1. ENVI3410 : Lecture 8 Ken Carslaw
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 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)

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
Supersaturation (%)
Drop conc’n (cm-3)
Ave’ radius (mm)
Liquid water content
(g m-3)
(S = %RH-100)

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

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”
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

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-Coalescence30 25 20
time (mins) 15 10 5
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 = % !
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)