Multi-angle Imaging SpectroRadiometer

Abstract: The Multi-angle Imaging SpectroRadiometer (MISR) instrument is scheduled for launch aboard the first of the Earth Observing System (EOS) spacecraft, EOS-AM1. MISR will provide global, radiometrically calibrated, georectified, and spatially coregistered imagery at nine discrete viewing angles and four visible/near-infrared spectral bands. Algorithms specifically developed to capitalize on this measurement strategy will be used to retrieve geophysical products for studies of clouds, aerosols, and surface radiation. This paper provides an overview of the as-built instrument characteristics and the application of MISR to remote sensing of the Earth.

Abstract: A new semiempirical model to describe the bidirectional reflectance of arbitrary natural surfaces using only three parameters has been developed. This model successfully accounts for the observed variability of reflectance measurements in laboratory and field conditions, ranging from bare soil to full canopy cover, in both the visible and the near-infrared bands. Coupled with a simple atmospheric radiation transfer model, this model has been inverted against actual NOAA/advanced very high resolution radiometer (AVHRR) data from several desert sites in northern Africa. This procedure allows the retrieval of surface properties and average amounts of atmospheric constituents (aerosol optical thickness and water vapor) for the duration of the measurement period. Further work is required to expand the usability of the coupled model to other locations and shorter periods of time, but the paper demonstrates the feasibility of inverting a coupled surface-atmosphere model against existing AVHRR data and documents the current limits of this approach.

Abstract: Aerosols are believed to play a direct role in the radiation budget of Earth, but their net radiative effect is not well established, particularly on regional scales. Whether aerosols heat or cool a given location depends on their composition and column amount and on the surface albedo, information that is not routinely available, especially over land. Obtaining global information on aerosol and surface radiative characteristics, over both ocean and land, is a task of the Multi-angle Imaging SpectroRadiometer (MISR), an instrument to be launched in 1998 on the Earth Observing System EOS-AM1 platform. Three algorithms are described that will be implemented to retrieve aerosol properties globally using MISR data. Because of the large volume of data to be processed on a daily basis, these algorithms rely on lookup tables of atmospheric radiative parameters and predetermined aerosol mixture models to expedite the radiative transfer (RT) calculations. Over oceans, the "dark water" algorithm is used, taking full advantage of the nature of the MISR data. Over land, a choice of algorithms is made, depending on the surface types within a scene-dark water bodies, heavily vegetated areas, or high-contrast terrain. The retrieval algorithms are tested on simulated MISR data, computed using realistic aerosol and surface reflectance models. The results indicate that aerosol optical depth can be retrieved with an accuracy of 0.05 or 10%, whichever is greater, and some information can be obtained about the aerosol chemical and physical properties.

Abstract: In this paper, Multi-angle Imaging SpectroRadiometer (MISR) aerosol product attributes are described, including geometry and algorithm performance flags. Actual retrieval coverage is mapped and explained in detail using representative global monthly data. Statistical comparisons are made with coincident aerosol optical depth (AOD) and Angstrom exponent (ANG) retrieval results from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. The relationship between these results and the ones previously obtained for MISR and MODIS individually, based on comparisons with coincident ground-truth observations, is established. For the data examined, MISR and MODIS each obtain successful aerosol retrievals about 15% of the time, and coincident MISR-MODIS aerosol retrievals are obtained for about 6%-7% of the total overlap region. Cloud avoidance, glint and oblique-Sun exclusions, and other algorithm physical limitations account for these results. For both MISR and MODIS, successful retrievals are obtained for over 75% of locations where attempts are made. Where coincident AOD retrievals are obtained over ocean, the MISR-MODIS correlation coefficient is about 0.9; over land, the correlation coefficient is about 0.7. Differences are traced to specific known algorithm issues or conditions. Over-ocean ANG comparisons yield a correlation of 0.67, showing consistency in distinguishing aerosol air masses dominated by coarse-mode versus fine-mode particles. Sampling considerations imply that care must be taken when assessing monthly global aerosol direct radiative forcing and AOD trends with these products, but they can be used directly for many other applications, such as regional AOD gradient and aerosol air mass type mapping and aerosol transport model validation. Users are urged to take seriously the published product data-quality statements.