Download Lecture 6 Cloud Microphysics 2010

Cloud Microphysics
SOEE3410 : Lecture 6
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 6
• Drop formation – factors controlling drop number
and size
• Rain formation – what is needed?
• The ice phase
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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
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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.
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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.
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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)
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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
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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)
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Increase in humidity in a rising air
parcel
pw
% RH  100. 0
pw
Partial pressure of water in the air
Saturation vapour pressure over pure water
water pressure
100% RH line
>100% RH above
the 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




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Droplet “activation”
sea salt
ammonium
sulphate
>100% RH
(supersaturation)
needed to form drops
• 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
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
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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
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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
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Droplet number vs. aerosol size and
number
• Fixed updraught
speed
See Pringle et al., ACP,
http://tinyurl.com/39rwk3r
log(N)
Solid contours = CDN; colours = aerosol mass (mg m-3)
ENVI3410 : 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
80
as droplets
grow,
suppresses
60
new droplets
80
80
60
60
40
40
40
40
20
20
20
20
0
0
0
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)
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
0
0.1 0.2
Liquid water content
(g m-3)
1
Diffusional Droplet Growth
Droplets grow by diffusion of water vapour
dr
S
r

dt const
(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
typical drop
r=10, V=1
NaCl 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
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Diffusional Droplet Growth
dr
S
r

dt const
• Leads to narrowing of droplet size distribution,
but not observed
Diffusion only
Observed
• Possible reasons:
Ndrop
Ndrop
– Giant CN
– Supersaturation fluctuations
– Mixing
cloud top
cloud base
Diameter
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
cloud base
cloud top
Diameter
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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
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“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
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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
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Droplet Evolution with CollisionCoalescence
30
25
20
15
10
5
0
10-3
10-2
10-1
100
Radius (cm)
10 mm
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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)
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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
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Ice formation Processes
Water drops readily supercool below 0oC without freezing
See http://www.youtube.com/watch?v=0JtBZGXd5zo
Between
–10 oC
and –39 oC
Result = very
few crystals
Contact nucleation
freezing
Immersion
freezing
(Rate proportional
to drop volume)
Deposition nucleation
(reverse sublimation)
Below –39 oC
Result =
complete freezing
Homogeneous
of all drops
freezing
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The Growth Advantage of Ice
Crystals
At –20 oC 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
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Atmospheric Ice Nuclei
Concentrations
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Effect of Freezing on Cloud
Development
• Intensification of rain
• Release of latent heat aloft (giving further
buoyancy)
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