Cloud Microphysics

Liz Page

NWS/COMET

Hydromet 00-2

23 February 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 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