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Cloud Development & Precipitation

Cloud Development & Precipitation. Importance of Clouds. Release heat to atmosphere Help regulate energy balance Indicate physical processes. Atmospheric Stability. Clouds form as air rises and cools Adiabatic processes: change in temperature without giving or removing

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Cloud Development & Precipitation

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  1. Cloud Development &Precipitation

  2. Importance of Clouds • Release heat to atmosphere • Help regulate energy balance • Indicate physical processes

  3. Atmospheric Stability • Clouds form as air rises and cools • Adiabatic processes: change in temperature without giving or removing • Dry rate = 10°C/1000m • Moist rate = 6°C/1000m • Stability is a state of equilibrium in terms of atmospheric movement; no vertical movement occurs

  4. Warm air rises or is unstable Cool air sinks or is stable Compare air parcel lapse rate to environmental lapse rate Stable environment Environmental lapse rate less than moist lapse rate If an air parcel is forced it will spread horizontally and form stratus clouds Usually a cool surface (radiation, advection) Inversion: warm over cool Determining Stability

  5. Determining Stability • A Conditionally Unstable Atmosphere • Moist adiabatic lapse rate is less than the environmental lapse rate which is less than the dry adiabatic lapse rate • Stable below cloud unstable above cloud base • Atmosphere usually in this state

  6. Cloud Development and Stability • Clouds develop as an air parcel rises and cools below the dew point. • Usually a trigger or process is needed to initiate the rise of an air parcel.

  7. Cloud Development and Stability • Convection • Differential land surface heating creates areas of high surface temperature. • Air above warm land surface heats, forming a ‘bubble’ of warm air that rises or convection. • Cloud base forms at level of free convection.

  8. Cloud Development and Stability • Topography • Orographic uplift (occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down) • Orographic clouds • Windward, leeward, rain shadow • Lenticular clouds (lens shaped) • Topic: Stability & Windy Afternoons • Warm, clear days, windiest part of the day in the afternoon

  9. Precipitation Processes • Precipitation is any form of water that falls from a cloud and reaches the ground. • How do cloud drops grow? • When air is saturated with respect to a flat surface it is unsaturated with respect to a curved droplet of water. • Super saturated

  10. Precipitation Processes • Cloud Seeding • Inject cloud with small particles that act as condensation nuclei, starting the precipitation process • NEED CLOUDS: seeding does not generate clouds • Cold clouds with a low seed ration best • Dry ice, silver iodide

  11. Precipitation Types • Rain: falling drop of liquid water • Drizzle less than 0.5 mm • Virga • Cloudburst • Snow: frozen water falling from sky (crystal or flake) • Most precipitation starts as snow • Freezing level, snow and cloud appearance, fall streaks, drifting snow, blizzard • A blanket of snow is a good insulator

  12. Precipitation Types • Topic: Tear Drops • Raindrops not tear shaped • Shape is size dependent • Less than 2 mm = sphere • Greater than 2 mm = flattened sphere

  13. Precipitation Types • Sleet: Air below freezing, then travels through a layer of air above freezing, begins to melt and then falls through a layer of air below freezing just above the ground surface • Freezing Rain: Ground surface is freezing; as rain hits the surface it freezes

  14. Precipitation Types • Snow Grains: solid equivalent of drizzle, no bounce or shatter • Snow Pellets: larger than grains, bounce, break, crunch underfoot • Graupel: ice particle accumulation with rime • Hail: graupel act as embryo in intense thunderstorm, grow through aggregation as pushed up by updraft.

  15. Measuring Precipitation • Instruments • Rain gauge: standard, tipping bucket, weighing • Snow: average depth at 3 locations, 10:1 water equivalent • Doppler Radar • Transmitter generates energy toward target, returned energy measured and displayed • Brightness of echo = amount/intensity of rain • Doppler: measures speed of horizontal rain

  16. Measuring Precipitation • Measuring from space • Specific satellites designed to assess clouds, atmospheric moisture, and rain • TRMM • CloudScat

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