How Precipitation Forms

Forms of Precipitation

Precipitation Measurements

Intentional Weather Modification

Inadvertent Weather Modification

3/04/03

Outline for Lecture 10

standard atmosphere 300mb 30,000 ft

standard atmosphere 500mb 18,000 ft

standard atmosphere 700mb 10,000 ft

standard atmosphere 850mb 5,000 ft

How Precipitation FormsImportant Facts:

-cloud droplets are tiny(20 micrometers).-many condensation nuclei are present. -tiny particles fall moreslowly than large ones.-a cloud droplet’s diameter must grow~200 times to reach a raindrop’s diameter.-to attain the volume ofa rain droplet, the clouddroplet increases a million times in volume.

•The Bergeron Process (Cold Clouds)

•The Collision-Coalescence Process (Warm Clouds)

Precipitation Formation Mechanisms

The Bergeron process describes how rain or snow forms when the cloud temperature is below freezing.

This process where ice crystals grow at the expense of cloud droplets is called the Ice Crystal Process. It is also named after the Norwegian researcher who discovered it (Tor Bergeron, there were others).

Temperature RH wrt* H2O(liq) RH wrt H2O(ice)

0°C 100% 100%

-05°C 100% 105%

-10°C 100% 110%

-15°C 100% 115%

-20°C 100% 121%

*wrt = with respect to

Three important properties of water droplets:

1. Cloud droplets do not freeze at 0.°C

2. Supercooled (water in the liquid state below 0°C) water droplets will freeze immediately if agitated sufficiently or when they come in contact with freezing nuclei (a crystalline structure similar to ice)

3. The saturation vapor pressure with respect to ice is lower than the saturation vapor pressure with respect to liquid water.

Ice

Saturation with Respect to Ice and Water

•Vapor pressure is the pressure due to water vapor molecules when the evaporation rate is equal to the condensation rate.•Because of the crystalline structure of ice, water molecules are not able to break free from an ice surface as easily than from a water surface. •Therefore, the saturation vapor pressure with respect to an ice surface would be less than the saturation vapor pressure with respect to a liquid water surface at a given temperature.

Liquid Water

Growth Process by Bergeron Process

Coexistence of supercooled water droplets and ice crystals is essential to precipitation process in the Mid latitudes.

-10°C

-10°C

-40°C20,000 ft

3,000 ft

1,000 ft Water Droplets

Supercooled water droplets and ice crystals

Ice crystals top

middle

lower

-20°C7,000 ft

Gravitational Force = Frictional Force

Terminal Velocity occurs when: F = 0(when Fgravity = Ffriction )

· 4/3 · r3 · g = · r2 · v ·

4/3 · g · r = v

mass · gravity = area · velocity ·

(velocity is a function of r)

Fall Velocity for Cloud droplets as a function of size

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6

Diameter of Cloud droplets (millimeter)

Fal

l Vel

oci

ty (

km/h

r)

From: Table 5-2

Maximum Fall Distance Before Evaporation

1.0E-06

1.0E-04

1.0E-02

1.0E+00

1.0E+02

1.0E+04

1.0E+06

0 500 1000 1500 2000 2500 3000

drop diamter (um)

max

fall

dist

ance

(m)

From Table 5-A

•As droplets fall they collide with smaller droplets and coalesce.

• after collecting ~1 million clouddroplets the particle is large enoughto fall without evaporating.

•Because there are a large number ofcollisions needed, clouds with greatvertical extent are typically produceprecipitation by this process.

The Process from Warm Clouds:The Collision-Coalescence Process

Mist drizzle rain/sleet

0.005-0.05 mm

less than 0.5mm

0.5 – 5 mm

Approximate size of types of Precipitation

Forms of Precipitation

(Rain, Snow, Sleet and Glaze, Hail)

Droplet size determines the type of precipitation.

Rain is the term for drops of water that fall from a cloudand have a diameter of 0.5 millimeter (mm). Drizzle and misthave smaller droplets.

Rain mostly occurs in nimbostratus clouds and cumulonimbusclouds. These clouds are capable of producing cloudbursts.

Most rain starts as snow or ice crystals; as the snow falls throughthe cloud it melts. Drizzle is a fine uniform water droplet with adiameter less than 0.5 mm.

Rain

Temperature Profile for Rain

Snow is precipitation in the form of ice crystals (snowflakes) or more often, aggregates if ice crystals. The size and structure of the crystals is a function of the temperature at which they form.

When air temperatures are cold the moisture content is verysmall. This results in the formation of very light fluffy snowmade up of six sided ice crystals.

When conditions are warmer, the ice crystals join together intolarger clumps consisting interlocked aggregates of crystals.

Snow

Temperature Profile for Snow

Sleet and Glaze

Sleet is a wintertime phenomenon that refers to the fall of small particles of ice that are clear to translucent.

Sleet forms when rainpasses through a coldlayer of air and freezesinto ice pellets. Thisoccurs most often in the winter when warmair is forced over a layerof cold air.

Temperature Profile for Sleet and Glaze

HailHail is precipitation in the form of hard, rounded pellets orirregular lumps of ice. The layers of ice accumulate as the hailstone travels up and down in a strong convective cloud.

Hailstones begin as smallice pellets that grow by addingsupercooled water droplets asthey move through the cloud. Asthe ice crystal cycles up and downin the cloud the hailstones increasein size until they are forced out bya downdraft or become heavyenough to fall out.

This large hailstone fell from a tornadic supercell northeast of Breckenridge, TX. The stones left a path miles wide littered with three to four and one half inch stones. The hail fell from a dark cloud base to the south of a developing mesocyclone. No rain or thunder occurred in the immediate area while the stones were coming down.

Hail

4”

Rime

Rime is a deposit of ice crystals formed by the freezing of super cooled fog or cloud droplets on objects whose surface temperature is below freezing. When rime forms on trees, it covers them with ice feathers; in windy conditions only the windward surfaces will accumulate the layer of rime.

Table 5-4

Precipitation Measurements

Standard Instruments:

Simple rain gauge, standard rain gauge, tipping bucket gauge, weighing gauge

Radar Summary

Good estimates of rainfall amounts can be obtain from radar summaries. Radar is less reliable for frozen precipitation. Coverage still sparse in many locations.

Tropical Rain Measuring Mission (TRMM)

Idealized Global Precipitation Representation

As of late 1997, measurements of the global distribution of rainfall at the Earth's surfacehad uncertainties of the order of 50% and theglobal distribution of vertical profiles of precipitation was far less well determined.

TRMM will provide the firstspaceborne rain radar andmicrowave radiometric datathat will measure the verticaldistribution of precipitationover the tropics in a band between ± 35' in latitude. Such information will greatlyenhance our understandingof the interactions betweenthe sea, air and land masses which produce changes in global rainfall and climate.TRMM observations will alsohelp improve modeling of tropical rainfall processes andtheir influence on global circulation leading to betterpredictions of rainfall and itsvariability at various time scales.

Tropical Rainfall Measuring Mission (TRMM)

Intentional Weather Modification

The deliberate human intervention to influence atmospheric processes that constitute the weather.

Much of the motivation to attempt weather modification techniqueswas based on anecdotal evidence. Resultswere never conclusive.

Hail cannons where popular in the late1800’s, with the belief that injection of smokeparticles into developing clouds could increase the ice nuclei and reduce the sizeof hailstones or suppress hail altogether.

Charles M. Hatfield (1876-1958)

Lots of people in Southern California knew about Hatfield, who by then rated folk hero status. He called himself the Moisture Accelerator, but others knew him best as The Rainmaker. Since 1902 when his first experiments with chemicals and "evaporating tanks" dampened his father's ranch near Oceanside, Hatfield had pleased people in Los Angeles and farmers in the Central and San Joaquin valleys by fulfilling hundreds of their rainmaking contracts.

The Moisture Accelerator!

1946 Vincent J. Schafer discovered that dry ice dropped into supercooled clouds spurred the growth of ice crystals whicheither induced precipitation or dispersed fog or clouds.

Cloud Seeding

In order for cloud seeding to trigger precipitation, conditions mustbe just right.

• Clouds must be present; seeding cannot create clouds.• A portion of the clouds must contain supercooled water.

One method assumes that the clouds are lacking in freezing nucleiand adding them will stimulate precipitation by the Bergeron process. One must be careful not to overseed as this will produce too many, too small ice crystals.

Cloud Seeding

Precipitation is a complicatedprocess. After fifty years of cloud seeding experimentation and studythe scientific community is stilllacking a full understanding of precipitation processes.

Cloud Seeding

Frost Prevention

Temperature zonation duringan inversion. Temperature increases with height to the topof the inversion and then decreases. Frost protectiontechniques use the warmer air above the crop asa heat source.

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