MISR abstract

Diner, D. J., Martonchik, J. V., Kahn, R. A., Crean, K. A., Nelson, D. L., Gaitley, B. J., Smyth, M. M., Pinty, B.; Gobron, N. (2003). Improvement in the MISR aerosol product over land. EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #13213.

The Multi-angle Imaging SpectroRadiometer (MISR) is one of five science instruments aboard the polar-orbiting Terra spacecraft. Among the objectives of the experiment is the global retrieval of amounts and particle properties of tropospheric aerosols. MISR observes the Earth in reflected sunlight, and its typical data collection mode is to observe the Earth globally at nine different view zenith angles, ranging from 70 deg. forward to 70 deg. backward along the spacecraft track, in four spectral bands (446, 558, 672, and 866 nm) with a crosstrack ground spatial resolution of 275 m - 1.1 km. After these 36 channels of imaging data are radiometrically calibrated, georectified, and averaged to a uniform resolution of 1.1 km, they are then automatically analyzed to determine aerosol properties at a resolution of 17.6 km x 17.6 km. Given the surface location of many aerosol sources, mapping of global and regional aerosol amounts and properties over land is of great interest for climate and environmental studies. In particular, the ability to deal with the broad variety of underlying surface conditions is essential to achieving global coverage. At the heart of the retrieval is an algorithm that derives a representation of the angular shape of the surface bidirectional reflectance in terms of empirical orthogonal functions (EOF's). A major attribute of the MISR algorithm is that it requires no assumption about the absolute reflectance of the land surface, and in principle there are no restrictions on the type of surface over which the algorithm can be applied, as long as contrast is present in the scene. In particular, MISR exhibits the ability to retrieve optical depth over bright targets such as deserts, snow-covered regions, and urban areas. In practice, its success in reproducing trends in optical depth is offset by spatial noise in the retrievals, and the presence of sporadic blunders. This algorithmic noise translates into errors in the retrieved surface reflectances, particularly at the extreme off-nadir angles, and is often manifested as anomalous angular shapes of the retrieved surface hemispherical-directional reflectance factor (HDRF), especially in the blue band. This result has been turned to advantage, in that a positive "feedback" between the characteristics of the retrieved angular and spectral surface reflectances provides the means to stabilize the retrievals. This feedback is formulated in terms of empirical constraints on the angular and spectral shape similarity of HDRF among the various instrument channels. Adding such constraints to the existing algorithm results in higher quality of the MISR aerosol and surface products, both in terms of reduced noise and improved correlation of spectral optical depths with independent sunphotometer measurements from AERONET. These algorithm refinements have been implemented in the operational product generation software, and constitute one step toward formal validation. Global and regional product examples will be presented.

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