Microphysics of Cold Clouds

1
Microphysics of Cold Clouds
2
Microphysics of Cold Clouds

• Reading
• Wallace Hobbs
• pp 232 245

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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

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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

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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

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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

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Growth of Ice Crystals

• Growth by Depostion
• Growth by Riming
• Growth by Aggregation

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Growth by Deposition

• Ice Crystals Grow by Vapor Diffusion
• Mixed Phase Cloud

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Thermodynamics Review

• Equilibrium Curve
• Water Vapor vs. Liquid Water

es
Equilibrium
Pressure
Temperature
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Thermodynamics Review

• Supercooled Liquid Water (SLW)
• Absence of ice

Equilibrium with Liquid Water
esw
Pressure
0.01oC
Temperature
12
Thermodynamics Review

• What about water vapor vs. ice?

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Thermodynamics Review

• Equilibrium Curve for Ice

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Thermodynamics Review

• Mixed Phase Cloud

-12oC
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Thermodynamics Review

• Saturated With Respect to Liquid Water

Equilibrium with Liquid Water
esw
Pressure
5 mb
-12oC
Temperature
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Thermodynamics Review

• Supersaturated With Respect to Ice

Equilibrium with Liquid Water
esw
Pressure
Equilibrium with Ice
5 mb
4.7 mb
esi
-12oC
Temperature
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Thermodynamics Review

• Supersaturations of Up to 20
• Compare to 1 in Warm Clouds

Equilibrium with Liquid Water
esw
Pressure
Equilibrium with Ice
5 mb
4.7 mb
esi
-12oC
Temperature
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Growth by Deposition

• Cloud Droplets Evaporate at the Expense of Ice
  Crystals

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Growth by Deposition

• Cloud Droplets Evaporate at the Expense of Ice
  Crystals

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Growth by Deposition

• Cloud Droplets Evaporate at the Expense of Ice
  Crystals

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Growth by Deposition

• Cloud Droplets Evaporate at the Expense of Ice
  Crystals

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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
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Growth by Deposition

• Flux of Water Vapor is Normal to Surface

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Growth by Deposition

• Ice Crystals Arent Always Round

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Growth by Deposition

• Vapor Diffuses to Sharp Points From Many
  Directions
• Points Grow More Rapidly

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Growth by Deposition

• Analogy
• Electric Field Around a Charged Conductor of
  Irregular Shape
• Can Be Determined Experimentally in Laboratory

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Growth by Deposition

• Diffusional Capacitance

C Capacitance eo permittivity of free
space 8.85 x 10-12 C2 N-1 m-2
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Growth by Deposition

• For a Sphere

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Growth by Deposition

• General Form
• Capacitance (C) Determined Experimentally

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Growth by Deposition

• Simplify
• Vapor Pressure Away from Crystal Is Not Very
  Different At Crystal Surface

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Growth by Deposition

• Simplify
• Ice Crystal Is Not Too Small

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Growth by Deposition

• Simplify

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Growth by Deposition

• Rate of Growth Depends on
• Shape of Ice Crystal
• Supersaturation
• Other Temperature Dependent Factors

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Growth by Deposition

• Maximum in GiSi at -15oC
• Difference Between esi and es
• Most Rapid Growth

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Ice Crystal Habits

• Variables
• Temperature
• Primary
• Supersaturation
• Secondary
• Electric Field
• Minor

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Ice Crystal Habits

• Basic Habits
• Plates

Hexagonal Plate
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Ice Crystal Habits

• Basic Habits
• Prisms (or Columns)

Column
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Ice Crystal Habits

• Basic Habit Changes Three Times with Decreasing
  Temperature

Prisms
Prisms
Plates
Plates
0
-5
-10
-15
-20
-25
-30
-35
TEMPERATURE (oC)
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Ice Crystal Habits

• Thickness Decreases with Increasing
  Supersaturation

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Ice Crystal Habits

• Classification Scemes
• International Commission on Snow and Ice (1951)
• Nakaya (1954)
• Magano Lee (1966)

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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

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International Commission on Snow and Ice (1951)

• Simple

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Nakaya (1954)

• Seven Major Grouping of Snow Crystals
• 41 Individual Morphological Types

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Magano Lee (1966)

• Most Complete
• Extension of Nakaya
• 80 Different Morphological Types

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Magano Lee (1966)
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Ice Crystal Habits

• Significant Crystals
• Plates
• Prisms

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Significant Crystals

• Plates
• Dendrite
• Prettiest
• Fastest Growing
• -15oC
• Stellar Dendrites

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Significant Crystals

• Plates
• Dendrite Forms
• Sectored Plate

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Significant Crystals

• Plates
• Hexagonal Plates
• High Terminal Velocity
• Graupel Embryo

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Significant Crystals

• Columns
• Needles
• Form in Strong Electric Fields

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Significant Crystals

• Columns
• Hollow Columns
• Capped Columns
• Bullet Combos

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Ice Crystal Habits

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

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Growth of Ice Crystals

• Growth by Depostion
• Growth by Riming
• Growth by Aggregation

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Growth by Riming

• Riming
• The process by which ice crystals grow through
  the collision, collection and freezing of
  supercooled water drops

Fred Remer, 2002
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Growth by Riming
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Growth by Riming

• Freezes on Contact

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Growth by Riming

• Freezes on Contact

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Growth by Riming

• Collection efficiency of ice crystal habits vary

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Growth by Riming

• Terminal Velocity of Ice Crystal Increases

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Growth by Riming

• Difficult to Distinguish Original Ice Crystal

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Growth by Riming

• Graupel
• Heavily rimed ice crystals
• Often called snow pellets
• Diameter lt 5 mm

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Growth by Riming

• Graupel
• Shapes
• Conical
• Hexagonal
• Lump or Irregular

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Growth by Riming

• Graupel
• Serves as Hail Embryo

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Growth by Riming

• Hailstone
• Diameter gt 5 mm
• Small Hail (Diam. lt 6.4 mm)
• Large Hail (Diam. gt 6.4 mm)

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Growth by Riming

• Hailstone
• Growth
• Dry Growth
• Wet Growth

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Hailstone Growth

• Dry Growth
• Mostly Riming

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Hailstone Growth

• Wet Growth
• Accretion of Liquid Water and Freezing

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Hailstone Growth

• Both Processes Can Occur at Different Periods of
  a Hailstones Life Time

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Growth of Ice Crystals

• Growth by Depostion
• Growth by Riming
• Growth by Aggregation

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Growth by Aggregation

• Aggregation
• The process of clumping together of ice crystals
  following collision as they fall to form
  snowflakes

Glossary of Meteorology
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Growth by Aggregation
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Growth by Aggregation

• Terminal Fall Speeds
• Adhesion

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Growth by Aggregation

• Terminal Fall Speeds
• Columns
• Plates

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Growth by Aggregation

• Terminal Fall Speeds
• Columns
• Increases with Length
• Needles
• .5 to .7 ms-1

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Growth by Aggregation

• Terminal Fall Speeds
• Plates
• Independent of Diameter
• Similar Fall Speeds
• Unlikely to Collide

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Growth by Aggregation

• Riming Greatly Enhances Collisions
• Various Fall Speeds

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Growth by Aggregation

• Adhesion
• Type of Ice Crystals
• Temperature

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Growth by Aggregation

• Adhesion
• Type of Ice Crystals
• More Intricate Crystals Become Entwined Upon
  Collision

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Growth by Aggregation

• Adhesion
• Temperature
• Ice Crystals Become Sticky Between 5 to 0oC

0oC
80
Cold Cloud Process

• Cloud Droplets Grow at the Expense of Ice Crystals

Wegener (1911) Bergeron (1933) Findeisen (1938)
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Cold Cloud Process

• Deposition Can Account for Precipitation Sized
  Ice Particles

1 mm
30 min.
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Cold Cloud Process

• Deposition Cannot Account for Precipitation Sized
  Rain Drops

1 mm
.3 ms-1
0oC
260 mm
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Cold Cloud Process

• Riming and Aggregation Produces Precipitation
  Sized Rain Drops

1 mm
1 ms-1
0oC
460 mm
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Cold Cloud Process

• Riming and Aggregation Produces Precipitation
  Sized Rain Drops

1 cm
1 ms-1
0oC
2 mm