1. Chapters 5 and 6 Cloud and Aerosol Physics
Goals:
Understand …
Homogeneous and heterogeneous nucleation of cloud droplets
The central role of aerosol in cloud physics
Precipitation formed in warm and cold clouds
Saturation and super saturation in the atmosphere
Super cooling of cloud droplets
Formation of ice crystals in cirrus clouds and ice fog
Cloud electrification
Figures, etc, from Wallace and Hobbs unless otherwise stated.

2. Aerosol

3. Atmosphere as a Photochemical Reactor: OH The Hydroxyl Radical

4. Section 5.4: Tropospheric Aerosol
SOLAR
INFRARED

5. Aerosol Number Distributions for air source: Continental (red curve) Marine (blue curve) and urban polluted (black curve)
Aitken nuclei: Particle counted with a very high super saturation, 100% or so, also known as condensation nuclei (CN).
Cloud condensation nuclei (CCN) are large CN that are counted with supersaturations of only around a percent or so, as found in the atmosphere.

6. Particle Surface Area and Dynamics (rough idea)
Continental (red curve) Marine (blue curve) and urban polluted (black curve)
What fraction are CCN at various supersaturations?

7. UNR CIMEL SUNPHOTOMETER AEROSOL DATA
AEROSOL SPECTRAL OPTICAL DEPTH

8. UNR CIMEL SUNPHOTOMETER AEROSOL DATA
Coarse mode
‘Retrieved’ Aerosol Volume Distributions
Fine mode

9. Chemistry of Stratospheric Sulfate Layer (strong volcanic eruptions): A form of ‘natural’ geoengineering, aerosol scatter solar radiation to space
Aerosol removed by tropospheric folds and jet stream dynamics: locations where stratospheric air is forced to mix down to the troposphere.

10. Stratospheric Aerosol Optical Depth at wavelength 1 micron (40 km to 2 km above tropopause)
Volcanos

11. Tropospheric Folds

12. Chapter 6: Cloud And Aerosol Microphysics
CCN size is 25x too large

13. Summary In Two Images Water droplet coalescence to form raindrops.
Ice crystals/snowflakes at top; Mid level mixed clouds, ice consumes water droplets
Melt to raindrops near the surface
from

14. Cloud Types

15. Courtesy: Steve Platnick, NASA
Homogeneous Nucleation of Droplets;
Kelvin’s Equation
Cloud Condensation Nuclei.
Warm Clouds.
Growth of Drops by Condensation
Atmospheric Aerosols
Heterogeneous Nucleation of Droplets;
Köhler Curves
Growth of Drops by Collisions.
Ice Nuclei and Ice Crystal in Clouds
Growth of Ice Particles in Clouds
Cold Cloud Processes
Courtesy ?
Warm Cloud Processes
Courtesy: Steve Platnick, NASA

16. Chapter 6: First topic: Homogeneous nucleation of cloud droplets (not how most cloud droplets are formed)
Droplets with R < r evaporate:
with R > r grow since the energy diminshes. NOTE: e > es is needed for growth.
Growth begets growth,
shrink begets droplet shrink:
Unstable equilibrium

17. Another View …

18. How Many Water Molecules in a Droplet?

19. Relative Humidity and Supersaturation at Equilibrium Above a Droplet (with respect to a flat surface of water)
Unrealistically large … super saturations this high are not found
Cloud droplets can’t get past the small size this way:
Need another mechanism

20. Relative Humidity and Supersaturation at Equilibrium Above a Droplet (with respect to a flat surface of water)
Unrealistically large … super saturations this high are not found (broader range of size)
Cloud droplets can’t get past the small size this way:
Need another mechanism
Relative probability molecules are in the vapor phase for a flat surface compared with a curved surface of radius r and area S.

21. Relative Humidity and Supersaturation at Equilibrium Above a Droplet (with respect to a flat surface of water)
Inside water molecules ‘hold’ the surface molecules less strongly for droplets compared with a flat surface.

22. How to Overcome Problem of High Supersaturation Needed for Homogeneous Nucleation

23. Nucleation s.v.p. over curved surface > s.v.p. over flat surface
For typical molecule, r ~ 0.6 nm, would need RH=740% (SS=640%) In atmosphere maximum SS observed is ~1% Condensation must be on particles with r ~ 100nm
Radius
Name
Conc. (cm-3) SS
< 0.1 µm
Aitken nuclei
10,000 1% µm
large nuclei
100
0.1%
> 1 µm
giant nuclei 1
0.01%

24. Haze and RH Hysteresis

25. Haze and RH Hysteresis 150 nm diameter dry aerosol:
Start dry at 30% RH:
Increase to 80% RH:
Deliquescence:
Growth for RH>80%:
Decrease RH to 38%:
Efflorescence:
Dry for RH<38%

26. Raoult’s Law (Wikipedia)

27. Limitations of Raoult’s Law

28. Raoult’s Law Example: Fewer water molecules are above a solution droplet

29. From Köhler, salty water
Köhler Theory: Add dissolved ions to reduce the number of water vapor molecules escaping the solution droplet (impure water droplet)
From Lord Kelvin, pure water
From Raoult’s Law
From Köhler, salty water

30. Köhler Curves for Different Salts: NaCl and (NH4)2SO4: 1 ion unit = 1 million ions
Note the change of scale above 100%

31. Haze Droplet and Activated Cloud Droplet for Ambient RH=100.4%

32. Another Look at Köhler Curves
slides from

33. 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

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

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

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

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

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

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

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

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

42. 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

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

44. Droplets growing on smaller nuclei behave as before

45. Look at largest nucleus

46. g
10-18g
10-17g

47. g
10-18g
10-17g

48. g
10-18g
10-17g

49. g
10-18g
10-17g

50. g
10-18g
10-17g

51. g
10-18g
10-17g
Droplet keeps growing!

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

53. Cloud Condensation Nuclei (CCN): Measurement Method, Thermal Diffusion Cloud Chamber

54. Cloud Condensation Nuclei (CCN): Typical Measurements
Data from Hudson and Yun, JGR, 2002 (DRI)

55. Warm Clouds (everywhere > 0 C)

56. Measurements of Cloud Droplets
Schematic of pod mounted instrument

57. Warm Cloud Microphysics Example

58. Warm Cloud Comparison

59. Cloud Aerosol Indirect Effect: Clouds with the same liquid water path, but more CCN and more cloud droplets, have higher albedo than clouds with lesser CCN and fewer cloud droplets. (Twomey hypothesis)

60. Cloud Effective Radius From Satellite Retrievals
Generally smaller cloud droplets over land than over ocean.

61. Ship Tracks (ships emit huge numbers of CCN): Bright lines from numerous small cloud droplets.
Often present off the California coast

62. Cloud Liquid Water Content (LWC): Adiabatic LWC

63. Entrainment of Dry Air Causes Local Evaporation and Reduced Cloud LWC

64. Nonattainment of Adiabatic LWC

65. Growth of Cloud Droplets in Warm Clouds: Condensation and Collision-Coalescence

66. Growth of Cloud Droplets in Warm Clouds: Condensational Growth Near Cloud Base

67. Small and Large Droplet Fall Speed

68. Terminal velocity of drops
For laminar flow, at terminal velocity
drag force=weight

69. Faster Falling Droplets Collect Slower Moving Little Ones
Giant CCN and/or turbulent mixing may produce large collector drops.
Precipitation falls out when collector drop terminal fall speed exceeds the updraft.
Large collector droplets can break into smaller ones during impacts.

70. Cold Clouds: Ice Crystal Microphysics

71. Key Idea: Vapor pressure for water is greater than for ice below 0 C.
From

72. Ice Nuclei are Often Much Less Prevalent than Ice Crystals

73. Ice Multiplication

74. Cloud Structure