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Meteo 3: Chapter 8

Meteo 3: Chapter 8. Stability and Cloud Types Read Chapter 8. Parcel Theory. Parcel: volume of air assumed to behave separately from the air surrounding it Environment: everything outside the air parcel. Hydrostatic Balance means small vertical motions.

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Meteo 3: Chapter 8

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  1. Meteo 3: Chapter 8 Stability and Cloud Types Read Chapter 8

  2. Parcel Theory • Parcel: volume of air assumed to behave separately from the air surrounding it • Environment: everything outside the air parcel

  3. Hydrostatic Balance means small vertical motions

  4. Hydrostatic Balance can be violated on small space and time scales…requires instability (buoyancy)

  5. Role of Stability in Cloud Formation • We already have relationships to determine if net condensation is occurring (e.g. LCL, vapor pressure vs. saturation vapor pressure) • If we can determine how air moves vertically, we can determine whether clouds are likely to form and how deep they might be!

  6. Concept of Buoyancy

  7. Lifting a Parcel of Air

  8. Unstable equilibrium: Tparcel > Tenvironment

  9. Stable equilibrium: Tparcel < Tenvironment

  10. Neutral equilibrium • A parcel is given a push upward and comes to a stop after a short rise (Tparcel = Tenvironment) • The key to determining stability is comparing a parcel’s temperature to the atmospheric temperature at the altitude of interest

  11. Dry Adiabatic Lapse Rate (DALR)

  12. More on DALR • An unsaturated sinking air parcel warms at 10°C/km • These temperature changes result from molecules in the parcel doing work for parcel expansion when rising and gaining kinetic energy via compression when sinking • Adiabatic: no exchange of heat energy between a parcel and its surroundings

  13. Even more • Environmental lapse rate is what is measured by a weather balloon • Determined locally by height above ground and temperature advection, among other things • On average, is ~6.5°C/km • If temperature decreases with height, lapse rates are positive • If lapse rate is negative (temperature increases with height), an inversion is present

  14. Moist Adiabatic Lapse Rate (MALR)

  15. Importance of MALR • Since rising, saturated air parcels cool at a slower rate, the parcel has a better chance of staying warmer than its environment longer, thus has a better chance of being positively buoyant • Not constant

  16. Conditional Instability

  17. Simple Model of Stability • Instability results when an air parcel is warmer than its environment (warm below cool ideal) • Solar heating & low-level warm advection increase instability…sometimes lapse rate near ground is greater than DALR = superadiabatic • Cooling ground increases stability (radiational cooling/evaporational cooling) • Upper-level cooling (warming) increases (decreases) instability • Warm, moist low levels and chilly mid-levels are an ideal setup for an unstable atmosphere (thunderstorms)

  18. Warm below, cold above

  19. Cloud Classification

  20. High Clouds • Composed of ice crystals • Cirrostratus can lead to formation of halo around sun or moon

  21. Middle clouds • If cirrostratus lower and thicken into altostratus, that’s a good sign precipitation may be on the way • Alto-, strato- & cirro- cumulus form from destabilization of cloudy region in a generally stable environment

  22. Low clouds • Stratiform clouds to north of warm front where overrunning is occurring • “Nimbus” means rain

  23. Cumuliform clouds • Form via convection • “Clouds of vertical development” • Can last for minutes-few hours • Cumulonimbus produce thunderstorms, tornadoes, hail • Updrafts range from tens of cm/s (cumulus) to tens of m/s (cumulonimbus)

  24. Wave Clouds • Indicate a stable atmosphere

  25. Smoke as a stability clue

  26. Summertime afternoons and wind

  27. How does precipitation form? • Liquid water can become supercooled => drops of water exist as liquid with temperatures below 0°C • At temperatures >-40°C, tiny particles are needed to start ice crystal formation (ice nuclei) • Ice, liquid, and vapor coexist in clouds • Equilibrium vapor pressure over ice is less than the equilibrium vapor pressure over water at the same sub-freezing temperature

  28. Saturation is reached over ice but not over water, thus ice crystals grow at the expense of liquid drops….eventually become large and fall Bergeron Process

  29. Occurs in “warm” clouds Falling ice crystals melt into rain Bigger raindrops fall faster than smaller ones Larger drops collide with smaller drops Grow as all or part of smaller drop’s water coalesces with larger drop Collision-Coalescence Process

  30. Virga

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