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Microphysics of Cold Clouds

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Presentation on theme: "Microphysics of Cold Clouds"— Presentation transcript:

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 explain why ice crystals grow at the expense of water drops Be able to list the factors that determine the rate that ice crystals grow by deposition Be able to recall the temperature at which ice crystals grow the fastest by deposition in a mixed phase cloud

4 Microphysics of Cold Clouds
Objectives Be able to list the factors that determine ice crystal habit Be able to recall the two basic types of ice crystals Be able to identify ice crystal classification schemes

5 Microphysics of Cold Clouds
Objectives Be able to identify significant ice crystal habits Be able to define riming Be able to describe the positive feedback that riming has on water mass accretion Be able to define graupel

6 Microphysics of Cold Clouds
Objectives Be able to recall the importance of graupel in hail formation Be able to identify the size cut-off between hail and graupel Be able to describe the two modes of hail growth Be able to define aggregation

7 Microphysics of Cold Clouds
Objectives Be able to describe the two factors that determine aggregation Be able to generally comment on the formation of rain from the cold cloud process

8 Growth of Ice Crystals Growth by Depostion Growth by Riming
Growth by Aggregation

9 Growth by Deposition Ice Crystals Grow by Vapor Diffusion
Mixed Phase Cloud

10 Thermodynamics Review
Equilibrium Curve Water Vapor vs. Liquid Water es Equilibrium Pressure Temperature

11 Thermodynamics Review
Supercooled Liquid Water (SLW) Absence of ice Equilibrium with Liquid Water esw SLW Pressure 0.01oC Temperature

12 Thermodynamics Review
What about water vapor vs. ice?

13 Thermodynamics Review
Equilibrium Curve for Ice Temperature Pressure Equilibrium with Liquid Water esw with Ice 0.01oC esi

14 Thermodynamics Review
Mixed Phase Cloud -12oC

15 Thermodynamics Review
Saturated With Respect to Liquid Water Equilibrium with Liquid Water esw Pressure 5 mb -12oC Temperature

16 Thermodynamics Review
Supersaturated With Respect to Ice Equilibrium with Liquid Water esw Pressure 5 mb Equilibrium with Ice 4.7 mb esi -12oC Temperature

17 Thermodynamics Review
Supersaturations of Up to 20% Compare to 1% in Warm Clouds Equilibrium with Liquid Water esw Pressure 5 mb Equilibrium with Ice 4.7 mb esi -12oC Temperature

18 Growth by Deposition Cloud Droplets Evaporate at the Expense of Ice Crystals

19 Growth by Deposition Cloud Droplets Evaporate at the Expense of Ice Crystals

20 Growth by Deposition Cloud Droplets Evaporate at the Expense of Ice Crystals

21 Growth by Deposition Similar to Growth of Water Drop m = mass of ice
t = time r = radius of ice rv,0 = vapor density adjacent to droplet surface rv,oo = vapor density adjacent to droplet surface

22 Growth by Deposition Flux of Water Vapor is Normal to Surface

23 Growth by Deposition Ice Crystals Aren’t Always Round

24 Growth by Deposition Vapor Diffuses to Sharp Points From Many Directions Points Grow More Rapidly

25 Growth by Deposition Analogy
Electric Field Around a Charged Conductor of Irregular Shape Can Be Determined Experimentally in Laboratory

26 Growth by Deposition Diffusional Capacitance C = Capacitance
eo = permittivity of free space = 8.85 x C2 N-1 m-2

27 Growth by Deposition For a Sphere

28 Growth by Deposition General Form
Capacitance (C) Determined Experimentally

29 Growth by Deposition Simplify
Vapor Pressure Away from Crystal Is Not Very Different At Crystal Surface

30 Growth by Deposition Simplify Ice Crystal Is Not Too Small

31 Growth by Deposition Simplify

32 Growth by Deposition Rate of Growth Depends on Shape of Ice Crystal
Supersaturation Other Temperature Dependent Factors

33 Growth by Deposition Maximum in GiSi at -15oC Most Rapid Growth
Difference Between esi and es Most Rapid Growth -10 -20 -30 -40 TEMPERATURE (oC) GiSi (kg s-1 m-1) 1 2 3 4

34 Ice Crystal Habits Variables Temperature Supersaturation
Primary Supersaturation Secondary Electric Field Minor

35 Ice Crystal Habits Basic Habits Plates Hexagonal Plate

36 Ice Crystal Habits Basic Habits Prisms (or Columns) Column

37 Ice Crystal Habits Basic Habit Changes Three Times with Decreasing Temperature Prisms Prisms Plates Plates -5 -10 -15 -20 -25 -30 -35 TEMPERATURE (oC)

38 Ice Crystal Habits Thickness Decreases with Increasing Supersaturation

39 Ice Crystal Habits Classification Scemes
International Commission on Snow and Ice (1951) Nakaya (1954) Magano & Lee (1966)

40 International Commission on Snow & Ice (1951)
Seven Principle Snow Crystal Types Plates Stellar Crystals Caped Columns Columns Needles Spatial Dendrites Irregular Forms Three Additonal Types of Frozen Precipitation Graupel Ice Pellets Hail

41 International Commission on Snow and Ice (1951)
Simple

42 Nakaya (1954) Seven Major Grouping of Snow Crystals
41 Individual Morphological Types

43 Magano & Lee (1966) Most Complete Extension of Nakaya
80 Different Morphological Types

44 Magano & Lee (1966)

45 Ice Crystal Habits Significant Crystals Plates Prisms

46 Significant Crystals Plates Dendrite Prettiest Fastest Growing -15oC
Stellar Dendrites

47 Significant Crystals Plates Dendrite Forms Sectored Plate

48 Significant Crystals Plates Hexagonal Plates High Terminal Velocity
Graupel Embryo

49 Significant Crystals Columns Needles Form in Strong Electric Fields

50 Significant Crystals Columns Hollow Columns Capped Columns
Bullet Combos

51 Ice Crystal Habits Ice Crystal May Grow Several Different Habits
Depends on Supersaturation and Temperature

52 Growth of Ice Crystals Growth by Depostion Growth by Riming
Growth by Aggregation

53 Growth by Riming Riming
“The process by which ice crystals grow through the collision, collection and freezing of supercooled water drops” Fred Remer, 2002

54 Growth by Riming

55 Growth by Riming Freezes on Contact

56 Growth by Riming Freezes on Contact

57 Growth by Riming Collection efficiency of ice crystal habits vary

58 Growth by Riming Terminal Velocity of Ice Crystal Increases

59 Growth by Riming Difficult to Distinguish Original Ice Crystal

60 Growth by Riming Graupel Heavily rimed ice crystals
Often called snow pellets Diameter < 5 mm

61 Growth by Riming Graupel Shapes Conical Hexagonal Lump or Irregular

62 Growth by Riming Graupel Serves as Hail Embryo

63 Growth by Riming Hailstone Diameter > 5 mm
Small Hail (Diam. < 6.4 mm) Large Hail (Diam. > 6.4 mm)

64 Growth by Riming Hailstone Growth Dry Growth Wet Growth

65 Hailstone Growth Dry Growth Mostly Riming

66 Hailstone Growth Wet Growth Accretion of Liquid Water and Freezing

67 Hailstone Growth Both Processes Can Occur at Different Periods of a Hailstone’s Life Time

68 Growth of Ice Crystals Growth by Depostion Growth by Riming
Growth by Aggregation

69 Glossary of Meteorology
Growth by Aggregation Aggregation “The process of clumping together of ice crystals following collision as they fall to form snowflakes” Glossary of Meteorology

70 Growth by Aggregation

71 Growth by Aggregation Terminal Fall Speeds Adhesion

72 Growth by Aggregation Terminal Fall Speeds Columns Plates

73 Growth by Aggregation Terminal Fall Speeds Columns
Increases with Length Needles .5 to .7 ms-1

74 Growth by Aggregation Terminal Fall Speeds Plates
Independent of Diameter Similar Fall Speeds Unlikely to Collide

75 Growth by Aggregation Riming Greatly Enhances Collisions
Various Fall Speeds

76 Growth by Aggregation Adhesion Type of Ice Crystals Temperature

77 Growth by Aggregation Adhesion Type of Ice Crystals
More Intricate Crystals Become Entwined Upon Collision

78 Growth by Aggregation Adhesion Temperature
Ice Crystals Become Sticky Between –5 to 0oC 0oC

79 Cold Cloud Process Cloud Droplets Grow at the Expense of Ice Crystals
Wegener (1911) Bergeron (1933) Findeisen (1938)

80 Cold Cloud Process Deposition Can Account for Precipitation Sized Ice Particles 1 mm 30 min.

81 Cold Cloud Process Deposition Cannot Account for Precipitation Sized Rain Drops 1 mm .3 ms-1 0oC 260 mm

82 Cold Cloud Process Riming and Aggregation Produces Precipitation Sized Rain Drops 1 mm 1 ms-1 0oC 460 mm

83 Cold Cloud Process Riming and Aggregation Produces Precipitation Sized Rain Drops 1 cm 1 ms-1 0oC 2 mm

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