Cloud Microphysics ENVI 3410 Lecture 8 Ken Carslaw

Cloud Microphysics ENVI 3410 : 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

Content of Lecture 8 • Drop formation – factors controlling drop number and size • Rain formation – what is needed? • The ice phase ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Recommended Reading for This Lecture • A Short Course on Cloud Physics, R. R. Rogers and M. K. Yau, 3 rd ed. , Butterworth-Heinemann – Some very readable chapters – Physics L-0 Rog (Reference, short, long) • Several cloud physics books in the library worth flicking through ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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. ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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) – 2 nd indirect effect of aerosols on climate • Ice formation affects latent heat release, precipitation intensity, cirrus properties, etc. ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Condensation Nuclei Starting 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) • ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Increase in humidity in a rising air parcel water pressure 100% RH line Droplets form Air initially at 70% RH Air rises, cools, RH increases 100% RH (saturation, dew point) Droplets grow, remove water vapour temperature ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Droplet “activation” sea salt ammonium sulphate • 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 ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Droplet “activation” Typically 1000 -10000 cm-3 Typically 100 -1000 cm-3 growth maximum supersaturation in cloud equates to minimum radius of activation ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Factors affecting droplet number • Aerosol particle size } – larger particles activate at lower humidities • Particle chemical composition – Some substances are more ‘hygroscopic’ Human activities affect these • Aerosol particle number concentration – Simple • Cloud-scale updraught speed – Higher speed = more drops ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Droplet number vs. aerosol size and number • Fixed updraught speed log(N) Solid contours = CDN; colours = aerosol mass (mg m-3) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics Diameter 1

Droplet Evolution Above Cloud Base updraught = 2. 0 ms-1 Height above cloud base (m) updraught = 0. 5 ms-1 80 60 Decreasing supersat’n as 80 droplets grow, suppresses 60 new droplets 80 80 60 60 40 40 20 20 0 0 0. 4 0. 6 Supersaturation (%) 0 200 400 0 2 4 6 Drop conc’n (cm-3) Ave’ radius (mm) (S = %RH-100) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 0 0. 1 0. 2 Liquid water content (g m-3) 1

Diffusional Droplet Growth (S = %RH-100) Droplets grow by diffusion of water vapour Radius time 1 2. 4 s 2 130 s 4 1000 s 10 2700 s 20 2. 4 hr 30 4. 9 hr 40 12. 4 hr transition drop r=50, V=27 typical drop r=10, V=1 Na. Cl particle (10 -14 g mass); initial radius = 0. 75 micron; RH = 100. 05%; p = 900 mb; T = 273 K large drop r=50, V=27 . typical CN r=0. 1, V=10 -4 typical raindrop: r=1000, V=650 ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Diffusional Droplet Growth • Leads to narrowing of droplet size distribution, but not observed Diffusion only Observed • Possible reasons: Ndrop – Giant CN – Supersaturation fluctuations – Mixing cloud top cloud base Diameter ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics cloud base cloud top Diameter 1

Definition of “Precipitation-Sized” Droplet • How big must a droplet be before it can be considered a “raindrop” Initial radius Distance fallen 1 mm 2. 0 mm 3 mm 0. 17 mm 10 mm 2. 1 cm 30 mm 1. 69 m 0. 1 mm 208 m 0. 15 mm 1. 05 km Distance a drop falls before evaporating. Assumes isothermal atmosphere with T=280 K, RH=80% Definition of a drizzle drop ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

“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 collisioncoalescence ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Collision and Coalescence Rates “wake” effects Almost all collisions result in coalescence Collision-Coalescence efficiency reduced because small drops are swept round the larger one Coalescence very inefficient below about 20 mm Therefore droplet distribution broadening needed ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Droplet Evolution with Collision. Coalescence 30 25 s) in 20 m e ( 15 tim 10 5 0 10 -3 10 -2 10 -1 100 Radius (cm) 10 mm ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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 ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Ice formation Processes Between – 10 o. C and – 39 o. C Result = very few crystals Contact nucleation freezing Immersion freezing (Rate proportional to drop volume) Deposition nucleation (reverse sublimation) Below – 39 o. C Result = complete freezing of all Homogeneous drops freezing ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

The Growth Advantage of Ice Crystals At – 20 o. C at 100% RH Sice = 24% Air is Marginally supersaturated with respect to liquid water in a rising cloud thermal Compare with typical Sliq = 0. 05 -0. 5% ! Highly supersaturated with respect to ice Few crystals grow at expense of drops Subsequent growth from accretion and aggregation ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Atmospheric Ice Nuclei Concentrations ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Effect of Freezing on Cloud Development • Intensification of rain • Release of latent heat aloft (giving further buoyancy) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1