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CLOUD PHYSICS

CLOUD PHYSICS. COVERAGE. General aspects of cloud & precipitation formation Condensation Nuclei Growth of water droplets Microphysical properties of clouds Condensation plus Coalescence

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CLOUD PHYSICS

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  1. CLOUD PHYSICS

  2. COVERAGE • General aspects of cloud & precipitation formation • Condensation Nuclei • Growth of water droplets • Microphysical properties of clouds • Condensation plus Coalescence • Ice phase Nucleation, Ice Nuclei, Diffusional growth on ice. nuclei, further growth by accretion & aggregation • Types of precipitation processes • Weather Modification Cloud Physics

  3. CLOUD PHYSICS • Cloud physics is the study of physical processes that lead to the formation, growth and precipitation of clouds • Clouds are composed of microscopic droplets of water, tiny crystals of ice, or both • Under suitable conditions, the droplets combine to form precipitation, where they may fall to the earth Cloud Physics

  4. CLOUD PHYSICS • Precise mechanics of how a cloud forms and grows is not completely understood, but scientists have developed theories explaining the structure of clouds by studying the microphysics of individual droplets • Advances in weather radar and satellite technology have also allowed the precise study of clouds on a large scale Cloud Physics

  5. CLOUD • Defined as a visible aggregate of minute particles of water or ice or both, in the free air • Clouds form in the sky, develop, take different shapes and dissolve • Each process is an indication of some physical state or process in the atmosphere • Size of water droplets is about 0.02 mm in diameter Cloud Physics

  6. COMPOSITION OF CLOUDS • Clouds below freezing level are fully composed of water droplets • Clouds just above freezing level are composed of super-cooled water droplets • Clouds at further higher level are composed of super-cooled water drops and ice crystals • Clouds above - 40ºC are composed wholly of ice crystals Cloud Physics

  7. CLOUD FORMATION Free Convection Topography Low-level Convergence Fronts Cloud Physics

  8. FORMATION OF CLOUD DROPLETS • Phase changes of water are basic to cloud microphysics • Possible changes are as follows: Vapour  liquid (condensation, evaporation) Liquid  solid (freezing, melting) Vapour  solid (deposition, sublimation) Cloud Physics

  9. FORMATION OF CLOUD DROPLETS • Changes in the direction from left to right in this pattern correspond to increasing molecular order • A droplet to form by condensation from the vapour, surface tension must be overcome by a strong gradient of vapour pressure Cloud Physics

  10. When the air reaches saturation, condensation of water vapor into tiny cloud droplets may begin In order for water vapor to condense, particles in the atmosphere called cloud condensation nuclei (CCN) are required Without CCN, a relative humidity of several hundred percent would be needed to keep a tiny cloud droplet from evaporating away FORMATION OF CLOUD DROPLETS Cloud Physics

  11. Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles (typically 0.2 µm) Common types :- Dust or Clay, Soot or black carbon from grassland or forest fires Sea salt from ocean wave spray Soot from factory smokestacks or internal combustion engines Sulfate from volcanic activity Organic particles from land surfaces Gas-to-particle conversion particulates CLOUD CONDENSATION NUCLEI Cloud Physics

  12. CCN Nuclei activated at super saturations less than a few per cent (S < 1.02) are called CCN Sea Salt is the principal CCN and Sulfurous and Nitrous acids take the second place Size 0.2 -1 µm Cloud Physics

  13. FEATURES OF CCN’s • Typical diameter of :- Raindrop ~ 2 mm Cloud droplet ~ 0.02 mm Cloud condensation nucleus ~ 0.0002 mm • Number of cloud condensation nuclei in the air can be measured and ranges between around 100 to 1000 per cubic cm • Total mass of CCNs injected into atmosphere has been estimated at 2x1012 kg over a year's time • Large concentrations of particulates are also responsible for haze in areas with lower humidity Cloud Physics

  14. CLOUD DROPLETS TO RAINDROPS Raindrop is 106 bigger than a cloud droplet 106 bigger 106 bigger Cloud Physics

  15. FEATURES OF CCN’s • Ability of these different types of particles to form cloud droplets varies according to their size and also their exact composition, as the hygroscopic properties of these different constituents are very different • Sulfate and sea salt, readily absorb water whereas soot, organic carbon and mineral particles do not Cloud Physics

  16. FEATURES OF CCN’s • This is made even more complicated by the fact that many of the chemical species may be mixed within the particles (in particular the sulfate and organic carbon) • Additionally, while some particles (such as soot and minerals) do not make very good CCN, they do act as very good ice nuclei in colder parts of the atmosphere Cloud Physics

  17. TYPES OF CLOUDS • Clouds formed in nature can be grouped into different categories depending on the process by which they develop • Two types of processes by which clouds form:- • Warm Clouds • Cold Clouds Cloud Physics

  18. WARM CLOUDS Temperature -10ºC Temperature 0ºC Warm Cloud Cloud Physics

  19. WARM CLOUD CHARACTERISTICS • Cloud tops below freezing level • Composition (Microphysics) - Consists of water droplets • Particle radius - ~ 0.01 mm • Main process – Collision Coalescence • Few large drops form => then the cloud grows Cloud Physics

  20. COLD CLOUDS Temperature -10ºC Temperature 0ºC Cold Cloud Cloud Physics

  21. COLD CLOUD CHARACTERISTICS • Cloud tops above freezing level • Composition (Microphysics) - consists of Ice, crystals and super cooled water droplets • Particle radius - ~ 0.25 mm • Main process – Bergeron process • Ice crystals grow at expense of cloud droplets Cloud Physics

  22. COLD CLOUD CHARACTERISTICS • Cold Cloud - Cloud extending above the 0°C isotherm • Super Cooleddroplets - Water droplets at temp < 0°C • Mixed Cloud - Cloud consisting of both ice particles & super cooled droplets • Glaciated Cloud - Cloud consisting of ice only Cloud Physics

  23. COLD CLOUD CHARACTERISTICS • In mixed cloud, equilibrium vapour pressure over ice is less than that over water at same temperature • Therefore, ice crystal grows by depositionof vapour provided by the evaporation of water droplets • Ice crystals can also grow by colliding with super cooled droplets which then freeze onto them - accretion or riming • Heavily rimed ice crystal is known as Graupel Cloud Physics

  24. COLD CLOUD CHARACTERISTICS • Hailstones represents an extreme case of growth by riming • Frozen water particles that can be "re-cycled" through vigorous convection during which they collect super cooled water droplets and freeze them • When ice crystals collide with one another, they can "stick" together to form larger crystals - Aggregation Cloud Physics

  25. COLD CLOUD CHARACTERISTICS • Precipitation from cold clouds form first by the process of deposition and then through the rimingand aggregation processes so that the ice crystals can grow to sizes heavy enough to fall through the upward current (updraft) Cloud Physics

  26. CLOUD FORMATION • Cloud formation: As air parcels rise, cool and condense • In warm clouds, droplets can grow by :- • condensation in a supersaturated environment and • colliding and coalescing with other cloud droplets • Cloud droplets can also form with aid of cloud condensation nuclei in an unsaturated environment (RH > 90%) • If a cloud extends above 0°C line, it is called cold cloud Cloud Physics

  27. CLOUD FORMATION • Water droplets can exist in clouds as super-cooled droplets even below 0°C • Cloud temperatures frequently need to get below -10°C for any significant number of ice particles to form • Cloud containing both ice particles and super-cooled droplets: Mixed cloud • Cold cloud consisting entirely of ice, is said to be glaciated • Since cloud droplets are small, they are hardest to freeze Condensation nuclei help the cloud droplets to freeze Cloud Physics

  28. CONCEPTS OF CLOUD FORMATION • Condensation acts too slow to produce rain Several days required for condensation Clouds produce rain in less than 1 hour • Warm clouds (no ice) Collision-Coalescence Process • Cold clouds (with ice) Ice Crystal Process Accretion-Splintering-Aggregation Cloud Physics

  29. Ordinary cloud droplets are extremely small Average size is 20 microns (0.002 cm) If the cloud droplet is in equilibrium with its surroundings, the size of the droplet does not change because the water molecules condensing onto one droplet will be exactly balanced by those evaporating GROWTH OF CLOUD DROPLETS • If it is not in equilibrium, the droplet size will either increase or decrease depending on whether condensation or evaporation predominates Cloud Physics

  30. MECHANISM OF GROWTH OF CLOUD DROPLET Condensation Collision & Coalescence Bergeron Process Cloud Physics

  31. Clouds do not necessarily mean precipitation Many clouds never produce precipitation So why do some clouds produce precipitation while others do not? PROCESS OF PRECIPITATION Cloud Physics

  32. CONDENSATION • At a given temperature air can hold a certain amount of water vapour and no more • When the air holds the maximum amount of water vapour which it can hold at that temperature, the air is said to be saturated Cloud Physics

  33. CONDENSATION • Temperature at which saturation occurs is called the dew point • When air is cooled below the dew point temperature, condensation takes place • Conversion of water vapour into liquid water is called condensation Cloud Physics

  34. Curvature Effect Solute Effect CONDENSATION Cloud Physics

  35. Figure shows a cloud droplet and a flat water surface both in equilibrium More vapor molecules surround the small droplet, therefore it has a greater equilibrium vapor pressure This occurs because water molecules are less strongly attached to a curved water surface, so they evaporate more readily CURVATURE EFFECT Cloud Physics

  36. CURVATURE EFFECT Smaller the droplet, greater its curvature, higher the super saturation required for equilibrium Cloud Physics

  37. Saturation water vapor pressure depends on the curvature of the water surface Larger the curvature the easier it is for the water molecules to leave the surface of the liquid water Saturation vapor pressure for small droplets is higher therefore they require more vapor to keep their size CURVATURE EFFECT Cloud Physics

  38. Small droplets don’t make it as raindrops When air is saturated with respect to a flat surface, it is unsaturated with respect to a curved droplet of water, and the drop evaporates Bottom line: the smaller the droplet, the more difficult it is to grow CURVATURE EFFECT Cloud Physics

  39. To prevent evaporation, the air surrounding the cloud droplets must be supersaturated (RH > 100%) For droplets near 20 microns, the RH must be near 100.1% to keep it from evaporating The figure shows that a droplet near 1 microns will grow larger as the RH approaches 101% PROCESS OF PRECIPITATION • However RH, even in clouds, is rarely greater than 101% • Question: How do small cloud droplets of less than 1 microns grow to the size of an average cloud droplet? Cloud Physics

  40. For the cloud droplets to grow, it is necessary for the water droplets to form around condensation nuclei rather than a pure water droplet Most cloud condensation nuclei (CCN) are hygroscopic and therefore have a connection to water vapor These CCN reduce the saturation vapor pressure needed for condensation to occur Therefore, a droplet containing salt can be in equilibrium with its environment when the atmospheric RH is much lower than 100% However, RH closer to 100% mean water vapor molecules attach to the droplet at a faster rate SOLUTE EFFECT Cloud Physics

  41. When CCN is dissolved in the water droplet, a solution is formed We know that the equilibrium vapour pressure reduces when salt is dissolved in liquid water The reduction is expressed by :- e = 1 - CM es SOLUTE EFFECT If water vapour condenses on the solution, M will decrease and e will approach es Cloud Physics

  42. Therefore, if an average sea salt CCN is dissolved in a spherical drop of water of size > 1µ, the solution will be diluted and the effect is insignificant Therefore Solute effect is dominant in smaller droplets SOLUTE EFFECT Cloud Physics

  43. The solute effect and the curvature effect both work together to dictate how a droplet will grow At small sizes, the solute effect dominates At large sizes, the solution becomes more diluted and the curvature effect dominates SOLUTE EFFECT Cloud Physics

  44. COMBINED EFFECT: SOLUTE & CURVATURE

  45. When droplets are small they grow slowly because the curvature effect dominates As the droplets grow, they eventually reach the point where the solute effect dominates and then the droplet grows rapidly Over land masses there are a lot of CCN and each of new droplets compete for existing water vapor Over the ocean there are fewer CCN and there is a lot of vapor for each droplet In either case we now have large number of droplets that are cloud sized and we have a cloud PROCESS OF PRECIPITATION Cloud Physics

  46. PRECIPITATION PROCESSES CONDENSATION Condensation process by itself is entirely too slow to produce rain If condensation was sole process, it would take several days to grow raindrop sized droplets It takes a nucleus about 1 sec to grow to 10µ 2 min to grow to 100µ 3 hrs to grow to 1mm a day to grow to 3 mm Cloud Physics

  47. PRECIPITATION PROCESSES CONDENSATION Observations show that clouds can develop and begin to produce rain in less than an hour. There must be some other process or processes that accounts for the growth of cloud droplets into raindrops Based on radar, photographs, and aircraft observations there are two methods that can account for the growth of raindrops from cloud droplets Cloud Physics

  48. Noted in southern US and in tropics that cloud tops have temperatures rarely below 0C Process starts when just one large cloud droplet forms This large droplet reaches its terminal velocity Velocity where gravity and air friction balance each other out and the droplet no longer accelerates As this one large droplet falls, it collides with other droplets PRECIPITATION PROCESSES: COLLISION & COALESCENCE Cloud Physics

  49. COALESCENCE • Coalescence is the process by which two or more droplets or particles merge during contact to form a single droplet (or bubble)Its role is crucial in the formation of rain • As droplets are carried by updrafts and downdrafts in a cloud, they collide and coalesce to form larger droplets. • When the droplets become too large to be sustained on the air currents, they begin to fall as rain • Adding to this process, the cloud may be seeded with ice from higher altitudes either by the cloud tops reaching −40 C or cloud being seeded by ice from cirrus clouds Cloud Physics

  50. Big water drops fall faster than small drops As big drops fall, they collide with smaller drops Some of the smaller drops stick to the big drops Drops can grow by this process in warm clouds with no ice Occurs in warm tropical clouds COLLISION-COALESCENCE Area swept is smaller than area of drop small raindrop Collection Efficiency 10-50% Cloud Physics

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