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Precipitation Chapter 7 Precipitation Processes • Precipitation is any form of water that falls from a cloud and reaches the ground. • How do cloud drops grow? – When air is saturated with respect to a flat surface it is unsaturated with respect to a curved droplet of water….the smaller droplet evaporates more rapidly …Curvature Effect – At equilibrium, the vapor pressure over a curved droplet of water is greater than that over a flat surface To keep tiny cloud droplets in equilibrium with surrounding air, the air must be supersaturated …RH> 100% The curved line represents the relative humidity needed to keep a droplet in equilibrium with its environment. For a given droplet size, the droplet will evaporate and shrink when the relative humidity is less than that given by the curve. The droplet will grow by condensation when the relative humidity is greater than the value on the curve. • Collision & Coalescence – Plays a significant role in clouds with tops warmer than -15C (5 F) – Droplets of different sizes collide and coalesce into larger droplets Collision and coalescence. (a) In a warm cloud composed only of small cloud droplets of uniform size, the droplets are less likely to collide as they all fall very slowly at about the same speed. Those droplets that do collide, frequently do not coalesce because of the strong surface tension that holds together each tiny droplet. (b) In a cloud composed of different size droplets, larger droplets fall faster than smaller droplets. Although some tiny droplets are swept aside, some collect on the larger droplet’s forward edge, while others (captured in the wake of the larger droplet) coalesce on the droplet’s backside. A cloud droplet rising then falling through a warm cumulus cloud can grow by collision and coalescence, and emerge from the cloud as a large raindrop. “Warm Cloud” means all levels are above freezing Most important factor in production of raindrops is cloud’s liquid water content. Other factors: • range of droplet size • cloud thickness • updrafts • electrical charges Ice-Crystal (Bergeron) Process Wegener-Bergeron-Findeisen process ○ Occurs in cold clouds - mixture of ice & water ○ Ice crystals grow at expense of surrounding water droplets ○ Saturation vapor pressure greater over water as compared to ice. In a saturated environment, the water droplet and the ice crystal are in equilibrium, as the number of molecules leaving the surface of each droplet and ice crystal equals the number returning. The greater number of vapor molecules above the liquid indicates, however, that the saturation vapor pressure over water is greater than it is over ice. The difference in saturation vapor pressure between supercooled water and ice at different temperatures. The ice-crystal (Bergeron) process. (1) The greater number of water vapor molecules around the liquid droplet causes water molecules to diffuse from the liquid droplet toward the ice crystal. (2) The ice crystal absorbs the water vapor and grows larger, while (3) the water droplet grows smaller. Precipitation Processes • Cloud Seeding began in 1940’s – Inject cloud with small particles that act as condensation nuclei, starting the precipitation process. – NEED CLOUDS: seeding does not generate clouds – Cold clouds with a low seed ration best – Dry ice, silver iodide Natural Seeding Ice crystals falling from a dense cirriform cloud into a lower nimbostratus cloud. This photo was taken at an altitude near 6 km (19,700 ft) above western Pennsylvania. At the surface, moderate rain was falling over the region. Natural seeding by cirrus clouds may form bands of precipitation downwind of a mountain chain. Precipitation in Clouds • Starts quickly • Most precipitation formed through accretion • Many times rain starts out as ice How ice crystals grow and produce precipitation in clouds with a low liquid-water content and a high liquid-water content. Precipitation Types • Rain: 6mm max falling drop of liquid water – Drizzle less than 0.5 mm – Virga – evaporates before reaching ground Which of the three drops drawn here represents the real shape of a falling raindrop? Snow Most precipitation starts as snow Freezing level, snow & cloud appearance, fall streaks, drifting snow, blizzard A blanket of snow is a good insulator Snow scatters sunlight more effectively than rain. Consequently, when you look toward the sun, the region of falling precipitation looks darker above the melting level than below it. The dangling white streamers of ice crystals beneath these cirrus clouds are known as fallstreaks. The bending of the streaks is due to the changing wind speed with height. Figure 7.17 Common ice crystal forms (habits). • Sleet: air below freezing, then travels through a layer of air above freezing, begins to melt and then falls through a layer of air below freezing just above the ground surface. • Freezing Rain: ground surface is freezing as rain hits the surface it freezes. An accumulation of rime ice forms on tree branches as supercooled fog droplets freeze on contact in the below-freezing air. A heavy coating of freezing rain (glaze) covers Syracuse, New York, during January, 1998, causing tree limbs to break and power lines to sag. Average annual number of days with freezing rain and freezing drizzle over the United States. (NOAA) Vertical temperature profiles (solid red line) associated with different forms of precipitation. • Snow Grains: solid equivalent of drizzle, no bounce or shatter • Snow Pellets: larger than grains, bounce, break, crunch underfoot • Graupel: ice particle accumulation with rime • Hail: graupel acts as embryo in intense thunderstorm, grow through aggregation as pushed back up by updraft. The formation of snow pellets. In the cold air of a convective cloud, with a high liquidwater content, ice particles collide with supercooled cloud droplets, freezing them into clumps of icy matter called graupel. Upon reaching the relatively cold surface, the graupel is classified as snow pellets. A snowflake becoming a rimed snowflake, then finally graupel (a snow pellet). The accumulation of small hail after a thunderstorm. The hail formed as supercooled cloud droplets collected on ice particles called graupel inside a cumulonimbus cloud. This whopping hailstone fell on Vivian, South Dakota, on July 23, 2010. It had a record diameter of 8 inches, weighed a record 1.94 pounds and had a circumference of 18.6 inches. Stepped Art Measuring Precipitation • Instruments – Rain gauge: standard, tipping bucket, weighing • Snow: average depth at 3 locations, 10:1 water equivalent Radar ( Radio Detection and Ranging) Transmitter generates energy toward target, returned energy measured and displayed 1 cm wavelength – cloud droplets 3-10cm wavelengths - larger - Precipitation ○ Brightness of echo = amount/intensity of rain Stepped Art Doppler Radar • 1990’s replaced conventional • Has capacity to measure speed of rain moving toward and away from antenna • “ see” rotation and tornado generating storms (a) Doppler radar display showing precipitation intensity over North Carolina for August 27, 2011, as Hurricane Irene moves onshore. (b) Doppler radar display showing 1-hour rainfall estimates over North Carolina for August 27, 2011. Notice that in some places Doppler radar estimated that more than 1.50 inches of rain had fallen in one hour. (NOAA/NWS Doppler radar display showing 1-hour rainfall amounts over Oklahoma for April 24, 1999. Measuring Precipitation • Measuring from space – Specific satellites designed to assess clouds, atmospheric moisture, and rain ○ TRMM Tropical Rainfall Measuring Mission: – Precipitation Radar – Microwave Imager – Visible and Infrared Scanner ○ CloudSat – 2006 , 700km orbit – Cloud Profiling Radar – vertical profiling A satellite and radar image of Hurricane Humberto obtained by the TRMM satellite on September 13, 2007. Precipitation rates (lowest in blue, highest in dark red) were obtained by the satellite’s Precipitation Radar and Microwave Imager. The rainfall estimates are overlain on the infrared image of the storm. The National Weather Services across the United States are upgrading to better radar technology that can provide meteorologists better radar data that can improve warnings and lead time. The upgrade, called Dual Polarization (aka dual-pol), will be installed in all 160 National Weather Service offices. The standard Doppler radar transmits and receives information horizontally. With the dual-pol upgrade, meteorologists will be able to transmit and receive information in both horizontally and vertically in the atmosphere. These upgrades will improve the accuracy and identification of precipitation types, precipitation estimates, and even be able to view tornadoes better. These upgrades will continue across the United States and all offices should have the dual polarization technology installed by May 2013.