Lecture 13: PrecipitationLecture 13: Precipitation

W & H: Sections 6.4 and 6.5W & H: Sections 6.4 and 6.5

Cloud Droplets vs. Raindrops Cloud Droplets vs. Raindrops

ExerciseExercise

What is the ratio of the volume of a typical What is the ratio of the volume of a typical raindrop to the volume of a typical cloud raindrop to the volume of a typical cloud droplet?droplet? rrdropletdroplet = .01 mm = .01 mm

rrraindropraindrop= 1 mm= 1 mm

Answer: 10Answer: 1066

Diffusional GrowthDiffusional Growth

In condensation, water vapor molecules In condensation, water vapor molecules migrate toward the droplet by migrate toward the droplet by diffusiondiffusion

To grow a raindrop by diffusion would take To grow a raindrop by diffusion would take weeks!weeks!

There must be a faster way!There must be a faster way!

Growth by CollectionGrowth by Collection

Droplets collide and coalesce, forming a Droplets collide and coalesce, forming a larger dropletlarger droplet

Problem: How can droplets collide?Problem: How can droplets collide?

If all droplets were the same size, probability of collisions would be very small.

CollisionsCollisions

Requires large “starter droplets” Requires large “starter droplets”

Have larger terminal velocities than Have larger terminal velocities than smaller droplets smaller droplets

Large droplets overtake smaller ones and collide with them.

The droplets may coalesce, forming a larger droplet.

As large droplet falls, its speed and size As large droplet falls, its speed and size increaseincrease

rate of collection of smaller droplets rate of collection of smaller droplets increasesincreases

Result: Growth rate of collector drop Result: Growth rate of collector drop increases rapidly.increases rapidly.

t = 0

t = t

t = 2t

t = 3t

Diffusional Growth vs. CollectionDiffusional Growth vs. Collection

Diffusional Growth

Growth by collection

Growth of Ice ParticlesGrowth of Ice Particles

Diffusional growth (deposition)Diffusional growth (deposition)

Accretion (riming)Accretion (riming)

AggregationAggregation

Diffusional GrowthDiffusional Growth

Consider a mixed population of supercooled Consider a mixed population of supercooled droplets and ice crystalsdroplets and ice crystals Concentration of droplets >> number of crystalsConcentration of droplets >> number of crystals

Ambient vapor pressure Ambient vapor pressure e es,ws,w > e > es.is.i

ice crystals will grow rapidly ice crystals will grow rapidly

Diffusional Growth of CrystalDiffusional Growth of Crystal

For example, T = -10For example, T = -10CC ees,ws,w = 2.87 hPa; e = 2.87 hPa; es,is,i = 2.60 hPa = 2.60 hPa

Ice crystal finds itself in a highly super-Ice crystal finds itself in a highly super-saturated environmentsaturated environment

%11060.2

87.2100)( iceRH

super-saturation with respect to ice = 10%

Result: rapid growth

Evaporation of DropletsEvaporation of Droplets

As ice crystals grow, they deplete water As ice crystals grow, they deplete water vaporvapor

vapor pressure falls below evapor pressure falls below es,ws,w

droplets begin to evaporatedroplets begin to evaporate

es,i es,w

ambient vapor pressure

InitiallyInitially

es,i es,w

ambient vapor pressure

LaterLater

Air is super-saturated with respect to ice, but sub-saturated with respect to water

es,i es,w

ambient vapor pressure

EventuallyEventually

No droplets left; ice-crystals stop growing

Ice crystal growing at expense of surrounding supercooled droplets. Fig. 6.36 in W & H

Crystal ShapesCrystal Shapes

Crystal shapes determined by temperature Crystal shapes determined by temperature and supersaturationand supersaturation

W & H: Fig. 6.40.

Hexagonal Plates

Column

Dendrite

Sector Plate

Bullet Rosette

Accretion (Riming)Accretion (Riming)

Ice particles collide with super-cooled Ice particles collide with super-cooled dropletsdroplets

Droplets freeze onto ice crystalsDroplets freeze onto ice crystals Produces a rimed ice crystalProduces a rimed ice crystal

Rimed ice crystals

W & H, Fig. 6.41

Graupel

AggregationAggregation

Clumping together of ice crystalsClumping together of ice crystals (This is how snowflakes are formed)(This is how snowflakes are formed)

Precipitation InitiationPrecipitation Initiation

1.1. Drop growth by collectionDrop growth by collection

Growth of ice crystals by diffusion, Growth of ice crystals by diffusion, accretion, and aggregation.accretion, and aggregation.

#1 is dominant in the tropics (T > 0#1 is dominant in the tropics (T > 0C)C)

1 & 2 are important in the middle latitudes1 & 2 are important in the middle latitudes

Precipitation TypesPrecipitation Types

Rain, snow, sleet & freezing rain, hailRain, snow, sleet & freezing rain, hail

In cold clouds, precipitation starts as snow In cold clouds, precipitation starts as snow in cloudin cloud

Precipitation at surface depends on Precipitation at surface depends on temperatures below the cloudtemperatures below the cloud Can get rain, snow, sleet, or freezing rainCan get rain, snow, sleet, or freezing rain

Sleet FormationSleet Formation

HailHail

Forms in cumulonimbus cloudsForms in cumulonimbus clouds

Starts as small ice crystalStarts as small ice crystal

Ice crystal moves through region of Ice crystal moves through region of supercooled water & grows by accretionsupercooled water & grows by accretion

AMS GlossaryAMS Glossary

Hail GrowthHail Growth

Weather Radar: PurposesWeather Radar: Purposes

1.1. Detection of precipitationDetection of precipitation

2.2. Detection of tornadoesDetection of tornadoes

Detection of PrecipitationDetection of Precipitation

Radar transmits microwavesRadar transmits microwaves

Strength of return signal depends on Strength of return signal depends on precipitation intensity precipitation intensity

Radar unit does a 360Radar unit does a 360 scan at various scan at various elevation angleselevation angles Called a “volume scan”Called a “volume scan”

ExampleExample

A 14-level volume scan mode

Base vs. Composite ReflectivityBase vs. Composite Reflectivity

Base reflectivity just shows the lowest Base reflectivity just shows the lowest angle scanangle scan

Composite shows the strongest echo from Composite shows the strongest echo from any levelany level

ComparisonComparison http://http://

www.srh.noaa.gov/srh/jetstream/doppler/comwww.srh.noaa.gov/srh/jetstream/doppler/comprefl.htmprefl.htm

Sample Reflectivity DisplaySample Reflectivity Display

Velocity DisplayVelocity Display

Shows radial velocity of precipitation Shows radial velocity of precipitation particlesparticles

Uses Doppler effectUses Doppler effect Frequency of return signal is different from Frequency of return signal is different from

frequency of transmitted signal. frequency of transmitted signal.

Straight-Line MotionStraight-Line Motion

On velocity display, red indicates motion away from the radar

Green indicates motion toward the radar

ExampleExample

RadarStorm movement

Detection of RotationDetection of Rotation

Radar

Motion toward radar

Motion away from radar

NWS TutorialNWS Tutorial

http://http://www.srh.noaa.gov/srh/jetstream/doppwww.srh.noaa.gov/srh/jetstream/doppler/doppler_intro.htmler/doppler_intro.htm