Test of Ground Reflectance Derived Using the AATSR Dual-view Algorithm
Pekka Kolmonen(1), Larisa Sogacheva(2), Anu-Maija Sundström(2) and Gerrit de Leeuw(1,2)
(1) Finnish Meteorological Institute, Erik Palmenin Aukio 1, 00560 Helsinki, Finland
(2) University of Helsinki, Gustaf Hällströmin katu 2, 00014 University of Helsinki, Finland
The dual-view algorithm for the retrieval of optical properties of aerosols utilizing the AATSR/ATSR-2 top-of-atmosphere (TOA) reflectance is described in Veefkind et al. (1998). The AATSR/ATSR-2 instrument has two views: forward and near nadir. The treatment of the ground reflectance is based on the so-called k-assumption where the ratio (k) of the ground reflectance of the two views is independent of the wavelength. Using this assumption, together with an aerosol model which is a mixture of two aerosol types, an iterative method can be used for the computation of aerosol optical depth (AOD) over land. After the AOD is determined the ground reflectance can be computed by subtracting the atmospheric reflectance from the measured TOA reflectance. The atmospheric reflectance is the sum of the aerosol reflectance and the Rayleigh reflectance.
In this study the k-assumption was tested. The hypothesis here states that the ground reflectance should not change during short time periods. In addition, the ground reflectance should not depend on the retrieved AOD. Other relevant test of the k-assumption would be to study the spectral behavior of k. This is to be done in future work.
In the retrievals the aerosol model was a mixture of monomodal sulfate and dust aerosol types. AOD at 555 nm wavelength was retrieved at three dates in July 2005 over Ispra in Italy. Ispra was chosen since an AERONET site is located there. The AERONET level 2.0 data was used to assure the quality of the retrieved AOD. The retrieved AOD and ground reflectance were averaged over a 10x10 km2 area. Surface albedo was computed by taking the average of the forward and nadir ground reflectance. This procedure approximates the integration of the bidirectional reflectance distribution function.
When AOD from the retrievals and from the AERONET data was compared, the values of the retrieved AOD were on average 10 % lower than the values of the reference AOD from AERONET. The determined surface albedo compared favorably with other findings at Ispra (Clerici and Melin, 2008). One of the values of the retrieved AOD was twice higher than the other two. The surface albedo for the highest value of AOD was about 5 % higher than for the lower values of AOD. The values of surface albedo corresponding to the lower values of AOD were 16 days apart. The surface albedo was practically equal for these dates.
Based on the tests it can be concluded that with a constant AOD the ground reflectance remains also constant. When the ground reflectance is computed from two retrievals having different AOD, there may be discrepancies in the ground reflectance. This should, however be confirmed with more extensive testing as the discrepancy was here too small to draw final conclusions. Also, a more careful choosing of the aerosol model can enhance the quality of the retrieved AOD which can lead to changes in the ground reflectance results.
Clerici M. and Melin F. Atmos. Chem. Phys. Discuss, 8, 3227-3285, 2008.
Veefkind J. P. et al., Geophysical Research Letters, 25, 3135-3138, 1998.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,