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MET 60

MET 60. Topic 5 Clouds and Cloud Microphysics (and precipitation processes). Cloud types. Ci. Cc. Cs. Ac. As. Ns. Sc. St. Cu. Cu. Cb. Cloud Microphysics Things to cover: When does condensation occur? How does condensation occur? Laboratory observations (in text)

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MET 60

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  1. MET 60 Topic 5 Clouds and Cloud Microphysics (and precipitation processes) MET 60 topic 05

  2. Cloud types Ci MET 60 topic 05

  3. Cc MET 60 topic 05

  4. Cs MET 60 topic 05

  5. Ac MET 60 topic 05

  6. As MET 60 topic 05

  7. Ns MET 60 topic 05

  8. Sc MET 60 topic 05

  9. St MET 60 topic 05

  10. Cu MET 60 topic 05

  11. Cu MET 60 topic 05

  12. Cb MET 60 topic 05

  13. MET 60 topic 05

  14. Cloud Microphysics Things to cover: • When does condensation occur? • How does condensation occur? • Laboratory observations (in text) • In-situ observations (in text) • Modeling of clouds (in text) MET 60 topic 05

  15. When does condensation occur? • When RH  100%, right? • Wrong! • More complicated… • Answer is related to… MET 60 topic 05

  16. How does condensation occur? There are two ways: (i) without help Homogeneous nucleation MET 60 topic 05

  17. (ii) with help Heterogeneous nucleation MET 60 topic 05

  18. Homogeneous nucleation Where vapor molecules collide and form tiny embryonic liquid water droplets. Held together by surface tension. Given enough molecules, i.e., high RH values, nucleation can happen… MET 60 topic 05

  19. High RH values? We can define supersaturation where RH > 100% In the lab with clean air and pure water, we need RH >> 100% (say 400%) Depends on if condensation is relative to a flat or curved surface Curved surface = surface of embryonic droplet MET 60 topic 05

  20. Such high RH values are NOT OBSERVED Max observed supersaturations are up to about 1%  Homogeneous nucleation does not contribute to cloud formation MET 60 topic 05

  21. Heterogeneous nucleation Where condensation occurs around an aerosol particle which serves as the nucleus of the water droplet. aerosol particle (can be soluble or not) Liquid water droplet MET 60 topic 05

  22. Atmospheric aerosol We can take samples of air and see what aerosols are present (e.g., Aitken nucleus counter) http://www.sciencemuseum.org.uk/objects/meteorology/1908-193.aspx MET 60 topic 05

  23. Take a sample of moist air • Expand rapidly → cooling → supersaturation • Vapor will condense onto aerosol particles → “cloud” • Allow droplets to settle onto a surface • Count droplets e.g., with microscope Things to look for: • Amount (number of particles) • Size of particles • Type of particles (salt? dust? chemical?) MET 60 topic 05

  24. Figure 5.7 → aerosol sizes Notes: we’re looking at a few things in Cht. 5 be familiar with sizes & ranges Figure 5.8 → a “formation” mechanism! Aerosol distributions from MODIS… http://modis-atmos.gsfc.nasa.gov/MOD04_L2/sample.html MET 60 topic 05

  25. Some of these aerosols serve as Cloud Condensation Nuclei “CCN” MET 60 topic 05

  26. Observations of CCN Depends where you look! Over land? Over ocean? Near surface? Higher up? Day/night? MET 60 topic 05

  27. Typical particle sizes and numbers Figure 5.9 → both (all aerosol) Notes: • Range of sizes is 10-2 m to 100 m • MANY more small particles than large ones • Aitken nuclei have diameter < 0.2 m MET 60 topic 05

  28. Figure 6.5 looks just at CCN More CCN in air over land than over ocean clean Arctic air 30 per cm3 Marine air 100 per cm3 polluted Continental air 300 per cm3 MET 60 topic 05

  29. More CCN near the surface in continental air 5x reduction in CCN from boundary layer to “free atmosphere” (which starts at  1 km) But maritime air… number density roughly constant up to cloud base Also, daily max at 6pm (makes sense) MET 60 topic 05

  30. Origins of CCNs Continental air… Source = land Soil and dust particles Volcanic injection Anthropogenic sources Table 5.3 MET 60 topic 05

  31. Origins of CCNs Marine air… Salt particles MET 60 topic 05

  32. Recent stuff • Fine dust particles from the Gobi • Blown east of the Pacific towards us http://seattletimes.nwsource.com/ABPub/2006/12/27/2003498327.pdf MET 60 topic 05

  33. MET 60 topic 05

  34. Warm versus Cold Clouds A warm cloud has T > 0C throughout A cold cloud has T < 0C in part or all • e.g., cirrus (Ci) would have T < 0C throughout (cold cloud) • e.g., cumulonimbus (Cb) would have T < 0C in the upper part (cold cloud) • e.g., status (St) would have T > 0C throughout (warm cloud) MET 60 topic 05

  35. Almost all our rain (in San Jose) comes from cold clouds • e.g., Friday’s rain (11/27/09) 11/2809 00Z OAK sounding: MET 60 topic 05

  36. OAK, 11/28/09 @ 00Z cloud FZL  800 hPa MET 60 topic 05

  37. Warm Clouds Three things to monitor in a warm cloud: • Liquid Water Content (LWC) grams per cubic meter • Cloud droplet concentration Number per cc • Droplet size distribution histogram MET 60 topic 05

  38. How to sample droplets? Impaction studies • Fly through a cloud • Impact droplets on oil-covered slides etc. • Preserve droplets in oil for later studies Other e.g., scattering (p.216) MET 60 topic 05

  39. How to sample droplets cont… Measure LWC via a heated wire (p. 216) Measure optical depths via optical methods (p.216) MET 60 topic 05

  40. Measurements! Figure 6.6→ aircraft measurements of • w (vertical velocity component) • LWC • droplet size spectra MET 60 topic 05

  41. Cold Clouds A cold cloud contains: • Liquid water droplets (T > 0) • Supercooled water (T < 0) • Ice particles MET 60 topic 05

  42. Recall from Thermo that SVP over water > SVP over ice Fig. 3.9 Thus – vapor will migrate towards ice rather than liquid droplets Fig. 6.36 = extreme case MET 60 topic 05

  43. Conclusion = ice particles are VIP in growth process leading to precipitation • Ice crystals form and start to grow via… • Homogeneous nucleation? • Happens when T  -40C MET 60 topic 05

  44. Hetergeneous nucleation? • Requires nucleation sites (just like CCN) • Called ice nuclei • Fewer in number than CCN • Need to have ice-like structure • See http://www.edinformatics.com/interactive_molecules/ice.htm MET 60 topic 05

  45. In a study: 87% of snow crystals collected on the ground contained clay mineral particles @ center • of which  ½ were kaolinite, which is from soil • In urban air, 108 aerosol particles per liter • Of which ONE can act as an ice nucleus @ temp. -20C !!! • Figure 6.31 • Figure 6.35 if time! MET 60 topic 05

  46. Cloud Climatology Q: How much of the globe is cloud covered? Q: What types of clouds? Q: Why do we care? • Role in radiation budget • Role in climate • Clouds have a net COOLING effect • (based on satellite studies) MET 60 topic 05

  47. We also care because of clouds’ role in climate change! Warmed atmosphere & ocean • More vapor in atmosphere • More clouds (maybe?) • But which coulds? • Low? • High? MET 60 topic 05

  48. Observations of cloud cover: http://isccp.giss.nasa.gov/climanal.html Notes: • “cloud amounts vary by about 1-3% compared to a mean value of 66.7%” • “cloud top pressures vary by about 10-40 mb compared to a mean value of 578.1 mb” • “cloud top temperatures vary by about 1-4 K compared to a mean value of 261.7 K” • “cloud optical thicknesses vary by about 0.1-0.4 compared to a mean value of 3.4” • “surface temperatures vary by about 2-3 K compared to a mean value of 288.4 K” MET 60 topic 05

  49. Look at: • Global cloud cover variation (“cloud amount” in Part 1) • In Part 7, play with different cloud types • Cu – 11.6% • Sc – 12% • St – 1.75% • Ci – 13.4% • Cb – 2.89% MET 60 topic 05

  50. Look at: • VIS-IR low cloud cover: 25.49% • VIS-IR low cloud cover: 18.65% • VIS-IR low cloud cover: 22.23% MET 60 topic 05

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