a mushroom shape Drops eventually flatten Drops split when frictional drag overcomes the surface tension of water Splitting ensures a maximum drop size of about 5 mm and the continuation of the collision-coalescence process

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Graupel and Hail Graupel are ice crystals that undergo extensive riming Lose six sided shape and smooth out Either falls to the ground or provides a nucleus for hail Hail are concentric layers of ice around graupel Up and downdraft interactions in thunderstorms Great Plains = highest frequency of hail events

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Hail Frequency in the U.S.

Sleet and Freezing Rain Sleet begins as ice crystals which melt into rain through a mid-level inversion before solidifying in colder near surface air Freezing Rain forms similarly to sleet, but does not fully refreeze

Twomey effect, 1974

Cloud Seeding The objective is to convert some of the supercooled droplets in a cool clouds to ice and cause precipitation by the Bergeron process. Two primary methods are used to trigger the precipitation process. Dry ice is used to lower cloud temperature to a freezing point in order to stimulate ice crystal production leading to the Bergeron process. Silver iodide initiates the Bergeron process by directly acting as freezing nuclei. Under ideal conditions, seeding may enhance precipitation by about 10%.

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Measuring Snow Accumulated snow measured Water equivalent of snow = 10 to 1 ratio Automated snow pillows Convert weight to water equivalent