cloud microphysics envi3410 : lecture 8 ken carslaw lecture 2 of a series of 5 on clouds and climate...
Post on 20-Dec-2015
217 views
TRANSCRIPT
Cloud MicrophysicsENVI3410 : 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
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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, 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
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)– 2nd 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 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)
•
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
temperature
wate
r pre
ssure
100% RH line
Air initially at 70% RH
Air rises, cools, RH increases
100% RH (saturation, dew point)Droplets form
Droplets grow, remove water vapour
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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
ammoniumsulphate
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Droplet “activation”
Typically100-1000 cm-3
Typically 1000-10000 cm-3
maximum supersaturation in cloud equates to minimum radius of activation
growth
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– Simple
• Cloud-scale updraught speed– Higher speed = more drops
}Humanactivitiesaffect these
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Droplet number vs. aerosol size and number
• Fixed updraught speed
log(N)
DiameterSolid contours = CDN; colours = aerosol mass (g m-3)
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Droplet Evolution Above Cloud Base
Hei
ght a
bove
clo
ud b
ase
(m)
Supersaturation (%)
0
20
40
60
80
0 0.4 0.6Drop conc’n (cm-3)
0
20
40
60
80
0 200 400Ave’ radius (m)
0
20
40
60
80
0 2 4 6Liquid water content (g m-3)
0
20
40
60
80
0 0.1 0.2
updraught = 0.5 ms-1
updraught = 2.0 ms-1
Decreasingsupersat’n as droplets grow, suppresses new droplets
(S = %RH-100)
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Diffusional Droplet Growth
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
NaCl particle (10-14 g mass); initial radius = 0.75 micron; RH = 100.05%; p = 900 mb; T = 273 K
.typical CNr=0.1, V=10-4
large dropr=50, V=27
typical dropr=10, V=1
typical raindrop: r=1000, V=650
transition dropr=50, V=27
Droplets grow by diffusion of water vapour
const
S
dt
drr (S = %RH-100)
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Diffusional Droplet Growth
• Leads to narrowing of droplet size distribution, but not observed
• Possible reasons:– Giant CN
– Supersaturation fluctuations
– Mixing
const
S
dt
drr
Ndrop
Diameter
Ndrop
Diameter
cloud base
cloud top
cloud base
cloud top
Diffusion only Observed
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
Distance fallen
1 m 2.0 m
3 m 0.17 mm
10 m 2.1 cm
30 m 1.69 m
0.1 mm 208 m
0.15 mm 1.05 km
Distance a drop falls before evaporating.Assumes isothermal atmosphere withT=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
Some large drops initiate collision-coalescence
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Collision and Coalescence Rates
Coalescence very inefficientbelow about 20 mTherefore droplet distributionbroadening needed
Collision-Coalescence efficiencyreduced because smalldrops are swept roundthe larger one
Almost all collisionsresult in coalescence
“wake” effects
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Droplet Evolution with Collision-Coalescence
05
1015
2025
30
time (
mins)
10-3 10-2 10-1 100
Radius (cm)
10 m
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)
– 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
Contact nucleationfreezing
Immersionfreezing
(Rate proportionalto drop volume)
Deposition nucleation(reverse sublimation)
Between–10 oC and –39 oC
Result = very few crystals
Homogeneousfreezing
Below –39 oC
Result = complete freezing of all drops
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
The Growth Advantage of Ice Crystals
At –20 oC at 100% RH Sice = 24%
Compare with typical Sliq = 0.05-0.5% !
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 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)