Precipitationand

Intro to Radar

ATS 351Lecture 7

October 19, 2009

Droplet Formation Recall the two types of nucleation

Homogeneous Nucleation Water molecules come together to form a cloud

droplet Heterogeneous Nucleation

Requires a cloud condensation Nuclei (CCN)

Heterogeneous Nucleation

Droplet Growth

Once a cloud droplet forms, there are 2 ways it can grow into precipitation

Growth by condensation Growth by Collision and Coalescence

Growth by condensation Very slow process

Growth by Collision and Coalescence More realistic mechanism

Collision and Coalescence

Coalescence occurs in clouds with tops warmer than 5°F (-15°C)

The greater the speed of the falling droplet, the more air molecules the drop encounters

Important factors for droplet growth High liquid water content within the cloud Strong and consistent updrafts Large range of cloud droplet sizes Vertically thick cloud Terminal velocity Droplet electric charge and cloud electric field

Collision and Coalescence

Homogeneous nucleation of ice

Freezing of pure water Enough molecules in the droplet must join together in

a rigid pattern to form an ice embryo The smaller the amount of pure water, the lower the

temperature at which water freezes Supercooled droplets

Water droplets existing at temperatures below freezing

1 ice crystal to 106 liquid droplets at -10°C Homogeneous nucleation (freezing) occurs at temperatures of –

40°C Vapor deposition

From vapor to solid Not likely to be sufficient in our atmosphere

Ice nuclei

Ice crystals (IN) form in subfreezing air on particles called ice nuclei

Ice nuclei are rare; only 1 out of 10 million aerosols is an effective ice nuclei

Fewer sources than CCN Desert and arid regions: silicate particle (dominant) Clay particles: for temperatures between –10 and –20°C Volcanic emissions Combustion products Bacteria

Oceans are NOT good sources of IN

IN requirements Insolubility

If soluble, cannot maintain molecular structure requirement for ice

Size Must be comparable, or larger than, that of a critical ice

embryo (typically 0.1 microns) Chemical bond

Must have similar hydrogen bonds to that of ice available at its surface

Crystallographic Similar lattice structure to that of ice (hexagonal)

Active Site Pits and steps in their surfaces

Growth mechanisms• Vapor deposition

Saturation vapor pressure over water greater than over ice Temperature affects saturation vapor pressure over ice the same

way that it affects saturation vapor pressure over liquid When ice and liquid coexist in cloud, water vapor evaporates

from drop and flows toward ice to maintain equilibrium Ice crystals continuously grow at the water droplet’s expense The process of precipitation formation in cold parts of clouds by

ice crystal diffusional growth at the expense of liquid water droplets is known as Bergeron process

Growth mechanisms

Diffusional growth alone not sufficient for precipitation formation

• Accretion/Riming Ice crystals collide with supercooled

droplets, which freeze upon impact Forms graupel (snow pellets) May fracture or split as falls, producing

more ice crystals

Growth mechanisms

Graupel from Accretion

Accretion of ice from ocean spray

Growth mechanisms• Aggregation

Collision of ice crystals with each other and sticking together

Clump of ice crystals referred to as a snowflake

Common in temperatures near freezing where there may be some liquid water on the surface of the crystal

Differing temperatures can cause aggregates to grow into different shapes

Precipitation Types

Rain - drop greater than 0.5 mm Rarely larger than mm because collisions break

them up What is the shape of a raindrop?

Drizzle - < 0.5 mm Usually from stratus

Snow - small ice of many forms Fallstreaks (like virga, but from cirrus) Flurries (no accumulation) Snow squalls Blizzard - winds > 30 kts

Precipitation Types

Sleet - tiny ice pellets formed from refreezing of rain drops

Translucent (unlike graupel), < 5 mm Freezing rain/drizzle - freezes upon

contact with the surface Can be extremely damaging Knocks out power Pulls down tree branches

Both are common along warm fronts

Damage from freezing rain

Precipitation Types Virga - any precipitation that evaporates before hitting the

surface

Graupel

Ice crystals falls through cloud, accumulating supercooled water droplets that freeze upon impact. Thus, graupel is an example of growth by accretion/riming.

Creates many tiny air spaces These air bubbles act to keep the density low and scatter

light, making the particle opaque

When ice particle accumulates heavy coating of rime, it’s called graupel

Hail An extreme example of growth by accretion Hailstones form when either graupel particles or large frozen

drops grow by collecting copious amounts of supercooled water Graupel and hail stones carried upward in cloud by strong

updrafts and fall back downward on outer edge of cloud where updraft is weaker

Hail continues to grow through updrafts until it’s so large that it eventually falls out bottom of cloud

Hail growth As hailstone collects supercooled drops which freeze on surface,

latent heat released, warming the surface of the hailstone Dry Growth

At low growth rates (caused by lower liquid water contents), this heat dissipates into surrounding air, keeping surface of stone well below freezing and all accreted water is frozen

Wet Growth If a hailstone collects supercooled drops beyond a

critical rate or if the cloud water content is greater than a certain value, latent heat release will warm surface to 0°C

Prevents all accreted water from freezing Surface of hailstone covered by layer of liquid water

Hail layers Alternating dark and light layers Wet growth

solubility of air increases with decreasing temperature so little air dissolved in ice during wet growth

Ice appears clear Dry growth

Hailstone temperature close to environmental temperature so at cold temperatures, large amount of air dissolved

Ice appears opaque

Hail Descriptors

Size (inches) Name0.25 Pea0.75 Quarter1.00 Golf Ball

1.75 Tennis Ball2.50 Baseball2.75 Grapefruit4.00 Giant> 4.00 Ruler measured

• RAdio Detection And Ranging• Transmits a microwave into the

atmosphere and measures the return power– 10, 5, 3 cm typical

• Size chosen depends on use

TransmitterTransmit/Receive

SwitchReceiver Display

Antenna

• Pulse of microwave energy sent out (emitted from antenna to parabolic dish reflector), dish focuses energy into beam

• Beam travels through atmosphere• If the beam hits an object, then some of the energy

is reflected back to the radar• Return power measured• Data processed to a visual display

• Radar measures the intensity of the returned signal, the frequency of the returned signal, and the elapsed time from the transmission of the pulse

• Energy beam travels at the speed of light– Knowledge of the elapsed time allows the

computation of the distance from the radar site

• Frequency uses doppler shift to determine movement

• Only a fraction of the emitted energy gets returned from reflection

• amplified and measured in decibels (dbz), Reflectivity– 1dbz = 10 log(p2/p1)

– p2 = power received at radar, varies

• Reflectivity is dependent upon the size of the object

• In meteorology, the objects are precipitation particles

• The return power (reflected beam) is dependent on the number of particles present, and the size of the particles

• Particle diameter^6 dependence• Number^1 dependence• Larger drops lead to larger reflectivities• Reflectivity mostly based on particle size

• Drizzle: 20 - 25 dBz• Light rain: 25 – 35 dBz• Heavy Rain: 35 – 50 dBz• Thunderstorm Heaviest Rainfall: >50

dBz• Light Snowfall: 15 – 25 dBz• Heavy Snowfall: 25 – 35 dBz

• How much rain falls to the surface in a given hour

• R=inches/hour

• a and b are constants• Higher reflectivity generally corresponds

to higher rainfall rates

Z e=aRb