Cloud Microphysics SOEE3410 : Lecture 4 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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Aims of Lecture 4 Understand: What determines the number and size of drops in a cloud The two main processes that can initiate rain in clouds

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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 Short article from ISCCP

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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.

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Microphysics and Climate Cloud drop number (CDN) influences cloud albedo, or reflectivity (next lecture) –“1 st 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.

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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) Typical CN concentrations (100-10,000 cm -3 ). Typical CCN ( cm -3 ) CN are composed of –Salt particles from sea spray –Natural material (inorganic and organic mixtures) –Human pollution (sulphuric acid particles, etc)

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Cloud Formation Either: Air rises and cools to saturation (100% RH) and then supersaturation (>100% RH) –By adiabatic expansion Air cools by radiative energy loss or advection over a cold surface (fogs)

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Increase in humidity in a rising air parcel temperature water vapour concentration 100% RH line Air initially at 70% RH Air rises, cools, RH increases 100% RH (saturation, dew point) Droplets form (RH>100%) Droplets grow, remove water vapour      

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Droplet “activation” Typically cm -3 Typically cm -3 maximum supersaturation in cloud equates to minimum radius of activation growth

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Droplet number vs. aerosol size and number Fixed updraught speed log(N) Diameter Solid contours = CDN; colours = aerosol mass (  g m -3 )

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Droplet Evolution Above Cloud Base Height above cloud base (m) Supersaturation (%) Drop conc’n (cm-3) Ave’ radius (  m) Liquid water content (g m-3) updraught = 0.5 ms -1 updraught = 2.0 ms -1 Decreasing supersat’n as droplets grow, suppresses new droplets (S = %RH-100)

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Diffusional Droplet Growth Radiustime 12.4 s 2130 s s s hr hr hr NaCl particle ( g mass); initial radius = 0.75 micron; RH = %; p = 900 mb; T = 273 K. typical CN r=0.1, V=10 -4 large drop r=50, V=27 typical drop r=10, V=1 typical raindrop: r=1000, V=650 transition drop r=50, V=27 Droplets grow by diffusion of water vapour (S = %RH-100)

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Diffusional Droplet Growth Leads to narrowing of droplet size distribution, but not observed Possible reasons: –Giant CN –Supersaturation fluctuations –Mixing N drop Diameter N drop Diameter cloud base cloud top cloud base cloud top Diffusion onlyObserved

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Definition of “Precipitation-Sized” Droplet How big must a droplet be before it can be considered a “raindrop” Initial radius Distance fallen 1  m2.0  m 3  m 0.17 mm 10  m 2.1 cm 30  m 1.69 m 0.1 mm208 m 0.15 mm1.05 km Distance a drop falls before evaporating. Assumes isothermal atmosphere with T=280 K, RH=80% Definition of a drizzle drop

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 “Warm Rain” Formation Rain formation without ice phase Additional process needed to grow droplets to precipitation size Collision and coalescence Narrow distributions not very efficient for collision Some large drops initiate collision- coalescence 20  m droplets needed to initiate rapid collision- coalescence

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Droplet Evolution with Collision- Coalescence time (mins) Radius (cm) 10  m

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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  m) –Large vertical extent (=long growth/collision times)

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Ice formation Processes Contact nucleation freezing Immersion freezing (Rate proportional to drop volume) Between –10 o C and –39 o C Result = very few crystals Homogeneous freezing Below –39 o C Result = complete freezing of all drops

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 The Growth Advantage of Ice Crystals At –20 o C at 100% RH S ice = 24% Compare with typical S liq = % ! Few crystals grow at expense of drops Air is Marginally supersaturated with respect to liquid water in a rising cloud thermal Highly supersaturated with respect to ice Subsequent growth from accretion and aggregation

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Atmospheric Ice Nuclei Concentrations

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Effect of Freezing on Cloud Development Intensification of rain Release of latent heat aloft (giving further buoyancy)

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Questions for lecture 4 Define what is meant by the activation of cloud drops Why do more cloud drops form in clouds with higher updraught speeds? When you leave this lecture, what sort of clouds are visible and what is their typical droplet concentration? Based on the figure on slide 12, what droplet concentration would occur in an atmosphere with a) an aerosol concentration of 50 cm-3 of diameter 0.05  m and b) 200 cm -3 and 0.1  m diameter? Explain the difference. Explain why condensation of water on growing droplets is not enough to initiate rain