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Cloud Microphysics

Cloud Microphysics. ENVI3410 : Lecture 8 Ken Carslaw. Lecture 2 of a series of 5 on clouds and climate Properties and distribution of clouds Cloud microphysics and precipitation Clouds and radiation Clouds and climate: forced changes to clouds

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Cloud Microphysics

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  1. Cloud Microphysics ENVI3410 : Lecture 8 Ken Carslaw Lecture 2 of a series of 5 on clouds and climate • Properties and distribution of clouds • Cloud microphysics and precipitation • Clouds and radiation • Clouds and climate: forced changes to clouds • Clouds and climate: cloud response to climate change

  2. Content of Lecture 8 • Drop formation – factors controlling drop number and size • Rain formation – what is needed? • The ice phase

  3. Recommended Reading for This Lecture • A Short Course on Cloud Physics, R. R. Rogers and M. K. Yau, 3rd ed., Butterworth-Heinemann • Some very readable chapters • Physics L-0 Rog (Reference, short, long) • Several cloud physics books in the library worth flicking through

  4. What is Cloud Microphysics? • Properties of a cloud on the micro-scale (i.e., micrometres) • Includes droplet concentrations, sizes, ice crystal formation, droplet-droplet interactions, rain drop formation, etc.

  5. Microphysics and Climate • Cloud drop number (CDN) influences cloud albedo (next lecture) • Ist indirect effect of aerosols on climate • CDN/size influences precipitation efficiency (and therefore cloud lifetime/distribution and cloud fraction) • 2nd indirect effect of aerosols on climate • Ice formation affects latent heat release, precipitation intensity, cirrus properties,etc.

  6. Microphysical Processes • Drop formation • What determines the number and size of drops? • Drop spectrum broadening (collision and coalescence) • How do some drops grow to precipitation-sized particles in the time available? • Ice formation • Ice phase processes (riming, accretion, etc)

  7. Condensation NucleiStarting Point for Drop Formation • Droplets form by condensation of water vapour on aerosol particles (condensation nuclei, CN) at very close to 100% RH • Without CN, humidities of >300% are required for drop formation • Droplets form on some (a subset of) CN • Cloud Condensation Nuclei (CCN) • CN are composed of • Salt particles from sea spray • Natural material (inorganic and organic mixtures) • Human pollution (sulphuric acid particles, etc)

  8. Cloud Formation Either: • Air rises and cools to saturation (100% RH) and then supersaturation (>100% RH) • Adiabatic expansion • Air cools by radiative energy loss or advection over a cold surface (fogs)

  9. Increase in humidity in a rising air parcel 100% RH line  Air initially at 70% RH water pressure  Air rises, cools, RH increases Droplets form  100% RH (saturation, dew point)  Droplets grow, remove water vapour   temperature  

  10. Droplet “activation” • Small particles require higher humidities because surface tension of small droplets increases the pressure of water vapour over their surface • Consequence: droplets form on large particles first sea salt ammonium sulphate

  11. Droplet “activation” Typically 1000-10000 cm-3 Typically100-1000 cm-3 growth maximum supersaturation in cloud equates to minimum radius of activation

  12. Factors affecting droplet number } • Aerosol particle size • larger particles activate at lower humidities • Particle chemical composition • Some substances are more ‘hygroscopic’ • Aerosol particle number concentration • Simple • Cloud-scale updraught speed • Higher speed = more drops Human activities affect these

  13. Droplet number vs. aerosol size and number • Fixed updraught speed log(N) Solid contours = CDN; colours = aerosol mass (mg m-3) Diameter

  14. Droplet Evolution Above Cloud Base updraught = 2.0 ms-1 updraught = 0.5 ms-1 Decreasing supersat’n as droplets grow, suppresses new droplets 80 80 80 80 60 60 60 60 Height above cloud base (m) 40 40 40 40 20 20 20 20 0 0 0 0 0 0.4 0.6 0 200 400 0 2 4 6 0 0.1 0.2 Ave’ radius (mm) Supersaturation (%) Drop conc’n (cm-3) Liquid water content (g m-3) (S = %RH-100)

  15. Diffusional Droplet Growth (S = %RH-100) Droplets grow by diffusion of water vapour transition drop r=50, V=27 large drop r=50, V=27 typical drop r=10, V=1 . typical CN r=0.1, V=10-4 typical raindrop: r=1000, V=650 NaCl particle (10-14 g mass); initial radius = 0.75 micron; RH = 100.05%; p = 900 mb; T = 273 K

  16. Diffusional Droplet Growth • Leads to narrowing of droplet size distribution, but not observed • Possible reasons: • Giant CN • Supersaturation fluctuations • Mixing Diffusion only Observed Ndrop Ndrop cloud top cloud base cloud base cloud top Diameter Diameter

  17. Definition of “Precipitation-Sized” Droplet • How big must a droplet be before it can be considered a “raindrop” Distance a drop falls before evaporating. Assumes isothermal atmosphere with T=280 K, RH=80% Definition of a drizzle drop

  18. “Warm Rain” Formation • Rain formation without ice phase • Additional process needed to grow droplets to precipitation size • Collision and coalescence • Two processes: collision rate and coalescence rate Narrow distributions not very efficient for collision Some large drops initiate collision-coalescence

  19. Collision and Coalescence Rates “wake” effects Almost all collisions result in coalescence Coalescence very inefficient below about 20 mm Therefore droplet distribution broadening needed Collision-Coalescence efficiency reduced because small drops are swept round the larger one

  20. Droplet Evolution with Collision-Coalescence 30 25 20 time (mins) 15 10 5 0 10-3 10-2 10-1 100 Radius (cm) 10 mm

  21. Summary of “Warm Cloud” Microphysics • Precipitation is favoured in clouds with • Large liquid water content (i.e., deep cumulus) • Broad drop spectrum • Large drops (must be larger than ~20 mm) • Large vertical extent (=long growth/collision times)

  22. Precipitation Formation Through Ice Processes Ice forms on ice nuclei (IN) • Silicates (soil dust, etc.) • Clays • Fungal spores • Combustion particles (soot, etc.) • Other industrial material

  23. Ice formation Processes Between –10 oC and –39 oC Result = very few crystals Immersion freezing (Rate proportional to drop volume) Contact nucleation freezing Deposition nucleation (reverse sublimation) Below –39 oC Result = complete freezing of all drops Homogeneous freezing

  24. The Growth Advantage of Ice Crystals At –20 oC at 100% RH Sice = 24% Compare with typical Sliq = 0.05-0.5% ! Air is Marginally supersaturated with respect to liquid water in a rising cloud thermal Highly supersaturated with respect to ice Few crystals grow at expense of drops Subsequent growth from accretion and aggregation

  25. Atmospheric Ice Nuclei Concentrations

  26. Effect of Freezing on Cloud Development • Intensification of rain • Release of latent heat aloft (giving further buoyancy)

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