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NATS 101 Lecture 8a Vertical Stability QuickTime™ and a decompressor are needed to see this picture. NATS 101 Lecture 8a Vertical Stability QuickTime™ and a decompressor are needed to see this picture. 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 Unstable Rock never returns to starting point Conditionally Unstable Rock never returns if rolled past top of initial hill Ahrens, Fig 5.1 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 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 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 Conditionally Unstable Ahrens, Fig 5.9 Conditionally Unstable QuickTime™ and a decompressor are needed to see this picture. http://en.wikipedia.org/wiki/File:CloudOverUtah.jpg Environmental Lapse Rate (ELR) 6.5o C/km 6.0o C/km 10.0o C/km ELR is the Temp change with height that is recorded by a weather balloon ELR average is 6.5o C/km and thus ELR is frequently conditionally unstable! ELR is often absolutely unstable in a thin layer just above dry ground on hot, sunny days Ahrens, Meteorology Today 5th Ed. Lapse Rates and Cumulus Types Shallow Moderate Deep 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. 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 NATS 101 Lecture 8b Precipitation Processes QuickTime™ and a decompressor are needed to see this picture. 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 Cloud Droplets to Raindrops 106 larger 106 larger Ahrens, Fig. 5.17 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? Drop Size-Shape and Air Resistance sphere Air Resistance Terminal parachute Small Radius < 2.0 mm Large Radius 2.0-4.0 mm Gravitational = mg 2 A v2 mg = (1/2)=K(1/2) ρair AKvρTair Air Resistance vT = [2 mKg->/(K A) ]1/2 object K = 0.5 sphere; 2.0ρair irregular vT = [ 4/3 rdrop g (ρwtr)/(ρair) ]1/2 for sphere Terminal Fall Speeds Terminal Fall Speed (cm/s) (Gravitational Force=Air Resistance) 1.E+03 1.E+02 1.E+01 1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 6.5 m/sec 1.0 cm/sec 0.1 m/sec 0.0002 0.02 0.1 0.2 1 Diameter (millimeters) CCN Cloud Droplets -> Drizzle 1 km in 1010 sec 1 km in 105 -103 sec 2 5 Small -> Large Raindrops 1 km in 102 sec Collision-Coalescence Not all drops in collection path adhere Ahrens Fig 5.18b Collection Efficiency 10-50% 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 Warm Cloud Precipitation Ahrens, Fig. 5.19 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 Mixed Water-Ice Clouds glaciated region Ahrens, Fig. 5.20 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 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. SVP near Droplets and Ice More Vapor Less Vapor Ahrens Fig. 5.21 SVP is higher over supercooled water drops than ice 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, o Effect is maximized near -15 C they begin to fall where SVP water-ice is largest Ahrens, Fig. 5.22 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 Splintering Aggregation (Riming) a.k.a. Bergeron Process after the meteorologist who first recognized importance of the process 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 Examples of Precipitation Types Type Drizzle Size < 0.5 mm Rain 0.5 - 5 mm Freezing Rain 0.5 - 5 mm Sleet 0.5 - 5 mm Snow 1 - 2 mm Hail 5 to 10 cm or larger Description Small uniform drops that fall from stratus clouds Size of drops generally vary from one place to another Rain that freezes on contact with object Ice particles from raindrops that freeze during descent Aggregated ice crystals that remain frozen during descent Hard pellets of ice from cumulonimbus clouds Temp Profiles for Precipitation Snow - Temp colder than 0oC (almost) everywhere Ahrens Met. Today 9th Ed. 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 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 Next Class Assignment Air 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