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22 results on '"North, P.R.J."'

9. Earth observation for land-atmosphere interaction science

Author

Marconcini, M., Fernàndez-Prieto, D., Reissell, A., Ellis, M., Blyth, E.M., Burrows, J.P., de Leeuw, G., Gerard, F.F., Houweling, S., Kaminski, T., Krol, M., Muller, J-P., North, P.R.J., Pinty, B., Plummer, S., Quegan, S., Reichstein, M., Remedios, J.J., Roberts, G.J., Shvidenko, A., Scipal, K., Sobrino, J.A., Teuling, A.J., van der Werf, G.R., Marconcini, M., Fernàndez-Prieto, D., Reissell, A., Ellis, M., Blyth, E.M., Burrows, J.P., de Leeuw, G., Gerard, F.F., Houweling, S., Kaminski, T., Krol, M., Muller, J-P., North, P.R.J., Pinty, B., Plummer, S., Quegan, S., Reichstein, M., Remedios, J.J., Roberts, G.J., Shvidenko, A., Scipal, K., Sobrino, J.A., Teuling, A.J., and van der Werf, G.R.

Abstract

The European Space Agency (ESA), iLEAPS (Inte-grated Land Ecosystem-Atmosphere Processes Study, i.e. the land-atmosphere core project of the International Geosphere-Biosphere Programme), and the European Geosciences Union (EGU) jointly organized the “Earth Observation for Land-Atmosphere Interaction Science” Conference, which took place from 3rd to 5th November 2010 at the Italian premises of ESA in Frascati (Rome). The event represented an attempt to effectively draw together Earth-observation (EO) and Earth-system scientists investigating land-atmosphere processes in order to better understand the current gaps in science and derive recommendations to advance in the use of EO technology in the context of this important topic. Around 200 people from more than 30 countries worldwide met and discussed for three intensive days. This paper reports keypoints and the main recommendations of the Symposium for each of the key Themes addressed during the Conference.

Published
2011

10. The third RAdiation transfer Model Intercomparison (RAMI) exercise: Documenting progress in canopy reflectance models

Author

Widlowski, J.L., Taberner, M., Pinty, B., Bruniquel-Pinel, V., Disney, M.I., Fernandes, R., Gastellu-Etchegorry, J.P., Gobron, N., Kuusk, A., Lavergne, T., LeBlanc, S., Lewis, P.E., Martin, E., Mõttus, M., North, P.R.J., Qin, W., Robustelli, M., Rochdi, N., Ruiloba, R., Thompson, R., Verhoef, W., Verstraete, M.M., Xie, D., Widlowski, J.L., Taberner, M., Pinty, B., Bruniquel-Pinel, V., Disney, M.I., Fernandes, R., Gastellu-Etchegorry, J.P., Gobron, N., Kuusk, A., Lavergne, T., LeBlanc, S., Lewis, P.E., Martin, E., Mõttus, M., North, P.R.J., Qin, W., Robustelli, M., Rochdi, N., Ruiloba, R., Thompson, R., Verhoef, W., Verstraete, M.M., and Xie, D.

Abstract

[1] The Radiation Transfer Model Intercomparison ( RAMI) initiative benchmarks canopy reflectance models under well-controlled experimental conditions. Launched for the first time in 1999, this triennial community exercise encourages the systematic evaluation of canopy reflectance models on a voluntary basis. The first phase of RAMI focused on documenting the spread among radiative transfer (RT) simulations over a small set of primarily 1-D canopies. The second phase expanded the scope to include structurally complex 3-D plant architectures with and without background topography. Here sometimes significant discrepancies were noted which effectively prevented the definition of a reliable "surrogate truth,'' over heterogeneous vegetation canopies, against which other RT models could then be compared. The present paper documents the outcome of the third phase of RAMI, highlighting both the significant progress that has been made in terms of model agreement since RAMI-2 and the capability of/need for RT models to accurately reproduce local estimates of radiative quantities under conditions that are reminiscent of in situ measurements. Our assessment of the self-consistency and the relative and absolute performance of 3-D Monte Carlo models in RAMI-3 supports their usage in the generation of a "surrogate truth'' for all RAMI test cases. This development then leads ( 1) to the presentation of the "RAMI Online Model Checker'' (ROMC), an open-access web-based interface to evaluate RT models automatically, and ( 2) to a reassessment of the role, scope, and opportunities of the RAMI project in the future.

Published
2007

21. A method to convert AVHRR Normalized Difference Vegetation Index time series to a standard viewing and illumination geometry

Author

Los, S.O., North, P.R.J., Grey, W.M.F., and Barnsley, M.J.

Subjects
*TREES, *BOTANY, *ATMOSPHERE, *OPTICAL polarization
Abstract

Abstract: The bi-directional reflectance distribution function (BRDF) alters the seasonal and inter-annual variations exhibited in Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI) data and this hampers the detection and, consequently, the interpretation of temporal variations in land-surface vegetation. The magnitude and sign of bi-directional effects in commonly used AVHRR data sets depend on land-surface properties, atmospheric composition and the type of atmospheric correction that is applied to the data. We develop an approach to estimate BRDF effects in AVHRR NDVI time series using the Moderate Resolution Imaging Spectrometer (MODIS) BRDF kernels and subsequently adjust NDVI time series to a standard illumination and viewing geometry. The approach is tested on NDVI time series that are simulated for representative AVHRR viewing and illumination geometry. These time series are simulated with a canopy radiative transfer model coupled to an atmospheric radiative transfer model for four different land cover types—tropical forest, boreal forest, temperate forest and grassland – and five different atmospheric conditions – turbid and clear top-of-atmosphere, turbid and clear top-of-atmosphere with a correction for ozone absorption and Rayleigh scattering applied (Pathfinder AVHRR Land data) and ground-observations (fully corrected for atmospheric effects). The simulations indicate that the timing of key phenological stages, such as start and end of growing season and time of maximum greenness, is affected by BRDF effects. Moreover, BRDF effects vary with latitude and season and increase over the time of operation of subsequent NOAA satellites because of orbital drift. Application of the MODIS kernels on simulated NVDI data results in a 50% to 85% reduction of BRDF effects. When applied to the global 18-year global Normalized Difference Vegetation Index (NDVI) Pathfinder data we find BRDF effects similar in magnitude to those in the simulations. Our analysis of the global data shows that BRDF effects are especially large in high latitudes; here we find that in at least 20% of the data BRDF errors are too large for accurate detection of seasonal and interannual variability. These large BRDF errors tend to compensate, however, when averaged over latitude. [Copyright &y& Elsevier]

Published
2005
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22. Forest ecosystem chlorophyll content: implications for remotely sensed estimates of net primary productivity.

Author

Dawson, T.P., North, P.R.J., Plummer, S.E., and Curran, P.J.

Subjects
*CHLOROPHYLL, *REMOTE sensing, *FOREST ecology
Abstract

Demonstrates that the Normalized Difference Vegetation Index (NDVI) is highly sensitive to both canopy foliar and understorey chlorophyll content, which could account for significant errors in remotely sensed estimates of the fraction of incident photosynthetically-active radiation (fPAR). Use of a coupled leaf-canopy model of radiative transfer.

Published
2003
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