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