Bernard Pinty

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.

Carbon cycle data assimilation with a generic phenology model

Photosynthesis by terrestrial plants is the main driver of the global carbon cycle, and the presence of actively photosynthesizing vegetation can now be observed from space. However, challenges remain when translating remotely sensed data into carbon fluxes. One reason is that the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR), which documents the presence of photosynthetically active vegetation, relates more directly to leaf development and leaf phenology than to photosynthetic rates. Here, we present a new approach for linking FAPAR and vegetation-to-atmosphere carbon fluxes through variational data assimilation. The scheme extends the Carbon Cycle Data Assimilation System (CCDAS) by a newly developed, globally applicable and generic leaf phenology model, which includes both temperature and water-driven leaf development. CCDAS is run for seven sites, six of them included in the FLUXNET network. Optimization is carried out simultaneously for all sites against 20 months of daily FAPAR from the Medium Resolution Imaging Spectrometer on board the European Space Agencys ENVISAT platform. Fourteen parameters related to phenology and 24 related to photosynthesis are optimized simultaneously, and their posterior uncertainties are computed. We find that with one parameter set for all sites, the model is able to reproduce the observed FAPAR spanning boreal, temperate, humid-tropical, and semiarid climates. Assimilation of FAPAR has led to reduced uncertainty (by >10%) of 10 of the 38 parameters, including one parameter related to photosynthesis, and a moderate reduction in net primary productivity uncertainty. The approach can easily be extended to regional or global studies and to the assimilation of further remotely sensed data. (Less)

Using 1-D models to interpret the reflectance anisotropy of 3-D canopy targets: issues and caveats

This paper evaluates 1) to what extent one-dimensional (1-D) models can be used to represent the magnitude and directionality of the surface reflectance field of heterogeneous canopy targets at different spatial resolutions, and 2) whether this usage results in significant biases in the estimation of the corresponding state variables. It will be seen that when both the 1-D and three-dimensional (3-D) models account for all features of the measured radiation field, then-in the absence of further information regarding the nature and structure of the target-the use of a 3-D model may amount to an over-interpretation of the available data. The simplified surface structure formulation contained within the 1-D model, on the other hand, may affect the values of the state variables that such models will retrieve. This is because the shape of the reflectance anisotropy of the 3-D target is almost always different from that of a structurally homogeneous (1-D) canopy with the same state variable values but no foliage clumping. By consequence the 1-D canopies that are capable of mimicking the bell (or bowl) shaped reflectance anisotropy of 3-D targets will tend to feature lower leaf area index, higher soil albedo and, in particular, predominantly erectophile (or plagiophile) leaf normal distributions.

Generating 275-m Resolution Land Surface Products From the Multi-Angle Imaging SpectroRadiometer Data

This paper shows how to reconstruct the original 275-m resolution data of the Multi-angle Imaging SpectroRadiometer (MISR) instrument in the 24 spectrodirectional global mode channels that are spatially averaged to 1.1 km on-board the Terra platform, with negligible loss of information relative to images acquired in native-resolution local mode. Standard approaches to improve the spatial resolution of products rely on one (typically panchromatic) high-resolution (HR) image to sharpen multiple spectral images. In the case of the MISR-HR package described here, three of the 12 available HR channels are combined to regenerate each of the 24 reduced-resolution channel to its native resolution. The accurate and rigorously reconstructed spectral bidirectional reflectance data allow sensitive and physically meaningful land surface attributes to be recovered at a spatial resolution appropriate to document the spatial heterogeneity of the land surface and relevant for climate and environment studies. MISR has been in continuous operation since February 2000 and provides global coverage in at most nine days (depending on latitude). This technique allows the generation of quantitative information to monitor change and model ecosystem function virtually anywhere and at any time during the last decade. The potential is demonstrated for a savanna landscape in South Africa.

The impact of multi‐angular measurements on the accuracy of land‐surface Albedo retrieval: Peliminary results for the proposed ESA LSPIM mission

This paper documents, through a sensitivity study, the retrieval accuracy of the albedo and the bidirectional reflectance factor (BRF) field as a function of the number of observations and their angular distribution. A BRF data base is generated for different surface types, solar positions, and wavelengths. A realistic amount of random noise is added to approximate actual measurement conditions. A parametric BRF model is inverted against the noisy data strings of variable lengths and subsequently used in a forward mode to reconstruct the BRF field and estimate the albedo. The retrieval accuracy is estimated by comparing the original and the reconstructed fields and values. For a single observation, the retrieval accuracy based on the assumption of a Lambertian surface reflectance is compared to the one based on the assumption of a typical BRF field.

The IWMMM-2 Conference: Background, Achievements and Recommendations.

The Second International Workshop on Multiangular Measurements and Models (IWMMM-2) was held in Ispra, Italy, on September 15-17, 1999. This international event was jointly sponsored by the European Network for the development of Advanced Models to interpret Optical Remote Sensing data over terrestrial environments (ENAMORS) supported by the European Commission DG-XII under the Fourth Framework Programme, the Space Applications Institute (SAI) of the European Commission DGJoint Research Centre (JRC), the US National Aeronautics and Space Administration (NASA) and the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia. The meeting was attended by over 100 scientists from Western and Eastern Europe, the United States, Australia, New Zealand, Canada, China and Japan. These participants, listed in Annex 1, were involved or interested in modeling the anisotropy of the radiation field, in multiangular measurements, or in the exploitation of these tools and data in their own applications. This 3-day meeting addressed three main objectives: (1) to establish the state of the art in direct and inverse modeling techniques, including those proposed for the operational exploitation of earlier instruments