Lecture 13: PrecipitationCloud Droplets vs. RaindropsExerciseDiffusional GrowthGrowth by CollectionSlide 6CollisionsSlide 8Slide 9Slide 10Diffusional Growth vs. CollectionGrowth of Ice ParticlesSlide 13Diffusional Growth of CrystalEvaporation of DropletsInitiallyLaterEventuallySlide 19Crystal ShapesSlide 21Accretion (Riming)Slide 23AggregationPrecipitation InitiationPrecipitation TypesSleet FormationHailHail GrowthSlide 30Weather Radar: PurposesDetection of PrecipitationExampleBase vs. Composite ReflectivitySample Reflectivity DisplayVelocity DisplayStraight-Line MotionSlide 38Detection of RotationNWS TutorialLecture 13: PrecipitationLecture 13: PrecipitationW & H: Sections 6.4 and 6.5W & H: Sections 6.4 and 6.5Cloud Droplets vs. Raindrops Cloud Droplets vs. RaindropsExerciseExerciseWhat 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 mmAnswer: 10Answer: 1066Diffusional GrowthDiffusional GrowthIn 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!Growth by CollectionGrowth by CollectionDroplets collide and coalesce, forming a Droplets collide and coalesce, forming a larger dropletlarger dropletProblem: How can droplets collide?Problem: How can droplets collide?If all droplets were the same size, probability of collisions would be very small.CollisionsCollisionsRequires large “starter droplets” Requires large “starter droplets” Have larger terminal velocities than Have larger terminal velocities than smaller droplets smaller dropletsLarge 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 increasesincreasesResult: Growth rate of collector drop Result: Growth rate of collector drop increases rapidly.increases rapidly.t = 0t = tt = 2t t = 3tDiffusional Growth vs. CollectionDiffusional Growth vs. CollectionDiffusional GrowthGrowth by collectionGrowth of Ice ParticlesGrowth of Ice ParticlesDiffusional growth (deposition)Diffusional growth (deposition)Accretion (riming)Accretion (riming)AggregationAggregationDiffusional GrowthDiffusional GrowthConsider 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 crystalsAmbient vapor pressure Ambient vapor pressure  e es,ws,w > e > es.is.i  ice crystals will grow rapidly ice crystals will grow rapidlyDiffusional Growth of CrystalDiffusional Growth of CrystalFor example, T = -10For example, T = -10CCees,ws,w = 2.87 hPa; e = 2.87 hPa; es,is,i = 2.60 hPa = 2.60 hPaIce 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 growthEvaporation of DropletsEvaporation of DropletsAs 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 evaporatees,i es,wambient vapor pressureInitiallyInitiallyes,i es,wambient vapor pressureLaterLaterAir is super-saturated with respect to ice, but sub-saturated with respect to wateres,i es,wambient vapor pressureEventuallyEventuallyNo droplets left; ice-crystals stop growingIce crystal growing at expense of surrounding supercooled droplets. Fig. 6.36 in W & HCrystal ShapesCrystal ShapesCrystal shapes determined by temperature Crystal shapes determined by temperature and supersaturationand supersaturationW & H: Fig. 6.40.Hexagonal PlatesColumnDendriteSector PlateBullet RosetteAccretion (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 crystalRimed ice crystalsW & H, Fig. 6.41GraupelAggregationAggregationClumping together of ice crystalsClumping together of ice crystals(This is how snowflakes are formed)(This is how snowflakes are formed)Precipitation InitiationPrecipitation Initiation1.1.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 > 0C)C) 1 & 2 are important in the middle latitudes1 & 2 are important in the middle latitudesPrecipitation TypesPrecipitation TypesRain, 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 rainSleet FormationSleet FormationHailHailForms 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 GlossaryHail GrowthHail GrowthWeather Radar: PurposesWeather Radar: Purposes1.1.Detection of precipitationDetection of precipitation2.2.Detection of tornadoesDetection of tornadoesDetection of PrecipitationDetection of PrecipitationRadar 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