Cloud Microphysics Liz Page NWS/COMET Hydromet 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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4 Vapor Pressure Dalton’s Law of Partial Pressure Saturation Vapor Pressure Saturation is a dynamic process

Dalton’s Law of Partial Pressure  t  d  e Total pressure = partial pressure of dry air + partial pressure of water vapor e = vapor pressure e s = 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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7 Condensation and Cloud Formation Cloud Cloud Condensation Nuclei –Dust –Salt Particles from Sea Spray –Natural Aerosols –Human Created Pollution

8 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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10 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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15 Collision and Coalescence Consists of two steps –Will the droplets collide? –If so, will they coalesce?

16 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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18 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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22 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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24 Precipitation formation through ice processes Ice forms on Ice Nuclei (IN) –Silicates –Clays –Combustion products –Industrial products

25 Nucleation of Ice IN activate as a function of temperature Contact nucleation Homogeneous nucleation Warm cloud tops (greater than -10 o C) rarely have ice

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27 Ice Crystal Growth Ice crystals grow by –Vapor deposition –Accretion of cloud droplets –Aggregation

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31 Ice Particle Multiplication Three processes –Fracture –Splintering during riming –Fragmentation of large drops during freezing