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NATS 101 Lecture 13 Precipitation Processes

NATS 101 Lecture 13 Precipitation Processes. Supplemental References for Today’s Lecture. Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology . 462 pp. McGraw-Hill. (ISBN 0-697-21711-6)

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NATS 101 Lecture 13 Precipitation Processes

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  1. NATS 101Lecture 13Precipitation Processes

  2. Supplemental References for Today’s Lecture 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)

  3. Review: Vertical Stability Rising unsaturated air, and all sinking air Temp changes at DAR of 10oC/km Dew Point (DP) changes at rate of 2oC/km Rising saturated air Latent Heating Mitigates Adia. Cooling Temp and DP cool at MAR of 6oC/km Water Vapor Condenses into Liquid

  4. Review: Vertical Stability Vertical Stability Determined by ELR Conditionally Unstable (MAR < ELR < DAR) Temp Difference between Environmental Air and Air Parcel, and the Depth of Conditionally Instability Controls Vertical Extent and Severity of Cumulus TODAY’s Material

  5. 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

  6. Radiative Fog: An Example of a Stratiform Cloud that Forms in Absolutely Stable Air Fig. 5-4, p.114

  7. Absolutely Unstable: Middle Rock Unstable air accelerates 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

  8. Cumulus Cloud Above a Smoke Plume from Forest Fire: Example of Cumuliform Cloud Forming in the Absolutely Unstable Air that Is Immediately Above the Fire Fig. 5-6, p.115

  9. Conditionally Unstable: Lower Rock Ahrens, Fig 5.7

  10. Cumulus Congestus: Example of Cumuliform Clouds Developing in Conditionally Unstable Air Fig. 5-11, p.119

  11. 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.

  12. 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

  13. Terminal Fall Speeds(Upward Suspension Velocity) CCN 1 km in 1010 sec Cloud Droplets -> Drizzle 1 km in 105 sec Small-Large Raindrops 1 km in 102 sec

  14. 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 Area swept is smaller than area of drop small raindrop Collision-Coalescence Collection Efficiency 10-50%

  15. 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

  16. 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

  17. 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.

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

  19. 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

  20. 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

  21. 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

  22. Examples of Precipitation Types

  23. 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

  24. 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

  25. Assignment for Next Lecture • Topic – Atmospheric Pressure • Reading - Ahrens pg 141-148 • Problems - 6.1, 6.7, 6.8

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