Physical Geography Lecture 08 - Precipitation, Air Masses, and Storms 110216
Lecture 13: Precipitation
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Transcript of Lecture 13: Precipitation
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Lecture 13: PrecipitationLecture 13: PrecipitationW & H: Sections 6.4 and 6.5W & H: Sections 6.4 and 6.5
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Cloud Droplets vs. Raindrops Cloud Droplets vs. Raindrops
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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
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Diffusional GrowthDiffusional Growth
In condensation, water vapor molecules In condensation, water vapor molecules migrate toward the droplet by migrate toward the droplet by diffusiondiffusionTo 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!
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Growth by CollectionGrowth by Collection
Droplets collide and coalesce, forming a Droplets collide and coalesce, forming a larger dropletlarger dropletProblem: How can droplets collide?Problem: How can droplets collide?
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If all droplets were the same size, probability of collisions would be very small.
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CollisionsCollisions
Requires large “starter droplets” Requires large “starter droplets” Have larger terminal velocities than Have larger terminal velocities than smaller droplets smaller droplets
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Large droplets overtake smaller ones and collide with them.
The droplets may coalesce, forming a larger droplet.
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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 increasesincreasesResult: Growth rate of collector drop Result: Growth rate of collector drop increases rapidly.increases rapidly.
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t = 0
t = t
t = 2t
t = 3t
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Diffusional Growth vs. CollectionDiffusional Growth vs. Collection
Diffusional Growth
Growth by collection
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Growth of Ice ParticlesGrowth of Ice Particles
Diffusional growth (deposition)Diffusional growth (deposition)Accretion (riming)Accretion (riming)AggregationAggregation
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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
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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
%11060.287.2100)( iceRH
super-saturation with respect to ice = 10%
Result: rapid growth
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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
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es,i es,w
ambient vapor pressure
InitiallyInitially
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es,i es,w
ambient vapor pressure
LaterLater
Air is super-saturated with respect to ice, but sub-saturated with respect to water
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es,i es,w
ambient vapor pressure
EventuallyEventually
No droplets left; ice-crystals stop growing
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Ice crystal growing at expense of surrounding supercooled droplets. Fig. 6.36 in W & H
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Crystal ShapesCrystal Shapes
Crystal shapes determined by temperature Crystal shapes determined by temperature and supersaturationand supersaturation
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W & H: Fig. 6.40.
Hexagonal Plates
Column
Dendrite
Sector Plate
Bullet Rosette
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Accretion (Riming)Accretion (Riming)
Ice particles collide with super-cooled Ice particles collide with super-cooled dropletsdropletsDroplets freeze onto ice crystalsDroplets freeze onto ice crystals Produces a rimed ice crystalProduces a rimed ice crystal
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Rimed ice crystals
W & H, Fig. 6.41
Graupel
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AggregationAggregation
Clumping together of ice crystalsClumping together of ice crystals (This is how snowflakes are formed)(This is how snowflakes are formed)
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Precipitation InitiationPrecipitation Initiation
1.1. Drop growth by collectionDrop growth by collectionGrowth 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
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Precipitation TypesPrecipitation Types
Rain, snow, sleet & freezing rain, hailRain, snow, sleet & freezing rain, hailIn cold clouds, precipitation starts as snow In cold clouds, precipitation starts as snow in cloudin cloudPrecipitation 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
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Sleet FormationSleet Formation
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HailHail
Forms in cumulonimbus cloudsForms in cumulonimbus cloudsStarts as small ice crystalStarts as small ice crystalIce crystal moves through region of Ice crystal moves through region of supercooled water & grows by accretionsupercooled water & grows by accretionAMS GlossaryAMS Glossary
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Hail GrowthHail Growth
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Weather Radar: PurposesWeather Radar: Purposes
1.1. Detection of precipitationDetection of precipitation2.2. Detection of tornadoesDetection of tornadoes
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Detection of PrecipitationDetection of Precipitation
Radar transmits microwavesRadar transmits microwavesStrength 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”
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ExampleExample
A 14-level volume scan mode
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Base vs. Composite ReflectivityBase vs. Composite Reflectivity
Base reflectivity just shows the lowest Base reflectivity just shows the lowest angle scanangle scanComposite shows the strongest echo from Composite shows the strongest echo from any levelany levelComparisonComparison http://http://
www.srh.noaa.gov/srh/jetstream/doppler/comwww.srh.noaa.gov/srh/jetstream/doppler/comprefl.htmprefl.htm
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Sample Reflectivity DisplaySample Reflectivity Display
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Velocity DisplayVelocity Display
Shows radial velocity of precipitation Shows radial velocity of precipitation particlesparticlesUses 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.
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Straight-Line MotionStraight-Line MotionOn velocity display, red indicates motion away from the radar
Green indicates motion toward the radar
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ExampleExample
RadarStorm movement
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Detection of RotationDetection of Rotation
Radar
Motion toward radar
Motion away from radar
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NWS TutorialNWS Tutorial
http://http://www.srh.noaa.gov/srh/jetstream/doppwww.srh.noaa.gov/srh/jetstream/doppler/doppler_intro.htmler/doppler_intro.htm