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NATS 101 Lecture 10 Vertical Stability Precipitation

Explore concepts of stability, saturation, and buoyancy in meteorology to understand vertical air movements over a mountain and precipitation processes. Concepts such as DAR, MAR, ELR, and cloud formation are explained with practical examples.

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NATS 101 Lecture 10 Vertical Stability Precipitation

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  1. NATS 101Lecture 10Vertical StabilityPrecipitation

  2. ice mixture liquid Ahrens, Fig 4.29

  3. Moist Flow over a Mountain +10C +2C DAR -6C -6C MAR saturated -6C -6C MAR unsaturated +10C +2C DAR unsaturated -10C-2C DAR +10C +2C DAR Ahrens, Fig 5.12 These concepts can be applied to understand Temp and DP changes for moist flow over a mountain

  4. Brain Burners Rising unsaturated (clear) air, and all sinking air Temperature changes at Dry Adiabatic Rate (DAR) of 10oC/km Dew point changes at rate of 2oC/km Rising saturated (cloudy) air Temperature cools atMoist Adiabatic Rate(MAR) of6oC/km Dew point decreases at rate of6oC/km

  5. Archimedes’ Principle • Archimedes' principle is the law of buoyancy. It states that"any body partially or completely submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body." • The weight of an object acts downward, and the buoyant force provided by the displaced fluid acts upward. If the density of an object is greater/less than the density of water, the object will sink/float. • Demo: Diet vs. Regular Soda (last lecture). http://www.onr.navy.mil/focus/blowballast/sub/work2.htm

  6. Concept of Stability StableRock always returns to starting point UnstableRock never returns to starting point Conditionally UnstableRock never returns if rolled past top of initial hill Ahrens, Fig 5.1

  7. Absolutely Stable: Top Rock Stable air strongly resists upward motion External force must be applied to an air parcel before it can rise Clouds that form in stable air spread out horizontally in layers, with flat bases-tops Ahrens, Fig 5.3

  8. Absolutely Unstable: Middle Rock Unstable air does not resist upward motion Clouds in unstable air stretch out vertically Absolute instability is limited to very thin layer next to ground on hot, sunny days Superadiabatic lapse rate Ahrens, Fig 5.5

  9. Conditionally Unstable: Lower Rock Ahrens, Fig 5.7

  10. Environmental Lapse Rate (ELR) ELR is the Temp change with height that is recorded by a weather balloon 6.5o C/km 6.0o C/km ELR is 6.5o C/km, on average, and thus is conditionally unstable! 10.0o C/km ELR is absolutely unstable in a thin layerjust above the ground on hot, sunny days Ahrens, Meteorology Today 5th Ed.

  11. Summary: Key Concepts I Rising unsaturated air, and all sinking air Temp changes at DAR of 10oC/km DP changes at rate of 2oC/km Saturation occurs with sufficient lifting Rising saturated air Latent Heating Mitigates Adia. Cooling Temp and DP cools at MAR of 6oC/km Note that MAR is always less than DAR

  12. Summary: Key Concepts II Vertical Stability Determined by ELR Absolutely Stable and Unstable Conditionally Unstable Temp Difference between ELR and Air Parcel, and Depth of Layer of Conditionally Instability Modulates Vertical Extent and Severity of Cumulus

  13. Precipitation Processes

  14. Supplemental References for Precipitation processes Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN 0-697-21711-6) Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6)

  15. Cloud Droplets to Raindrops A raindrop is 106 bigger than a cloud droplet Several days are needed for condensation alone to grow raindrops Yet, raindrops can form from cloud droplets in a less than one hour What processes account for such rapid growth? 106 bigger 106 bigger Ahrens, Fig. 5.15

  16. Terminal Fall Speeds(upward suspension velocity) CCN Cloud Droplets-Drizzle Small-Large Raindrops

  17. 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 Collision-Coalescence 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%

  18. As cloud droplet ascends, it grows larger by collision-coalescence Cloud droplet reaches the height where the updraft speed equals terminal fall speed As drop falls, it grows by collision-coalescence to size of a large raindrop Warm Cloud Precipitation Updraft (5 m/s) Ahrens, Fig. 5.16

  19. Mixed Water-Ice Clouds Clouds that rise above freezing level contain mixture of water-ice Mixed region exists where Temps > -40oC Only ice crystals exist where Temps < -40oC Mid-latitude clouds are generally mixed glaciated region Ahrens, Fig. 5.17

  20. SVP over Liquid and Ice SVP over ice is less than over water because sublimation takes more energy than evaporation If water surface is not flat, but instead curves like a cloud drop, then the SVP difference is even larger So at equilibrium, more vapor resides over cloud droplets than ice crystals Ahrens, Meteorology Today 5th Ed.

  21. SVP near Droplets and Ice Ahrens, Fig. 5.18 SVP is higher over supercooled water drops than ice

  22. Ice Crystal Process Since SVP for a water droplet is higher than for ice crystal, vapor next to droplet will diffuse towards ice Ice crystals grow at the expense of water drops, which freeze on contact As the ice crystals grow, they begin to fall Effect maximized around -15oC Ahrens, Fig. 5.19

  23. Accretion-Aggregation Process Small ice particles will adhere to ice crystals Supercooled water droplets will freeze on contact with ice snowflake ice crystal Ahrens, Fig. 5.17 Accretion (Riming) Splintering Aggregation Also known as the Bergeron Process after the meteorologist who first recognized the importance of ice in the precipitation process

  24. Summary: Key Concepts 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

  25. Examples of Precipitation Types

  26. Temp Profiles for Precipitation Ahrens, Meteorology Today 5th Ed. Snow - Temp colder than 0oC everywhere (generally speaking!) Sleet - Melting aloft, deep freezing layer near ground Freezing Rain - Melting aloft, shallow freezing layer at ground Rain - Deep layer of warmer than 0oC near ground

  27. Summary: Key Concepts Precipitation can take many forms Drizzle-Rain-Glazing-Sleet-Snow-Hail Depending on specific weather conditions Radar used to sense precipitation remotely Location-Rate-Type (liquid v. frozen) Cloud drops with short wavelength pulse Wind component toward and from radar

  28. Assignment • Topic - Precipitation Processes • Reading - Ahrens p121-134 • Problems - 5.14, 5.16, 5.17 • Topic – Atmospheric Pressure • Reading - Ahrens pg 141-148 • Problems - 6.1, 6.7, 6.8

  29. Assignment for Next Lecture

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