John V. Martonchik

Multi-angle Imaging SpectroRadiometer (MISR) instrument description and experiment overview

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.

Synergistic algorithm for estimating vegetation canopy leaf area index and fraction of absorbed photosynthetically active radiation from MODIS and MISR data

A synergistic algorithm for producing global leaf area index and fraction of absorbed photosynthetically active radiation fields from canopy reflectance data measured by MODIS (moderate resolution imaging spectroradiometer) and MISR (multiangle imaging spectroradiometer) instruments aboard the EOS-AM 1 platform is described here. The proposed algorithm is based on a three-dimensional formulation of the radiative transfer process in vegetation canopies. It allows the use of information provided by MODIS (single angle and up to 7 shortwave spectral bands) and MISR (nine angles and four shortwave spectral bands) instruments within one algorithm. By accounting features specific to the problem of radiative transfer in plant canopies, powerful techniques developed in reactor theory and atmospheric physics are adapted to split a complicated three-dimensional radiative transfer problem into two independent, simpler subproblems, the solutions of which are stored in the form of a look-up table. The theoretical background required for the design of the synergistic algorithm is discussed. Large-scale ecosystem modeling is used to simulate a range of ecological responses to changes in climate and chemical composition of the atmosphere, including changes in the distribution of terrestrial plant communities across the globe in response to climate changes. Leaf area index (LAI) is a state parameter in all models describing the exchange of fluxes of energy, mass (e.g., water and CO 2), and momentum between the surface and the planetary boundary layer. Analyses of global carbon budget indicate a large terrestrial middle- to high-latitude sink, without which the accumulation of carbon in the atmosphere would be higher than the present rate. The problem of accurately evaluating the exchange of carbon between the atmosphere and the terrestrial vegetation therefore requires special attention. In this context the fraction of photosynthetically active radiation (FPAR) absorbed by global vegetation is a key state variable in most ecosystem productivity models and in global models of climate, hydrology, biogeochemestry, and ecology (Sellers et al., 1997). Therefore these variables that describe vegetation canopy structure and its energy absorption capacity are required by many of the EOS Interdisciplinary Projects (Myneni et al., 1997a). In order to quantitatively and accurately model global dynamics of these processes, differentiate short-term from long-term trends, as well as to distinguish regional from global phenomena, these two

Multiangle Imaging Spectroradiometer (MISR) global aerosol optical depth validation based on 2 years of coincident Aerosol Robotic Network (AERONET) observations

[1] Performance of the Multiangle Imaging Spectroradiometer (MISR) early postlaunch aerosol optical thickness (AOT) retrieval algorithm is assessed quantitatively over land and ocean by comparison with a 2-year measurement record of globally distributed AERONET Sun photometers. There are sufficient coincident observations to stratify the data set by season and expected aerosol type. In addition to reporting uncertainty envelopes, we identify trends and outliers, and investigate their likely causes, with the aim of refining algorithm performance. Overall, about 2/3 of the MISR-retrieved AOT values fall within [0.05 or 20% x AOT] of Aerosol Robotic Network (AERONET). More than a third are within [0.03 or 10% x AOT]. Correlation coefficients are highest for maritime stations (∼0.9), and lowest for dusty sites (more than ∼0.7). Retrieved spectral slopes closely match Sun photometer values for Biomass burning and continental aerosol types. Detailed comparisons suggest that adding to the algorithm climatology more absorbing spherical particles, more realistic dust analogs, and a richer selection of multimodal aerosol mixtures would reduce the remaining discrepancies for MISR retrievals over land; in addition, refining instrument low-light-level calibration could reduce or eliminate a small but systematic offset in maritime AOT values. On the basis of cases for which current particle models are representative, a second-generation MISR aerosol retrieval algorithm incorporating these improvements could provide AOT accuracy unprecedented for a spaceborne technique.

Estimation of vegetation canopy leaf area index and fraction of absorbed photosynthetically active radiation from atmosphere‐corrected MISR data

The multiangle imaging spectroradiometer (MISR) instrument is designed to provide global imagery at nine discrete viewing angles and four visible/near-infrared spectral bands. This paper describes an algorithm for the retrieval of leaf area index (LAI) and fraction of photosynthetically active radiation absorbed by vegetation (FPAR) from atmospherically corrected MISR data. The proposed algorithm is designed to utilize all the information provided by this instrument, using a two-step process. The first step involves a comparison of the retrieved spectral hemispherically integrated reflectances with those determined from the model which depend on biome type, canopy structure, and soil/understory reflectances. The biome/canopy/soil/understory models that pass this comparison test are subject to the second step, which is a comparison of their directional reflectances at the MISR angles to the retrieved spectral directional reflectances. This procedure, however, can produce multiple acceptable solutions. The measure theory is used to specify the most probable values of LAI and FPAR using the set of all acceptable solutions. Optimization of the retrieval technique for efficient global processing is discussed. This paper is the second of a two-part set describing a synergistic algorithm for producing global LAI and FPAR fields from canopy reflectance data provided by the MODIS (moderate resolution imaging spectroradiometer) and MISR instruments.

MISR Aerosol Product Attributes and Statistical Comparisons With MODIS

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.

Regional aerosol retrieval results from MISR

Examples of aerosol retrieval results, derived from the Multi-angle Imaging SpectroRadiometer (MISR) on the Earth Observation Science (EOS) Terra platform, are shown and the performance of the retrieval algorithms are discussed, following the first 18 months of operational data processing. A number of algorithm modifications were implemented, based on an analysis of aerosol retrieval results during this period, and these changes are described. Two cloud-screening algorithms, the angle-to-angle smoothness and angle-to-angle correlation tests, which were used in the preprocessing phase of the analyses are also described. The aerosol retrieval examples cover a wide variety of conditions, both over land and water. Particular aerosol types include dust clouds, forest fire and volcanic plumes, and localized dense haze. Finally, some ideas are discussed for additional improvement of the MISR aerosol data product, based on the experience gained in analyzing multiangle data and the associated geophysical products.

Determination of land and ocean reflective, radiative, and biophysical properties using multiangle imaging

Knowledge of the directional and hemispherical reflectance properties of natural surfaces, such as soils and vegetation canopies, is essential for classification studies and canopy model inversion. The Multi-angle Imaging SpectroRadiometer (MISR), an instrument to be launched in 1998 onboard the EOS-AM1 platform, will make global observations of the Earths surface at 1.1-km spatial resolution, with the objective of determining the atmospherically corrected reflectance properties of most of the land surface and the tropical ocean. The algorithms to retrieve surface directional reflectances, albedos, and selected biophysical parameters using MISR data are described. Since part of the MISR data analyses includes an aerosol retrieval, it is assumed that the optical properties of the atmosphere (i.e. aerosol characteristics) have been determined well enough to accurately model the radiative transfer process. The core surface retrieval algorithms are tested on simulated MISR data, computed using realistic surface reflectance and aerosol models, and the sensitivity of the retrieved directional and hemispherical reflectances to aerosol type and column amount is illustrated. Included is a summary list of the MISR surface products.

Aerosol source plume physical characteristics from space‐based multiangle imaging

Models that assess aerosol effects on regional air quality and global climate parameterize aerosol sources in terms of amount, type, and injection height. The multiangle imaging spectroradiometer (MISR) aboard NASAs Terra satellite retrieves total column aerosol optical thickness (AOT), and aerosol type over cloud-free land and water. A stereo-matching algorithm automatically retrieves reflecting-layer altitude wherever clouds or aerosol plumes have discernable spatial contrast, with about 500-m accuracy, at 1.1-km horizontal resolution. Near-source biomass burning smoke, volcanic effluent, and desert dust plumes are observed routinely, providing information about aerosol amount, particle type, and injection height useful for modeling applications. Compared to background aerosols, the plumes sampled have higher AOT, contain particles having expected differences in Angstrom exponent, size, single-scattering albedo, and for volcanic plume and dust cloud cases, particle shape. As basic thermodynamics predicts, thin aerosol plumes lifted only by regional winds or less intense heat sources are confined to the boundary layer. However, when sources have sufficient buoyancy, the representative plumes studied tend to concentrate within discrete, high-elevation layers of local stability; the aerosol is not uniformly distributed up to a peak altitude, as is sometimes assumed in modeling. MISR-derived plume heights, along with meteorological profile data from other sources, make it possible to relate radiant energy flux observed by the moderate resolution imaging spectroradiometer (MODIS), also aboard the Terra spacecraft, to convective heat flux that plays a major role in buoyant plume dynamics. A MISR climatology of plume behavior based on these results is being developed.

A review of reflectance nomenclature used in remote sensing

Field and laboratory radiometers have long been used to characterize the multiangle reflectance properties of homogeneous surface targets. Within the last few years space‐based multiangle spectroradiometers have been deployed, including the Along‐Track Scanning Radiometer‐2 (ATSR), the Polarization and Directionality of the Earths Reflectances (POLDER) instrument, and the Multiangle Imaging SpectroRadiometer (MISR). Other scanning instruments, such as the Moderate Resolution Imaging Spectrometer (MODIS) make use of successive passes to build up multiangle observations. With global monitoring from space, as well as the continuation of traditional observations in the field, advances are being made in research and applications which use these data. There comes a need, with this growth, to standardize reflectance terminology. Success in this endeavor will ensure the integrity of the data products, and facilitate the intercomparison of results. This paper reviews the reflectance terminology that is currently in use by the remote sensing community, including bidirectional reflectance distribution function, bidirectional reflectance factor, hemispherical‐directional reflectance factor, directional‐hemispherical reflectance, hemispherical‐directional reflectance, and bihemispherical reflectance (albedo).

MISR aerosol optical depth retrievals over southern Africa during the SAFARI‐2000 Dry Season Campaign

This paper presents, for the first time, retrievals of aerosol optical depths from Multi-angle Imaging Spectro-Radiometer (MISR) observations over land. Application of the MISR operational algorithm to data taken over southern Africa during the SAFARI-2000 dry season campaign yields results that compare favorably with coincident surface-based measurements taken by the AERONET radiometer network.