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Lecture 13: PrecipitationLecture 13: Precipitation
W & H: Sections 6.4 and 6.5W & H: Sections 6.4 and 6.5
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?
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.
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
LaterLater
Air is super-saturated with respect to ice, but sub-saturated with respect to water
Crystal ShapesCrystal Shapes
Crystal shapes determined by temperature Crystal shapes determined by temperature and supersaturationand supersaturation
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
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
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
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”
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
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
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