Microphysics of Cold Clouds

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

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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
  

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Cold Cloud

• Some Part of Cloud Extends Above 0oC

0oC
8
Mixed Phase Cloud

• Contains Both Liquid Water Drops and Ice Particles

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Glaciated Cloud

• Consists Entirely of Ice Particles

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Supercooled Liquid Water

• Liquid Water That Exists at Temperatures Colder
  Than 0oC

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Supercooled Liquid Water

• Liquid Water Molecule
• Hydrogen Bonds

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Supercooled Liquid Water

• Metastable State
• Want to Freeze

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Fusion

• Ice
• Water Molecules Arranged in Lattice

Fusion
Liquid Water
Ice
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Fusion

• Homogeneous Nucleation
• Heterogeneous Nucleation

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Homogeneous Nucleation

• Water Molecules Arrange Themselves into a Lattice

Ice Embryo
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Homogeneous Nucleation

• Embryo Grows by Chance Aggregation

Ice Embryo
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Homogeneous Nucleation

• Ice Nucleus Is in Constant Flux
• Molecule Come Go

Ice Embryo
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Homogeneous Nucleation

• Chance Aggregation Increases with Decreasing
  Temperature

Ice Embryo
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Homogeneous Nucleation

• Chance Aggregation Also Depends on Drop Size (or
  Surface Tension)

Ice Embryo
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Homogeneous Nucleation

• Ice Embryo Must Reach Critical Size Before Fusion
  Occurs Spontaneously

Ice Embryo
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Homogeneous Nucleation

• Spontaneous Nucleation Occurs At

5 mm
-39oC
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Homogeneous Nucleation

• Spontaneous Nucleation Occurs At

5 mm
40 mm
-36oC
-39oC
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Homogeneous Nucleation

• Spontaneous Nucleation Rarely Occurs in the
  Atmosphere

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Fusion

• Homogeneous Nucleation
• Heterogeneous Nucleation

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Heterogeneous Nucleation

• Fusion Is Aided by Foreign Substances Called Ice
  Nuclei

Ice Nuclei
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Heterogeneous Nucleation

• Ice Nuclei Provide a Surface for Liquid Water to
  Attach

Ice Nuclei
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Heterogeneous Nucleation

• Ice Embryo Starts At a Larger Size

Ice Nuclei
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Heterogeneous Nucleation

• Fusion Occurs at Much Warmer Temperature

Ice Nuclei
-39oC
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Heterogeneous Nucleation Modes

• Condensation Followed by Freezing
• Contact
• Immersion
• Deposition

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Heterogeneous Nucleation Modes

• Condensation Followed by Freezing
• Ice nuclei acts as CCN, then freezes droplet

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Heterogeneous Nucleation Modes

• Contact
• Water droplet freezes instantaneously upon
  contact with ice nuclei

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Heterogeneous Nucleation Modes

• Immersion
• Causes freezing after becoming embedded within
  droplet

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Heterogeneous Nucleation Modes

• Deposition
• Ice forms directly from vapor

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Heterogeneous Nucleation Modes

• One particle may nucleate in different ways

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Heterogeneous Nucleation Modes

• Temperature at which nucleation occurs also
  depends on mode

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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
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Ice Nuclei

• Crystal Structure Similar to Ice
• Nucleate at warmer temperatures

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Ice Nuclei

• Pure Substances
• Minerals
• Organic Material

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Ice Nuclei
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Ice Nuclei

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

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Ice Nuclei

• Measurement Techniques
• Cloud Chambers
• Filter Systems

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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

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Ice Nuclei

• High Degree of Variablity
• Order of Magnitude

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Ice Nuclei

• Observed Concentrations (Active)

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Ice Nuclei

• Fletcher Relationship
• 1/L _at_ -20oC
• Decreases by order of magnitude every 4oC

100
10
Concentration (/L)
1
.1
.01
.001
-10
-20
-15
-25
Temperature (oC)
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Ice Nuclei

• Fletcher Relationship

100
10
1
Concentration (/L)
.1
.01
.001
-10
-20
-15
-25
a .6 N concentration per liter DT cha
nge 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
-20
-15
-25
Temperature (oC)
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Ice in Cloud

• Observed Concentrations
• Much Higher Than Ice Nuclei Concentrations

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Ice in Cloud

• Depends Upon
• Temperature
• Age of Cloud
• Type of Cloud

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Ice in Cloud

• Observed Concentrations
• Increases with Decreasing Temperature

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Ice in Cloud

• Probability of ice crystals

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Ice in Cloud

• Probability of ice crystals increases with age of
  cloud
  
• Old clouds
• Glaciated
• Higher concentrations
• Young Clouds
• Water droplets

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Ice in Cloud

• Higher probability of ice crystals in stratus
  than in cumulus of same cloud top temperature

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Ice in Cloud

• Higher ice crystal concentrations in clouds with
  broad spectra

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Ice in Cloud

• Discrepancy Between
• Ice Nuclei Concentration
• Observed Ice Crystal Concentration

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Ice in Cloud

• Primary Method
• Heterogeneous Nucleation

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Ice in Cloud

• Seconday Method
• Ice Multiplication Process
• Fracture of Ice Crystals
• Splintering of Freezing Drops

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Ice Multiplication Process

• Fracture of Ice Crystals

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Ice Multiplication Process

• Splintering of Freezing Drops