SP P P Latitude (deg)
1
Lidar Ratios for Dust Aerosols Derived from Retrievals of CALIPSO Visible Extinction Profiles Constrained by Optical Depths from MODIS-Aqua and CALIPSO/CloudSat Ocean Surface Reflectance Measurements. Stuart A. Young, CSIRO Marine and Atmospheric Research, Aspendale, VIC, Australia. ([email protected]) Damien B. Josset, Science Systems and Applications Inc., Hampton, VA, USA. ([email protected]) Mark A. Vaughan, NASA Langley Research Center, Hampton, VA, USA. ([email protected]) Introduction CALIPSO’s (Cloud Aerosol Lidar Infrared Pathfinder Satellite Observations) analysis algorithms generally require the use of tabulated values of the lidar ratio in order to retrieve aerosol extinction and optical depth from measured profiles of attenuated backscatter. However, for any given time or location, the lidar ratio for a given aerosol type can differ from the tabulated value. To gain some insight as to the extent of the variability, we here calculate the lidar ratio for dust aerosols using aerosol optical depth constraints from two sources. Daytime measurements are constrained using Level 2, Collection 5, 550-nm aerosol optical depth measurements made over the ocean by the MODIS (Moderate Resolution Imaging Spectroradiometer) on board the Aqua satellite, which flies in formation with CALIPSO. We also retrieve lidar ratios from night-time profiles constrained by aerosol column optical depths obtained by analysis of CALIPSO and CloudSat backscatter signals from the ocean surface [Ref. 1]. Theory For correctly calibrated signals, the particulate lidar ratio, S P , can be related to the particulate optical depth of a layer, t P , multiple-scattering correction factor, h, and integral over the depth of the layer of the attenuated, particulate backscatter, g’ P , by the following equation (adapted from Ref. [2]): 1 exp 2 /2 P P P S ht hg However, as the layers studied here are in contact with the surface, it is not possible to evaluate g’ P directly from the attenuated backscatter signal and, therefore, it is also not possible to derive S P analytically from (1). It must, therefore, be obtained by an iterative process during the retrieval of the particulate extinction and backscatter profiles. (1) Method 1. Select overpass regions where strong dust layers have advected over ocean, 2. Average CALIPSO Level-1 attenuated backscatter data to 5-km horizontal resolution, 3. Calculate average MODIS (or ocean surface reflectance method) aerosol optical depth (AOD) over the CALIPSO 5-km path, 4. Assume all aerosol resides in surface layer, 5. Assume lidar ratio and h are constant throughout the layer, 6. Renormalize averaged attenuated backscatter profiles above the layer top to account for attenuation by the overlying atmosphere, 7. Retrieve particulate extinction profile though the layer, adjusting lidar ratio iteratively until the retrieved optical depth matches that from MODIS or the surface reflectance measurements. (i.e. a constrained retrieval). (c) References 1. Josset, D., J. Pelon, A. Protat, and C. Flamant (2008), New approach to determine aerosol optical depth from combined CALIPSO and CloudSat ocean surface echoes, Geophys. Res. Lett., 35, L10805, doi:10.1029/2008GL033442. 2. Fernald, F. G., B. M. Herman and J. A. Reagan (1972). Determination of Aerosol Height Distributions by Lidar, J. Appl. Meteor. 11, 482-489. (b) (c) Results Particulate backscatter (row d), extinction (e) and lidar ratio (g) retrievals obtained using AOD (f) constraints from MODIS on January 1 st and 3 rd 2007 are presented below in columns 1 and 2 respectively. Results obtained using ocean surface reflectance (SODA) measurements on January 2 nd and 4 th 2007 appear in columns 3 and 4. The 532- nm attenuated backscatter data appear in row (a) (with CALIPSO’s orbit shown in the inset) and CALIPSO’s Cloud-Aerosol Vertical Feature Mask in (b) and Aerosol Type Mask in (c). The occasional black stripes and missing data from the lower panels result from AOD constraints that were unavailable over the land or in cloudy regions. During the period shown dust was being advected over the ocean from the African continent. The mean lidar ratios for January 1st – 3rd are in the range 35 – 45 sr and compare well with CALIPSO’s standard value for dust of 40 sr. On January 4th, the mean value is around 53 sr. The higher value can be explained by the mixing of smoke with the dust to create “polluted dust” (see panel (c) and can be compared with CALIPSO’s standard value of 55 sr. 0.001 0.01 0.1 1 10 532-nm Particulate Extinction (/km) 14 16 18 20 22 24 26 28 0 1 2 3 4 5 (e) (c) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 14 16 18 20 22 24 26 28 SODA AOD constraint CALIOP 5-km column AOD product (f) 0.0001 0.001 0.01 0.1 1 532-nm Particulate Backscatter (/km/sr) 14 16 18 20 22 24 26 28 0 1 2 3 4 5 (d) (b) 30 35 40 45 50 55 60 14 16 18 20 22 24 26 28 Latitude (degrees) Retrieved Sp, Zcal = 2.5 km CALIOP 5-km Column-effective Sp (g) (a) 0.01 0.1 1 10 532-nm Particulate Extinction (/km) 8 10 12 14 16 18 20 22 0 1 2 3 4 5 0.0001 0.001 0.01 0.1 1 532-nm Particulate Backscatter (/km/sr) 8 10 12 14 16 18 20 22 0 1 2 3 4 5 (a) 0 0.5 1 1.5 2 2.5 8 10 12 14 16 18 20 22 MODIS AOD constraint CALIOP 5-km column AOD product (f) 0 10 20 30 40 50 60 70 80 90 8 10 12 14 16 18 20 22 Latitude (degrees) Retrieved Sp, Zcal = 3.5 km CALIOP 5-km Column-effective Sp (g) MODIS AOD (a) (b) (c) 0.0001 0.001 0.01 0.1 532-nm Particulate Backscatter (/km/sr) 8 10 12 14 16 18 20 22 24 26 0 1 2 3 4 5 Altitude (km) (d) 0.01 0.1 1 10 Latitude (degrees) 532-nm Particulate Extinction (/km) 8 10 12 14 16 18 20 22 24 26 0 1 2 3 4 5 Altitude (km) (e) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 8 10 12 14 16 18 20 22 24 26 MODIS AOD constraint CALIOP 5-km column AOD product (f) 0 10 20 30 40 50 60 70 8 10 12 14 16 18 20 22 24 26 Latitude (degrees) Retrieved Sp, Zcal = 2.5 km CALIOP 5-km Column effective Sp (g) (a) (b) (c) 1e-006 1e-005 0.0001 0.001 0.01 0.1 1 532-nm Particulate Backscatter 8 8.5 9 9.5 10 10.5 0 2 4 6 8 10 12 14 (d) 0.0001 0.001 0.01 0.1 1 10 532-nm Particulate Extinction (/km) 8 8.5 9 9.5 10 10.5 0 2 4 6 8 10 12 14 (e) 0 10 20 30 40 50 60 8 8.5 9 9.5 10 10.5 Latitude (degrees) Retrieved Sp, Zcal = 3.5 km CALIOP 5-km Column-effective Sp (g) 0 0.5 1 1.5 2 2.5 3 8 8.5 9 9.5 10 10.5 SODA AOD constraint CALIOP 5-km column AOD product (f)
Transcript of SP P P Latitude (deg)
Lidar Ratios for Dust Aerosols Derived from Retrievals of CALIPSO Visible Extinction
Profiles Constrained by Optical Depths from MODIS-Aqua and CALIPSO/CloudSat Ocean
Surface Reflectance Measurements.
Stuart A. Young, CSIRO Marine and Atmospheric Research, Aspendale, VIC, Australia. ([email protected])
Damien B. Josset, Science Systems and Applications Inc., Hampton, VA, USA. ([email protected])
Mark A. Vaughan, NASA Langley Research Center, Hampton, VA, USA. ([email protected])
IntroductionCALIPSO’s (Cloud Aerosol Lidar Infrared
Pathfinder Satellite Observations) analysis
algorithms generally require the use of tabulated
values of the lidar ratio in order to retrieve aerosol
extinction and optical depth from measured profiles
of attenuated backscatter. However, for any given
time or location, the lidar ratio for a given aerosol
type can differ from the tabulated value. To gain
some insight as to the extent of the variability, we
here calculate the lidar ratio for dust aerosols using
aerosol optical depth constraints from two sources.
Daytime measurements are constrained using Level
2, Collection 5, 550-nm aerosol optical depth
measurements made over the ocean by the MODIS
(Moderate Resolution Imaging Spectroradiometer)
on board the Aqua satellite, which flies in
formation with CALIPSO. We also retrieve lidar
ratios from night-time profiles constrained by
aerosol column optical depths obtained by analysis
of CALIPSO and CloudSat backscatter signals
from the ocean surface [Ref. 1].
TheoryFor correctly calibrated signals, the particulate lidar
ratio, SP, can be related to the particulate optical
depth of a layer, tP, multiple-scattering correction
factor, h, and integral over the depth of the layer of
the attenuated, particulate backscatter, g’P, by the
following equation (adapted from Ref. [2]):
1 exp 2 / 2P P PS ht hg
However, as the layers studied here are in contact
with the surface, it is not possible to evaluate g’Pdirectly from the attenuated backscatter signal and,
therefore, it is also not possible to derive SP
analytically from (1). It must, therefore, be obtained
by an iterative process during the retrieval of the
particulate extinction and backscatter profiles.
(1)
Method1. Select overpass regions where strong dust layers
have advected over ocean,
2. Average CALIPSO Level-1 attenuated
backscatter data to 5-km horizontal resolution,
3. Calculate average MODIS (or ocean surface
reflectance method) aerosol optical depth
(AOD) over the CALIPSO 5-km path,
4. Assume all aerosol resides in surface layer,
5. Assume lidar ratio and h are constant
throughout the layer,
6. Renormalize averaged attenuated backscatter
profiles above the layer top to account for
attenuation by the overlying atmosphere,
7. Retrieve particulate extinction profile though the
layer, adjusting lidar ratio iteratively until the
retrieved optical depth matches that from
MODIS or the surface reflectance
measurements. (i.e. a constrained retrieval).
(c)
References1. Josset, D., J. Pelon, A. Protat, and C. Flamant
(2008), New approach to determine aerosol
optical depth from combined CALIPSO and
CloudSat ocean surface echoes, Geophys. Res.
Lett., 35, L10805,
doi:10.1029/2008GL033442.
2. Fernald, F. G., B. M. Herman and J. A. Reagan
(1972). Determination of Aerosol Height
Distributions by Lidar, J. Appl. Meteor. 11,
482-489.
(b)
(c)
(d)
(e)
ResultsParticulate backscatter (row d), extinction (e) and
lidar ratio (g) retrievals obtained using AOD (f)
constraints from MODIS on January 1st and 3rd
2007 are presented below in columns 1 and 2
respectively. Results obtained using ocean surface
reflectance (SODA) measurements on January 2nd
and 4th 2007 appear in columns 3 and 4. The 532-
nm attenuated backscatter data appear in row (a)
(with CALIPSO’s orbit shown in the inset) and
CALIPSO’s Cloud-Aerosol Vertical Feature Mask
in (b) and Aerosol Type Mask in (c).
The occasional black stripes and missing data from
the lower panels result from AOD constraints that
were unavailable over the land or in cloudy regions.
During the period shown dust was being advected
over the ocean from the African continent.
The mean lidar ratios for January 1st – 3rd are in
the range 35 – 45 sr and compare well with
CALIPSO’s standard value for dust of 40 sr. On
January 4th, the mean value is around 53 sr. The
higher value can be explained by the mixing of
smoke with the dust to create “polluted dust” (see
panel (c) and can be compared with CALIPSO’s
standard value of 55 sr.
0.001 0.01 0.1 1 10
Latitude (deg)
Altitu
de (km
)
532-nm Particulate Extinction (/km)
14 16 18 20 22 24 26 28
0
1
2
3
4
5
(e)
(c)
0 0.2 0.4 0.6 0.8
1 1.2 1.4 1.6 1.8
14 16 18 20 22 24 26 28
SODA AOD constraint
CALIOP 5-km column AOD product(f)
0.0001 0.001 0.01 0.1 1
Latitude (deg)
Altitu
de (km
)
532-nm Particulate Backscatter (/km/sr)
14 16 18 20 22 24 26 28
0
1
2
3
4
5
(d)
(b)
30
35
40
45
50
55
60
14 16 18 20 22 24 26 28
Latitude (degrees)
2007-01-02 02-45-03 Lat 27.998 to 13.515 Lon -16.182 to -19.585
Retrieved Sp, Zcal = 2.5 km
CALIOP 5-km Column-effective Sp(g)
(a)
0.01 0.1 1 10
Latitude (deg)
Altitu
de (km
)
532-nm Particulate Extinction (/km)
8 10 12 14 16 18 20 22
0
1
2
3
4
5
0.0001 0.001 0.01 0.1 1
Latitude (deg)
Altitu
de (k
m)
532-nm Particulate Backscatter (/km/sr)
8 10 12 14 16 18 20 22
0
1
2
3
4
5
(a)
0
0.5
1
1.5
2
2.5
8 10 12 14 16 18 20 22
MODIS AOD constraint
CALIOP 5-km column AOD product(f)
0 10 20 30 40 50 60 70 80 90
8 10 12 14 16 18 20 22
Latitude (degrees)
Retrieved Sp, Zcal = 3.5 km
CALIOP 5-km Column-effective Sp(g)
MODIS AOD
(a)
(b)
(c)
0.0001 0.001 0.01 0.1
Latitude (degrees)
Altitu
de (km
)
532-nm Particulate Backscatter (/km/sr)
8 10 12 14 16 18 20 22 24 26
0
1
2
3
4
5
Alt
itu
de (
km
) (d)
0.01 0.1 1 10
Latitude (degrees)
Altitu
de (km
)
532-nm Particulate Extinction (/km)
8 10 12 14 16 18 20 22 24 26
0
1
2
3
4
5
Alt
itu
de (
km
)
(e)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
8 10 12 14 16 18 20 22 24 26
Aero
so
l O
pti
cal D
ep
th
MODIS AOD constraint
CALIOP 5-km column AOD product(f)
0
10
20
30
40
50
60
70
8 10 12 14 16 18 20 22 24 26
Lid
ar
Rati
o S
p (
sr)
Latitude (degrees)
Retrieved Sp, Zcal = 2.5 km
CALIOP 5-km Column effective Sp
(g)
(a)
(b)
(c)
1e-006 1e-005 0.0001 0.001 0.01 0.1 1
2007-01-04 02-32-44 Lat 10.751 to 7.518 Lon -17.096 to -17.796
Altitu
de (k
m)
532-nm Particulate Backscatter
8 8.5 9 9.5 10 10.5
0
2
4
6
8
10
12
14
(d)
0.0001 0.001 0.01 0.1 1 10
2007-01-04 02-32-44 Lat 10.751 to 7.518 Lon -17.096 to -17.796
Altitu
de (k
m)
532-nm Particulate Extinction (/km)
8 8.5 9 9.5 10 10.5
0
2
4
6
8
10
12
14
(e)
0
10
20
30
40
50
60
8 8.5 9 9.5 10 10.5
Latitude (degrees)
Retrieved Sp, Zcal = 3.5 km
CALIOP 5-km Column-effective Sp
(g)
0
0.5
1
1.5
2
2.5
3
8 8.5 9 9.5 10 10.5
SODA AOD constraint
CALIOP 5-km column AOD product(f)
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