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? r droplet =.01 mm r droplet =.01 mm r raindrop = 1 mm r raindrop = 1 mm Answer: 10 6

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  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 Concentration of droplets >> number of crystals Ambient vapor pressure  e s,w > e s.i  ice crystals will grow rapidly  ice crystals will grow rapidly

14. Diffusional Growth of Crystal For example, T = -10  C e s,w = 2.87 hPa; e s,i = 2.60 hPa e s,w = 2.87 hPa; e s,i = 2.60 hPa Ice crystal finds itself in a highly super- saturated environment  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 e s,w  vapor pressure falls below e s,w  droplets begin to evaporate  droplets begin to evaporate

16. e s,i e s,w ambient vapor pressure Initially

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

18. e s,i e s,w ambient vapor pressure Eventually 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. W & H: Fig. 6.40. Hexagonal Plates Column Dendrite Sector Plate Bullet Rosette

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

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

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

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

29. Hail Growth

31. Weather Radar: Purposes 1.Detection of precipitation 2.Detection of tornadoes

32. 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” Called a “volume scan”

33. Example A 14-level volume scan mode

34. 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 http://www.srh.noaa.gov/srh/jetstream/doppler /comprefl.htm http://www.srh.noaa.gov/srh/jetstream/doppler /comprefl.htm http://www.srh.noaa.gov/srh/jetstream/doppler /comprefl.htm

35. Sample Reflectivity Display

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

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

38. Example Radar Storm movement

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

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