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PRECIPITATION

COALESCENCE MODEL

Only effective with droplets greater than 40 microns in diameter

Caused by collision among droplets

Use of giant nuclei

Probably quite common in tropical oceans because of giant particles of sea salt

May also occur associated with turbulent mixing between the cloud and its drier environment. For example, dry air injected into a cloud near its top will evaporate cloud droplets until the mixed air reaches saturation. If this mixed blob of air rises and cool the water vapor available is then shared by fewer droplets, allowing them to grow to a larger size than they would otherwise

BERGERON FINDEISEN THEORY OF PRECIPITATION

Named for Scandinavian meteorologist Tor Bergeron who first described the process in 1939.

Process applied to cold clouds which are at temps below 0 C

Based on the fact that the relative humidity of air is greater with respect to an ice surface than with respect to a water surface at the same temperature. This is because water droplets vaporize more readily from liquid water than from solid ice at temps below freezing because water is more energetic in the liquid form than in the solid phase.

-this is particularly true for temps between -5C and -25C where the difference exceeds .2 mb

-if ice crystals and super cooled water droplets exist together in a cloud (which is common from -10 C to -20 C) the latter tend to evaporate and direct deposition takes place from the vapor to the ice crystals

Small water droplets can be supercooled to -40C before spontaneous freezing occurs (due to the fact that the water must slow down its motion enough to arrange itself in the proper form for freezing

-however, ice crystals generally predominate in clouds where temps are lower than -22 C

Sources of Freezing Nuclei in Storms

Two sources of ice forming and freezing nuclei exist in the atmosphere.

l. derived from the background aerosol present in the atmosphere

2. produced by the storm itself from local soil particles aerosolized by wind turbulence

-may be several orders of magnitude greater than 1

-fraction in a given size range of aerosol particles acting as

ice forming nuclei increases with size of particles

Freezing nuclei are less numerous than condensation nuclei but some are active at higher temperatures

Threshold temps at which various substances produce ice crystals

kaolinite -8

carbon dioxide 0

silver iodide -4

lead iodide -6

magnetite -8

illite -9

montmorillinite -16

As the ice crystals grow larger and heavier they begin to fall at an accelerating rate and as the larger crystals fall, they collide and coalesce with supercooled water droplets and with the smaller ice crystals in their path, thereby growing still larger. Eventually the ice crystals become heavy enough to fall out of the cloud. If air temps are below freezing most of the way to the ground the crystals reach the surface in the form of snowflakes. If the air below the cloud is above freezing, snowflakes melt and fall as rain.

Deposition nuclei cause direct freezing while freezing nuclei promote the freezing of water droplets.

The difference in vapor pressure causes water vapor molecules to move (diffuse) from the liquid drops toward the ice crystals. The removal of vapor molecules reduces the vapor pressure above the droplets. Since the droplet is now out of equilibrium with its surroundings, it evaporates to replenish the diminished supply of water vapor above it. This provides a continuous source of moisture for the ice crystals, which absorb the water vapor and grow rapidly by depositions.

In some clouds ice crystals might collide with supercooled droplets which freeze on contact and stick together, a process called accretion or timing. The icy matter (rime) that forms is called graupel. As the graupel falls it may fracture of splinter into tiny ice particles when it collides with cloud droplets. these splinters may then go on themselves to become new graupel, which in turn may produce more splinters

When several cloud layers are present in atmosphere natural seeding may be important. If ice crystals fall from high level cirrostratus or altostratus (releaser cloud) into nimbostratus (spender cloud) composed of supercooled water droplets, the latter can grow very rapidly and lead to long periods of much precip (common in winter cyclonic storms)

Once ice crystals have formed they grow readily by deposition from vapor

l. Number of ice crystals grow because small splinters become detached and serve as fresh sources

2. ice crystals readily aggregate on collision and tens of crystals may form a snowflake (esp at temps of 0 to -5) because thin films of water on the surface freeze when contact is made, bindingthem together

3. eventually the crystals fall

COLLISION- COALESCENCE

Once the droplets or crystals have reached about 40 microns they

may begin to fall and collide with other small droplets and crystals in their path

-larger drops fall faster (higher terminal velocity)and therefore catch up and overtake other smaller droplets

-however the collection efficiency is very slight when the particle is less than 40 microns (10 percent efficiency) but increases to 50 percent at 60 microns

-process is enhanced if droplets have opposite electrical charges

Coalescence is also enhanced if cloud is thick. A very large cloud droplet of 200 microns falling in still air will take 12 minutes to travel through a 500 m thick cloud and over an hour if the cloud is 2500 m thick. Rising air currents in a forming cloud slow the rate at which droplets fall and a thick cloud with strong updrafts will maximize the time cloud droplets spend in the cloud and hence the size to which they can grow

Most important factor is liquid water content of cloud.. Other significant factors are

1. relative droplet size

2. electrical charge

3. cloud thickness

4. updrafts of the cloud

PRECIPITATION

Minimum size for a raindrop to reach surface

diameter .01 inches

typical .05 inches

max .2 inches

Diameter of a raindrop is about 1/6 of splash mark on surface

TERMINAL VELOCITY

water droplets or ice crystals that compose clouds are so minute that they remain suspended indefinitely unless they vaporize or undergo considerable growth

-terminal velocity is speed of downward movement

-for cloud droplets this terminal velocity is usually less than 1 cm/second

Speed of a falling cloud droplet or ice crystal in calm air is regulated by two forces

1. force of gravity which accelerates tHE particle downward

2. an opposing force caused by the resistance of the air through which the particle descends

As the particle accelerates downward it meets with increasing air resistance and eventually the resistance force equals the force of gravity and particle drifts downward at a constant speed known as the terminal velocity

Generally terminal velocity increases with the size of the particle and the larger the particle the more vigorous the updraft must be in order to keep the particle in suspension

It takes about one million cloud droplets to form a single raindrop about 2 mm in diameter

Types of Precipitation

l. Rain

-precipitation in the form of liquid drops

-most start out as snowflakes or hailstones

-most are from 1 to 6 mm in diameter

-raindrops from warm clouds are usually smaller than those from cold clouds and are usually less than 2 mm in diameter unless there is extraordinary convection

-regardless of size of cloud drops size on reaching ground

depends on

l. amount of evap on descent

2. frictional effect will break up drops

-descending air means larger drops

2. Drizzle

-light intensity rain barely reaching ground

-usually falls from low status because there is no time

for coalescence

3. Mist

-when droplets evaporate before ground is reached

-condensation occurs in rising air that has cooled to sub freezing temperature, hexagonal ice crystals form

-may exist in isolated forms of may coalesce to form snowflakes in varying sizes and shapes

4. Sleet

-water droplets falling from clouds encountering cold air layer

near ground and solidify

-essentially frozen rain

-usually occurs beneath a warm front

5. Hail

-product of violent convection

-rounded or jagged lumps of ice, often characterized by concentric internal layering resembling the internal structure of an onion

-develops within intense thunderstorms as strong convection currents carry ice pellets upward into the middle and upper reaches of a thunderhead

-along the way the ice pellet grows by collecting supercooled water droplets

-hailstones eventually become too heavy to be supported by convective updrafts and fall to earth.

-these hailstones are large enough to survive the trip to the ground as ice even though surface temps are well above freezing

-most hailstones are harmless granules of ice less than 1 cm in diameter called graupel but can be larger

6. Glaze

-also called freezing rain

-occurs when rain hits a cold ground surface

-usually occurs below a warm front aloft

7. Rime

-freezing fog

-forms as a thick frosty deposit when objects with less then 0 C temps encounter a fog (i.e. ships and planes)

-as aircraft begin to fly in supercooled water clouds exposed surfaces are iced up, especially the leading edges of wings

-surfaces collect masses of ice and freezing rain by the impact freezing of supercooled cloud droplets

-weight becomes a problem and also the distortion of the aerofoil shape

-changing elevation usually solves the problem quickly as does the use of heating systems or even flexing the leading edges of wings or other crucial parts

8. Snow

-develops directly from water vapor that deposits directly as a solid, bypassing the liquid state

-forms first as a six sided hexagonal crystal

-crystal grows fastest at 6 points because they are more

exposed to water vapor

-snow pellets (graupel)

-supercooled droplets freeze on ice crystals

-transition between sleet and snow

-snow grains originate like drizzle except that they are frozen and are usually less than 1 mm

8. virga-rain evaporating before it reaches the surface

Problems of Measuring Precipitation at the Surface

Types of Precipitation Gages