From Aerosols to Cloud Microphysics

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3 Νοε 2013 (πριν από 4 χρόνια και 1 μήνα)

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From Aerosols to Cloud Microphysics

Paolo Laj

Laboratoire de Glaciologie et Géophysique de
l’Environnement


Grenoble
-

France

Clouds and the global Energy budget (SW
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

(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 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

(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



100
m
m
-

3mm



Ice crystals


<15cm/s



~1 m
-
3



100
m
m
-

6mm



Raindrops


<30cm/s



100
-
1000 cm
-
3


1
m
m
-
100
m
m


Cloud droplets


Terminal
velocity


Number
concentration


Size
(diameter)


Shape


Hydrometeor

Different 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/m
3

.

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
properties

Cloud albedo effect
(Twomey effect)

Cloud Albedo and cloud microphysical
properties

Cloud Geometry

Question : what does this
equation tells us ?

LWP and Cloud Optical depth

Adiabatic assumption

Qext = extinction coefficient


LWP= Liquid Water Path (g m
-
2
)


Reff= effective radius

Cloud Albedo and Cloud Optical depth

Question : implications of the R/


dependency‿

a= empirical coefficient

g = assimetry parameter (0.85
for clouds)

Cloud 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 Microphysics

Pixel
-
average cloud spherical albedo as a function of vertical
cloud LWP, for three satellite overpasses

Cloud 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 ?

Question : what can you say about this picture ?

Aerosol activation to cloud droplets

CNs and CCNs

Higher hygroscopic fraction

Lower hygroscopic fraction

smaller size

ERCA School Grenoble
-

January2002









Equilibrium between aqueous solution and humid air

Curvature (Kelvin) Effect
: the
saturation vapour pressure
increases with increasing
curvature

Solute (Raoult) Effect
: the
presence of solutes in the
drop decreases the saturation
vapour pressure

Cloud droplet formation

The 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 II

Kelvin Effect

The vapor pressure for a

solution drop is less than that

for a plane of pure water


The vapor pressure required

to maintain equilibrium

decreases

as the drop radius

decreases.


This is opposite of the effect
for curvature.


Cloud droplet formation III

Raoult Effect

We can combine the effects of curvature and
solution. This curve, represented by

the thick line at the right, is the K
öhler 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 III

Raoult + 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

Modified Kolher Equation to include the effects of
slightly soluble organic compounds

Derived parameter
Growth Factor


GF = D
p
(@90%RH)/D
0

Measurement of HGF: Principle of
Tandem
-
DMA

Measurement of CCNs

Measurement of HGF: Principle of
Tandem
-
DMA

A simplified view of the
Atmospheric Aerosols

Hygroscopic growth of laboratory
aerosol mixtures

Classic growth theory
(soluble fraction)

Neglecting hydrophilic
organic material and surface
tension effect


Zdanoski
-
Stokes
-
Robinson
(ZSR) approach

GF = (


GF䄳+


GFB3+
…)
1/3


Neglecting non
-
linearity of
organic/inorganic mixture on
water activity and suface
tension


Interstitial Phase

(RJI)

Condensed Phase

CVI

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 I

The original Sampler

Cloud Sampler I

Passive Sampler

Cloud Sampler III

Active String collector

Cloud Droplet Dynamics Overal Losses

20µm

5 m s
-
1

Analyzer

Settling velocity: 1
-
2 cm s
-
1

Stopping distance: 0.5 cm

Relaxation time: 0.001 s
-
1

Stokes number: 1
-
2

Evaporation time : 1
-
5 s

50
-
80%

5
-
15%

60
-
80%

Interstitial Phase

(RJI)

Condensed Phase

CVI

Microphysics

Condensed Phase

(cloud impactor)

Interstitial + Condensed
Phases (Whole air)

In
-
situ

Characterisation of scavenging

Question : How to characterize the scavenged aerosol fraction ?

Sampling cloud droplets

Principle 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….

Sellegri et al., 2003

Estimating the Indirect Effect




Cloud 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 broadband

Radiative 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 Effect




Cloud 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 broadband

Radiative 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 instrumentation

Long
-
term observations

Global coverage

Direct measurment

Fine scale

1,2,3,4D measurments

Long
-
term observations

Limited spatial coverage

Indirect observation

1D sampling

Short
-
term observations

Noise

Indirect measurement

+

-

A need for a multiscale approach

Modelling the cloud
albedo 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….

THank you for your attention