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Lecture 13: Precipitation

Lecture 13: Precipitation. W & H: Sections 6.4 and 6.5. Cloud Droplets vs. Raindrops . Exercise. What is the ratio of the volume of a typical raindrop to the volume of a typical cloud droplet? r droplet = .01 mm r raindrop = 1 mm Answer: 10 6. Diffusional Growth.

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Lecture 13: Precipitation

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  1. Lecture 13: Precipitation W & H: Sections 6.4 and 6.5

  2. Cloud Droplets vs. Raindrops

  3. Exercise • What is the ratio of the volume of a typical raindrop to the volume of a typical cloud droplet? • rdroplet = .01 mm • rraindrop= 1 mm • Answer: 106

  4. Diffusional Growth • In condensation, water vapor molecules migrate toward the droplet by diffusion • To grow a raindrop by diffusion would take weeks! • There must be a faster way!

  5. Growth by Collection • Droplets collide and coalesce, forming a larger droplet • Problem: How can droplets collide?

  6. If all droplets were the same size, probability of collisions would be very small.

  7. Collisions • Requires large “starter droplets” • Have larger terminal velocities than smaller droplets

  8. Large droplets overtake smaller ones and collide with them. The droplets may coalesce, forming a larger droplet.

  9. As large droplet falls, its speed and size increase  rate of collection of smaller droplets increases • Result: Growth rate of collector drop increases rapidly.

  10. t = 0 t = t t = 2t t = 3t

  11. Diffusional Growth vs. Collection Diffusional Growth Growth by collection

  12. Growth of Ice Particles • Diffusional growth (deposition) • Accretion (riming) • Aggregation

  13. Diffusional Growth • Consider a mixed population of supercooled droplets and ice crystals • Concentration of droplets >> number of crystals • Ambient vapor pressure  es,w > es.i  ice crystals will grow rapidly

  14. For example, T = -10C es,w = 2.87 hPa; es,i = 2.60 hPa Ice crystal finds itself in a highly super-saturated environment Diffusional Growth of Crystal  super-saturation with respect to ice = 10% Result: rapid growth

  15. Evaporation of Droplets • As ice crystals grow, they deplete water vapor  vapor pressure falls below es,w  droplets begin to evaporate

  16. Initially es,i es,w ambient vapor pressure

  17. Later es,i es,w ambient vapor pressure Air is super-saturated with respect to ice, but sub-saturated with respect to water

  18. Eventually es,i es,w ambient vapor pressure No droplets left; ice-crystals stop growing

  19. Ice crystal growing at expense of surrounding supercooled droplets. Fig. 6.36 in W & H

  20. Crystal Shapes • Crystal shapes determined by temperature and supersaturation

  21. Hexagonal Plates Column Dendrite Sector Plate Bullet Rosette W & H: Fig. 6.40.

  22. Accretion (Riming) • Ice particles collide with super-cooled droplets • Droplets freeze onto ice crystals • Produces a rimed ice crystal

  23. Rimed ice crystals Graupel W & H, Fig. 6.41

  24. Aggregation • Clumping together of ice crystals • (This is how snowflakes are formed)

  25. Precipitation Initiation • Drop growth by collection • Growth of ice crystals by diffusion, accretion, and aggregation. #1 is dominant in the tropics (T > 0C) 1 & 2 are important in the middle latitudes

  26. Precipitation Types • Rain, snow, sleet & freezing rain, hail • In cold clouds, precipitation starts as snow in cloud • Precipitation at surface depends on temperatures below the cloud • Can get rain, snow, sleet, or freezing rain

  27. Sleet Formation

  28. Hail • Forms in cumulonimbus clouds • Starts as small ice crystal • Ice crystal moves through region of supercooled water & grows by accretion • AMS Glossary

  29. Hail Growth

  30. Weather Radar: Purposes • Detection of precipitation • Detection of tornadoes

  31. Detection of Precipitation • Radar transmits microwaves • Strength of return signal depends on precipitation intensity • Radar unit does a 360 scan at various elevation angles • Called a “volume scan”

  32. Example A 14-level volume scan mode

  33. Base vs. Composite Reflectivity • Base reflectivity just shows the lowest angle scan • Composite shows the strongest echo from any level • Comparison • http://www.srh.noaa.gov/srh/jetstream/doppler/comprefl.htm

  34. Sample Reflectivity Display

  35. Velocity Display • Shows radial velocity of precipitation particles • Uses Doppler effect • Frequency of return signal is different from frequency of transmitted signal.

  36. Straight-Line Motion On velocity display, red indicates motion away from the radar Green indicates motion toward the radar

  37. Example Storm movement Radar

  38. Radar Detection of Rotation Motion away from radar Motion toward radar

  39. NWS Tutorial • http://www.srh.noaa.gov/srh/jetstream/doppler/doppler_intro.htm

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