aerosol properties near clouds
DESCRIPTION
Aerosol properties near clouds. Alexander Marshak and Tamás Várnai NASA GSFC and UMBC JCET R . Cahalan, F. Evans, G. Wen, W. Yang. Aerosol measurements near clouds are important. Motivation: Help satellite studies of aerosol-cloud interactions - PowerPoint PPT PresentationTRANSCRIPT
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Alexander Marshak and Tamás Várnai
NASA GSFC and UMBC JCET
R. Cahalan, F. Evans, G. Wen, W. Yang
Aerosol properties near clouds
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Motivation:• Help satellite studies of aerosol-cloud interactions• Aerosol remote sensing near clouds is challenging• Excluding areas near-cloud risks biases in aerosol data
Aerosol measurements near clouds are important
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NE Atlantic Ocean, MODIS Terra2000-2007, September 14-29
MODIS reflectances increase near clouds
Várnai and Marshak (2009)
Reflectance increase may come from:•Aerosol changes (e.g., swelling in humid air)•Undetected cloud particles•Instrument imperfections•3D radiative effects
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CALIOP backscatter and particle size
Far from clouds (> 5km)
global night data over oceanJuly 8 – Aug 7, 2007
Fraction of cloud-free vertical profiles
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CALIOP backscatter and particle size
Close to clouds (< 5km)
global night data over oceanJuly 8 – Aug 7, 2007
Fraction of cloud-free vertical profiles
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Enhancements occur over all oceans
MODIS 0.47 µm reflectance (R) enhancement
Median reflectance enhancement
Rd<5km – Rd>5km
CALIOP 532 nm backscatter (β) enhancement
Enhancement of median backscatter integrated up to 3
km (sr-1)
βd<5km – βd>5km
One year, 11/06 – 10/07
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MODIS can add cloud information
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1D & 2D cloud masks yield similar enhancements
CALIOP 532 nm backscatter (β532)
integrated up to 3 km altitude
CALIOP color ratio (β1064 / β532)(closely related to particle size)
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OrbitR3D (0.47 µm) R3D-R1D (0.47 µm)
3D-related increases should be asymmetric
3D effect: enhancementeverywhere
(outside shadows)
LES liquid water path
Distance to cloud (km)
CALI
OP 5
32 n
m b
acks
catte
r
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MODIS, 0.47 µm MODIS, 0.86 µm
Várnai and Marshak (2009)
Asymmetry stronger at shorter l: 3D is important
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Orbit
Várnai and Marshak (2009)
Strong increase near thick clouds agrees with 3D
MODIS reflectances near clouds of various optical thicknesses
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Simple model to correct for 3D enhancement
Marshak et al. (2008)
Rcorrected = RMODIS - ΔR(τRayleigh,Freflected)
Need this
CERES
RT model (τ, CF, re)Input from CERES and MODISOcean BRDFCorrelated-k for BB
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Test shows assumption works for radiances
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Test scene near New Zealand
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MODIS cloud products
Average(τ)=9Stddev(τ)=9
Average(Re)=18 μmStddev(Re) =5 μm
300 km 300 km
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Albedo and AOT
Low level water clouds Average AOT ~0.13
300 km 300 km
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Correction reduces some AOT values
Big change
Small changeOriginal 0.47 µm AOT Corrected 0.47 µm AOT
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Correction reduces fraction of small aerosols
Wavelength
Scat
terin
g cloudlarge aerosolsmall aerosolair molecules
Smal
l fra
ctio
n, c
orre
cted
Small fraction, original
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• Clouds are surrounded by a wide (>10 km) transition zone of enhanced particle size and light scattering. This transition zone needs to be considered in studies of aerosol radiative effects and aerosol-cloud interactions.
• 3D radiative processes play an important role in enhancing clear sky solar reflectance near clouds. A simple two-layer model shows promise for considering these processes in passive satellite remote sensing.
• A synergy of passive (MODIS, CERES and WFC) and active remote sensing (CALIOP) can help better understand and measure aerosol properties near clouds.
Summary
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Cloud fraction affects typical distance to clouds
Annual median distance to clouds below 3 km
Annual mean cloud fraction
Median distance to clouds (km)
Mean cloud fraction
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Cloud mask affects increase mainly near clouds
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Median relative near-cloud enhancements
MODIS CALIOP
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CALIOP: increases occur below cloud top
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Várnai and Marshak (2010)
CALIOP: increases occur below cloud top
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Increases suggest large changes in AOT