Understanding Cloud Microphysics: Nucleation and Droplet Formation

1. Lecture 11 Cloud Microphysics Wallace and Hobbs – Ch. 6 Ignore most of the math – concentrate on descriptive conclusions and graphs

2. Cloud Types

3. Outline Cooling Supersaturation Droplet Formation Droplet Growth Precipitation Formation

4. Nucleation • Usually refers to the initial formation of a droplet • More general definition: AMS Glossary • Homogeneous nucleation • Droplet spontaneously forms in pure air • No particles present • Heterogeneous nucleation • Droplets form on particles called cloud condensation nuclei (CCN)

5. Homogeneous Nucleation • Formation of a curved water surface requires energy  maintenance of a small droplet requires large supersaturations

6. RH to Form Droplet of Radius r RH W & H, Fig. 6.2 112% Such large RHs do not occur in nature. r (m) 0.01

7. Heterogeneous Nucleation • Hygroscopic CCN are particularly effective condensation initiators • Generally made of soluble salts • When droplet forms, solution has a much lower vapor pressure than pure water  Condensation begins when RH < 100% • Droplet growth requires supersaturations of less than 1% • Such supersaturations are achieved in updrafts

8. Köhler Curves • Give the equilibrium droplet size for a given RH.

9. “Saturation ratio” = RH/100 Köhler Curves 10-19 g 10-18g 10-17g Numbers indicate mass of dissolved salt (NaCl) Suppose RH = 100.1%

10. 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius

11. 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius

12. 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius

13. 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius

14. 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius

15. Typical cloud droplet radius 10-19 g 10-18g 10-17g Droplets grow until they reach equilibrium radius

16. Droplet Growth • If ambient RH < value at peak of curve, droplets stop growing when much smaller than typical cloud drop • They are called haze droplets

17. 10-19 g 10-18g 10-17g Suppose RH = 100.3%

18. 10-19 g 10-18g 10-17g Droplets growing on smaller nuclei behave as before

19. 10-19 g 10-18g 10-17g Look at largest nucleus

20. 10-19 g 10-18g 10-17g

21. 10-19 g 10-18g 10-17g

22. 10-19 g 10-18g 10-17g

23. 10-19 g 10-18g 10-17g

24. 10-19 g 10-18g 10-17g

25. 10-19 g 10-18g 10-17g Droplet keeps growing!

26. Droplet “Activation” • If ambient RH > peak value, droplet grows indefinitely • Once droplet has gotten “over the hump”, it is said to be activated.

27. Slowing of Growth • Rate of droplet growth decreases as droplets grow • Let r = droplet radius • It can be shown that

28. Depletion of Water Vapor • Also, growth of large number of droplets reduces supersaturation • Result: Droplet radius tends to level off at about 10m • Fall velocity of such a droplet is < 1 cm-1  droplets tend to be carried upward • Droplets must be much larger to actually fall

29. Microphysical Parameters • Liquid water content (LWC) • grams of liquid water per m3 of cloud • Droplet concentration, N • Number of droplets per cm3 • Mean droplet size, • Usually given in m • Not independent – knowledge of any two determines the third

30. Relationship Between Microphysical Parameters where L is the density of liquid water. See W & H, p. 217 for typical values of microphysical parameters

31. Supercooled Water • Definition: Liquid water with T < 0C • Freezing • Homogeneous nucleation occurs at -40C! • Heterogeneous nucleation occurs in presence of a freezing nucleus • (Typically occurs at temps much higher than -40C)

32. Freezing Point • Common experience: Water freezes at 0C • This works when mass of water >> cloud droplet • Only one nucleation event is required to freeze entire mass • Such an event is virtually certain for masses of water normally encountered • Cloud droplets very small • Probability of a nucleation event at 0C is small • Probability increases as temperature falls

33. Ice Crystals • When T < 0C, ice crystals can form directly from vapor • Homogeneous nucleation requires unrealistically large super-saturations • Heterogeneous nucleation occurs on particles called deposition nuclei

34. Ice Nuclei • General name for various types of nuclei • e.g., freezing nuclei, deposition nuclei • Relatively rare • 1 particle in 108 suitable!

35. Nucleation Temps Substance Temp. (C) -9 Kaolinite Silver Iodide -4 Bacteria! -3 Source: Table 9.1 in A Short Course in Cloud Physics, 3rd Ed. Rogers, R. and M. Yau. Pergamon Press, 293 pp.

36. Supercooled Water and Ice • Let es,w(T) be the saturation vapor pressure over liquid water at temperature T • Let es,i(T) be the saturation vapor pressure over ice at temperature T • es,i(T) < es,w(T) for T < 0C

38. es,i vs. es,w (Source: Smithsonian Meteorological Tables, 6th Ed.)