Guest lecture by Prof Kathleen Schiro 12 October 2020 Cloud Droplet Formation Review of how condensation occurs Curvature effect Cloud condensation nuclei Solute effect 33 x 10 5 Jkg 25 x 10 ID: 1021820
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1. Cloud PhysicsCloud formation and precipitation processesGuest lecture by Prof. Kathleen Schiro12 October 2020
2. Cloud Droplet FormationReview of how condensation occursCurvature effectCloud condensation nucleiSolute effect
3. 3.3 x 105 J/kg2.5 x 106 J/kgLatent heat (L) of water phase changes
4. Condensation and droplet formationCondensation occurs when e = es. One key mechanism for decreasing temperature is lifting the parcel of air to higher elevation. As a parcel of air is lifted to higher elevations, it becomes cooler (adiabatic expansion) and es decreases. The term lifting condensation level (LCL) is often used to describe this elevation. Broadly speaking, there are 3 types of lifting mechanisms: Orographic lifting: When air is forced to go over mountainsFrontal surface lifting: When warm, moist air air is forced above cooler airConvection: Warm, moist air rises because it is less dense than its surroundings *Condensation can also occur due to radiative cooling, horizontal advection, mixing
5. EFFECT OF DROPLET CURVATUREON SATURATIONSATURATION(RH=100%)SUPERSATURATION(RH>100%)If the surface is curved, then the amount of bonding that can go on between any one water molecule on the surface and its neighbors is reduced.
6. CURVATURE EFFECTWhen air is saturated with respect to a flat surface, it is unsaturated with respect to a curved droplet of waterSmaller droplets have a more rapid rate of evaporationDROPLETS GROWDROPLETS SHRINKRate of evaporation > rate of condensationRate of condensation > rate of evaporationsupersaturationKelvin Equation is the equilibrium vapor pressure over a curved surface of pure water is the saturation vapor pressure over a flat surface of pure water is the water surface tension is the number of moles of liquid water per unit volume is the universal gas constant is the temperature
7. CURVATURE EFFECTWhen air is saturated with respect to a flat surface, it is unsaturated with respect to a curved droplet of waterSmaller droplets have a more rapid rate of evaporationTo keep tiny cloud droplets in equilibrium with the surrounding air, the air must be supersaturatedFor pure water vapor, condensation to form droplets does not occur spontaneously until RH is much higher than 100% (300-400%) However, the observed RH, even in clouds, rarely exceeds 101%DROPLETS GROWDROPLETS SHRINKRate of evaporation > rate of condensationRate of condensation > rate of evaporationsupersaturation
8. 75% of total mass direct from natural or anthropogenic sourcesWind-generated dust (20%)Sea spray (40%)Forest fires (10%)Combustion and other industry (5%)25% of total mass from conversion of gaseous constituents to small particles by photochemical and other chemical processes.SO2, NO2, NH3Atmospheric Aerosols and Cloud Condensation Nuclei
9. Aerosols can be categorized according to their affinity for water.HydrophobicNucleation is difficult and requires even higher super-saturation.NeutralSame supersaturation as homogeneous nucleation.HygroscopicMuch lower supersaturation required.Atmospheric Aerosols and Cloud Condensation Nuclei
10. Cloud Condensation Nuclei (CCN) are hygroscopic aerosols that are necessary for cloud formationCCN provide a surface to encourage condensation of water vapor moleculesIf vapor pressure of solute is less than that of the solvent, the vapor pressure of solution (droplet) is reduced.Bonding with solute makes it more difficult for water molecules to evaporate A solution droplet can be in equilibrium at a much lower supersaturation than a pure water droplet of the same size.There are about 10 times more CCN in continental than maritime air masses Atmospheric Aerosols and Cloud Condensation Nuclei
11. SupersaturationSuper-saturationCURVATURE AND SOLUTE EFFECTSA: r = 0.4 microns: saturated relative to pure water,RH=100.42%B: r = 0.4 microns: w/ 10–15g (solute), A issupersaturated (equilibrium at 99.6%)C: 10–16g in solution; r=0.075 microns; RH=99.8%supersaturated...grows to r=0.15 microns (D)E: 10–16g in solution; r=0.2 microns; RH=99.8%unsaturated (equilibrium at 100.4% (F))...evaporates to r=0.15 microns (D)G: supersaturated relative to both pure and solutecurves...would grow rapidly
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13. Condensation is a necessary but insufficient condition for precipitation formation.
14. Precipitation FormationCollision and CoalescenceBergeron processHydrometeor types (rain, snow, sleet, freezing rain, hail)
15. Collision-Coalescence ProcessAs soon as a cloud droplet becomes large enough, raindrops start to fall. Terminal velocity: constant fall speed of a particle achieved through a balance of gravitational and drag forcesLarger drops will have a faster terminal velocity than small drops so the large drops will fall faster. is mass is acceleration due to gravity is density of fluid through which object is falling is area of the object is the drag coefficient
16. Collision-Coalescence ProcessAs soon as a cloud droplet becomes large enough, raindrops start to fall. Terminal velocity: constant fall speed of a particle achieved through a balance of gravitational and drag forcesLarger drops will have a faster terminal velocity than small drops so the large drops will fall faster. Collisions occur and droplets merge.
17. Collision-Coalescence ProcessThe collision-coalescence mechanism is most efficient in environments where cloud water content is large and large cloud droplets are plentiful (e.g. tropics).Depends strongly on droplet size distribution, cloud water content/column moisture, updraft strength.Drops make it to the ground whenThe drop is large enoughThe cloud base is low enoughThe air between the cloud base and the ground is moist.
18. Pure liquid water will not freeze above -40oC, and so we get supercooled water. Supercooled water droplets will freeze almost instantly if they come into contact with a solid particle that resembles an ice crystal (freezing nucleus)
19. Cloud TemperatureCloud ContentsAbove 0 C (32 F)Liquid Water -10 to 0 C (12-32 F)Supercooled Water -40 to -10 C (-4-14 F)Supercooled Water and Ice Crystals Coexist Below -40 C (-4 F)Mainly Ice Crystals
20. Bergeron (Ice Crystal) ProcessWhen a cloud is comprised of both Supercooled Water and Ice Crystals, The Bergeron Process is the dominant process producing precipitation (diffusional growth).The saturation vapor pressure over a water surface is greater than that over an ice surface at the same subfreezing temperature.More vapor evaporating from liquid droplets than ice. Excess vapor deposits directly onto ice.
21. (N: 135–146;A: Chap. 7)Bergeron (Ice Crystal) Process
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23. VAPOR PRESSURE DIFFERENCE BETWEEN LIQUID WATER AND ICE
24. The crystal habit, or shape, of a growing ice crystal is determined by the temperature and associated saturation vapor pressure difference between ice and supercooled water.
25. Snowflakes result from the aggregation of ice crystals created by the Bergeron process. Snowflakes have a very low density, and hence fall relatively slowly through the air (≈ 1 m s−1). Graupel consists of ice crystals or snowflakes which have encountered additional supercooled liquid. When such droplets impact these particles, they freeze instantly, a process called riming. Riming tends to fill in the gaps between crystals, resulting in particles with densities higher than that of snowflakes, but less than that of pure ice. Hail is produced when graupel encounters especially high concentrations of supercooled water. Formed in cumulonimbus clouds where vigorous updrafts permit its suspension and growth.When the freezing level is far enough above the surface, most precipitation particles melt and thus form raindrops before they reach the surface, with the exception of large hailstones. Pbs.comwikipedia.comDenver Post
26. weather.gov