Optical properties
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Optical properties Satellite observation
?T,H2O…
From dust microphysical properties to dust hyperspectral infrared remote
sensingClémence Pierangelo(1), Michael Mishchenko(2), Alain Chédin(1) (1) Laboratoire de Météorologie Dynamique - Institut Pierre Simon Laplace, Ecole Polytechnique
(2) Goddard Institute for Space Sciences – [email protected]
Dust optical properties at infrared wavelengthsDust optical properties at infrared wavelengths
Radiative transfer computations with aerosols at infrared wavelengthsRadiative transfer computations with aerosols at infrared wavelengths
Remote sensing of dust with hyperspectral infrared sounders : Application to AIRSRemote sensing of dust with hyperspectral infrared sounders : Application to AIRS
IPCC 2001: dust radiative forcing poorly known. Since then, most studies focus on the visible wavelength, whereas the closure of the Earth radiative balance also needs knowledge of the dust effect on terrestrial and atmospheric infrared radiation (3.5 to 15 µm), and computations of IR forcing from visible or near-IR measurements are not reliable enough.
For realistic values of re (1 to 3µm), moderate impact on , g and Qext normalized (max 30%).
Why studying dust in the infrared? Advantages:• night and day detection• sensitivity to dust vertical distribution : altitude retrieval• sensitivity to dust mineralogical composition : might be retrieved• over deserts
Limitations: • Spatial resolution (20km)• Aerosol refractive indices poorly known + high spectral dependency• High sensitivity to temperature and gas profiles•No direct validation
Optical depth Layer altitude
BT 177 (8.14µm)– BT 165 (9.33µm)(K.)Simulations for 100 tropical atmospheric situations (Same aerosol properties)
The aerosol impact itself depends on the atmospheric situation
First component to the signal: the temperature and the water vapor profiles.
The error in the retrieval caused by the use of spherical particles is below 10% for the AOD, and still lower for the altitude
Very strong effect of the refractive index on the AOD!!! But, with the data sets “dust”, “mineral” or “SHADE”, the retrieved AOD could not be greater than 0.5… way to exclude these models
Purpose: modelling of the maximum effect of shape: spheroids with aspect ratio=2 (effect stronger than a mixture of spheroids with several aspect ratios)
9.5 m 9.5 m9.5 m
3.75 mCext
3.75 mg
3.75 m
Optical properties are greater for oblate or prolate spheroids than spheres. The maximum impact is about 10%.
Weak impact of the aspect ratio on the phase function because the size parameter is relatively small.Note that the impact of the phase function on the radiance at satellite level is not as crucial as in the visible (no reflected solar radiation).
INFRARED VISIBLE/NEAR-IR
REFRACTIVE INDEX
+++ High variability of the refractive indices with the wavelength and with the dust model
+ Dependency of the refractive indices to the wavelength pretty well known
SIZE DISTRIBUTION
++ Moderate impact +++ High impact
SHAPE+ Small impact (no impact on phase function for longwave)
++ Moderate impact on phase function with effect on the solar reflected radiance
Simulations of AIRS (Advanced Infrared Sounder-AQUA) brightness temperatures for 324 channels with a code coupling the line-by-line « 4A » and DISORT
The impact of size is greater in the visible than in the IR.The ratio of IR to visible extinction increases with dust size.
Huge variability of refractive indices with wavelength + with data set…
Aerosol optical depth (AOD) and altitude impact: a few K. Aerosol size and shape impact: a few tenth
of K.
Aerosol microphysical properties-Size distribution-Refractive index at each wavelength-Shape
Aerosol optical properties-Extinction cross-section Cext
/ efficiency Qext
-Single-scattering albedo -Phase function or asymetry parameter g
-Mie code-T Matrix code
Imaginary part Real part… and big variability of optical properties with data set too!(“SHADE” model probably not realistic)
Qext g
gNorm. Qext
Norm. Qext
Wavelength (µm)
Qext
Wavelength (µm)
Refractive index is a more problematic issue than size or shape
1. effective radius
3. Refractive indices
2. shape
1. Dust altitude and 10 µm AOD (LUT retrieval)
2. Optical depth and altitude 3. Size and shape
size
BT (
3µ
m-1
µm
)
shapeB
T (
sph
ero
ids-
sph
ere
s)
1. Atmospheric situation
3. Dust effective radius
Method: Look-Up-Tables (LUT) built for 8 AIRS channels, several dust altitudes and 10 µm AOD, one aerosol model (OPAC) and almost 600 atmospheric situations (Pierangelo et al., ACP, in press)
Method: Channel 165 (1072cm-1) sensitive to size, not to shape
2. Validation of the LUT approachValidation of the LUT
retrieved atmospheric situation is performed comparing its surface temperature (SST) and water vapor content (WVC) to MODIS / SSMI observations.
RetrievalAtmosphere, dust AOT, dust altitude
BT 165 (re)
calculated BT 165
observed
Validation with simulations
Results: April-May 2003
The effective radius of Saharan dust decreases from 2.8 µm to 1.2 µm with transport.
The error in the retrieval caused by the use of one fixed size distribution is below 10% for the AOD, and still lower for the altitude
Robustness of the retrieval to dust microphysical properties
aspect ratio aspect ratio
OPA
C
Volz
SH
AD
E
dust
min
era
l
OPA
C
Volz
SH
AD
E
dust
min
era
l
Input AOD Output AOD
Effective radius (µm)
Effective radius (µm)
Siz
eR
ef.
in
dic
es
Sh
ap
e
AIRS 10 µm AOD
AIRS dust altitude
MODIS 0.55 µm AOD
Apr
il 2
003
Sep
tem
ber
2003
Aug
ust 2
003
July
200
3Ju
ne 2
003
May
200
3
Microphysical propertiesReid et al., 2003
3.75 m
9.5 m
Phase function
285 303 SST (K)
MO
DIS
nig
ht S
ST
AIR
S n
ight
SS
T
July 2003
0 60WVC (mm)
AIR
S n
ight
WV
C
SS
MI
WV
C
nig
ht+
day
July 2003