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  2. Chapter Five Cloud Development & Precipitation

  3. Goal for this Chapter We are going to learn answers to the following questions: • Why there any instabilities in the atmosphere? • How can we make the atmosphere more stable? • Why cloud droplets seldom reach the ground? • How rain drops are produced? • How does the ice crystal process forms precipitation? • What is cloud seeding? • Difference between freezing rain and sleet? • How does Doppler radar measure intensity of rain? • Why heavy showers fall from cumuliform while steady precipitation is derived from stratiform clouds?

  4. Atmospheric stability • A rising parcel of air expands and cools, while a sinking parcel is compressed and warms • When air is in stable equilibrium, after being moved up or down, tends to come back to its original position • Adiabatic Process: A process in which there is no transfer of heat between the air parcel and its surroundings (compression --- warming & expansion --- cooling) • Dry adiabatic rate: Rate of change of temp in a rising or descending unsaturated air parcel; ~10°C/1000 m in elevation • Moist adiabatic rate:Rate of change of temp in a rising or descending saturated air parcel; ~6°C/1000 m in elevation

  5. Concept of equilibrium

  6. What happens to a rising air?? • Rising air----- cools ----- RH increases as the air temp approaches the dew-point temp-----if air cools to its dew point temp, RH ~100%----- further air lifting leads to condensation ----- cloud forms -----latent heat is released ----- • Stable Air: If the rising air is colder than its surrounding air, then, it is heavier and will sink back to its original position – stable air strongly resists upward vertical motion. If clouds form in rising air, cloud will spread horizontally in relatively thin layers – cirrostratus, altostratus, nimbostratus or stratus clouds

  7. Dry adiabatic rate; unsaturated air cools @10°C/1000m

  8. Absolute stable atmosphere when rising air parcel is colder and heavier than surrounding air

  9. Stable Air – contd. • Atmosphere is stable when lapse rate is small • The cooling of surface air could be due to: • Nighttime radiational cooling of the surface • Influx of cold air from other region brought by wind • Air moving over a colder surface • The air is generally most stable in the early morning around sunrise • Subsidence Inversion: Inversion produced by compressional warming – the adiabatic warming of a layer of sinking air • Presence of inversion near the ground fog, haze, & asso-ciated pollutants are kept close to the surface

  10. Cold surface air produces a stable atmosphere that inhibits vertical motions – fog & haze are kept close to the ground

  11. Unstable Air • When air temperature decreases rapidly as we move up, air becomes unstable • The warming of air may be due to: • Daytime solar heating of the surface • An influx of warm air brought in by the wind • Air moving over a warm surface • As the surface air warms during the day, the air becomes more unstable – most unstable during summer months and when there is much temp fluctuation in a day • Sinking air produces warming and a more stable atmosphere while rising air produces cooling and unstable atmosphere

  12. Unstable atmosphere – rising air parcel is warmer and lighter than the surrounding air

  13. How stability of air affects the type of clouds formed • Unsaturated Air parcel if forced to rise ---- expands and cools at the dry adiabatic rate --- cools until dew point – now RH is 100% --- further lifting results in condensation and the formation of cloud --- The elevation above which the cloud first forms is called condensation level • Conditionally unstable atmosphere (or conditional instability): When the environmental lapse rate is less than the dry adiabatic rate but greater than the moist adiabatic rate, conditional instability exists. • Level of free convection: Level at which a lifted parcel of air becomes warmer than the surrounding in a conditionally unstable atmosphere

  14. Unstable Air. Warmth from the forest fire heats the air, causing insta. near the surface; warm, less dense air bubbles upward, expanding & cooling as it rises – rises air cools to dew point, condensation begins & cumulus cloud forms

  15. Conditionally unstable air – when unsaturated stable air is lifted to a level where it becomes saturated and warmer than the air surrounding air

  16. Cloud Development and stability • Some surface heats up quickly --- air in contact warms --- hot ‘bubble’ of air (thermal) rises --- undergoes expansion & cooling when it rises --- Two things can happen: i) thermal mixes with cooler air and looses its identity and air vertical movement slows down; ii) air keeps cooling until it reaches to its saturation point --- moisture will condense --- thermal becomes visible as a cumulus cloud • Outside of a cumulus cloud, there is downward movement of air because i) evaporation around the outer edge of the cloud makes the air cooler and denser; & ii) completion of the convection current started by the thermal

  17. How clouds form: a) surface heating & convection; b) forced lifting along topographic barriers; c) convergence of surface air; d) forced lifting along weather fronts

  18. Cumulus cloud formation from the hot air rising from earth’s surface – around the cloud, air is sinking

  19. Why Cumulus clouds appear-disappear-reappear • Cumulus clouds grow – shuts off surface heating and upward convection --- without continual supply of air, cloud disappears --- heating and upward convection starts again

  20. Topography and Clouds • Large air masses rise when approaching a mountain chain --- this leads to cooling & if the air is cool, clouds form --- Orographic clouds---during this condensation, latent heat is released • Temperature at the leeward side is higher (loss of heat in the upwind side); dew point temp on the leeward side is lower than the windward side • Drier air in the leeward side; More rain in upwind side and rain shadow (low precipitation) in the leeward side

  21. Rain shadow, Orographic uplift & cloud development

  22. Formation of lenticular clouds: Moist air rises in the upwind side of the wave, it cools and condenses, producing cloud; in the downwind side, air sinks and warms – the cloud evaporates

  23. Precipitation Processes • Average diameter cloud droplets ~ 0.02 mm • Typical raindrop size ~ 2 mm • Growth of cloud droplets by condensation is slow to produce rain; clouds can develop and begin to rain in less than an hour • 1 million average size cloud droplets will make a average size raindrop – Other processes?? • Two important processes on how rain is produced • Collision-Coalescence Process • Ice-crystal (or Bergeron) process

  24. Relative sizes of raindrops, cloud droplets, & condensation nuclei

  25. Collision & Coalescence: a) warm cloud composed only of small cloud droplets of uniform size; b) different size droplets

  26. Collision & Coalescence – contd. • In clouds warmer than -15°C(5 °F), collision between droplets play a significant role • Larger drops may form on larger condensation nuclei (salt particles or through random collision droplets; turbulent mixing between cloud and drier environment) • Amount of air resistance depends on the size of the drop and its rate of fall --- speed of falls increases until the air resistance = gravity – Terminal velocity – Larger drops means less evaporation also • Coalescence: Merging of droplets by collision • Forces that hold together tiny droplet together are so strong that if the droplets collide with another droplet, they would not stick together

  27. How surface area depends on the size

  28. Collision & coalescence – contd. • Rising air currents slow the rate at which drops fall --- thick cloud with strong updrafts will maximize the time droplets spend in a cloud --- the bigger size droplets • When the fall velocity of the drop > updraft velocity, droplet slowly descends; when it reaches the bottom of the cloud, size ~ 5 mm --- typically occur in a rain shower originating in the warm, convective cumulus clouds • Factors in the production of raindrops • Cloud’s liquid water content (most important) • Range of droplet sizes, cloud thickness, updrafts of the cloud, electric charge of the droplets and the electric field in the cloud

  29. Cloud droplet rising & then falling through a warm cumulus cloud by growth and coalescence

  30. Ice crystal Process • Bergeron process of rain formation: A process that produces precipitation; involves tiny ice crystals in a supercooled cloud growing larger at the expense of the surrounding liquid droplets • Ice crystals and liquid cloud droplets must coexist in clouds at below freezing • Accretion or riming of ice crystals: Ice crystals grow larger by colliding with the supercooled liquid droplets; the droplets freeze into ice and stick to the ice crystal

  31. Distribution of ice and water in a cumulonimbus cloud

  32. Water droplets and ice crystal are in equilibrium; water vapor molecules > liquid is saturation vapor pressure over water is greater than it is over ice

  33. Cloud Seeding & Precipitation • Cloud Seeding: Inject a cloud with small particles that will act as nuclei, so that cloud particles will grow large enough to fall to the surface as precipitation • Silver iodide is used: has a crystalline structure similar to ice crystal, as it acts as an effective ice nucleus at temp. of -4°C (25 °F) and lower • Important factors in cloud-seeding experiment: Type of cloud, its temperature, moisture content, droplet size distribution, and updraft velocities in the cloud • Cloud seeding in certain instances may lead to more precipitation; in others, to less precipitation, and in still others, to no change in precipitation amounts; • Can avoid hail storms --- very important use

  34. Natural seeding by cirrus clouds may lead to precipitation downwind

  35. Precipitation Types • Rain (Meteorology definition!): falling drop diameter  0.5 mm • Drizzle: Water drop diameter < 0.5 mm • Most drizzle falls from stratus clouds; also, rain passing through undersaturated zone and undergo evaporation leading to smaller-sized droplets – drizzle • Virga: Precipitation that falls from a cloud but evaporates before reaching the ground • Raindrops that reach the earth’s surface are seldom larger than ~6mm as collision between raindrops tend to break them apart into many smaller drops

  36. Virga: Streaks of Falling precipitation evaporates before reaching the ground

  37. Raindrops < 2mm nearly spherical; >2mm, elliptical

  38. Precipitation Types – Contd. • Snow: Much of the precipitation reaching the ground begins as snow • During summer, freezing level is usually high & snowflakes falling from a cloud melt before reaching the surface • During winter, freezing level is much lower, and falling snowflakes have a better chance of survival • Snowflakes can fall ~300 m below the freezing level before completely melting • Fallstreaks: Falling ice crystals that evaporate before reaching the ground • Ice crystals have been observed falling at temp ~-47°C

  39. Ice crystals beneath cirrus clouds

  40. Precipitation Types – contd. • When snowflakes fall through very cold air with a low moisture content, they do not readily stick together & powdery flakes of ‘dry’ snow accumulates on ground • Flurries: Light snow showers that fall intermittently for short duration; often from developing cumulus clouds • Snow Squall: A more intense snow showers (comparable to summer rain showers); usually form from cumuliform clouds • Ground Blizzard: Drifting + Blowing snow after snow fall ended • Blizzard: Weather with low temp & >30 knot winds bearing large amounts of fine, dry, powdery snow

  41. Sleet & Freezing Rain • Sleet: Partially snowflake (or cold raindrop) passing through warmer air undergoes partial melting; when it again goes through subfreezing surface layer of air, partially melted snowflake or cold raindrop turns back into a tiny transparent ice pellet, called, sleet • Freezing Rain: Supercooled liquid drops upon striking a cold surface, form a thin veneer of ice – this form of precipitation is called freezing rain • Freezing drizzle: If the water droplets are small, then, it is called freezing drizzle • Rime: White/Milky granular deposit of ice formed by the rapid freezing of supercooled water drops when they come in contact with an object in below-freezing air

  42. Sleet – partially snowflake (cold droplet) freezes into a pellet of ice before reaching the ground

  43. Accumulation of rime on tree branches

  44. Ice storm caused tree limbs to break & Power lines to sag

  45. Snow grains, pellets and hail • Snow grains: Small, opaque grains of ice (equivalent of drizzle); fall from stratus clouds • Snow Pellet: White, opaque grains of ice of the size of rain drop • Hail: Pieces of ice either transparent or partially opaque, ranging in size from that of small peas to that of golf balls or larger; biggest size in US 757 g & 14 cm diam.; • Single hailstorm can damage in minutes; annual loss hundreds of millions of $ in US; • Hail is produced in a cumulonimbus cloud when large frozen raindrops that grow by accumulating supercooled liquid droplets

  46. Hail – contd. • Graupel: Ice particles between 2-5 mm in diameter that form in a cloud often by the process of accretion • For a hail to grow to the size of golf ball, it must remain for 5-10 minutes in the cloud • Ice crystals of appreciable size that can’t be supported by rising air, begin to fall – Hail • Largest form of precipitation occurs during the warmest time of the year (due to strong updraft that keeps the crystal to become bigger) • Preventing hailstorm--- cloud seeding --- excessive nuclei prevents from growing

  47. Accumulation of small hail after a thunderstorm