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8. Precipitation

8. Precipitation. 8.1 Precipitation processes 8.2 Precipitation types 8.3 Measuring Precipitation. 8.1 Precipitation Processes. How do cloud droplets grow larger?. Curvature effect ( 곡률효과 ). Smaller cloud droplets exhibit a greater curvature, which causes a

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8. Precipitation

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  1. 8. Precipitation 8.1 Precipitation processes 8.2 Precipitation types 8.3 Measuring Precipitation

  2. 8.1 Precipitation Processes How do cloud droplets grow larger?

  3. Curvature effect (곡률효과) Smaller cloud droplets exhibit a greater curvature, which causes a more rapid of evaporation: curvature effect When air is saturated with respect to a flat surface, it is unsaturated with respect to a curved droplet of pure water -> droplet evaporates As droplets become larger, the effect of curvature lessens. (<20m)

  4. Solute effect (용질효과) Relative humidities rarely become greater than 101 percent, how do tiny cloud droplets of less than 1 m grow to the size of an average cloud droplets? Condensation begin on hygroscopic condensation nuclei when the relative humidity is well below 100 %..

  5. Collision and coalescence process In clouds with tops warmer than -15°C Terminal velocity: The particular falling speed for any give object at which the drag forces and buoyant forces by the fluid equals the gravitational force acting on the object Drag force Buoyant force Gravitational force

  6. Larger drops fall faster than smaller drops. Large droplets overtake and collide with smaller drops in their path Merging of cloud droplets by collision: coalescence (병합)

  7. An important factor influencing cloud droplet growth by the collision process : the amount of time the droplet spend in the cloud Warm cloud: clouds that have above freezing temperature at all levels precipitation forms by the collision and coalescence process Raindrop< 5mm

  8. The most important factor in the production of rain drop: liquid water content Other factors • The range of droplet sizes • The cloud thickness • The updrafts of the cloud • The electric charge of the droplets and • The electric field in the cloud

  9. Ice-crystal (Bergeron) process Extremely important in the middle and high latitude where clouds extend upward into regions where the air temperature is well below freezing. –cold clouds Supercooled (과냉각수적): water droplets existing at temperatures below freezing. T<-40°C only ice crystals Why are there so few ice crystals in the middle of cloud, even though temperatures there are well below freezing? The smaller the amount of pure water, the lower the temperature at which water freezes.

  10. Ice crystal may form in the subfreezing air on particles called ice nuclei. • The number of ice-forming nuclei available in the atmosphere is small. • It is difficult to find substances in nature that have a lattice structure similar to ice. -> Ice-forming nuclei are rare compared to cloud condensation nuclei. Contact freezing: freezing nuclei cause supercooled droplets to freeze when they collide with them

  11. Ice-crystal grow larger at the expense of the surrounding water droplet

  12. Accretion: the process of ice crystals growing larger as they collide with super-cooled cloud droplet Aggregation: the process of ice crystals colliding then sticking together the end product- snowflake

  13. Cloud seeding and precipitation The primary goal in cloud seeding: to inject a cloud with small particles that will act as nuclei The first ingredient in any seeding project: the presence of cloud Cloud seed: dry ice (드라이 아이스), silver iodide (AgI, 요오드화 은) Silver iodide • ice crystals form when silver iodide crystals come in contact with super-cooled liquid droplets • ice crystals grow in size as water vapor deposits onto the silver iodide crystal

  14. Precipitation in clouds Cloud forms -> initiated by either the collision-coalescence or the ice-crystal process-> precipitation growth is by accretion

  15. 8.2 Precipitation types Rain : falling drop of liquid water with a diameter (D) equal to or greater than 0.5 mm Drizzle: D < 0.5 mm Virga: evaporating streak of precipitation

  16. Are raindrops tear shaped? 2. Raindrop less than 2 mm in diameter 3. Raindrop greater than 2mm in diameter : Air pressure against the drop is greatest on the bottom and least on the sides

  17. snow Fallstreak: ice crystals and snowflakes falling from high cirrus clouds

  18. Snowflakes that fall through moist air that is slightly above freezing slowly melt as they descend -> a thin flim of water forms on the edged of the flakes -> it acts like glue when other snowflakes come in contact with it.

  19. Sleet and freezing rain snowflakes fall into warmer air -> melting -> it fall through subfreezing air -> partially melted snow flake or cold raindrop turns back into ice: sleet (진눈깨비) When super-cooled raindrop reach the cod surface, it Immediately freeze, forming a thin Veneer of ice: freezing rain (어는비)

  20. Rime(상고대): when supercooled cloud or fog droplets strike an object whose temperature is below freezing, the tiny droplet freeze, forming an accumulation of white or milky granular ice

  21. Snow grains and snow pellets Snow grain: small, opaque grains of ice the solid equivalent of drizzle fairly flat or elongated, with diameter generally less than 1 mm Upon striking a hard surface, they neither bounce or shatter Snow pellets: white, opaque grains of ice with diameters less than 5 mm brittle, crunchy and bounce upon hitting a hard surface fall as showers especially from cumulus congestus clouds

  22. Hail Hail : pieces of ice, either transparent or partially opaque, in size from that of small peas to that of golf balls or larger is produced in a cumulonimbus cloud

  23. Dry growth regime: If the growing hailstone enters a region inside the storm where the liquid water content is relatively low, super-cooled droplets will freeze immediately on the stone, producing a coating of white or opaque rime ice Wet growth regime: If the growing hailstone enters a region inside the storm where the liquid water content is high, super-cooled droplets no longer freeze on impact because the surface temperature remains a 0°C due to the release of latent heat. Clear ice form

  24. 8.3 Measuring precipitation Instruments Rain gauge: any instruments that can collect and measured rainfall

  25. Doppler radar and precipitation RADAR: RAdio Detection And Ranging transmit microwave receive scattered energy by target elapsed time between transmission and reception indicates the target’s distance

  26. Measuring precipitation from space • TRMM satellite is able to measure rainfall • intensity. • The onboard precipitation radar is capable of • detecting rainfall rates down to about • 0.7 mm per hour

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