numerical simulations of optical properties of nonspherical dust aerosols using the t-matrix method...
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Numerical simulations of optical properties of nonspherical dust
aerosols using the T-matrix method
Numerical simulations of optical properties of nonspherical dust
aerosols using the T-matrix method
Hyung-Jin [email protected] of Earth and Atmospheric SciencesGeorgia Institute of Technology
November 14, 2008
Graduate student symposium
Motivation: Importance of Mineral dustMotivation: Importance of Mineral dust
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Mineral wind-blown dust is the most abundant aerosol species that originates from diverse sources throughout the world
Assessment of radiative impact of dust remains highly uncertain because of the complexity associated with dust emission, variable transport and aging process
From: IPCC 2007
Dust particles play an important role in the Earth’s radiation budget.
Direct radiative forcing: absorption, scattering Indirect radiative forcing: affect clouds as CCN, IN
SeaWiFS image
10 source of dust
Satellite remote sensing provides the best tool for studying dust
Key limitation: passive remote sensing gives only column-integrated (2D) view
Based on the our observation of CALIPSO for Asian dust in spring 2007
The volume depolarization ratio shows high value over the source
During mid-range transport the depolarization ratio of Asian dust can remain as high as ~0.35 or be much lower (0.1~0.15) than that in the source region.
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The CALIPSO space lidar provides a new capability for improved understanding of dust impacts: measures the vertical distribution of aerosols over the whole Earth for 24 hours
=> provides 3D view determines height-resolved aerosol types:
=> measures the linear depolarization ratio δa which is indicative of the nonspherical particles (such as dust)
ex) Liu et al.(2008) found that δa remained constant during long-range transport of a Saharan dust outbreak => explained by little changes in the dust size distribution and shapes
Motivation : Previous study & Observation Motivation : Previous study & Observation
For passive remote sensing, many previous studies calculated the dust optical properties using
the T-matrix method, which approximates the shape of dust particles as spheroids.
=> How do changes in dust microphysical properties (size spectra, shape, and
composition) affect the optical properties (lidar ratio, depolarization ratio, and single
scattering albedo) measured by the CALIPSO lidar?
GoalsGoals
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Perform a modeling of the optical properties of nonspherical
dust particles to aid in the interpretation of CALIPSO data.
How do changes in dust microphysical properties (size spectra, shape, and
composition) affect the optical properties (lidar ratio, depolarization ratio,
and single scattering albedo) measured by the CALIPSO lidar?
Can we reproduce the observed optical properties of Asian dust from
CALIPSO using the T-matrix method?
ApproachApproach
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Observation Analyses & Previous Study(Optical properties)
T-matrix method(Dust particle = Spheroid)
randomly oriented nonspherical particles
• Reproduce the optical properties of Asian dust
Microphysical properties- Particle shape,- Size number distribution,- Refractive index
Optical properties- Single scattering albedo (ω0),- Lidar ratio (Sa),- Depolarization ratio (δa)
•Understanding of CALIPSO data
Based on a recent study by Lafon and Sokolick(2006), the refractive index of 1.56 + 0.003i at 532 nm (CALIPSO wavelength) was selected as representative for Asian dust.
Single scattering albedo: ω0 = Cs / Ce
Lidar ratio: Sa = 4π / ω0 P11(180°)
Depolarization ratio: δa
(δa = (P11(180°)-P22(180°))/(P11(180°)+ P22(180°))
Compare
Dust Particle
a
b
Spheroids are ellipsesrotated around one axis (b); if this axis (b) is the longer axis, they are called prolate, otherwise oblate.
Optical Modeling:Optical Modeling:
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3-2. Results
Input (Microphysical properties)
Refractive index: 1.56+0.003i at 532 nmSize distribution: lnσ2 = 0.5 - 0.1< r < 1 μm, rg1 = 0.5 μm for the fine mode - 0.1< r < 3 μm, rg1 = 1.0 μm for the coarse mode Aspect ratio: - Oblate: 1.05, 1.1, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0 - Prolate: 1/1.05, 1/1.1, 1/1.2, 1/1.4, 1/1.6, 1/1.8, 1/2.2, 1/2.4, 1/2.6, 1/2.8, 1/3.0
T-matrix method
Output (Optical properties for fine & coarse mode)
Extinction coefficient (Ce), Scattering coefficient (Cs),Single scattering albedo (ω0 = Cs / Ce)Phase function (P11(Θ))Asymmetry parameter (g)Lidar ratio (Sa) Linear depolarization ratio (δa)
Aspect ratio(ε’) 1.05 1.1 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Mixture1a 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Mixture2a 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083
Mix-
ture3 b
Fine 0.535 0.289 0.108 0.040 0.015 0.007 0.003 0.001 0.001 0.001
Coarse 0.103 0.234 0.218 0.157 0.101 0.065 0.041 0.027 0.018 0.026
Mixture4c 0.335 0.319 0.179 0.087 0.042 0.020 0.009 0.005 0.002 0.001
Mixture5c 0.141 0.173 0.230 0.219 0.123 0.060 0.029 0.014 0.006 0.003 0.001 0.001
5 Mixtures and 3 Cases
• Experiment: 100% Prolate (Wiegner et al., 2008)
• Varying proportion of fine and coarse mode - Case 1: 30% fine mode + 70% coarse mode - Case 2: 50% fine mode + 50% coarse mode - Case 3: 70% fine mode + 30% coarse mode
To see the relative distribution of each size mode
Mixtures- 1 & 2: by Dubovik et al.- 3 : by Wiegner et al.- 4 & 5: by Okada et al.
The comparison between fine & coarse mode and prolate & oblateThe comparison between fine & coarse mode and prolate & oblate
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Prolate vs. oblate spheroids: Distinct differences in all optical properties. Especially, the distributions of depolarization ratio
and lidar ratio in fine mode show different patterns.
Lidar ratio of prolate spheroids in coarse mode has higher values than that of oblate spheroids.
“Dust Mixture Experiment” : 100% prolate
Fine vs. coarse : Depolarization ratio in coarse mode changes little
with varying aspect ratio Single scattering albedo in fine mode has higher
values than that in coarse mode. Lidar ratio of prolate spheroids in coarse mode has
higher values than that in fine mode. Case 1(source area): 30% fine + 70% coarse mode Case 2(mid transport):
50% fine + 50% coarse mode Case 3(long transport):
70% fine + 30% coarse mode
a)
b)
c)
Results of “Dust Mixture Experiment”: Results of “Dust Mixture Experiment”:
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30% fine mode + 70% coarse mode
50% fine mode + 50% coarse mode
70% fine mode + 30% coarse mode
Coarser particles have lower values of ω0.
Dust may absorb more sunlight in the source area because of the relatively large fraction of coarse particles (ω0 : > 0.9 in Case 1).
Preferential removal of large particles during transport would result in less sunlight absorption (ω0 : > 0.93 in Case 3).
Lidar ratio varies with varying δa The CALIPSO lidar ratio for desert dust is
Sa=38.1 sr, which corresponds to δa ~ 0.24. δa higher than 0.3 are indicative of lower Sa.
None of cases can produce δa: <0.2 and >0.3 Limitations of the assumption on spheroid.
Range of δa: 0.21 ~ 0.28
Treatments of nonspherical dust particles as spheroids can reproduce some CALIPSO data.
Particle depolarization ratio (δa) has relatively low sensitivity to the size distribution.
The Liu’s conclusions are questionable..
• 1 & 2: Dubovik et al.
• 3 : Weigner et al.
• 4 & 5: Okada et al.
Summary and ConclusionSummary and Conclusion
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Our optical modeling with the T-matrix method showed
Treatments of nonspherical dust particles as spheroids can reproduce some
CALIPSO data.
However, this approach cannot provide the entire range of δa observed by
CALIPSO as well as ground-based lidars.
More realistic shapes of dust will need to be considered to improve the
interpretation of and aerosol retrievals from CALIPSO observations.
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Thank you!!