Precipitation Processes - Universitetet i oslo · producing rapid growth. –Aggregation occurs...

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Precipitation

Processes

Chapter 7 Lecture

Redina L. Herman

Western Illinois University

Understanding

Weather and

Climate

Seventh Edition

Frode Stordal, University of Oslo


• Introduction

– Drag, an opposing resistance, and gravity are two forces

acting on a falling object.

– Terminal velocity is the final speed obtained by an object

falling through the atmosphere, when friction with

surrounding air balances the force of gravity.

– Small objects fall slower than large objects.

– When cloud drops increase about 100 times larger, a

sufficient terminal velocity will overcome updrafts and rain

drops will fall.

Growth of Cloud Droplets


• Introduction



• Growth by Condensation

– Condensation nuclei form most cloud drops but after all the

available condensation nuclei have attracted water, any

further condensation can only occur on existing droplets.

– With so many droplets competing for a limited amount of

water, none can grow very large by condensation.

– Two other processes are responsible for further droplet

growth.



• Growth in Warm Clouds

– Warm clouds have temperatures above 0°C throughout and

can be found in the tropics and also the midlatitudes during

the warm season.

– Collision–coalescence causes precipitation of warm clouds.

– Collision–coalescence begins with large droplets, called

collector drops, which have high terminal velocities.

– As the collector drops fall, they overtake smaller droplets in its

path and provides the opportunity for collisions and

coalescence.



• Growth in Warm Clouds



• Growth in Cold and Cool Clouds

– Cold clouds have temperatures below 0°C and consist of

ice crystals.

– Cool clouds have temperatures above 0°C in the lower

range and subfreezing conditions in the higher range.

– Clouds may be composed of liquid water, supercooled water,

and/or ice.

– The coexistence of ice and supercooled water is critical to

the creation of cool cloud precipitation—the Bergeron

Process.




– Saturation vapor pressure of ice is less than that of

supercooled water and water vapor.

– During coexistence, water will sublimate directly onto ice.

– Ice crystals grow rapidly at the expense of supercooled

drops.

– Collisions between falling crystals and drops causes growth

through riming and aggregation.




– Riming occurs when liquid water freezes onto ice crystals

producing rapid growth.

– Aggregation occurs when the joining of multiple ice

crystals through the bonding of surface water builds ice

crystals to the point of overcoming updrafts.

– Collision combined with riming and aggregation allows the

formation of precipitation within 1/2 hour of initial

formation.



• Snow

– Snow is precipitation that reaches the surface without

melting and is formed by the Bergeron process, riming, and

aggregation.

– Snowflakes have a wide variety of shapes and sizes

depending on moisture content and temperature of the cloud.

Distribution and Forms of Precipitation


• Snow



• Rain



• Rain

– Raindrops begin with a spherical shape but as frictional

drag increases, it changes to a mushroom shape.

– Drops eventually flatten and split.

– Splitting results in a maximum drop size of about 5 mm

and the continuation of the collision–coalescence process.



• Rain



• Graupel and Hail

– Graupel are ice crystals that undergo riming and lose their

six-sided shape. It either falls to the ground or provides a

nucleus for hail.

– Hail forms as concentric layers of ice built around graupel as

it is carried around in the updrafts until it falls.

– Hailstones are very heavy and capable of tremendous

amounts of damage.

– The Great Plains and Canada have a high frequency of

hailstone events.



• Graupel and Hail


Precipitation Processes - Universitetet i oslo · producing rapid growth. –Aggregation occurs...

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