Direct Radiative Effect of aerosols over clouds and clear skies determined using CALIPSO and the...
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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
Slide 2
Effect of aerosol layer on TOA SW radiation Surface albedo
Single scattering albedo (approx) DRE > 0 (warming) DRE < 0
(cooling) Coakley and Chylek (1974) 00 0.2 0.4 0.6 0.8 1.0 0.0 0.9
0.99 0.999
Slide 3
500 km MODIS Aqua RGB (enhanced) 13 Aug 2006 SE Atlantic MODIS
Aqua RGB (enhanced) 13 Aug 2006 SE Atlantic stratocumulus clouds
stratocumulus clouds biomass burning aerosol above cloud biomass
burning aerosol above cloud
Slide 4
Cloud-Aerosol Lidar and Infrared Pathfinder Satellite
Observations
Slide 5
CALIPSO lidar (CALIOP) Backscatter profiles at 532 and 1064 nm
Parallel and perpendicular polarization for 532 nm channel
Slide 6
Aerosol layers over clouds seen with CALIPSO over SE Atlantic
Ocean (13 Aug 2006)
Slide 7
Biomass burning fires (2006, monthly)
Slide 8
Direct radiative forcing (DRF) AEROCOM Models (Schulz et al.
2006)
Slide 9
Aerosol optical thickness retrieval methodologies
Slide 10
Measurements of Lidar ratio (S) From Anderson et al. (2000), J.
Geophys. Res.
Slide 11
Retrieval: color ratio method (CR) (Chand et al. 2006, J.
Geophys. Res.) CALIPSO data, integrated attenuated backscatter at
532 and 1064 nm ( 532 and 1064 ) Determine color ratio water 1064 /
532 from layers classified as cloud (z < 3 km) Unobstructed
liquid clouds should have = 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
Slide 12
Depolarization ratio method (DR) (Hu et al. 2007, Chand et al.
2007) Use depolarization of cloud layer, combined with its
integrated attenuated backscatter , to derive AOD of overlying
layer Extinction to backscatter ratio for water clouds (19 sr)
Self-calibration coefficient
Slide 13
Increasing Angstrom exponent assuming = 2 Comparison of DR and
CR aerosol optical depths assuming = 2 for CR method Daytime
results are similar
Slide 14
Cloud layer top heights
Slide 15
Angstrom exponent for layers above cloud
Slide 16
Aerosol optical depth for layers above cloud (by month 2006)
June July August Sep Oct Nov
Slide 17
Biomass burning fires (2006, monthly)
Slide 18
AOD and winds at 600 hPa -25 -20 -15 -10 -5 0 5 10 15
Slide 19
Determining the direct radiative effect of elevated aerosol
layers above the partly cloudy boundary layer (Chand et al. 2009,
Nature Geosciences)
Slide 20
Radiative transfer model DISORT radiative transfer model
Aerosol properties needed are AOD (from CALIPSO), single scattering
albedo ( =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)
Slide 21
Absorption of solar radiation by aerosols From Clarke and
Charlson, 1985, Science Single scattering albedo Aitken mode
particle conc. Scattering coefficient Absorption coefficient Single
scattering albedo a (10 -7 m -1 ) s (10 -6 m -1 )
Slide 22
Absorption: Single scattering albedo From Leahy et al. (2007),
Geophys. Res. Lett., 34, L12814, doi:10.1029/2007GL029697 Frequency
of occurrence 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1.0 single scattering
albedo at 550 nm Data from SAFARI-2000 field campaign
Slide 23
Wavelength dependence of From Bergstrom et al. (2007), Atmos.
Chem. Phys. Leahy et al. (2007) Aerosol is relatively more
absorptive at longer wavelengths
Slide 24
Effect of aerosol upon radiative fluxes AOD DRE (TOA)
Absorption Radiative forcing efficiency
Slide 25
RFE is determined primarily by cloud cover
Slide 26
Dependence on single scattering albedo Critical cloud fraction
increases strongly with SSA (brighter albedo required for positive
DRE)
Slide 27
Inter-model standard deviation of aerosol direct radiative
forcing (AEROCOM, Schulz et al. 2006)
Slide 28
Aerosol all-sky direct radiative forcing F all = F clr + C( F
cld - F clr ) Inter-model cloud cover variations explain 70% of the
variance in the aerosol direct radiative forcing (DRF) Model
prediction of cloud cover important for accurate quantification of
aerosol DRF 5-20 o S, 10 o E-10 o W F all
Slide 29
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
Slide 30
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?
Slide 31
Community Atmospheric Model (CAM) Simulations (Naoko Sakaeda,
Phil Rasch) Preliminary analysis of 20-year CAM simulations
Present-day AOD tuned to match CALIPSO measurements Figure shows
change in low cloud cover (biomass burning aerosols no biomass
burning aerosols) 0.20 0.15 0.10 0.05 0 -0.05
Slide 32
Slide 33
AEROCOM Models (Schulz et al. 2006) Direct radiative forcing
for cloudy skies