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

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

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

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

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

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

15. Typical cloud droplet radiusg
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. g
10-18g
10-17g
Suppose RH = 100.3%

18. Droplets growing on smaller nuclei behave as before

19. Look at largest nucleus

20. g
10-18g
10-17g

21. g
10-18g
10-17g

22. g
10-18g
10-17g

23. g
10-18g
10-17g

24. g
10-18g
10-17g

25. 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 Relatively rare
e.g., freezing nuclei, deposition nuclei
Relatively rare
1 particle in 108 suitable!

35. Nucleation Temps Substance Temp. (C) Kaolinite -9 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 T (C) es,i(hPa) es,w(hPa) 6.11 -5 4.02 4.21 -10 2.60
6.11 -5
4.02
4.21
-10
2.60
2.87
-15
1.65
1.91
-20
1.03
1.25
-25
0.63
0.81
-30
0.38
0.51
-35
0.22
0.31
-40
0.13
0.19
(Source: Smithsonian Meteorological Tables, 6th Ed.)