Malcolm Taberner

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.

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.

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...

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...

Spectral conversion of surface albedo derived from meteosat first generation observations

Comparison of surface albedos derived from spaceborne radiometers with different spectral bands requires, first of all, the conversion of these quantities into common spectral intervals. This letter proposes a spectral conversion method specifically dedicated to surface albedo derived in a large-band instrument such as the solar channel onboard the Meteosat first-generation radiometer. This new method accounts for the retrieval algorithm assumptions and radiometer spectral limitations that might have an impact on the retrieved surface albedo in such a large band. It is also shown that the proposed approach has no impact when surface albedo is derived in narrow bands and confirms the results of previously published spectral conversion methods.

Assessment of biophysical vegetation properties through spectral decomposition techniques

Abstract This article demonstrates the use of spectral decomposition for analyzing the spectral response of different semiarid vegetation species found throughout Mediterranean Europe. Using this technique, it is possible to decompose a spectral data set into a smaller number of significant factors that represent the key variables affecting vegetation spectral response. The results presented here show how spectral decomposition can be used to determine the intrinsic number and identity of the significant factors affecting the multispectral response. For the dataset investigated here, which comprises field spectral recorded over 1130 wavelengths, using a GER single field-of-view IRIS (SIRIS) spectroradiometer, it was found that a combination of just four factors was responsible for the majority of spectral variance. Interpretation of these factors was carried out by graphical analysis stepwise regeneration of the original spectra, and correlation with biophysical data. Considering the identity of these factors, it was found that the second most significant factor (factor 2) was strongly related to the proportion of directly irradiated green leaves within the field-of-view of the spectradiometer. In addition, it was found that the fourth most significant factor (factor 4) provided a good summary of the spectral response of the different samples in the region of strong chlorophyll absorption. This demonstrates the possibility of using spectral decomposition techniques, particularly in environments dominated by spectrally similar vegetation classes, to model the mixed spectral population as mixtures of fundamental biophysical parameters rather than as mixtures of the classes themselves.

Retrieval of land and sea brightness temperatures from mixed coastal pixels in passive microwave data

A technique is presented to separate uncontaminated land and sea brightness temperatures from mixed coastal pixels in 37-GHz vertically polarized passive microwave data from the Special Sensor Microwave Imager (SSM/I) instrument. Combining a mathematical model of the instrument response over several neighboring footprints with a GIS representation of the coastline yields a relationship between land and sea brightness temperatures and radiation measurements made at the satellite. Inverting this relationship allows separate land and sea brightness temperature values to be derived for each mixed coastal pixel in the original image. The technique has been successfully applied to 37-GHz vertically polarized SSM/I imagery for test areas covering the Gulf of Aden and the British Isles. Errors in the retrieved brightness temperatures were estimated to be of the order of 1-2 K.

Consolidating the Two-Stream Inversion Package (JRC-TIP) to Retrieve Land Surface Parameters From Albedo Products

The objective of this paper is to present a series of improvements on the Joint Research Centre Two-stream Inversion Package (JRC-TIP) that enhance its effectiveness to generate reliable surface products and associated uncertainties from surface albedo values. Lookup tables (LUTs) are built in the observation space from the JRC-TIP and are used to store solutions obtained from off-line dedicated procedures on selected sets of prior conditions. This new approach drastically limits the occurrence of questionable solutions, revealed by outliers in the retrievals, often associated with local instead of global minima and ensures that the retrieved values are insensitive to small variations in the input albedo values. This TIP table-based approach also reduces considerably the computing time requirement, which is a definite asset in the systematic application of the TIP against large data sets of surface albedo products.

Monitoring vegetation biomass of a coastal ecosystem using multidate optical satellite data

In order to effectively protect coastal ecosystems, objective information on the structural and functional characteristics of these systems must be obtained through regular monitoring. In recent years, more advanced technologies, such as satellite remote sensing, have arisen which are being utilized for the monitoring and management of coastal ecosystems in general, and vegetation in particular. This paper presents the results from monitoring the vegetation biomass of the Koycegiz Lagoon area, declared as a Specially Protected Area by the Turkish Government, using multi-temporal satellite imagery. Several problems need to be overcome when using imagery from different dates or from different sensors. These include; image registration, atmospheric variability, and, often, the lack of historical ground data. Furthermore, for monitoring purposes, techniques to overcome these problems should be robust and automatic allowing the database to be upgraded easily. The procedures we have developed include automatic registration (to subpixel accuracy), atmospheric normalization, and vegetation index (VI) calibration components. This was tested on multidate (1984, 1988, 1991, 1995 and 1996) LANDSAT-TM data. From this adjusted data set the performance of different vegetation indices, in this coastal environment was examined, and the vegetation trends analyzed.

Monitoring FAPAR over land surfaces with remote sensing data

Temporal changes of terrestrial vegetation have traditionally been monitored using empirical remote sensing tools, which are sensitive to perturbations as well as to the spectral properties of the sensor. Advances in the understanding of radiation transfer theory, and the availability of higher performance modern instruments, have led to the development of physically-based inverse methods to derive biogeophysical products. Jointly, these developments allow the retrieval of reliable, accurate information on the state and evolution of terrestrial environments. A series of optimized algorithms has been developed to document biogeophysical variables, and in particular to estimate the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) from a variety of optical instruments. As a result, monitoring managed (e.g., agriculture) or natural ecosystems will benefit from the availability of local, regional and global time series of remote sensing products such as FAPAR. This paper outlines the methodology and exhibits selected results in the form of temporal composites derived from the SeaWiFS sensor.