cloud microphysics liz page nws/comet hydromet 00-2 23 february 2000
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Cloud Microphysics
Liz Page
NWS/COMET
Hydromet 00-2
23 February 2000
2
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 is producing precipitation
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Vapor Pressure
• Dalton’s Law of Partial Pressure
• Saturation Vapor Pressure
• Saturation is a dynamic process
Dalton’s Law of Partial Pressure
tde
Total pressure = partial pressure of dry air + partial pressure of water vapor
e = vapor pressure
es = saturation vapor pressure [f(T) only]
= saturation ratio
S 100 = relative humidity of the parcel
Figure 2. Concept of Dalton’s Law of Partial Pressures and Relative Humidity
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Condensation and Cloud Formation
• Cloud 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 increase 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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Ice Particle Multiplication
• Three processes– Fracture– Splintering during riming– Fragmentation of large drops during freezing
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