Transcript Clouds and Climate
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
Content of Lecture 8
• Drop formation – factors controlling drop number and size • Rain formation – what is needed?
• The ice phase ENVI3410 : 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 ENVI3410 : 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.
ENVI3410 : 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.
ENVI3410 : 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) ENVI3410 : 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) ENVI3410 : 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) ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Increase in humidity in a rising air parcel
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 ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Droplet “activation”
sea salt ammonium sulphate ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics • 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 1
Droplet “activation” Typically 1000-10000 cm -3 Typically 100-1000 cm -3
growth maximum supersaturation in cloud equates to minimum radius of activation ENVI3410 : 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’ • Aerosol particle number concentration } Human activities affect these – Simple • Cloud-scale updraught speed – Higher speed = more drops ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Droplet number vs. aerosol size and number
• Fixed updraught speed log(N) Solid contours = CDN; colours = aerosol mass ( m g m -3 ) ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics Diameter 1
Droplet Evolution Above Cloud Base
updraught = 2.0 ms -1 updraught = 0.5 ms -1 80 60 Decreasing supersat’n as droplets grow, suppresses new droplets 80 60 80 60 80 60 40 20 40 20 40 20 40 20 0 0 0.4 0.6
Supersaturation (%) (
S
= %RH-100) 0 0 200 400 Drop conc’n (cm-3) 0 0 2 4 6 Ave’ radius ( m m) ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 0 0 0.1 0.2
Liquid water content (g m-3) 1
1 2 4 10 20 30 40
Diffusional Droplet Growth
Droplets grow by diffusion of water vapour
r dr S dt
const
(
S
= %RH-100) Radius time transition drop r=50, V=27 2.4 s 130 s 1000 s 2700 s 2.4 hr 4.9 hr 12.4 hr typical drop r=10, V=1 .
large drop r=50, V=27 typical CN r=0.1, V=10 -4 NaCl particle (10 -14 g mass); initial radius = 0.75 micron; RH = 100.05%; p = 900 mb; T = 273 K typical raindrop: r=1000, V=650 ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Diffusional Droplet Growth
r dr dt
S const
• Leads to narrowing of droplet size distribution, but not observed Observed • Possible reasons: Diffusion only N drop – Giant CN – Supersaturation fluctuations – Mixing cloud top cloud base N drop cloud base cloud top Diameter Diameter ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Definition of “Precipitation-Sized” Droplet
• How big must a droplet be before it can be considered a “raindrop” Initial radius 1 m m 3 m m 10 m m 30 m m 0.1 mm 0.15 mm Distance fallen 2.0 m m 0.17 mm 2.1 cm 1.69 m 208 m 1.05 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 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 ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics Some large drops initiate collision coalescence 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 m m Therefore droplet distribution broadening needed ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Droplet Evolution with Collision Coalescence
0 5 10 15 20 25 30 10 -3 10 -2 Radius (cm) 10 -1 10 m m 10 0 ENVI3410 : 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 m m) – Large vertical extent (=long growth/collision times) ENVI3410 : 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 ENVI3410 : 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) Below –39 o C Result = complete freezing of all drops Homogeneous freezing ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics Deposition nucleation (reverse sublimation) 1
The Growth Advantage of Ice Crystals
Air is Marginally supersaturated with respect to liquid water in a rising cloud thermal Highly supersaturated with respect to ice At –20 o C at 100% RH S ice = 24% Compare with typical S liq = 0.05-0.5% !
Few crystals grow at expense of drops Subsequent growth from accretion and aggregation ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Atmospheric Ice Nuclei Concentrations
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Effect of Freezing on Cloud Development
• Intensification of rain • Release of latent heat aloft (giving further buoyancy) ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1