From Aerosols to Cloud MicrophysicsFrom Aerosols to Cloud Microphysics
Paolo LajLaboratoire de Glaciologie et Géophysique de
l’Environnement
Grenoble - France
Clouds and the global Energy budget (SW Clouds and the global Energy budget (SW radiation)radiation)
Apollo 11 image of Africa & Europe
At any time, 30% of the Earth’s surface is covered by clouds
Some interesting numbers
Clouds increase the global reflection of solar radiation from 10 to 30%, reducing the amount of solar radiation absorbed by the Earth by about 44 W/m².
Clouds and the global Energy budget Clouds and the global Energy budget (LW radiation)(LW radiation)
Apollo 11 image of Africa & Europe
At any time, 30% of the Earth’s surface is covered by clouds
Some interesting numbers
Clouds increase the global reflection of solar radiation from 10 to 30%, reducing the amount of solar radiation absorbed by the Earth by about 44 W/m². This cooling is offset somewhat by the greenhouse effect of clouds which reduces the outgoing longwave radiation by about 31 W/m². Thus the net cloud forcing of the radiation budget is a loss of about 13 W/m²
Different kind of CloudsDifferent kind of Clouds
Question : which kind of hydrometeors ?
Low overcast clouds result in cooling (35 W m−2 ± 9 W m−2) Thin high clouds result in warming (20 W m−2 ± 8 W m−2)
Clouds and the redistribution of radiant energy within the atmosphere
Clouds and the global Energy budget Clouds and the global Energy budget (LW radiation)(LW radiation)
Objective of the lecture : 1- discuss the mechanisms by which anthropogenic activities may modify the Earth radiative budget
(Cloud Radiative Forcing)2- Focus on the aerosol/cloud interaction
Definition
What is an aerosol ?
Particles + Gases = Aerosols
<1m/s ~1 m-3 2mm-20mm Snowflakes
Up to 30m/s~1 m-3 1mm-50mm
Graupel and hail particles
<1m/s 1-100 l-1 100m- 3mm Ice crystals
<15cm/s ~1 m-3 100m- 6mm Raindrops
<30cm/s 100-1000 cm-31m-100m
Cloud droplets
Terminal velocity
Number concentration
Size (diameter)
Shape Hydrometeor
Different kind of hydrometeorsDifferent kind of hydrometeors
Size range of aerosols
Seoul, Korea, April 10, 2006
Dust in Seoul, Korea April 8, 2006
PM10 level reached 2,070 ug/m3 .
Black Carbon on snow
Enhancement of pixel-average cloud spherical albedo sph on April 5 relative to that on April 2, as a
function of LWP
Summary
1. Aerosol can scatter and absorb short-wave solar radiation
2. Aerosol can modify cloud microphysics and, in turn, change cloud reflectivity
3. Question: are these processes relevant in the global energy budget ?
Anthropogenic Radiative Forcing from IPCC
Question : what is behind the large uncertainty for the cloud albedo effect ?
More than one indirect effect…..
Question : how do we quantify the indirect effect ?
Cloud Albedo and cloud microphysical Cloud Albedo and cloud microphysical propertiesproperties
Cloud albedo effect (Twomey effect)
Cloud Albedo and cloud microphysical Cloud Albedo and cloud microphysical propertiesproperties
Cloud Geometry
3 4
9w
v
dR Rr
dN LWC
Question : what does this equation tells us ?
LWP and Cloud Optical depthLWP and Cloud Optical depth
cewrLWP 3
2
Adiabatic assumption
3
4ext
eff
Q LWP
r
Qext = extinction coefficient
LWP= Liquid Water Path (g m-2)
Reff= effective radius
Cloud Albedo and Cloud Optical depthCloud Albedo and Cloud Optical depth
Question : implications of the R/ dependency ?
1
Ra
g
a= empirical coefficientg = assimetry parameter (0.85 for clouds)
Cloud Albedo and Cloud MicrophysicsCloud Albedo and Cloud Microphysics
Aerosol influence on cloud albedo requires comparison
not of the albedo values themselves but of the
enhancement in albedo relative to that expected for
the same LWP
Question : can we measure it ? Which kind of clouds would you use ?
Cloud Albedo and Cloud MicrophysicsCloud Albedo and Cloud Microphysics
Pixel-average cloud spherical albedo as a function of vertical cloud LWP, for three satellite overpasses
Cloud Albedo and Cloud MicrophysicsCloud Albedo and Cloud Microphysics
Enhancement of pixel-average cloud spherical albedo sph on April 5 relative to that on April 2, as a
function of LWP
Enhancement against LWP shows maximum enhancement at intermediate values of LWP, for which sensitivity to increased cloud-drop number concentration is the greatest
Is LWP independent of CN ?Is LWP independent of CN ?
Question : what can you say about this picture ?
Aerosol activation to cloud dropletsAerosol activation to cloud droplets
CNs and CCNsCNs and CCNs
Higher hygroscopic fraction
Lower hygroscopic fractionsmaller size
ERCA School Grenoble- January2002
)(
2exp
33
3,
, Nws
Nswvs
w
asw
wsat
a
raM
rM
aRT
M
e
e
Equilibrium between aqueous solution and humid airEquilibrium between aqueous solution and humid air
Curvature (Kelvin) EffectCurvature (Kelvin) Effect: the saturation vapour pressure increases with increasing
curvature
Solute (Raoult) EffectSolute (Raoult) Effect: the presence of solutes in the
drop decreases the saturation vapour pressure
Cloud droplet formationCloud droplet formationThe Köhler theoryThe Köhler theory
The smaller the droplet, the greater the supersaturation (with respect to a flat surface) is needed to keep the
droplet from evaporating
Cloud droplet formation IICloud droplet formation IIKelvin EffectKelvin Effect
The vapor pressure for asolution drop is less than that
for a plane of pure water
The vapor pressure requiredto maintain equilibrium
decreases as the drop radiusdecreases.
This is opposite of the effect for curvature.
Cloud droplet formation IIICloud droplet formation IIIRaoult EffectRaoult Effect
We can combine the effects of curvature and solution. This curve, represented by
the thick line at the right, is the Köhler curve.
Initially the solution effect dominates, but as the drop gets bigger, the curvature effect
takes over.
When the drop is very large,neither effect dominates and the surface of
the drop, to the water molecules, appears as a flat surface.
Cloud droplet formation IIICloud droplet formation IIIRaoult + Kelvin EffectRaoult + Kelvin Effect
Question : what can we measure in the köhler equation ?
Köhler curves calculated for three aerosol dry sizes and
two different aerosol chemical compositions.
-inorganic aerosol with surface tension equal to that of pure
water (dotted lines).
-inorganic + organic aerosol and variable surface tension
(solid lines).
Effect of a lower surface tension on critical supersaturation due to
organic substances
S
SSS
SSC
SSCSSCSSCSSC
Nws
Nswvs
w
asw
wsat
a
MMX
raM
rM
aRT
M
e
e
)(
2exp
33
3,
,
Modified Kolher Equation to include the effects of slightly soluble organic compounds
DpDpDp
MonodisperseAerosol
D0
Dp
MonodisperseAerosol
D0Monodisperse
AerosolMonodisperse
Aerosol
D0
Derived parameter Derived parameter Growth FactorGrowth Factor
GF = DGF = Dpp(@90%RH)/D(@90%RH)/D00
DM
A 1
DM
A 1
DM
A 2
DM
A 2
CPC 1CPC 1 CPC 2CPC 2
10-15 %R
H
63Ni63Ni
RH – T°C
1
SelectionSelection
3
AnalyzerAnalyzer
3
AnalyzerAnalyzer
2
ConditioningConditioning
2
ConditioningConditioningINLETINLETINLETINLET
DpDp
MonodisperseAerosol
D0
Dp
MonodisperseAerosol
D0Monodisperse
AerosolMonodisperse
Aerosol
D0
Measurement of HGF: Principle of Measurement of HGF: Principle of Tandem-DMATandem-DMA
Measurement of CCNs
Measurement of HGF: Principle of Measurement of HGF: Principle of Tandem-DMATandem-DMA
A simplified view of the A simplified view of the Atmospheric AerosolsAtmospheric Aerosols
Hygroscopic growth of laboratory aerosol mixtures
Classic growth theory (soluble fraction) –Neglecting hydrophilic organic material and surface tension effect
Zdanoski-Stokes-Robinson (ZSR) approachGF = (A GFA3 + B GFB3 + …)1/3
Neglecting non-linearity of organic/inorganic mixture on water activity and suface tension
Interstitial Phase(RJI)
Condensed PhaseCVI
Microphysics
Condensed Phase(cloud impactor)
Interstitial + Condensed Phases (Whole air)
In-situ Characterisation of scavenging
Question : How to characterize the scavenged aerosol fraction ?
Cloud Sampler IThe original Sampler
Cloud Sampler IPassive Sampler
Cloud Sampler IIIActive String collector
Cloud Droplet Dynamics Overal LossesCloud Droplet Dynamics Overal Losses
20µm
5 m s-1
AnalyzerAnalyzer
Settling velocity: 1-2 cm s-1
Stopping distance: 0.5 cmRelaxation time: 0.001 s-1
Stokes number: 1-2Evaporation time : 1-5 s
50-80%5-15%
60-80%
Interstitial Phase(RJI)
Condensed PhaseCVI
Microphysics
Condensed Phase(cloud impactor)
Interstitial + Condensed Phases (Whole air)
In-situ Characterisation of scavenging
Question : How to characterize the scavenged aerosol fraction ?
stagnation p lane
counterflow (F3)
supply flow (F1)
return flow (F2)
Sampling cloud dropletsPrinciple of a Counter Flow Virtual Impactor
Hygroscopic properties of natural atmospheric aerosols
•Scavenging efficiency primarily related to size (Dusek et al., 2006)
•Size distribution alone explains 84 to 96% of the variation in CCN
•Variations of CCN activation with particle chemical composition observed but secondary role.
•Personal comment: I’am not fully convinced….
GMD (µm)0.1 1
scav
eng
ing
effic
ienc
ies
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Oxalateinorganic BC (750°C He+O2)OC1 (440°C He+O2)OC2 (650°C He)
Sellegri et al., 2003
Estimating the Indirect EffectCloud Properties
Macroscopic properties (horizontal and vertical distributions) Microphysical properties
Cloud base height Cloud fraction
Cloud top height Radar Doppler
Radar reflectivity
Aerosol Microphysical and chemical properties
Aerosol number concentration Aerosol particle size
Black carbon concentration Cloud condensation nuclei
Hygroscopic growth chemical composition
Particle size distribution Optical and radiative properties
Aerosol absorption Aerosol extinction Aerosol scattering
Backscattered radiation Optical depth
Radiometric measurements active (such as radar and lidar) and passive (such as broadband
radiometers and spectral sensors)
longwave broadbandRadiative heating rate longwave narrowband
…
Surface and column meteorology
Advective tendency Atmospheric moisture Atmospheric pressure
Atmospheric temperature Atmospheric turbulence
Horizontal wind Planetary boundary layer height
Precipitable water Radiative heating rate
Vertical velocity Virtual temperature
Pristine Air Mass
Estimating the Indirect EffectCloud Properties
Macroscopic properties (horizontal and vertical distributions) Microphysical properties
Cloud base height Cloud fraction
Cloud top height Radar Doppler
Radar reflectivity
Aerosol Microphysical and chemical properties
Aerosol number concentration Aerosol particle size
Black carbon concentration Cloud condensation nuclei
Hygroscopic growth chemical composition
Particle size distribution Optical and radiative properties
Aerosol absorption Aerosol extinction Aerosol scattering
Backscattered radiation Optical depth
Radiometric measurements active (such as radar and lidar) and passive (such as broadband
radiometers and spectral sensors)
longwave broadbandRadiative heating rate longwave narrowband
…
Surface and column meteorology
Advective tendency Atmospheric moisture Atmospheric pressure
Atmospheric temperature Atmospheric turbulence
Horizontal wind Planetary boundary layer height
Precipitable water Radiative heating rate
Vertical velocity Virtual temperature
Polluted air Mass
Question : where to find the ideal conditions ?
Complex instrumentationLong-term observations
Global coverage
Direct measurmentFine scale
1,2,3,4D measurmentsLong-term observations
Limited spatial coverage
Indirect observation
1D samplingShort-term observations
Noise
Indirect measurement
+ -
A need for a multiscale approach
Modelling the cloud Modelling the cloud albedo effectalbedo effect
Global decrease in cloud droplet effective radius caused by anthropogenic aerosols,
Global mean RF =0.52 W m–2 Over land = –1.14 W m–2
Over Oceans = –0.28 W m–2
One more problem: the ice phase
Anthropogenic effect of cloud dynamics
Conclusions
Inside a Cloud….Inside a Cloud….
THank you for your attention
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