The Aerosol Indirect Effect:

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The Aerosol Indirect Effect Can we measure
it? Reinout Boers, KNMI
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Contributions by Juan Acarreta, formerly at
KNMI John Gras, CSIRO, Australia Henk Klein
Baltink, KNMI Erik v Meijgaard, KNMI Ulrich
Loehnert, UMunchen Ela Grzeszczak, IGUWarsaw
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Structure of talk
1. What is the Aerosol Indirect Effect?
2. Technique to measure it
3. Measurements over the Southern Ocean
4. Measurements at Cabauw and Opportunities
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Definition of AIE according to IPPC report
Page 307 IPPC 2001 Indirect forcing by
aerosols is broadly defined as the overall
process by which aerosols perturb the
Earth-Atmosphere radiation balance by modulation
of cloud albedo and cloud amount
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Definition of AIE more precise
Indirect forcing by aerosols is defined as the
change in shortwave radiative energy at the top
of the atmosphere caused by aerosols that
modulate cloud reflectivity
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Definition of AIE even more precise
Indirect forcing by aerosols is defined as the
change in shortwave radiative energy at the top
of the atmosphere caused by anthropogenic
aerosols that modulate cloud reflectivity
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Definition of AIE even even more precise
Indirect forcing by aerosols is defined as the
change in shortwave radiative energy at the top
of the atmosphere caused by anthropogenic
aerosols that modulate cloud reflectivity since
the beginning of the Industrial Revolution
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In brief.
There is no precise definition that is used by
everyone
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Earth Radiation Budget
?F A
Cloud albedo
A OD
1
OD CDNC
Cloud optical depth
2
Warm cloud CDNC / size
Precip
OD CDNC,h
?F f,t
CDNC CCN
Cloud fraction f, Life time t, CDNC, Cloud depth
h
CCN
CCN Mass
Aerosol mass
Pathways of the AIE for warm clouds
Aerosol mass emission
Surface
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When presented with a time series of cloud
albedo, how much of the observed change is due to
the AIE?
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Cloud optical depth
OD CDNC
A OD
Warm cloud CDNC / size
Cloud albedo
CDNC CCN
?F A
CCN
CCN Mass
Earth Radiation Budget
Aerosol mass emission
Aerosol mass
Surface
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Optical depth
Optical depth is a measure of how much light
cloud droplets prevent from passing through an
atmospheric column. It is proportional to the
total surface area of columnal cloud droplets
exposed to the sun light
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Effective radius
Effective radius is defined as the fraction of
the third to the second moment of the size
distribution It is proportional to the radius of
cloud droplets
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For measuring the (1st) Aerosol Indirect Effect
over time, cloud droplet concentration
N(SO42-B)b is the key parameter to monitor
How are we going to do that globally??
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Procedure
Satellite radiance- ch 1
Satellite radiance- ch 2
Optical depth ?
Effective radius reff
Atmospheric model (single layered
cloud) ?A1N1/3h5/3, reffA2N-1/3h2/3
Droplet concentration N
Cloud depth h
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Australia
Tasmania
Modis box
MODIS Terra L2 data July 2000 July 2004
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For this region light and temperature dependent
DMS emission from the ocean surface is the
primary source of nss-sulphates acting as CCN in
the marine atmosphere
Seasonal Cycle in CCN, N, albedo
Low CCN
High CCN
Sea salt spray
DMS
Sea salt spray
Winter Ocean Surface
Summer
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Optical thickness Effective
radius derived from MODIS L2 Observations
2000-2004
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Retrievals of droplet concentration
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Retrieval of cloud depth
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Determine the sensitivity of albedo to changes in
N, h
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Respective contributions to ?A from N, h, ?0
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Conclusion Contribution to albedo
change Changes in ?0 60 Changes in
N 20 Changes in h 20 Even though the
sensitivity to changes in N is double that of the
sensitivity to changes in h!
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What can we do at Cabauw?
1) Conception of experiment to measure pathways
of the AIE 2) Retrieval of cloud droplet
concentration
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The Aerosol Indirect Effect is primarily a SW
radiative boundary layer cloud effect, so
requirements are

1. Accurate SW radiation measurements at top of
   atmosphere
2. A method to attribute the observed radiation to
   boundary layer cloud and aerosol processes

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Linking the measured SW radiation to boundary
layer clouds and aerosols
Use BL Observations with quantifiable errors
Compute Radiation profile
Long-term Observations plus occasional IOPS
Constrain to SW Surface and SW TOA fluxes
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Earth Radiation Budget
?F A
MSGBSRN
Cloud albedo
A OD
MSGSurface Remote sensing
Cloud optical depth
OD CDNC
Warm cloud CDNC / size
CDNC CCN
Tower Measurements
CCN
CCN Mass
TowerSurface Measurements
Aerosol mass
Aerosol mass emission
Cabauw Surface and Remote Sensing Observations of
Aerosol, Clouds and Radiation (CESAR partners
KNMI, TUDelft, TNO, ECN, WUR, ESA, RIVM)
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Earth Radiation Budget
?F A
Cloud albedo
A OD
Cloud optical depth
OD CDNC
Warm cloud CDNC / size
CDNC CCN
CCN
CCN Mass
Aerosol mass
Aerosol mass emission
Combined with Radiative Transfer Calculations
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Cabauw Site The site to do a long-term process
study of the Aerosol Indirect Effect
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The importance of Cabauw within Netherlands /
Europe

• A) Cabauw is within the centre of the Netherlands
  / Northwest Europe, therefore it is
  representative of the Netherlands and of
  Northwest Europe.

B) Cabauw is a continental / maritime site. A
great variety of air masses come across this
region depending on air mass history
C) No orography to confuse you
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Backtrajectories originating at an altitude of
500 m at Cabauw, and droplet concentrations for
seven days of the Merlin flights in September
2001,
172 cm-3
124 cm-3
Cabauw -1 hour -1 day (24 hours)
157 cm-3
180 cm-3
210 cm-3
270 cm-3
275 cm-3
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An experiment quantifying the Aerosol Indirect
Effect should consist of
Long-term continuous measurements at a single
site to obtain closure of the radiative budget
using in situ, ground-based and satellite-based
remote sensing observations AND
An number of Intensive Observation Periods for
combining aircraft in situ studies with
ground-based observations
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First step Integrated Profiling (IPT) and
Determining cloud microphysics from remote
sensing at Cabauw
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Integrated Profiling Technique (IPT) (University
of Munchen, U. Loehnert)Determining consistency
between radiative and physicals properties of the
atmospheric column1) Use multiwave length
microwave radiometer, radar, lidar to retrieve T,
LWC, water vapor profiles2) Make consistent with
radiation profiles
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Integrated Profiling Technique How good is this
technique in profiling? (Loehnert, v Meijgaard,
Klein Baltink, Boers)
Use Regional Climate Model (RCM) T, qv, ql
profiles to compute Tbs
Ingest Tbs plus lidar / radar data into IPT
procedure
Compare IPT against RCM output
IPT to compute T, qv, ql profiles
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Integrated Profiling Technique Results for LWP,
IWV
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Integrated Profiling Technique Results for T, q
proflies
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Integrated Profiling Technique Results for
radiative retrievals (preliminary)
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Retrieval of cloud droplet concentration (method
1, Klein Baltink / Boers)
Retrieve cloud boundaries from radar / lidar
Retrieve cloud base extinction from lidar
Retrieve liquid water path from microwave
radiometer
Combine to retrieve N
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Retrieval of cloud droplet concentration (method
2, Grzeszczak )
Retrieve cloud boundaries from radar / lidar
Retrieve in-cloud reflectivity from radar
Retrieve liquid water path from microwave
radiometer
Combine to retrieve N
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Retrieval of cloud droplet concentration (method
3, Dong, Boers, Klein Baltink)
Retrieve liquid water path from microwave
radiometer
Retrieve surface irradiance from pyranometers
Combine to retrieve N
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Cloud depthAdiabaticityLWPDropletConcen
tration
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Radar retrieval of droplet concentration
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Conclusions

1. It is possible to extract the Aerosol Indirect
   Effect from albedo observations by monitoring the
   cloud droplet concentration

• Over the Southern Ocean, albedo changes due to
  the AIE make up 20 of total change.

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Conclusions(2)

1. At Cabauw the AIE will be studied by combining
   radiative transfer modelling with integrated
   observations

• Procedures to retrieve T, q profiles using IPT
  are starting to work

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Conclusions(3)

• Procedures to retrieve droplet concentration from
  remote sensing observations are becoming mature(!)

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Can we now measure the AIE??
Yes.
But