1. Microphysics of Cold Clouds

2. Microphysics of Cold Clouds
Reading
Wallace & Hobbs
pp 232 – 245

3. Microphysics of Cold Clouds
Objectives
Be able to define cold clouds, mixed phase cloud and glaciated cloud
Be able to define supercooled liquid water
Be able to define homogeneous and heterogeneous nucleation

4. Microphysics of Cold Clouds
Objectives
Be able to generally describe the process of homogeneous nucleation
Be able to recall the relationship between homogeneous nucleation, temperature and drop size
Be able to generally describe why heterogeneous nucleation occurs at warmer temperatures

5. Microphysics of Cold Clouds
Objectives
Be able to describe the four different modes of heterogeneous nucleation
Be able to recall that the temperature of heterogeneous nucleation depends on the mode of nucleation
Be able to define ice nuclei

6. Microphysics of Cold Clouds
Objectives
Be able to identify the biggest source of ice nuclei
Be able to recall the Fletcher Relationship of ice nuclei size distribution
Be able to explain why concentrations of ice crystals in cloud exceed ice nuclei concentrations

7. Cold Cloud
Some Part of Cloud Extends Above 0oC
0oC

8. Mixed Phase Cloud Contains Both Liquid Water Drops and Ice Particles
0oC

9. Glaciated Cloud
Consists Entirely of Ice Particles
0oC

10. Supercooled Liquid Water
Liquid Water That Exists at Temperatures Colder Than 0oC
0oC

11. Supercooled Liquid Water
Liquid Water Molecule
Hydrogen Bonds

12. Supercooled Liquid Water
Metastable State
Want to Freeze
0oC

13. Fusion
Ice
Water Molecules Arranged in Lattice
Fusion
Liquid Water
Ice

14. Fusion
Homogeneous Nucleation
Heterogeneous Nucleation

15. Homogeneous Nucleation
Water Molecules Arrange Themselves into a Lattice
0oC
Ice
Embryo

16. Homogeneous Nucleation
Embryo Grows by Chance Aggregation
0oC
Ice
Embryo

17. Homogeneous Nucleation
Ice Nucleus Is in Constant Flux
Molecule Come & Go
0oC
Ice
Embryo

18. Homogeneous Nucleation
Chance Aggregation Increases with Decreasing Temperature
0oC
Ice
Embryo

19. Homogeneous Nucleation
Chance Aggregation Also Depends on Drop Size (or Surface Tension)
0oC
Ice
Embryo

20. Homogeneous Nucleation
Ice Embryo Must Reach Critical Size Before Fusion Occurs Spontaneously
0oC
Ice
Embryo

21. Homogeneous Nucleation
Spontaneous Nucleation Occurs At
5 mm
-39oC

22. Homogeneous Nucleation
Spontaneous Nucleation Occurs At
5 mm
40 mm
-36oC
-39oC

23. Homogeneous Nucleation
Spontaneous Nucleation Rarely Occurs in the Atmosphere
0oC

24. Fusion
Homogeneous Nucleation
Heterogeneous Nucleation

25. Heterogeneous Nucleation
Fusion Is Aided by Foreign Substances Called Ice Nuclei
0oC
Ice
Nuclei

26. Heterogeneous Nucleation
Ice Nuclei Provide a Surface for Liquid Water to Attach
0oC
Ice
Nuclei

27. Heterogeneous Nucleation
Ice Embryo Starts At a Larger Size
0oC
Ice
Nuclei

28. Heterogeneous Nucleation
Fusion Occurs at Much Warmer Temperature
0oC
Ice
Nuclei
-39oC

29. Heterogeneous Nucleation Modes
Condensation Followed by Freezing
Contact
Immersion
Deposition

30. Heterogeneous Nucleation Modes
Condensation Followed by Freezing
Ice nuclei acts as CCN, then freezes droplet

31. Heterogeneous Nucleation Modes
Contact
Water droplet freezes instantaneously upon contact with ice nuclei

32. Heterogeneous Nucleation Modes
Immersion
Causes freezing after becoming embedded within droplet

33. Heterogeneous Nucleation Modes
Deposition
Ice forms directly from vapor

34. Heterogeneous Nucleation Modes
One particle may nucleate in different ways

35. Heterogeneous Nucleation Modes
Temperature at which nucleation occurs also depends on mode
0oC

36. Glossary of Meteorology
Ice Nuclei
“Any particle that serves as a nucleus leading to the formation of ice crystals without regard to the particular physical processes involved in the nucleation.”
Glossary of Meteorology

37. Ice Nuclei Crystal Structure Similar to Ice
Nucleate at warmer temperatures

38. Ice Nuclei
Pure Substances
Minerals
Organic Material

39. Ice Nuclei

40. Ice Nuclei Terrestrial Source Biggest Contributor
Ice Nuclei Concentrations Decrease with Altitude
More Ice Nuclei in Continental Air

41. Ice Nuclei
Measurement Techniques
Cloud Chambers
Filter Systems

42. Ice Nuclei Measurement Problems Do not know mode of activation
Contact, Immersion or Condensation Followed by Freezing?
History of Aerosol
‘Trained’ or Preactivated Ice Nuclei
May Not Activate Until Later

43. Ice Nuclei
High Degree of Variablity
Order of Magnitude

44. Ice Nuclei Observed Concentrations (Active) 100 10 1 .1 .01 .001 -10
-20
-15
-25
Temperature (oC)
Concentration (#/L)

45. Ice Nuclei Fletcher Relationship 1/L @ -20oC
Decreases by order of magnitude every +4oC
100 10
Concentration (#/L) 1 .1
.01
.001
-10
-15
-20
-25
Temperature (oC)

46. Ice Nuclei Fletcher Relationship a = .6 N = concentration per liter
100 10 1
Concentration (#/L) .1
.01
.001
-10
-15
-20
-25
a = .6
N = concentration per liter
DT = change in temperature
Temperature (oC)

47. Ice Nuclei Typical aerosol concentration about 104 cm-3
One in 10 billion (108) aerosols acts as an ice nuclei (at -20oC)
100 10 1
Concentration (#/L) .1
.01
.001
-10
-15
-20
-25
Temperature (oC)

48. Ice in Cloud Observed Concentrations
Much Higher Than Ice Nuclei Concentrations
100 10 1 .1
.01
.001
-10
-20
-15
-25
Temperature (oC)
Concentration (#/L) -5

49. Ice in Cloud
Depends Upon
Temperature
Age of Cloud
Type of Cloud

50. Ice in Cloud Observed Concentrations
Increases with Decreasing Temperature
100 10 1 .1
.01
.001
-10
-20
-15
-25
Temperature (oC)
Concentration (#/L) -5

51. Ice in Cloud Probability of ice crystals 100 10 1 .1 .01 .001 -10 -20
-15
-25
Temperature (oC)
Concentration (#/L) -5

52. Ice in Cloud Probability of ice crystals increases with age of cloud
Old clouds
Glaciated
Higher concentrations
Young Clouds
Water droplets

53. Ice in Cloud
Higher probability of ice crystals in stratus than in cumulus of same cloud top temperature

54. Ice in Cloud
Higher ice crystal concentrations in clouds with broad spectra

55. Ice in Cloud Discrepancy Between Ice Nuclei Concentration
Observed Ice Crystal Concentration
100 10 1 .1
.01
.001
-10
-20
-15
-25
Temperature (oC)
Concentration (#/L) -5

56. Ice in Cloud
Primary Method
Heterogeneous Nucleation

57. Ice in Cloud Seconday Method Ice Multiplication Process
Fracture of Ice Crystals
Splintering of Freezing Drops

58. Ice Multiplication Process
Fracture of Ice Crystals

59. Ice Multiplication Process
Splintering of Freezing Drops