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Cloud Development and Precipitation: Understanding Atmospheric Stability and Measuring Precipitation

This chapter explores atmospheric stability and its influence on cloud development and precipitation processes. It also discusses different types of precipitation and methods for measuring precipitation.

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Cloud Development and Precipitation: Understanding Atmospheric Stability and Measuring Precipitation

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  1. Chapter 5: Cloud Development and Precipitation • Atmospheric Stability • Determining stability • Cloud development and stability • Precipitation processes • Precipitation types • Measuring precipitation

  2. Atmospheric Stability Q1: Why does the air rise on some occasions and not on others? Q2: Why does the size and shape of clouds vary so much when the air does rise?

  3. Atmospheric Stability • stable and unstable equilibria • air parcels: rising air parcel expands and cools • adiabatic process • dry adiabatic lapse rate (9.8 C/km) • moist adiabatic lapse rate (less than 9.8 C/km) • Stability does not control whether air will rise or sink.Rather, it controls whether rising air will continue to riseor whether sinking air will continue to sink.

  4. Determining Stability Stable condition: If a rising parcel’s T is colder than its environment, it is denser and would sink back. Unstable condition: If the rising parcel’s T is warmer, it is less dense and will continue to rise

  5. A Stable Atmosphere • environmental lapse rate • absolute stability • stabilizing processes surface cooling; air aloft warming (e.g., subsidence inversions) • Stable air provides excellent conditions for high pollution levels.

  6. An Unstable Atmosphere • absolute instability • destabilizin processes warming of surface air • superadiabatic lapse rates (>9.8 C/km) • Unstable air tends to be well-mixed.

  7. Conditionally Unstable Air • dry and moist adiabatic lapse rates • Conditional instability: environmental lapse rate between dry and moist lapse rates • Condensation level cloud base

  8. Cloud Development and Stability Layered clouds tend to form in a stable atmosphere; whereas cumuliform clouds tend to form in a Conditionally unstable atmosphere

  9. Cloud Development and Stability • surface heating and free convection • uplift along topography • widespread ascent • lifting along weather fronts

  10. Convection and Clouds • thermals • fair weather cumulus • Fair weather cumulus provide a visual marker of thermals. • Bases of fair-weather cumulus clouds marks the lifting condensation level, the level at which rising air first becomes saturated.

  11. Topography and Clouds • orographic uplift • rain shadow • The rain shadow works for snow too. Due to frequentwesterly winds, the western slope of the Rocky Mountainsreceives much more precipitation than the eastern slope.

  12. Precipitation Processes

  13. Collision and Coalescence Process • Warm clouds (above freezing T) • terminal velocity large drops fall faster than small drops • coalescence: the merging of cloud droplets by collision

  14. Ice Crystal Process • cold clouds (ice crystals and liquid drop coexist) • supercooled water droplets water drops below freezing T • saturation vapor pressures over liquid water is higher than over ice • Accretion ice crystals grow by colliding with supercooled water droplets to form graupel (or snow pellets)

  15. Fig. 5-22, p. 124

  16. Precipitation in Clouds • accretion • ice crystal process

  17. Cloud Seeding and Precipitation • cloud seeding inject (or seed) a cloud with small particles that will act as nuclei so that the cloud particles will grow large enough to fall to the surface as precipitation • silver iodide: as cloud-seeding agent because it has a crystalline structure similar to an ice crystal • It is very difficult to determine whether a cloud seedingattempt is successful. How would you know whetherthe cloud would have resulted in precipitation if it hadn’tbeen seeded?

  18. Precipitation Types

  19. Rain • Rain: drop diameter of 0.5 ~ 6mm • Drizzle: drop diameter < 0.5mm • Virga: rainfall not reaching surface • shower small to large medium droplet

  20. Snow • Snow • Fallstreaks: ice crystals and snowflakes from high cirrus clouds that usually do not reach surface • Dendrite: Snowflake shape depends on both temperature and relative humidity • Blizzard: low T and strong wind bearing large amounts of snow, reducing visibility to 1 few meters

  21. Sleet and Freezing Rain • Sleet: Sleet makes a ‘tap tap’ sound when falling on glass • freezing rain • rime

  22. Snow Grains and Snow Pellets • snow grains: snow equivalent of drizzle • snow pellets: larger and bounce on surface; formed as ice crystals collide with supercooled water droplets • Graupel: when snow pellets accumulate a heavy coating of rime, they are called graupel

  23. Hail • updraft cycles • accretion • A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.

  24. Measuring Precipitation

  25. Instruments • standard rain gauge: 0.01 inch interval ; trace • tipping bucket rain gauge: used in ASOS • It is difficult to capture rain in a bucket when thewind is blowing strongly.

  26. Doppler Radar and Precipitation • Radar radio detection and ranging • Doppler radar provide precip. area and intensity as well as horizontal speed of falling rain

  27. Fig. 5-40a,b, p. 136

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