The magnitude of different aerosol-cloud effects between ... · Köhler Curve (2) Raoult’s law:...
Transcript of The magnitude of different aerosol-cloud effects between ... · Köhler Curve (2) Raoult’s law:...
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Clouds
Ulrike Lohmann
Department of Physics and Atmospheric Science, Dalhousie University, Halifax, N. S., Canada
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Outline of this Lecture
• Overview of clouds
• Warm cloud formation
• Precipitation formation in warm clouds
• Ice cloud formation
• Summary of cloud microphysical processes
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Basic 10 Cloud Types in 4 Families
Cumulus : vertical development
Stratus : layered cloud
Cirrus : ice clouds
Low Base with vertical extent : Cu, Cb, Ns
Low Base and layered : (0-2 km) : St, Sc
Middle Altitude : (2-7 km) : As, Ac
High Altitude (Ice clouds) (5-13 km) : Ci, Cs, Cc
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Fog, Stratus, Stratocumulus, Nimbostratus
http://www.atmos.washington.edu/gcg/Atlas/
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Convective Clouds [http://www.atmos.washington.edu/gcg/Atlas]
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Middle Clouds [http://www.atmos.washington.edu/gcg/Atlas]
Subtype of Ac:
Castellanus
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High Clouds (Ice) [http://www.atmos.washington.edu/gcg/Atlas]
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[http://www.atmos.washington.edu/gcg/Atlas]
Orographic Clouds
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Clausius-Clapeyron Equation
des/dT = L es2/(Rv T2)
Rv = gas constant of water vapor
es = saturation vapor pressure over water/ice
L = latent heat of vaporization/sublimation
T = temperature
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Köhler Curve
consists of Raoult’s law and Kelvin equation:es (r)/ es(∞) = exp ({2 σ}/{ρw Rv T r}) = exp (a/r)
r = droplet radiuses (r) = saturation vapor pressure of droplet of size r es(∞) = saturation vapor pressure over a bulk surface of waterσ = surface tensionρw = water density
Saturation ratio Critical radius Number of molecules
0.12 µm
0.0126 µm
2 1.73 nm 730
0.52 nm
1.01 2.5 x 108
1.1 2.8 x 105
10 20
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Köhler Curve (2)
Raoult’s law: For a plane water surface the reduction in vapour pressure due to the presence of a non-volatile solute may be expressed:
e* (∞)/es(∞) = 1 – (3 ν m Mw)/(4 π Ms ρw r3) = 1 - b/r3
es* (∞) = saturation vapor pressure of bulk solution
Ms = molecular weight of the solute,
ms = mass of the solute,
ν = degree of dissociation.
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Köhler Curve (3)
Combination of Kelvin and Raoults equation (evaluated for e*(r)/es(r)) gives the Koehler curve:
e*(r)/es(∞) = (1 - b/r3) * exp(a/r) ~ 1 + a/r - b/r3
1. term: surface molecules possess extra energy2. term: solute molecules displacing surface water moleculesa ~ 3.3 10-7/T [m]b ~ 4.3 10-6 i Ms/ms [m3/mol]Ms = mass of salt [kg]ms = molecular mass of salt [kg/mol]
The critical radius rc and critical supersaturation Sc are given by:rc = (3b/a)1/2 , Sc = (4 a3/[27 b]) 1/2
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rc, Sc
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Cloud Droplet Formation
rc, Sc
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Observed Cloud Droplet Spectra
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Cloud Properties in a Developing Cloud
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Droplet Sizes [Rogers and Yau, 1989]
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Droplet Growth Equation
Before and after the droplet reaches the critical size, it grows by diffusion of water molecules from the vapor onto its surface.
r dr / dt = ( S – 1 ) / [ Fk + Fd ]
S = ambient saturation ratio
Fk= heat conduction term ~ L2 ρw / Rv K T2
Fd= vapor diffusion term = ρw Rv T / D es
D = diffusion of water vapor in air
K = thermal conductivity of air
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Droplet Growth Equation (2)
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Evolution of Droplet Population [Rogers and Yau, 1989]
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How Does Warm Rain Form?
Cloud droplets initially grow by condensation, then through collision-coalescence (sticking together).
Coalescence is not efficient when droplets < 14 µm
Problems in explaining observed droplet growth:
e.g., growth rate to a 14 µm droplet by condensation
~10-20 min
growth by collision-coalescence from 20 µm to 100 µm
~1 hour
i.e., the combined growth time is longer than the lifetime of small, precipitating cumulus clouds
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Cloud Spectrum Widening
Need to overcome an effective radius of 14 µm before the collision-coalescence process becomes effective
How?
Turbulence or giant CCN
Turbulence may help by providing overlapping eddies, different drop inertias, effects of shear
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Giant Cloud Condensation Nuclei
[Feingold et al. 1999]
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Can Sea Salt Cleanse Air Pollution over Oceans?
[Rosenfeld et al. 2002]
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Can Sea Salt Cleanse Air Pollution over Oceans?
[Rosenfeld et al. 2002]
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CollisionCollision--Coalescence ProcessCoalescence Processhttp://physics.uwstout.edu/wx/Notes/ch5notes.htm
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Warm Rain Formation
Collision efficiencies of water droplets of different size by gravitational settling
[Pruppacher and Klett, 1997]
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Turbulent Collision Efficiencies
[Charmaine Franklin et al. 2003]
Separation distance = 0.04 cm
E(gravitational) = 0.08 (theoretical values 0.03 – 0.11)
Collision Efficiency Collision Kernel x Collision Efficiency
Rλ Rλ
gravity1.7 x g
2.3 x g
1.8 x g
7.4 x g
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Rain Drop Spectra
[Willis, 1984]
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Warm Cloud Growth Regimes
[Cotton, personal notes]
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Different Ice Nucleation Mechanisms
Warmer
Colder
Contact Freezing
Immersion/Condensation
Deposition
T
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Different Ice Nucleation Mechanisms
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Requirements for Ice Nuclei (IN)
Insolubility: so that IN do not disintegrate under action of water
(mineral dust, black carbon)
Size: IN must be larger than > 0.1 µm, a critical ice embryo
(mineral dust)
Chemical bonds: complex organic molecules (aerobic bacteria) have hydrogen bonding groups similar to ice
(cholesterol)
Crystallographic resemblance to ice
(silver iodide AgI)
Active Sites: need pits and steps on ice nuclei
(dirty AgI works better for cloud seeding than clean AgI)
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Ice Nuclei Concentration vs. TemperatureIce Nuclei Concentration vs. Temperature
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Ice Nuclei Concentration vs. SizeIce Nuclei Concentration vs. Size
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Clausius-Clapeyron Equation
des/dT = L es2/(Rv T2)
Rv = gas constant of water vapor
es = saturation vapor pressure over water/ice
L = latent heat of vaporization/sublimation
T = temperature
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BergeronBergeron--FindeisenFindeisen ProcessProcesshttp://physics.uwstout.edu/wx/Notes/ch5notes.htm
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SeederSeeder--Feeder ProcessFeeder Process
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Mixed Phase CloudMixed Phase Cloud www.usd.edu/esci/figures
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Summary of Cloud Microphysical ProcessesSummary of Cloud Microphysical Processes
[Encyclopedia of Atmospheric
Sciences]