J.-L. Widlowski

Evaluation of fraction of absorbed photosynthetically active radiation products for different canopy radiation transfer regimes: Methodology and results using Joint Research Center products derived from SeaWiFS against ground-based estimations

[1] This paper discusses the quality and the accuracy of the Joint Research Center (JRC) fraction of absorbed photosynthetically active radiation (FAPAR) products generated from an analysis of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data. The FAPAR value acts as an indicator of the presence and state of the vegetation and it can be estimated from remote sensing measurements using a physically based approach. The quality of the SeaWiFS FAPAR products assessed in this paper capitalizes on the availability of a 6-year FAPAR time series over the full globe. This evaluation exercise is performed in two phases involving, first, an analysis of the verisimilitude of the FAPAR products under documented environmental conditions and, second, a direct comparison of the FAPAR values with ground-based estimations where and when the latter are available. This second phase is conducted following a careful analysis of problems arising for performing such a comparison. This results in the grouping of available field information into broad categories representing different radiative transfer regimes. This strategy greatly helps the interpretation of the results since it recognizes the various levels of difficulty and sources of uncertainty associated with the radiative sampling of different types of vegetation canopies.

Advanced vegetation indices optimized for up-coming sensors: Design, performance, and applications

This paper describes the implementation of a physical and mathematical approach to designing advanced vegetation indices optimized for future sensors operating in the solar domain such as the medium resolution imaging spectrometer (MERIS), the global imager (GLI), and the VEGETATION instrument, and proposes an initial evaluation of such indices. These optimized indices address sensor-specific issues such as dependencies with respect to the actual spectral response of the sensor as well as the natural sensitivity of remote sensing measurements to illumination and observing geometry, to atmospheric absorption and scattering effects, and to soil color or brightness changes. The derivation of vegetation index formulae optimized to estimate the same vegetation property fraction of absorbed photosynthetically active radiation (FAPAR) from data generated by different sensors allows the comparison of their relative performances compared with existing vegetation indices, both from a theoretical and experimental point of view and permits the creation of global products, as well as the constitution of long time series from multiple sensors.

Uniqueness of multiangular measurements. I. An indicator of subpixel surface heterogeneity from MISR

The recent availability of quasi-simultaneous multispectral and multidirectional measurements from space, as provided by the Multi-angle Imaging SpectroRadiometer (MISR) on board the Terra platform, offers new and unique opportunities to document the anisotropy of land surfaces at critical solar wavelengths. This paper presents simple physical principles supporting the interpretation of the anisotropy of spectral radiances exiting terrestrial surfaces in terms of a signature of surface heterogeneity. The shape of the anisotropy function is represented with two model parameter values which may be mapped and interpreted in their own right. The value of one of these parameters also permits identifying geophysical conditions where the surface heterogeneity becomes significant and where three-dimensional (3D) radiation transfer effects have to be explicitly accounted for. This paper documents these findings on the basis of results from a number of 3D radiation transfer model simulations. The latter are used to perform an extensive sensitivity study which includes issues related to the scale of investigation. A preliminary validation of these results, conducted with a dataset collected by the AirMISR instrument over the Konza prairie, is also discussed.

The state of vegetation in Europe following the 2003 drought

Multi‐annual time series of remote sensing data acquired over Europe from the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) and Medium Resolution Imaging Spectrometer (MERIS) instruments were analysed to assess the state of health of vegetation in Spring 2004, compared to previous years. These data show (1) the dramatic impact of the 2003 drought on a variety of land cover types in Europe and (2) the recovery of most ecosystems to a normal situation in early 2004.

Uniqueness of multiangular measurements. II. Joint retrieval of vegetation structure and photosynthetic activity from MISR

For pt.I see ibid., vol.40, no.7, p.1560-73 (2002). The Multi-angle Imaging SpectroRadiometer (MISR) instrument on board the Terra platform offers the capability of acquiring reflectance data on any Earth target in four spectral bands, from nine different directions, in at most seven minutes, at a spatial resolution adequate for the monitoring of the status of terrestrial surfaces. This paper describes the implementation of a physical and mathematical approach to design a simple two-dimensional algorithm dedicated to the interpretation of data collected by this instrument. One dimension fully exploits the spectral information in the blue, red and near-infrared bands while the other dimension capitalizes on the multiangular capability of MISR to assess the anisotropic behavior of terrestrial surfaces with respect to solar radiation. The spectral information is derived following an approach proposed for single angle instruments, such as the MEdium Resolution Imaging Spectrometer (MERIS), the Global Imager (GLI), the Sea-viewing Wide Field-of-view Sensor (SeaWIFS) and VEGETATION. The access to simultaneous multiangular observations from MISR allows extending this approach. This strategy delivers an estimate of the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR), which pertains to vegetation photosynthetic activity and is a measure of the presence and density of vegetation.

Characterization of Surface Heterogeneity Detected at the MISR/TERRA Subpixel Scale.

Vegetation structure can have a significant impact on the degree of anisotropy in the reflected radiation field. With the appropriate characterization of these effects, the analysis of multiangular data, such as provided by the Multi-angle Imaging SpectroRadiometer (MISR) instrument on board TERRA, can yield statistical information about the type of surface heterogeneity that exists at the subpixel scale.

Coupling Diffuse Sky Radiation and Surface Albedo

Abstract New satellite instruments have been delivering a wealth of information regarding land surface albedo. This basic quantity describes what fraction of solar radiation is reflected from the earth’s surface. However, its concept and measurements have some ambiguity resulting from its dependence on the incidence angles of both the direct and diffuse solar radiation. At any time of day, a surface receives direct radiation in the direction of the sun, and diffuse radiation from the various other directions in which it may have been scattered by air molecules, aerosols, and cloud droplets. This contribution proposes a complete description of the distribution of incident radiation with angles, and the implications in terms of surface albedo are given in a mathematical form, which is suitable for climate models that require evaluating surface albedo many times. The different definitions of observed albedos are explained in terms of the coupling between surface and atmospheric scattering properties. The ana...

Short communication: A model for deriving voxel-level tree leaf area density estimates from ground-based LiDAR

Forest canopy structure has long been known to be a major driver of the processes regulating the exchange of CO2 and water vapour between terrestrial ecosystems and the atmosphere. It is also an important driver of terrestrial vegetation dynamics. Information about fine-scale ecosystem structure is needed to better understand and predict how terrestrial ecosystems respond to and affect environmental change. LiDAR remote sensing from ground-based instruments is a promising technology for providing such information, and physically-based models are ideally suited to process the data and derive reliable products. While complex ray tracing algorithms have been developed to help in the interpretation of LiDAR data, none of these tools are currently widely available. In this paper we present the VoxLAD model; a parametric model using computational geometry that allows to compute estimates of leaf area density at the voxel scale on the basis of terrestrial LiDAR data. This modelling framework removes the need to compute the exact point of entry and exit into and out of the voxels for all individual laser pulses, and thus allows for easier usage. The model requires that each point in the LiDAR point cloud should be classified as wood, foliage, or noise. Here we provide the algorithmic details of the model, and demonstrate that the output of the model closely fits the output of a model using more complex ray tracing techniques. A parametric model (VoxLAD) for estimating leaf area density (LAD) from ground-based LiDAR is proposed.The LAD estimates agree well with those obtained from a complex ray-tracing model.The physically based VoxLAD model will improve accessibility of LiDAR processing methods.

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.

Global-Scale Comparison of MISR and MODIS Land Surface Albedos

Abstract The Moderate Resolution Imaging Spectroradiometer (MODIS) white-sky surface albedos are compared with similar products generated on the basis of the Multiangle Imaging SpectroRadiometer (MISR) surface bidirectional reflectance factor (BRF) model parameters available for the year 2005. The analysis is achieved using global-scale statistics to characterize the broad patterns of these two independent albedo datasets. The results obtained in M. Taberner et al. have shown that robust statistics can be established and that both datasets are highly correlated. As a result, the slight but consistent biases and trends identified in this paper, derived from statistics obtained on a global basis, should be considered sufficiently reliable to merit further investigation. The present paper reports on the zonal- and seasonal-mean differences retrieved from the analysis of the MODIS and MISR surface albedo broadband products. The MISR − MODIS differences exhibit a systematic positive bias or offset in the range...