Cloud Microphysics

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Cloud Microphysics

• Liz Page
• NWS/COMET
• Meteorology Primer/Hydromet
• 9 May 2000

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Introduction

• Meteorology and Hydrology are linked by the
  processes that produce precipitation
• A greater understanding of cloud microphysics
  will help determine which clouds will be most
  efficient in producing precipitation

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Vapor Pressure

• Daltons Law of Partial Pressure
• Saturation Vapor Pressure
• Saturation is a dynamic process

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Daltons Law of Partial Pressurert rd e

• Total pressure partial pressure of dry air
  partial pressure of water vapor e vapor
  pressure es saturation vapor pressure f(T)
  only S e / es saturation ratio S
  100 relative humidity of the parcelFigure 2.
  Concept of Daltons Law of Partial Pressures and
  Relative Humidity

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Condensation and Cloud Formation

• Cloud Condensation Nuclei
• Dust
• Salt Particles from Sea Spray
• Natural Aerosols
• Human Created Pollution

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Process of Cloud Formation

• Air rises and cools to saturation - most
  effective nuclei are activated
• Saturation vapor pressure decrease as parcel
  continues to rise and cool - the parcel becomes
  supersaturated
• More CCN activate at the higher humidity

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Cloud Droplet Growth by Condensation (Diffusion)

• Driven by the saturation vapor pressure
  difference
• Vapor is transported from higher to lower
  saturation vapor pressure

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Collision and Coalescence

• Consists of two steps
• Will the droplets collide?
• If so, will they coalesce?

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Collision and Coalescence

• Collisions begin at radius of 18 microns
• Collision efficiency increases as the size of the
  colliding drop increases

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Collision and Coalescence

• Not all collisions result in coalescence
• Coalescence is affected by turbulence, surface
  contaminants, electric fields and charges
• Broad droplet spectra favor more collisions

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Droplet Breakup and Multiplication

• Falling drops sweep out a cone-shaped volume
• Drops are unstable just after coalescence
• Droplet breakup broadens the spectra and limits
  the maximum size

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Precipitation formation through ice processes

• Ice forms on Ice Nuclei (IN)
• Silicates
• Clays
• Combustion products
• Industrial products

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Nucleation of Ice

• IN activate as a function of temperature
• Contact nucleation
• Homogeneous nucleation
• Warm cloud tops (greater than -10oC) rarely have
  ice

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Ice Crystal Growth

• Ice crystals grow by
• Vapor deposition
• Accretion of cloud droplets
• Aggregation

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Growth by Deposition
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Ice Particle Multiplication

• Three processes
• Fracture
• Splintering during riming
• Fragmentation of large drops during freezing

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