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NATS 101 Lecture 8a Vertical Stability

NATS 101 Lecture 8a Vertical Stability. NATS 101 Lecture 8a Vertical Stability. Severe thunderstorm near San Pedro River Valley, east of Tucson http://unfccc.int/files/inc/graphics/image/jpeg/calendar_06_11.jpg. Concept of Stability. Stable Rock always returns to starting point.

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NATS 101 Lecture 8a Vertical Stability

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  1. NATS 101Lecture 8aVertical Stability

  2. NATS 101Lecture 8aVertical Stability Severe thunderstorm near San Pedro River Valley, east of Tucsonhttp://unfccc.int/files/inc/graphics/image/jpeg/calendar_06_11.jpg

  3. 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

  4. 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. http://www.onr.navy.mil/focus/blowballast/sub/work2.htm

  5. Absolutely Stable 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 Ahrens, Fig 5.3

  6. Absolutely Unstable 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 or above forest fires Superadiabatic lapse rate Ahrens, Fig 5.6 Ahrens, Fig 5.8

  7. Ahrens, Fig 5.9 Conditionally Unstable

  8. Conditionally Unstable http://en.wikipedia.org/wiki/File:CloudOverUtah.jpg

  9. 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 average is 6.5o C/km and thus ELR is frequently conditionally unstable! 10.0o C/km ELR is often absolutely unstable in a thin layerjust above dry ground on hot, sunny days Ahrens, Meteorology Today 5th Ed.

  10. Lapse Rates and Cumulus Types Deep Moderate Shallow Ahrens, Meteorology Today 5th Ed. The ELR and depth of unstable layer modulates the type of Cu. As depth increases, the vertical extent of Cu generally increases. As temp difference between the air parcel and the environment increases, the updraft speed and severity of Cb typically increase.

  11. Take Home Points Vertical Stability Determined by ELR (Environmental Lapse Rate) Absolutely Stable and Unstable Conditionally Unstable Temp Difference between Air Parcel and ELR, along with Depth of Layer of Conditionally Instability Modulates Vertical Extent and Severity of Cumulus

  12. NATS 101Lecture 8bPrecipitation Processes Rim ice coats the observatory on the top of Mt. Washington, N.H. after a winter storm. Accretion is important in the formation of precipitation from cold clouds.http://www.craterranch.com/Mt_Washington/Mt_Washington_Images/IMG_2672.JPG

  13. 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 larger 106 larger Ahrens, Fig. 5.17

  14. sphere parachute Small Radius < 2.0 mm Large Radius 2.0-4.0 mm Drop Size-Shape and Air Resistance Air Resistance Terminal Air Resistance Gravitational = mg mg=(1/2) K ρair A vT2 vT = [2 m g /(K ρair A) ]1/2 vT = [ 4/3 rdrop g (ρwtr)/(ρair) ]1/2 for sphere Air Resistance= (1/2) K ρair A v2 K = 0.5 sphere; K -> 2.0 irregular object

  15. Terminal Fall Speeds(Gravitational Force=Air Resistance) 6.5 m/sec 1.0 cm/sec 0.1 m/sec CCN 1 km in 1010 sec Cloud Droplets -> Drizzle 1 km in 105 -103 sec Small -> Large Raindrops 1 km in 102 sec

  16. Big water drops fall faster than small drops As big drops fall, they collide with smaller drops Some of the smaller drops stick Collision-Coalescence Drops can grow by this process in warm clouds with no ice Occurs in warm tropical clouds Only 10-50% collisions cohere Collision-Coalescence Not all drops in collection path adhere Ahrens Fig 5.18b Collection Efficiency 10-50%

  17. 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 Ahrens, Fig. 5.19

  18. 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. 5.20

  19. SVP over Liquid and Ice SVP over ice is less than over water because it takes a H2O molecule more energy to sublimate than evaporation So at equilibrium, more vapor resides over cloud droplets than ice crystals Ahrens, Meteorology Today 5th Ed.

  20. SVP near Droplets and Ice More Vapor Less Vapor Ahrens Fig. 5.21 SVP is higher over supercooled water drops than ice

  21. Ice Crystal Process SVP for a water drop is too high for ice crystal, so vapor next to drop will diffuse towards ice Ice crystals grow at the expense of water drops, which freeze on contact Deposition As ice crystals grow, they begin to fall Effect is maximized near -15oC where SVP water-ice is largest Ahrens, Fig. 5.22

  22. 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. 5.23 Accretion (Riming) Splintering Aggregation a.k.a. Bergeron Process after the meteorologist who first recognized importance of the process

  23. Take Home Points 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 => Not efficient Cold clouds (with ice) Ice Crystal Process => Very efficient Accretion-Splintering-Aggregation

  24. Examples of Precipitation Types

  25. Temp Profiles for Precipitation Ahrens Met. Today 9th Ed. Snow - Temp colder than 0oC (almost) everywhere 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

  26. Summary: Key Concepts Precipitation can take many forms Drizzle-Rain-Glazing-Sleet-Snow-Hail Depends 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

  27. Next Class AssignmentAir Pressure, Surface Maps • Reading -Ahrens 3rd: 139-146 4th: 141-148 5th: 141-148 • Homework05 - D2L (Due Wednesday Mar. 3) 3rd-Pg 162: 6.1, 7, 8 4th-Pg 164: 6.1, 7, 8 5th-Pg 165: 6.1, 8, 9

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