Akihiro Yamazaki

Multiple scattering analysis of airborne POLDER image data over the sea

The authors have made a multiple scattering analysis of the airborne polarization and directionality of Earth reflectances (POLDER) data over the Mediterranean Sea, which was obtained in the Medimar campaign in 1991. Assuming an atmosphere-ocean system with an anisotropic Cox-Munk sea surface, the radiance and linear polarization in the upwelling radiation have been numerically evaluated for several typical aerosol models by using the adding and doubling method, in which multiple scatterings within the atmosphere and, multiple reflections at the atmosphere-ocean interface are fully taken into account. Major results based on this study are summarized as follows, (1) For the first time, the Medimar airborne POLDER data was analyzed from the linear polarization point of view by this study. Several typical aerosol models, namely, the Haze M, water-soluble, oceanic, maritime, continental and Junge-type models were fully tested against both the observed reflectance and linear polarization data at 0.85 /spl mu/m in the principal plane, as well as in the image form with various viewing angle information. (2) By examining various combinations of aerosols complex refractive indices and wind speeds, the authors found that only the water-soluble model and Junge-type models with /spl nu/=3.5, 4.0, and 4.5 can satisfy the observed reflectance data, but they also found that these models, satisfying the observed reflectance constraints, fail to satisfy the observed linear polarization features in the backward scattering directions. (3) Any aerosol models deduced from the reflectance data analysis should be tested against the polarization data. This study was the first such attempt and all the major aerosol models with a single-peaked size distribution function were rejected, as an appropriate model over the Mediterranean Sea at the Medimar experiment time because they all failed to satisfy the observed polarization data. (4) More studies are certainly needed to find an appropriate aerosol model which can satisfy both the observed reflectance and polarization features. Other types of aerosol models with a multipeaked size distribution function should be fully examined in the future.

The estimation of aerosol optical parameters from ADEOS/POLDER data

A cross calibration analysis between POLDER and OCTS sensors on board ADEOS satellite in terms of the space reflectance was made. Space reflectance values, computed from the POLDER and OCTS data (acquired simultaneously on 26 April 1997) for the same ocean target, were examined and we found that they are in good agreement in all bands, when the new in-flight calibration gain factors for each sensor are assumed. Furthermore, we retrieved aerosol optical parameters (i.e., the optical thickness, Angstrom exponent, and refractive index) by using several retrieving algorithms from using ADEOS/POLDERs directional space reflectance and polarization data in infrared bands simultaneously. Retrieved results were presented by the form of distribution maps. In addition, good and bad points of each retrieval algorithm were discussed. Finally, we proposed an improved method for retrieving aerosol optical parameters, which utilizes theoretical P-SR (polarization-space reflectance) diagrams at given viewing angles in an 865 nm band. The estimated aerosol optical parameters by our proposed method were compared with the simultaneously measured sky observation data. We found an excellent agreement between them.

Retrieval of aerosol optical properties over the ocean using multispectral polarization measurements from space

The optical thickness and Angstrom coefficient of the aerosols over the ocean near Japan were retrieved from polarization measurements in the visible and near-infrared wavelength regions by the Polarization and Directionality of the Earths Reflectances (POLDER) on board the Advanced Earth Observing Satellite (ADEOS) satellite. The optical thickness over the open sea in the Pacific Ocean was small (<0.2) whereas that over the Sea of Japan and near the Japan islands was large on 26 April 1997. The Angstrom coefficient also exhibited a large difference between over the open sea area and near the land. These results suggest that degree of polarization measurements are useful for retrieving aerosol optical properties over the ocean from space.

Retrieval algorithm for aerosol optical parameters from POLDER's directional reflectance and polarization data

We examined three different methods, namely, Method-I, -II, and -III for retrieving local aerosols optical parameters over the Japan Sea using the ADEOS/POLDER data by comparing with the validation data. Method-I, and Method-II use parameterized directional reflectance and polarization diagrams in two infrared bands, respectively. On the other hand, Method-III uses parameterized directional polarization- reflectance diagrams in a single infrared band. We found that Method-III gives the best agreement with the measured sky validation data. We also presented retrieved distribution maps for the aerosol optical thickness, and Angstrom exponent by Method-III and some discussions on further improvement for Method-III were given.

The estimation of aerosol models by the reflectance analysis of the airborne POLDER data

The authors have made the theoretical analysis of the reflectance data measured by the airborne POLDER sensor over the Mediterranean Sea in 1991. Several basic aerosol size distribution models, namely, a lognormal size distribution function, a modified gamma size distribution function, and a power law size distribution function by Junge were considered under the assumption of an atmosphere-ocean system with an anisotropic Cos-Munk reflecting sea surface model. The authors present several aerosol models satisfied observed reflectance data when appropriate wind speed is assumed.

An atmospheric correction algorithm for space remote sensing data and its validation

In atmospheric correction for space remote sensing images, the atmospheric optical thickness at the time of satellite observation is critically needed and it is highly desirable to estimate this value from the satellite measured data itself. The authors present an algorithm for estimating the meteorological range in the boundary layer from the measured satellite data. From the meteorological range value the atmospheric optical thickness can be found without difficulty. In this atmospheric correction algorithm the authors consider a one-dimensional vertically inhomogeneous radiative transfer model consisting of an atmosphere bounded by a flat Lambertian surface layer with a horizontally non-uniform albedo distribution. For the validation of atmospheric correction algorithm, they have made the simultaneous ground measurements of the surface reflectance and atmospheric optical thickness with the LANDSAT TM on Dec.12, 1996. They found a good agreement between the measured and estimated albedo values in TM band 3 and 4, but it is acceptable in band 2.

The estimation algorithm of aerosol optical thickness from space remote sensing data

In the atmospheric correction for space remote sensing images, the aerosol optical thickness at the satellite observation time is critically needed. It is highly desirable to estimate this value from the remote sensing data. The authors proposed a new algorithm for estimating the aerosol optical thickness from satellite image data alone. As for the algorithm validation, several simultaneous ground and sky measurements with LANDSAT/TM and ADEOS/AVNIR were conducted in 1996 and 1997. The authors found that the estimation error is less than 0.04 in the aerosol optical thickness by the proposed algorithm.

The analysis of the POLDER data

The ADvanced Earth Observing Satellite (ADEOS) was successfully launched by the National Space Development Agency of Japan (NASDA), last summer. The POLDER is one of eight sensors on board of the ADEOS and it has an ability to measure both the directional reflectance and polarization from the Earth atmosphere-surface system. Since the ADEOS/POLDER data sets are not yet available, the analysis results using the airborne POLDER data sets are given in this article. The authors have made analyses on both the Medimar campaign data over the Mediterranean Sea in 1991 and the La Crau campaign data over the land in the southeast of France in 1990.