- Email: [email protected]
Retrieval of spectral aerosol optical thickness from multi-wavelength space-borne sensors
Pergamon www.elsevier.com/locate/asr
Adv. Space Res. Vol. 29, No. 11, pp. 17651770,2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/02 $22.00 + 0.00 PII: SO273-1177(02)00108-4
RETRIEVAL OF SPECTRAL AEROSOL OPTICAL THICKNESS FROM MULTI-WAVELENGTH SPACE-BORNE SENSORS Wolfgang von Hoyningen-Huene,
Martin Freitag, and John P. Burrows
Institute of Environmental Physics, University of Bremen Kufsteiner Str. NWI, PO-Box 330440, D-28334 Bremen [email protected]
ABSTRACT Ground-based sun-photometer and sky-radiometer measurements have been used to derive the main radiative aerosol properties (spectral aerosol optical thickness, phase function and single scattering albedo) during INDOEX. The application of these parameters let determine look-up-tables (LUT) for a retrieval of the spectral aerosol optical thickness from top-of-atmosphere radiances. The application of the LUT gives estimations for the spectral aerosol optical thickness, comparable with ground based data. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
INTRODUCTION The consideration of the influence of atmospheric aerosol on global climate requires the detection of the tropospheric aerosol, its regional and global distribution in concentration and aerosol type as well as the temporal and seasonal variation. This can only be made by space-borne observations. Commonly space-borne retrievals of the aerosol optical thickness are made with one or two channels. These approaches cannot provide a reliable information about the spectral properties of the aerosol optical thickness. Either there is no spectral information available (one channel) or the spectral information is smoothed out by broad channel bands and is masked by gaseous absorbers (AVHRR). However the spectral slope of the aerosol optical thickness is an important tool to discriminate different aerosol types and to get their regional and global distribution. The use of multi-wavelength radiometers with narrow channel bands, as OCTS (ADEOS), SeaWIFS (Orbview2) or MOS (IRS-P3) enable the setup of look-up-tables for a constrained spectral aerosol retrieval. Over clean ocean surfaces with low surface reflectance in the 0.55 (0.412) - 0.87 pm wavelength range estimations of the spectral aerosol optical thickness and the resulting AngstrGm turbidity parameter can be determined unless’ small changes in the water reflectance causing errors in the spectral slope of the retrieved aerosol optical thickness. The determination of temporal and regional variability of the atmospheric aerosol requires space-borne techniques for a derivation of spectral aerosol properties. The Angstram turbidity parameter a (spectral slope) and p (turbidity coefficient) parametrize the spectral behavior of the aerosol optical thickness 6,(h)=P h”and indicate the spectral slope for different aerosol types. Closure experiments, as ACE-2 or INDOEX provide all ground-based data required and enable the test of retrieval algorithms for the determination of aerosol optical thickness from topof-atmosphere radiances.
METHOD In order to derive look-up-tables as relationships between the top-of-atmosphere radiance (TOA-radiance) and the aerosol optical thickness (AOT) radiative transfer calculations for the different spectral channels of the satellite radiometer (SeaWIFS) have to be performed. The calculation of the TOA radiance as a relationship of the AOT require the aerosol phase function and the single scattering albedo and are made for a given surface reflectance. Therefore the main task consists of the estimation of the input parameters from ground-based measurements. Here the CIRATRA approach (Coupled Inversion Radiation Transfer (Wendisch and von Hoyningen-Huene, 1994)) is
1765
1766
W. von Hoyningen-Huene
used, extended for the derivation of look-up-tables presented in the scheme of Figure 1.
et al.
for an aerosol retrieval. The main features of this approach are
Here ground-based measurements of spectral aerosol optical thickness, angular aureole brightness and angular sky radiance are connected by an inversion procedure to retrieve aerosol size distribution, aerosol phase function bwsures
wlrn
bfnn
1nn
nnecting Models .---------------------_7
Results
IO
ide
13
I
;;y$creaureole)
r
R
Implications on Scattering Theories
I /
i
X”‘-“-“-“Gn
RMSDT *--‘--------------y
I i
kc--_-_-__-____-__-------------------I Fig. 1. Overview over the main steps of the CIRATRA procedure. Upper dashed box presents the closures between the ground-based measurements to derive the aerosol parameters to perform radiative calculations for the satellite application, lower dashed box. and subsequent radiative transfer calculation of the angular sky radiance towards the sky radiometer. The refractive index is varied until the minimum RMSD between measured and calculated sky radiance is found. This case gives the refractive index, the columnar optical equivalent size distribution and the aerosol phase function together with a recommended scattering theory to be used. This approach do not require an a-priori aerosol model. Since in the case of INDOEX the RMSD between the calculated and measured sky radiance is found below 0.05, Mie-theory could be used as adequate scattering theory and no scattering theory for non-spherical particles had to be chosen. The main experimental data for INDOEX (in February/March 1999) are obtained from combined ground based sun and skyradiometer data from the AERONET station Kaashidhoo (Maldives, latitude=4.965 N, longitude=73.466 E).Totally cloud free situations are selected for spectral aerosol optical thickness and angular sky
Retrieval of Spectral Aerosol Optical Thickness
1767
radiance. The latter is used to derive the aureole brightness (scattering angles c 10’) too. The surface reflectance for the region of the Maldives is assumed to be clear ocean water. The spectral reflectance given by (Morel and Prieur, 1977) is assumed adding the Fresnel surface reflection of a wavy surface. The TOA-radiances are calculated for the 8 SeaWlFS channels and are compared with the SeaWIFS measurements. In the same way ground-based data from ACE-2 (June 1997, Portugal South-West coast) are used (von Hoyningen-Huene et al., 1998). The TOA radiance is calculated for the 8 channels of the OCTS radiometer on ADEOS, comparable the SeaWlFS wavelength.
RESULTS FROM AERONET SKYRADIOMETER For the spring period of INDOEX (February / March 1999) very consistent results were obtained. The normalized sky brightness functions derived from the angular sky radiances from the AERONET instrument in Kaashidhoo were very similar from day to day, so that one can conclude the aerosol type in the whole period is the same, c.f. Figure 2. For the INDOEX data in February/March 1999 in all cases Mie-theory could be used well to reproduce the measured sky brightness function with RMSD of about 3 %. From the case of the minimal RMSD the real part of the refractive index for the spring period was found with m&i = 1.54 f 0.05 (for LO.87 pm). The found average phase function is shown together with the normalized sky brightness functions in Figure 2. It has an average asymmetry parameter g = 0.60 10.02. If one is trying to fit aerosol models from the OPAC data base (Hess et all., 1998), to this results, the best fit is obtained with the model ‘continental polluted’ with 50 % humidity, however no coincidence is with the ‘marine’ model. This coincides with the phase function presented by (Rajeev et al., 2000), which is close to those presented here, especially within the range of scattering angles 100 - lSO”.The phase function, found by CIRATRA have a slightly increased side scattering, especially for scattering angles above 150“. The use of the normalized sky brightness function (normalized to the total scattering) in the CIRATRA approach removes unfortunately the effect of the single scattering albedo. So this parameter has to be assessed for the calculation of the look-up-tables. A first assumption was to use non-absorbing aerosol, unless that for continental out-flow situations at other places values of the single scattering albedo of 0.9 have been found. To compare satellite radiances of the overflights with calculated radiances assuming non-absorbing aerosol a correction factor of 0.92 had to be introduced, which can correspond to the single scattering albedo. This gives an estimation for the imaginary part k = 0.0077 + 0.0003 at 0.870 ym wavelength with less variation in the period. Taking the model ‘contienental polluted’ the single scattering albedo is 0.8 Il. The spectral slope of the aerosol optical thickness is well represented by a power law and therefore for a characterization the spectral aerosol optical thickness can be parametrized in kind of the Angstrom power law 6,(h)=j3 h”. The Angstrom Fig. 2. Normalized sky brightness functions horn almucantar spectral slope a of the spectral AOT varies measurements of the AERONET station Kaashidhoo (Maldives) between 0.9 and 1.5 with an average of a = 1.31 f and resulting average aerosol phase function for the 0.05. Such results seems to be characteristic for wavelength h = 0.87 urn. continental out-flow aerosol. p - the Angstrom turbidity parameter is temporally varying between 0.11 and 0.23 (for cloud free sections).
1768
W. van Hoyningen-Huene et al.
LUT -APPROACH AND COMPARISON Using the ground-based data look-up-table (LUT) for the variation of the TOA-radiance (normalized radiance: L,,, = L./(x E, cos z,)) due to the aerosol optical thickness in the 8 channels of SeaWIFS can be calculated using a clean water spectrum (Figure 3). This LUT enables an estimation of the spectral behavior of the aerosol optical thickness from the measured TOA radiance by Sea?WFS for this aerosol type of INDOEX represented by the ground based measurements, if cloud free conditions and a clear water surface exists. The LUT has a nearly linear relationship and Channel: 8 7 6 2 1 5 43 the crossing with the x-axis is the radiance for an 0.50 aerosol free atmosphere (Rayleigh scattering and 1 surface reflectance). Variations in the surface reflectance correspond with a shift of the curves i along the x-axis and can lead to under- or over 0.40 -1 estimations of the aerosol optical thickness. The application of these preliminary LUT enables an estimation of the spectral aerosol 0.30 optical thickness and its comparison with the ground-based data. Figure 4 presents as example retrieved AOT from TOA-radiances of several 0.20 SeaWIFS pixels around Kaashidhoo and the 1 position of the research vessel ‘Ron Brown’ for the overflight of 26. March 1999. The ground0.10 based measurements for Kaashidhoo are from the CIMEL instrument within the AERONET and on the RV ‘Ron Brown’ the data are obtained by a I Microtops sunphotometer and give the aerosol 0.00 I,,,, optical thickness before and after the over-flight. 0.00 0.05 0.10 0.15 0.20 0.25 In the short wave channels (than. 1 - 5) the norm Radiance thickness is retrieved aerosol optical Fig. 3. Look-up-table for INDOEX for a relationship between systematically lower than the ground based TOA-radiance and aerosol optical thickness for all 8 channels of This might be caused by measurements. SeaWIFS (Channels: 1 - 0.412 urn, 2 - 0.443 urn, 3 - 0.490 urn, unconsidered water contents compared with the 4 - 0.510 urn, 5 - 0.555 urn, 6 - 0.670 urn, 7 - 0.765 urn, 8 clear water spectrum used. The channels 6 - 8 are 0.865 urn).
5
1
,,,I
,I/,
I,,,
,,,,
INDOEX,
0
CIMEL Alpha Beta 0.10
1
4.966N
= 1.297 = 0.176 1
CIMEL
06:58
CIMEL
07:43
SeaWlFS Microlops
05~53
-
Microtops
07:OO
Ship (4.13N,
I
1
73.65E) SeaWlFS Alpha = 0.94 Beta = 0.209
MICROT Alpha = 1.18 Beta = 0.212
5
UTC
(Ship)
-
73.645E
Wavelength
06:58
(Kaash)
---
SeaWlFS Alpha = 0.97 Beta = 0.185 I
26.03.99,
---
A
1 Kaashidhoo
85,
SeaWlFS
I
I 1 .oo
I pm
Fig. 4. Spectral aerosol optical thickness: AERONET/CIMEL measurements (dashed lines), Microtops sunphotometer on board of the RV ‘Ron Brown’ (solid lines) and retrievals from SeaWIFS TOA-radiance for selected cloud free pixels around Kaashidhoo and the ship position (markers) for the over-flight of 26. March optical thickness). The different lines indicate the aerosol optical thickness before and after the over-flight.
Retrieval of Spectral Aerosol Optical Thickness
1769
in a sufficient agreement with the ground-based data. So the spectral slope determined by the SeaWIFS radiances with these LUT’s will be underestimated while the Angstrom turbidity coefficient is in a good agreement.
CONCLUSIONS Summarizing results from several overflights from INDOEX and ACE-2, made with the same method one find the retrieval can provide the spectral aerosol characteristics (spectral slope and turbidity coefficient) within some limits. Fig. 5 summarizes the results for all overflights. Tab. 1 gives the coincidence of the ground point with retrieved data from the satellite overflights.
&A(h) = p 1
-a
l.0
#y@
l,,’
“‘.
, q/y’ /’
9
4
*I’
I 0.00
INDOEX
#’
/'
,/’
=
0.83558
I
I
ACE-2 OCTS
,/‘:
ACE-2
Beta-Ground
0.00
0 I’ INDOEX 1999 SeaW IFS
0.10
’ Beta-Sat + 0.0218 I I I 0.20
0.30
Bela-Sat
0.00
1997
Alpha-Ground = 0,9653'Alpha_.Sat+ , , , , , , , ( , , , ,
I,,
0.00
0.50
1.00
0.1322 ,
,
1.50
Alpha_SeaWIFS
Fig. 5. Comparison of Angstrrjm turbidity parameters, spectral slope a and turbidity coefficient 0 for retrievals from TOA radiance from overpasses during INDOEX (SeaWIFS) and ACE-2 (OCTS) and ground-based measurements.
The largest deviation in the spectral slope cc is at the 10.03.99, where with the average parameters from Kaashidhoo a retrieval is made for the position of the RV ‘Ron Brown’ and a is overestimated by the satellite Date retrieval. The larger scattering for a is an expression for INDOEX 1Kaashidhoo 1Ron Brown unconsidered changes in the state of the ocean color and surface wind conditions. For the turbidity coefficient B a quite good correlation is obtained. Unless this reliable agreement between the retrieved and ground-based parameters there were different conditions between ACE-2 and INDOEX: INDOEX was characterized by a more turbid situation with continental outflow from the Indian sub-continent, which can be seen in the wind fields, presented by (Rajeev et al., 2000). In ACE-2 very clean conditions with marine aerosol have been typical for the overflights. In INDOEX the phase function could be OCTS obtained using Mie-theory and spherical particles, and X X 14.06.97 the model ‘continental polluted’ could fit the observed X 20.06.97 X data. However during ACE-2 a strong increased lateral scattering required the application of non-spherical particle scattering by the semi-empirical scattering theory of (Pollack and Cuzzi, 1980) and no standard aerosol model could be found to fit the data. In both cases the aerosol parameters used for the radiative transfer calculations have been derived from closure considerations Table 1. Overflights and available ground position for the comparisons in Fig.5.
1770
W. von Hoyningen-Huene
et al.
between different ground-based measurements, characterizing the ambient optical state. Look-up-tables aerosol properties enable to retrieve the spectral aerosol behavior under comparable conditions.
using such
ACKNOWLEDGEMENT The authors like to express their gratitude to Brent. N. Holben for the possibility to use the AERONET data for the used SeaWIFS overflights. The SeaWIFS Project (Code 970.2).and the DAAC (Code 902) at Goddard Space Flight Center is thanked for the support with the SeaWIFS data and Piotr Flatau and Krzysztof Markowicz for the Microtops radiometer measurements on board of the RV ‘Ron Brown’.
REFERENCES Hess, M., P. Koepke, and I. Schult, Optical properties of aerosols and clouds: The software package OPAC, Bull. Am. Met. Sot., 79, 831-844, 1998. Holben, B.N., T.F. Eck, D. Tanre, J.P. Buis, A. Setzer, E. Vermonte, J.A. Reagan, Y.J. Kaufman, T. Nakajima, F. Lavenu, I. Jankowiak, and A. Smimow, AERONET - a fedral instrument network and data archive for aerosol characterization, (From: http//spamer.gsfc.nasa.gov/valdesaire/valdesaire980206.html), 1998. Morel, A., and L. Prieur, Analysis of variation in ocean color, Limnology and Oceanography, 22,709-722, 1977. Pollack, J.B., and J.N. Cuzzi, Scattering by nonspherical particles of size comparable to the wavelength: a semiempirical theory and its application to tropospheric aerosols, J. Atmos. Sci., 37, 868-88 1, 1980. Fbjeev, K., V. Ramanathan, and J. Meywerk, Regional aerosol distribution and its long-range transport over the Indian Ocean, J. Geophys. Res., 105,2029-2043,200O. von Hoyningen-Huene, W., T. Schmidt, A. Herber, and A.M. Silva, Climete-relevant Columnar Aerosol Parameters Obtained During ACE-2 CLEARCOLUMN and Their Influence on the Shortwave Radiative Balance, J. Aeros. Sci., 29 (Sl), S269-S270, 1998. Wendisch, M., and W. von Hoyningen-Huene, Possibility of refractive index determination of atmospheric aerosol particles by ground based solar extinction and scattering measurements, Atmos. Environment, 28,785-795, 1994.
Adv. Space Res. Vol. 29, No. 11, pp. 17651770,2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/02 $22.00 + 0.00 PII: SO273-1177(02)00108-4
RETRIEVAL OF SPECTRAL AEROSOL OPTICAL THICKNESS FROM MULTI-WAVELENGTH SPACE-BORNE SENSORS Wolfgang von Hoyningen-Huene,
Martin Freitag, and John P. Burrows
Institute of Environmental Physics, University of Bremen Kufsteiner Str. NWI, PO-Box 330440, D-28334 Bremen [email protected]
ABSTRACT Ground-based sun-photometer and sky-radiometer measurements have been used to derive the main radiative aerosol properties (spectral aerosol optical thickness, phase function and single scattering albedo) during INDOEX. The application of these parameters let determine look-up-tables (LUT) for a retrieval of the spectral aerosol optical thickness from top-of-atmosphere radiances. The application of the LUT gives estimations for the spectral aerosol optical thickness, comparable with ground based data. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
INTRODUCTION The consideration of the influence of atmospheric aerosol on global climate requires the detection of the tropospheric aerosol, its regional and global distribution in concentration and aerosol type as well as the temporal and seasonal variation. This can only be made by space-borne observations. Commonly space-borne retrievals of the aerosol optical thickness are made with one or two channels. These approaches cannot provide a reliable information about the spectral properties of the aerosol optical thickness. Either there is no spectral information available (one channel) or the spectral information is smoothed out by broad channel bands and is masked by gaseous absorbers (AVHRR). However the spectral slope of the aerosol optical thickness is an important tool to discriminate different aerosol types and to get their regional and global distribution. The use of multi-wavelength radiometers with narrow channel bands, as OCTS (ADEOS), SeaWIFS (Orbview2) or MOS (IRS-P3) enable the setup of look-up-tables for a constrained spectral aerosol retrieval. Over clean ocean surfaces with low surface reflectance in the 0.55 (0.412) - 0.87 pm wavelength range estimations of the spectral aerosol optical thickness and the resulting AngstrGm turbidity parameter can be determined unless’ small changes in the water reflectance causing errors in the spectral slope of the retrieved aerosol optical thickness. The determination of temporal and regional variability of the atmospheric aerosol requires space-borne techniques for a derivation of spectral aerosol properties. The Angstram turbidity parameter a (spectral slope) and p (turbidity coefficient) parametrize the spectral behavior of the aerosol optical thickness 6,(h)=P h”and indicate the spectral slope for different aerosol types. Closure experiments, as ACE-2 or INDOEX provide all ground-based data required and enable the test of retrieval algorithms for the determination of aerosol optical thickness from topof-atmosphere radiances.
METHOD In order to derive look-up-tables as relationships between the top-of-atmosphere radiance (TOA-radiance) and the aerosol optical thickness (AOT) radiative transfer calculations for the different spectral channels of the satellite radiometer (SeaWIFS) have to be performed. The calculation of the TOA radiance as a relationship of the AOT require the aerosol phase function and the single scattering albedo and are made for a given surface reflectance. Therefore the main task consists of the estimation of the input parameters from ground-based measurements. Here the CIRATRA approach (Coupled Inversion Radiation Transfer (Wendisch and von Hoyningen-Huene, 1994)) is
1765
1766
W. von Hoyningen-Huene
used, extended for the derivation of look-up-tables presented in the scheme of Figure 1.
et al.
for an aerosol retrieval. The main features of this approach are
Here ground-based measurements of spectral aerosol optical thickness, angular aureole brightness and angular sky radiance are connected by an inversion procedure to retrieve aerosol size distribution, aerosol phase function bwsures
wlrn
bfnn
1nn
nnecting Models .---------------------_7
Results
IO
ide
13
I
;;y$creaureole)
r
R
Implications on Scattering Theories
I /
i
X”‘-“-“-“Gn
RMSDT *--‘--------------y
I i
kc--_-_-__-____-__-------------------I Fig. 1. Overview over the main steps of the CIRATRA procedure. Upper dashed box presents the closures between the ground-based measurements to derive the aerosol parameters to perform radiative calculations for the satellite application, lower dashed box. and subsequent radiative transfer calculation of the angular sky radiance towards the sky radiometer. The refractive index is varied until the minimum RMSD between measured and calculated sky radiance is found. This case gives the refractive index, the columnar optical equivalent size distribution and the aerosol phase function together with a recommended scattering theory to be used. This approach do not require an a-priori aerosol model. Since in the case of INDOEX the RMSD between the calculated and measured sky radiance is found below 0.05, Mie-theory could be used as adequate scattering theory and no scattering theory for non-spherical particles had to be chosen. The main experimental data for INDOEX (in February/March 1999) are obtained from combined ground based sun and skyradiometer data from the AERONET station Kaashidhoo (Maldives, latitude=4.965 N, longitude=73.466 E).Totally cloud free situations are selected for spectral aerosol optical thickness and angular sky
Retrieval of Spectral Aerosol Optical Thickness
1767
radiance. The latter is used to derive the aureole brightness (scattering angles c 10’) too. The surface reflectance for the region of the Maldives is assumed to be clear ocean water. The spectral reflectance given by (Morel and Prieur, 1977) is assumed adding the Fresnel surface reflection of a wavy surface. The TOA-radiances are calculated for the 8 SeaWlFS channels and are compared with the SeaWIFS measurements. In the same way ground-based data from ACE-2 (June 1997, Portugal South-West coast) are used (von Hoyningen-Huene et al., 1998). The TOA radiance is calculated for the 8 channels of the OCTS radiometer on ADEOS, comparable the SeaWlFS wavelength.
RESULTS FROM AERONET SKYRADIOMETER For the spring period of INDOEX (February / March 1999) very consistent results were obtained. The normalized sky brightness functions derived from the angular sky radiances from the AERONET instrument in Kaashidhoo were very similar from day to day, so that one can conclude the aerosol type in the whole period is the same, c.f. Figure 2. For the INDOEX data in February/March 1999 in all cases Mie-theory could be used well to reproduce the measured sky brightness function with RMSD of about 3 %. From the case of the minimal RMSD the real part of the refractive index for the spring period was found with m&i = 1.54 f 0.05 (for LO.87 pm). The found average phase function is shown together with the normalized sky brightness functions in Figure 2. It has an average asymmetry parameter g = 0.60 10.02. If one is trying to fit aerosol models from the OPAC data base (Hess et all., 1998), to this results, the best fit is obtained with the model ‘continental polluted’ with 50 % humidity, however no coincidence is with the ‘marine’ model. This coincides with the phase function presented by (Rajeev et al., 2000), which is close to those presented here, especially within the range of scattering angles 100 - lSO”.The phase function, found by CIRATRA have a slightly increased side scattering, especially for scattering angles above 150“. The use of the normalized sky brightness function (normalized to the total scattering) in the CIRATRA approach removes unfortunately the effect of the single scattering albedo. So this parameter has to be assessed for the calculation of the look-up-tables. A first assumption was to use non-absorbing aerosol, unless that for continental out-flow situations at other places values of the single scattering albedo of 0.9 have been found. To compare satellite radiances of the overflights with calculated radiances assuming non-absorbing aerosol a correction factor of 0.92 had to be introduced, which can correspond to the single scattering albedo. This gives an estimation for the imaginary part k = 0.0077 + 0.0003 at 0.870 ym wavelength with less variation in the period. Taking the model ‘contienental polluted’ the single scattering albedo is 0.8 Il. The spectral slope of the aerosol optical thickness is well represented by a power law and therefore for a characterization the spectral aerosol optical thickness can be parametrized in kind of the Angstrom power law 6,(h)=j3 h”. The Angstrom Fig. 2. Normalized sky brightness functions horn almucantar spectral slope a of the spectral AOT varies measurements of the AERONET station Kaashidhoo (Maldives) between 0.9 and 1.5 with an average of a = 1.31 f and resulting average aerosol phase function for the 0.05. Such results seems to be characteristic for wavelength h = 0.87 urn. continental out-flow aerosol. p - the Angstrom turbidity parameter is temporally varying between 0.11 and 0.23 (for cloud free sections).
1768
W. van Hoyningen-Huene et al.
LUT -APPROACH AND COMPARISON Using the ground-based data look-up-table (LUT) for the variation of the TOA-radiance (normalized radiance: L,,, = L./(x E, cos z,)) due to the aerosol optical thickness in the 8 channels of SeaWIFS can be calculated using a clean water spectrum (Figure 3). This LUT enables an estimation of the spectral behavior of the aerosol optical thickness from the measured TOA radiance by Sea?WFS for this aerosol type of INDOEX represented by the ground based measurements, if cloud free conditions and a clear water surface exists. The LUT has a nearly linear relationship and Channel: 8 7 6 2 1 5 43 the crossing with the x-axis is the radiance for an 0.50 aerosol free atmosphere (Rayleigh scattering and 1 surface reflectance). Variations in the surface reflectance correspond with a shift of the curves i along the x-axis and can lead to under- or over 0.40 -1 estimations of the aerosol optical thickness. The application of these preliminary LUT enables an estimation of the spectral aerosol 0.30 optical thickness and its comparison with the ground-based data. Figure 4 presents as example retrieved AOT from TOA-radiances of several 0.20 SeaWIFS pixels around Kaashidhoo and the 1 position of the research vessel ‘Ron Brown’ for the overflight of 26. March 1999. The ground0.10 based measurements for Kaashidhoo are from the CIMEL instrument within the AERONET and on the RV ‘Ron Brown’ the data are obtained by a I Microtops sunphotometer and give the aerosol 0.00 I,,,, optical thickness before and after the over-flight. 0.00 0.05 0.10 0.15 0.20 0.25 In the short wave channels (than. 1 - 5) the norm Radiance thickness is retrieved aerosol optical Fig. 3. Look-up-table for INDOEX for a relationship between systematically lower than the ground based TOA-radiance and aerosol optical thickness for all 8 channels of This might be caused by measurements. SeaWIFS (Channels: 1 - 0.412 urn, 2 - 0.443 urn, 3 - 0.490 urn, unconsidered water contents compared with the 4 - 0.510 urn, 5 - 0.555 urn, 6 - 0.670 urn, 7 - 0.765 urn, 8 clear water spectrum used. The channels 6 - 8 are 0.865 urn).
5
1
,,,I
,I/,
I,,,
,,,,
INDOEX,
0
CIMEL Alpha Beta 0.10
1
4.966N
= 1.297 = 0.176 1
CIMEL
06:58
CIMEL
07:43
SeaWlFS Microlops
05~53
-
Microtops
07:OO
Ship (4.13N,
I
1
73.65E) SeaWlFS Alpha = 0.94 Beta = 0.209
MICROT Alpha = 1.18 Beta = 0.212
5
UTC
(Ship)
-
73.645E
Wavelength
06:58
(Kaash)
---
SeaWlFS Alpha = 0.97 Beta = 0.185 I
26.03.99,
---
A
1 Kaashidhoo
85,
SeaWlFS
I
I 1 .oo
I pm
Fig. 4. Spectral aerosol optical thickness: AERONET/CIMEL measurements (dashed lines), Microtops sunphotometer on board of the RV ‘Ron Brown’ (solid lines) and retrievals from SeaWIFS TOA-radiance for selected cloud free pixels around Kaashidhoo and the ship position (markers) for the over-flight of 26. March optical thickness). The different lines indicate the aerosol optical thickness before and after the over-flight.
Retrieval of Spectral Aerosol Optical Thickness
1769
in a sufficient agreement with the ground-based data. So the spectral slope determined by the SeaWIFS radiances with these LUT’s will be underestimated while the Angstrom turbidity coefficient is in a good agreement.
CONCLUSIONS Summarizing results from several overflights from INDOEX and ACE-2, made with the same method one find the retrieval can provide the spectral aerosol characteristics (spectral slope and turbidity coefficient) within some limits. Fig. 5 summarizes the results for all overflights. Tab. 1 gives the coincidence of the ground point with retrieved data from the satellite overflights.
&A(h) = p 1
-a
l.0
#y@
l,,’
“‘.
, q/y’ /’
9
4
*I’
I 0.00
INDOEX
#’
/'
,/’
=
0.83558
I
I
ACE-2 OCTS
,/‘:
ACE-2
Beta-Ground
0.00
0 I’ INDOEX 1999 SeaW IFS
0.10
’ Beta-Sat + 0.0218 I I I 0.20
0.30
Bela-Sat
0.00
1997
Alpha-Ground = 0,9653'Alpha_.Sat+ , , , , , , , ( , , , ,
I,,
0.00
0.50
1.00
0.1322 ,
,
1.50
Alpha_SeaWIFS
Fig. 5. Comparison of Angstrrjm turbidity parameters, spectral slope a and turbidity coefficient 0 for retrievals from TOA radiance from overpasses during INDOEX (SeaWIFS) and ACE-2 (OCTS) and ground-based measurements.
The largest deviation in the spectral slope cc is at the 10.03.99, where with the average parameters from Kaashidhoo a retrieval is made for the position of the RV ‘Ron Brown’ and a is overestimated by the satellite Date retrieval. The larger scattering for a is an expression for INDOEX 1Kaashidhoo 1Ron Brown unconsidered changes in the state of the ocean color and surface wind conditions. For the turbidity coefficient B a quite good correlation is obtained. Unless this reliable agreement between the retrieved and ground-based parameters there were different conditions between ACE-2 and INDOEX: INDOEX was characterized by a more turbid situation with continental outflow from the Indian sub-continent, which can be seen in the wind fields, presented by (Rajeev et al., 2000). In ACE-2 very clean conditions with marine aerosol have been typical for the overflights. In INDOEX the phase function could be OCTS obtained using Mie-theory and spherical particles, and X X 14.06.97 the model ‘continental polluted’ could fit the observed X 20.06.97 X data. However during ACE-2 a strong increased lateral scattering required the application of non-spherical particle scattering by the semi-empirical scattering theory of (Pollack and Cuzzi, 1980) and no standard aerosol model could be found to fit the data. In both cases the aerosol parameters used for the radiative transfer calculations have been derived from closure considerations Table 1. Overflights and available ground position for the comparisons in Fig.5.
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between different ground-based measurements, characterizing the ambient optical state. Look-up-tables aerosol properties enable to retrieve the spectral aerosol behavior under comparable conditions.
using such
ACKNOWLEDGEMENT The authors like to express their gratitude to Brent. N. Holben for the possibility to use the AERONET data for the used SeaWIFS overflights. The SeaWIFS Project (Code 970.2).and the DAAC (Code 902) at Goddard Space Flight Center is thanked for the support with the SeaWIFS data and Piotr Flatau and Krzysztof Markowicz for the Microtops radiometer measurements on board of the RV ‘Ron Brown’.
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