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ATMO 336 Weather, Climate and Society Vertical Stability Precipitation Processes 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: 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 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 Conditionally Unstable: Lower Rock Ahrens, Fig 5.7 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 is 6.5o C/km, on average, and thus is conditionally unstable! ELR is absolutely unstable in a thin layer just above the ground on hot, sunny days Ahrens, Meteorology Today 5th Ed. 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 ATMO 336 Weather, Climate and Society Precipitation Processes Cloud Droplets to Raindrops 106 bigger 106 bigger Ahrens, Fig. 5.15 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? Terminal Fall Speeds Terminal Fall Speed (cm/s) (Upward Suspension Velocity) 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 0.0002 0.02 0.1 0.2 1 Diameter (millimeters) CCN Cloud Droplets -> Drizzle 1 km in 1010 sec 1 km in 105 sec 2 5 Small-Large Raindrops 1 km in 102 sec Collision-Coalescence Area swept is smaller than area of drop small raindrop 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 to the big drops Collision-Coalescence Drops can grow by this process in warm clouds with no ice Occurs in warm tropical clouds Warm Cloud Precipitation Updraft (5 m/s) Ahrens, Fig. 5.16 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. 5.17 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 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. SVP near Droplets and Ice Ahrens, Fig. 5.18 SVP is higher over supercooled water drops than ice Ice Crystal Process Effect maximized around -15oC Ahrens, Fig. 5.19 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 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 Splintering Aggregation (Riming) Also known as the Bergeron Process after the meteorologist who first recognized the importance of ice in the precipitation process 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 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 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 Weather Conditions Associated with Precipitation Types Gedzelman, The Science and Wonders of the Atmosphere 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