GEU 0047: Meteorology Lecture 7 Precipitation. Precipitation Types Precipitation: The deposit of...

GEU 0047: MeteorologyLecture 7

Precipitation

Precipitation Types

• Precipitation: The deposit of water (solid or liquid) falling from the clouds and reaching the ground. It includes the following types:– drizzle– rain– snow – sleet (雨夾雪 )– hail (冰雹 )– Glaze (also verglas; 雨淞 )

Water Droplet Sizes

Rates of condensation and evaporation control the increase and decrease in water droplet sizes.

Why “droplet” ? Why not “balls” ?• Saturation (equilibrium) vapor pressure varies with surface area

because the evaporation does also.

• Water molecules are less attached to a curved surface and therefore evaporate more readily.

• More water vapor is needed to reach equilibrium over a curved surface.

• This Curvature effect lessens when the diameter is larger than 10 μm.

Curvature Effect

Greater curvature means a higher Relative Humidity (R.H.) is

needed to achieve equilibrium over the larger surface area.

Smaller

droplets

evaporate more

easily.

Only large

droplets

can survive at

lower R.H.

Condensation Droplets• Typical scenario: adiabatic cooling of ascending air parcel.

• It passes the condensation level (i.e. LCL) and water vapor begins to condense on condensation nuclei, especially hydroscopic.

• Soon, all of the condensation nuclei have seeded tiny water droplets and condensation slows down. (Wet Haze)

• Condensation alone can produce water droplets on the order of ~ 20 m in radius. Further growth requires droplet-droplet interactions.

Solute Effect of Condensation Nuclei

Hydroscopic condensation nuclei allow droplet formation and

growth with less than 100% R.H. (as low as 75% R.H.)

They increase the likelihood of encounters between water

vapor and liquid droplets.

A solution (water and nuclei) also keeps water vapor from

evaporating so easily as it lowers the saturation vapor

pressure (i.e., solute effect).

Concept of Air Friction (review)

• Friction is dependent upon (1) droplet surface area, and (2) velocity.

• Frictional force ~ r2v2 Ff = k 4 r2 v2

As the velocity increases, so does the frictional force.

When the acceleration due to the frictional force is

equal to the gravity, no acceleration takes place.

When velocity stops changing, this is terminal velocity.

Physics of Terminal Velocity

FORCE = Weight - Air resistance

ma = mg - Ff Ff = k4r2v2

a = g - Ff/m

Acceleration occurs as long as the acceleration due to

gravity is greater than the acceleration due to the

frictional force.

Droplet weight (gravity) must overcome drag (air resistance).

The droplet mass must grow.

Terminal Velocity• Gravity increases faster than drag with radius. WHY? • Weight = 4/3 r3 depends on volume• Drag = 4 r2 kv2 depends on area

Vterminal = r1/2 weight = drag

• The terminal velocity increases as drop radius increases.• Both gravity and drag increase with radius, one because of

mass the other because of surface area. Weight is faster.

• This is even ignoring updrafts. So size does matter!

Drop Size• For a drop to fall, weight (r3) must overcome drag (r2).• Therefore, it has to grow to a large enough size.• Cloud drops are 1 million times smaller in total volume than

rain drops.

How Fast

A typical raindrop (2000 m, 6.5 m/s) falls 600x faster than

a typical cloud droplet (20 m, 0.01 m/s).

Using an average cloud height of 6500 meters and D = v t,

the time required for a drop to fall to the ground,

train = 6500 m/6.5 m/s = 1000 s ~ 16 minutes

tdroplet = 6500 m/0.01m/s = 160 hours

Occur in CALM air, ignoring wind or updrafts.

Collision/Coalescence

With a random distribution of sizes, droplets interact because

of their differences in terminal velocities.

N

Size

Clouds

Most clouds do not produce precipitation as condensation and

evaporation continue to work at similar rates on small droplets

within the clouds.

Condensation nuclei and condensation working alone would

take days to form large enough drops to fall to the ground.

Warm CloudsWarm clouds: Clouds that do not have ice crystal formation.

Collision/Coalescence

is the ONLY mechanism

for droplet growth.

Largest rain

drops hit the

ground first.

Stratus

Stratus: cloud droplets can be small

Very little vertical motions (updrafts), so small droplets

with small terminal velocities may reach the ground.

There is less time for collision and coalescence in the thin

stratus layer, so small drops are numerous. Drizzle and/or

misty rain (雨霧 ) is common.

Cumulus

Cumulus: cloud droplets can be very large.

Very large vertical motions (updrafts), so only large enough heavy drops can make it to the ground.

There is more time for collision and coalescence in the thick cumulus cloud with large vertical development, so large drops are numerous.

Towering cumulus can produce heavy rain and even hail.

Ice Nuclei

In order for non-homogeneous freezing to take place at higher

temperatures, ice nuclei that are structurally similar to ice must be present in order to initiate freezing.

Silver and lead iodide, cupric sulfide ( 硫酸銅 ), kaolinite (高嶺土 ) are examples.

Natural sources include clay soils, plant material, bacteria and

of course ice. Without these ice nuclei, saturated air will be

supercooled (often to ~ -40oC) before freezing would actually

take place.

Cloud Seeding

a form of weather modification (often refer to increase rainfall.)

Providing ice nuclei with the right geometry to facilitate

condensation, growth and precipitation. (CO2 drop; i.e., dry ice)

Cool Clouds

Colder clouds have ice nuclei which allow faster growth rates

for cloud droplets.

Water Versus Ice

es = o exp{L/Rv (1/To-1/T)} Saturation Vapor Pressure

Saturation Vapor Pressure

For equilibrium between condensation and evaporation, more

water vapor is needed over water than over ice.

(Curvature and Temperature are the difference)

Maximum SVP Difference

Supercooled water vs. Ice occurs when T ~ - 12 oC

Ice Crystals

Some water vapor

sublimates onto ice.

This removes water vapor

from the air and then causes

more evaporation of water

in order to establish

equilibrium.

BUT adding more water

vapor to the air, increases

ice crystal growth.

Bergeron ProcessEvaporation is larger near

droplet, condensation is

larger around ice nuclei.

Water diffuses from drop

to ice.

Removing water vapor

increases evaporation

around droplet.

Ice crystals grow at the

expense of water droplets.

Ice Crystal Growth

• Two mechanisms facilitate the growth of ice crystals.– Accretion: the growth of ice crystals as supercooled water

freezes onto them.

– Aggregation: the coalescense of ice crystals through adhesion (falling ice crystals collide and stick to other ice crystals).

Accretion

Most precipitation begins with

falling ice crystals as they have

much higher growth rates than

water droplets acting under

condensation alone.

The physical conditions those ice

crystals encounter as they fall

ultimately determines the type

of precipitation that reaches the

ground.

Aggregation

Growth of dendrite ice crystal

otherwise known as snow

through aggregation and

adhesion.

Dendrite (snow) is favored

because its growth rate is the

highest at the maximum

difference in S.V.P. between

water and ice near -12oC.

Snow Flakes• Dendrite Ice Crystals

Fracturing

Collision and fracturing provide

more ice nuclei.

Ratios of ice nuclei to droplets

1:100,000 precipitation

1:1 nothing

100,000:1 nothing

Even ice needs to get heavy

to fall.

Ice Crystal HabitsIce crystal growth

is dependent upon

pressure and

temperature.

Growth rates are

highly dependent

upon the S.V.P.

difference between

water and ice.

Precipitations• LIQUID

– Virga 幡狀雲 (wispy rain)– Mist– Drizzle– Rain

• ICE (Graupel:雪丸 )– Freezing Drizzle– Freezing (Icy) Rain (Glaze)– Sleet (雨夾雪 )– Snow– Hail

virga

sea mist

Rain or Snow?Melting Level is a function of season, higher up in summer.

Rain Profile• A deep warming layer always results in rain.

RainRain Intensities (inch/hour)

Light 0.01- 0.10

Moderate 0.11- 0.30

Heavy > 0.30

Rain Gauges• All official rain gauge instruments have 8-inch diameter

openings.

Simple Standard Advanced

Precipitation Frequency

Virga• Precipitation that evaporates before reaching the ground

Snow Profile• Temperatures well below freezing allow snow to reach the

ground.

Snowfall Intensity

• Snowfall intensity is not measured by accumulation as in the case of rainfall, usually.

• Intensity is measured by visibility (or lack thereof).

Heavy < 0.25 mile (~ 6 inch/day)

Moderate 0.25 - 0.50 mile

Light > 0.50 mile

Snowfall Frequency

Sleet Profile• A very deep freezing layer causes supercooled water

droplets to freeze into tiny ice pellets before reaching the ground.

Sleet Formation• The formation of sleet along a warm front.

Glaze Profile• A shallow freezing layer allows supercooled water droplets

to freeze upon contact creating an ice coating.

GlazeVery beautiful,

very heavy,

very dangerous,

very damaging.

Glaze

Glaze

Glaze

Graupel• The tiny ice pellets that collect from ice crystals, can form

very large raindrops (upon melting), very clumpy snow, and is the source for hail.

Hail Storm Frequency

HailGraupel ice that is allowed to

accumulate by accretion

and/or aggregation.

Associated with strong

updrafts.

The larger the hail, the stronger

and more severe the storms

associated with the strongest

updrafts.

Hail FormationThe tiny ice pellets

(graupel) can make

many trips up and

down in a large

cumulonimbus

cloud.

Each trip adds another

layer, like rings on a

tree for each season.

Rime ( 白霜 )• Supercooled water droplets (freezing fog or drizzle) can

create a winter wonderland when deposition takes place.

Summary