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GEU 0047: Meteorology Lecture 7 Precipitation

GEU 0047: Meteorology Lecture 7 Precipitation. Precipitation Types Precipitation: The deposit of water (solid or liquid) falling from the clouds and reaching

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  • GEU 0047: MeteorologyLecture 7Precipitation

  • Precipitation Types

    Precipitation: The deposit of water (solid or liquid) falling from the clouds and reaching the ground. It includes the following types:drizzlerainsnow sleet ()hail ()Glaze (also verglas; )

  • Water Droplet SizesRates of condensation and evaporation control the increase and decrease in water droplet sizes.

  • Why droplet ? Why not balls ?Saturation (equilibrium) vapor pressure varies with surface area because the evaporation does also.Water molecules are less attached to a curved surface and therefore evaporate more readily.More water vapor is needed to reach equilibrium over a curved surface.This Curvature effect lessens when the diameter is larger than 10 m.

  • Curvature EffectGreater curvature means a higher Relative Humidity (R.H.) isneeded to achieve equilibrium over the larger surface area.

    Smaller droplets evaporate more easily.

    Only large dropletscan survive at lower R.H.

  • Condensation DropletsTypical scenario: adiabatic cooling of ascending air parcel.

    It passes the condensation level (i.e. LCL) and water vapor begins to condense on condensation nuclei, especially hydroscopic.

    Soon, all of the condensation nuclei have seeded tiny water droplets and condensation slows down. (Wet Haze)

    Condensation alone can produce water droplets on the order of ~ 20 mm in radius. Further growth requires droplet-droplet interactions.

  • Solute Effect of Condensation Nuclei

    Hydroscopic condensation nuclei allow droplet formation andgrowth with less than 100% R.H. (as low as 75% R.H.)

    They increase the likelihood of encounters between watervapor and liquid droplets.

    A solution (water and nuclei) also keeps water vapor fromevaporating so easily as it lowers the saturation vaporpressure (i.e., solute effect).

  • Condensation RadiusSupersaturation versus condensation nuclei radius

  • Concept of Air Friction (review)

    Friction is dependent upon (1) droplet surface area, and (2) velocity.Frictional force ~ r2v2Ff = k 4 p r2 v2

    As the velocity increases, so does the frictional force.

    When the acceleration due to the frictional force is equal to the gravity, no acceleration takes place.

    When velocity stops changing, this is terminal velocity.

  • Physics of Terminal VelocityFORCE = Weight - Air resistance

    ma = mg - Ff Ff = k4pr2v2

    a = g - Ff/m

    Acceleration occurs as long as the acceleration due togravity is greater than the acceleration due to thefrictional force.Droplet weight (gravity) must overcome drag (air resistance). The droplet mass must grow.

  • Terminal VelocityGravity increases faster than drag with radius. WHY? Weight = r 4/3 p r3depends on volumeDrag = 4 p r2 kv2depends on area

    Vterminal = r1/2weight = drag

    The terminal velocity increases as drop radius increases.Both gravity and drag increase with radius, one because of mass the other because of surface area. Weight is faster.

    This is even ignoring updrafts. So size does matter!

  • Drop SizeFor a drop to fall, weight (r3) must overcome drag (r2).Therefore, it has to grow to a large enough size.Cloud drops are 1 million times smaller in total volume than rain drops.

  • How FastA typical raindrop (2000 mm, 6.5 m/s) falls 600x faster thana typical cloud droplet (20 mm, 0.01 m/s).

    Using an average cloud height of 6500 meters and D = v t, the time required for a drop to fall to the ground,

    train = 6500 m/6.5 m/s= 1000 s ~ 16 minutestdroplet = 6500 m/0.01m/s = 160 hours

    Occur in CALM air, ignoring wind or updrafts.

  • Collision/CoalescenceWith a random distribution of sizes, droplets interact becauseof their differences in terminal velocities.

  • CloudsMost clouds do not produce precipitation as condensation andevaporation continue to work at similar rates on small dropletswithin the clouds.

    Condensation nuclei and condensation working alone wouldtake days to form large enough drops to fall to the ground.

  • Warm CloudsWarm clouds: Clouds that do not have ice crystal formation.

    Collision/Coalescenceis the ONLY mechanism for droplet growth.

    Largest rain drops hit theground first.

  • StratusStratus: cloud droplets can be smallVery little vertical motions (updrafts), so small dropletswith small terminal velocities may reach the ground.There is less time for collision and coalescence in the thinstratus layer, so small drops are numerous. Drizzle and/ormisty rain () is common.

  • CumulusCumulus: cloud droplets can be very large.Very large vertical motions (updrafts), so only large enough heavy drops can make it to the ground.There is more time for collision and coalescence in the thick cumulus cloud with large vertical development, so large drops are numerous. Towering cumulus can produce heavy rain and even hail.

  • Ice NucleiIn order for non-homogeneous freezing to take place at highertemperatures, ice nuclei that are structurally similar to ice must be present in order to initiate freezing. Silver and lead iodide, cupric sulfide (), kaolinite () are examples.

    Natural sources include clay soils, plant material, bacteria andof course ice. Without these ice nuclei, saturated air will besupercooled (often to ~ -40oC) before freezing would actuallytake place.

  • Cloud Seedinga form of weather modification (often refer to increase rainfall.)Providing ice nuclei with the right geometry to facilitatecondensation, growth and precipitation. (CO2 drop; i.e., dry ice)

  • Cloud SeedingStatic mode: add ice crystals (silver iodide or dry ice) to cold clouds. Dynamic mode: enhance vertical air currents in clouds and thereby vertically process more water through the clouds. Usually a much larger number of ice crystals is added to the cloud than in the static mode. Hygroscopic mode: salt crystals are released into a cloud. These particles grow until large enough to cause precipitation. Clouds can be seeded from above with the help of airplanes or from the ground using rockets

  • Cool CloudsColder clouds have ice nuclei which allow faster growth ratesfor cloud droplets.

  • Water Versus Icees = eo exp{L/Rv (1/To-1/T)} Saturation Vapor Pressure

  • Saturation Vapor PressureFor equilibrium between condensation and evaporation, morewater vapor is needed over water than over ice.(Curvature and Temperature are the difference)

  • Maximum SVP DifferenceSupercooled water vs. Ice occurs when T ~ - 12 oC

  • Ice CrystalsSome water vaporsublimates onto ice.

    This removes water vapor from the air and then causesmore evaporation of water in order to establish equilibrium.

    BUT adding more water vapor to the air, increases ice crystal growth.

  • Bergeron ProcessEvaporation is larger neardroplet, condensation islarger around ice nuclei.

    Water diffuses from dropto ice.Removing water vapor increases evaporation around droplet.

    Ice crystals grow at theexpense of water droplets.

  • Ice Crystal Growth

    Two mechanisms facilitate the growth of ice crystals.Accretion: the growth of ice crystals as supercooled water freezes onto them.Aggregation: the coalescense of ice crystals through adhesion (falling ice crystals collide and stick to other ice crystals).

  • AccretionMost precipitation begins with falling ice crystals as they havemuch higher growth rates thanwater droplets acting undercondensation alone.

    The physical conditions those icecrystals encounter as they fallultimately determines the typeof precipitation that reaches theground.

  • AggregationGrowth of dendrite ice crystalotherwise known as snowthrough aggregation andadhesion.

    Dendrite (snow) is favored because its growth rate is thehighest at the maximumdifference in S.V.P. betweenwater and ice near -12oC.

  • Snow FlakesDendrite Ice Crystals

  • FracturingCollision and fracturing providemore ice nuclei.

    Ratios of ice nuclei to droplets1:100,000precipitation1:1 nothing100,000:1nothing

    Even ice needs to get heavy to fall.

  • Ice Crystal HabitsIce crystal growthis dependent uponpressure andtemperature.

    Growth rates arehighly dependentupon the S.V.P. difference betweenwater and ice.

  • PrecipitationsLIQUIDVirga(wispy rain)MistDrizzleRain

    ICE (Graupel:)Freezing DrizzleFreezing (Icy) Rain (Glaze)Sleet ()SnowHailvirgasea mist

  • Rain or Snow?Melting Level is a function of season, higher up in summer.

  • Rain ProfileA deep warming layer always results in rain.

  • RainRain Intensities (inch/hour)Light0.01- 0.10

    Moderate0.11- 0.30

    Heavy> 0.30

  • Rain GaugesAll official rain gauge instruments have 8-inch diameter openings. Simple Standard Advanced

  • Rain Radar IntensitydBZ = 10 log Z

    log Z = log (power)+2 log(range)+ constant

  • Rain Radar IntensitydBZ stands for decibels of Z, a meteorological measure of equivalent reflectivity (Z) of a radar signal reflected off a remote object. The common reference level for Z is 1 mm6 m-3, which is equal to 1 m3. It is related to the number of drops per unit volume and the sixth power of drop diameter.

  • Difference between dBZ(67,116)and vector(73,122) estimations for the center of typhoon Nari is about 7.8km

  • Precipitation Frequency

  • VirgaPrecipitation that evaporates before reaching the ground

  • Snow ProfileTemperatures well below freezing allow snow to reach the ground.

  • Snowfall Intensity

    Snowfall intensity is not measured by accumulation as in the case of rainfall, usually.

    Intensity is measured by visibility (or lack thereof).

    Heavy < 0.25 mile(~ 6 inch/day)Moderate0.25 - 0.50 mileLight > 0.50 mile

  • Snowfall Frequency

  • Sleet ProfileA very deep freezing layer causes supercooled water droplets to freeze into tiny ice pellets before reaching the ground.

  • Sleet FormationThe formation of sleet along a warm front.

  • Glaze ProfileA shallow freezing layer allows supercooled water droplets to freeze upon contact creating an ice coating.

  • GlazeVery beautiful,

    very heavy,

    very dangerous,

    very damaging.

  • Glaze

  • Glaze

  • Glaze

  • GraupelThe tiny ice pellets that collect from ice crystals, can form very large raindrops (upon melting), very clumpy snow, and is the source for hail.

  • Hail Storm Frequency

  • HailGraupel ice that is allowed toaccumulate by accretion and/or aggregation.

    Associated with strongupdrafts.

    The larger the hail, the strongerand more severe the stormsassociated with the strongestupdrafts.

  • Hail FormationThe tiny ice pellets(graupel) can make many trips up and down in a large cumulonimbuscloud.

    Each trip adds another layer, like rings on atree for each season.

  • Rime ()Supercooled water droplets (freezing fog or drizzle) can create a winter wonderland when deposition takes place.

  • Summary

    Precipitation: A deposit on the earth of hail, mist, rain, sleet, or snow; also, the quantity of water deposited.

    Glaze: A coating of ice, generally clear and smooth but usually containing some air pockets, formed on exposed objects by freezing of a film of super-cooled water deposited by rain, drizzle, fog, or possibly condensed from super-cooled water vapor. *,, *Cupric sulfide Kaolinite:*A cloud extends above 0C level**Aggregation: *Virga:Wispy:* *