Transcript Direct Radiative Effect of aerosols over clouds and clear skies determined using CALIPSO and the A-Train Robert Wood with Duli Chand, Tad Anderson, Bob.

Direct Radiative Effect
of aerosols over clouds and clear skies
determined using CALIPSO and the A-Train
Robert Wood
with Duli Chand, Tad Anderson, Bob Charlson
VOCALS RF04, 21 November 2008
Single scattering albedo (approx)
Effect of aerosol layer on TOA SW radiation
0.0
DRE > 0
(warming)
0.9
0.99
DRE < 0
(cooling)
0.999
00
Coakley and Chylek (1974)
0.2
0.4
0.6
Surface albedo
0.8
1.0
stratocumulus
clouds
biomass burning aerosol
above cloud
MODIS Aqua
RGB (enhanced)
13 Aug 2006
SE Atlantic
500 km
Cloud-Aerosol Lidar and Infrared Pathfinder
Satellite Observations
CALIPSO lidar (CALIOP)
• Backscatter profiles at 532 and 1064 nm
• Parallel and perpendicular polarization for 532
nm channel
Aerosol layers over clouds seen
with CALIPSO over SE Atlantic Ocean
(13 Aug 2006)
Biomass burning fires (2006, monthly)
Direct radiative forcing (DRF)
AEROCOM Models (Schulz et al. 2006)
Aerosol optical thickness retrieval
methodologies
Measurements of Lidar ratio (S)
From Anderson et al. (2000), J. Geophys. Res.
Retrieval: color ratio method (CR)
(Chand et al. 2006, J. Geophys. Res.)
• CALIPSO data, integrated attenuated backscatter at 532
and 1064 nm (g532 and g1064)
• Determine color ratio cwater = g1064/g532 from layers
classified as cloud (z < 3 km)
• Unobstructed liquid clouds should have c = 1, and so
deviations from this represent aerosols above clouds
• Use Beer-Lambert law to obtain AOD of aerosol layer:
=1 ideally, but use
unobstructed cloud to calibrate
Angstrom
exponent
Depolarization ratio method (DR)
(Hu et al. 2007, Chand et al. 2007)
• Use depolarization d of cloud layer, combined with
its integrated attenuated backscatter g, to derive
AOD of overlying layer
Extinction to backscatter ratio
for water clouds (19 sr)
Self-calibration coefficient
• Comparison of
DR and CR
aerosol optical
depths assuming
å = 2 for CR
method
Increasing
Angstrom
exponent
• Daytime results
are similar
assuming å = 2
Cloud layer top heights
Angstrom exponent for layers above cloud
Aerosol optical depth for layers above cloud
(by month 2006)
June
Sep
July
Oct
August
Nov
Biomass burning fires (2006, monthly)
AOD and winds at 600 hPa
-25
-20
-15 -10
-5
0
5
10
15
Determining the direct radiative effect
of elevated aerosol layers above the
partly cloudy boundary layer
(Chand et al. 2009, Nature Geosciences)
Radiative transfer model
• DISORT radiative transfer model
• Aerosol properties needed are AOD (from CALIPSO),
single scattering albedo (w=0.85, Leahy et al. 2006),
Angstrom exponent (CALIPSO), asymmetry factor (g =
0.62)
• Cloud properties are cloud optical depth and cloud
effective radius (MODIS), and cloud fraction
• Ocean surface albedo = 0.06
• Determine aerosol radiative effect for clear sky,
cloudy sky, and all-sky (Jul-Oct 2006/2007)

Absorption of solar radiation by aerosols
Single scattering
albedo
Single scattering albedo
a (10-7 m-1)
s (10-6 m-1)
Aitken mode
particle conc.
Scattering
coefficient
Absorption
coefficient
From Clarke and Charlson, 1985, Science
Frequency of occurrence
Absorption: Single scattering albedo
0.65
Data from SAFARI-2000
field campaign
0.7
0.75
0.8
0.85
0.9 0.95
single scattering albedo at 550 nm
1.0
From Leahy et al. (2007), Geophys. Res. Lett., 34, L12814, doi:10.1029/2007GL029697
Wavelength dependence of 
Aerosol is relatively more
absorptive at longer
wavelengths
Leahy et al. (2007)
From Bergstrom et al. (2007), Atmos. Chem. Phys.
Effect of aerosol upon radiative fluxes
AOD
Absorption
DRE (TOA)
Radiative forcing efficiency
RFE is determined primarily by cloud cover
Ju ly-O ctob er, 2006-2007
125
C F ice < = 0.05
D irect R F E (W m -2  -1 )
100
ssa= 0.85
g= 0.62
N = 337
75
TOA
a= -34.86
b= 86.12
r²= 0.96
50
25
A tm osp h ere
a= 59.30
b= 31.97
r²= 0.51
0
-25
-50
0.0
C crit
0.2
0.4
0.6
C lou d F raction w a ter
0.8
1.0
Dependence on single scattering albedo
• Critical cloud
fraction increases
strongly with SSA
(brighter albedo
required for
positive DRE)
Inter-model standard deviation of aerosol
direct radiative forcing
(AEROCOM, Schulz et al. 2006)
Aerosol all-sky direct radiative forcing
• DFall=DFclr + C(DFcld-DFclr)
5-20oS, 10oE-10oW
• Model prediction of
cloud cover important
for accurate
quantification of aerosol
DRF
DFall
• Inter-model cloud cover
variations explain 70%
of the variance in the
aerosol direct radiative
forcing (DRF)
Summary
• Novel method using color ratio of cloud targets used
to derive aerosol optical depth of elevated biomass
burning layers over clouds over SE Atlantic
• Radiative transfer calculations and MODIS cloud
optical properties data used to determine direct
radiative effect of elevated aerosol layers
– Remarkably linear dependence of RFE upon cloud
fractional coverage of low clouds (critical cloud fraction)
• Inter-model differences in DRF are not only related
to aerosol radiative properties, but are strongly
dependent upon model cloud cover
Questions
• What is the climate response to aerosol forcing by
biomass burning aerosols over the South Atlantic?
– Simulations using CAM (with Naoko Sakaeda, Phil Rasch)
• What are the global mean effects of aerosols above
clouds?
– Global analysis CALIPSO/DISORT. Use this to constrain
models (Duli Chand)
• Passive remote sensing of aerosols above clouds
using MODIS?
Community Atmospheric Model (CAM)
Simulations
(Naoko Sakaeda, Phil Rasch)
• Preliminary analysis of
20-year CAM simulations
• Present-day AOD tuned
to match CALIPSO
measurements
0.20
0.15
0.10
0.05
• Figure shows change in
low cloud cover (biomass
burning aerosols – no
biomass burning
aerosols)
0
-0.05
AEROCOM
Models
(Schulz et al. 2006)
Direct radiative
forcing for
cloudy skies