[Show abstract][Hide abstract] ABSTRACT: During the past decade, significant progress has been made towards the development of surface schemes for Numerical Weather Prediction (NWP) models. In order to simulate surface processes in such models adequately, a realistic description of surface properties is necessary. This is achieved using time series of NOAA (National Oceanic and Atmospheric Administration) AVHRR (Advanced Very High Resolution Radiometer) satellite data, and the method relies on an automatic clustering of multi-temporal NDVI (Normalized Difference Vegetation Index) maximum values. This leads to eleven vegetation classes where the forests are readily identified from a thorough analysis of visible reflectances in early summer. This intermediate step and look-up tables allow for a final mapping of the roughness length, leaf area index, fractional vegetation cover and minimum stomatal resistance. These are primary parameters of the land surface parameterization scheme ISBA used at Meteo-France and are updated from the seasonality of NDVI. In a last step, parameters are spatially averaged at the grid box scale according to aggregation rules. The reliability of the mapping is demonstrated through the improvements of the French NWP model in short-range forecasts.
International Journal of Remote Sensing 11/2010; 21(6):1183-1199. DOI:10.1080/014311600210137 · 1.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The MUREX (monitoring the usable soil reservoir experimentally) experiment was designed to provide continuous time series of field data over a long period, in order to improve and validate the Soil-vegetation-Atmosphere Transfer (SVAT) parameterisations employed in meteorological models. Intensive measurements were performed for more than three years over fallow farmland in southwestern France. To capture the main processes controlling land-atmosphere exchanges, the local climate was fully characterised, and surface water and energy fluxes, vegetation biomass, soil moisture profiles, surface soil moisture and surface and soil temperature were monitored. Additional physiological measurements were carried out during selected periods to describe the biological control of the fluxes. The MUREX data of 1995, 1996, and 1997 are presented. Four SVAT models are applied to the annual cycle of 1995. In general, they succeed in simulating the main features of the fallow functioning, although some shortcomings are revealed.Key words. Hydrology (evapotranspiration; soil moisture; water-energy interactions).
[Show abstract][Hide abstract] ABSTRACT: In the current policy frameworks the need for up-to-date and reliable information for entire Europe is well known. A step in this direction has been taken by the Pan-European Land Cover Monitoring (PELCOM) project. A concise classification methodologywas established for mapping major land cover types by using regional expertise in an integrated approach of NOAA-AVHRR satellite data and ancillary information. An important result was the 1-km pan-European land cover database. An extensive accuracy assessment has been implemented on basis of 40 interpreted Landsat-TM satellite images distributed over pan-Europe. The total average accuracy was 69.2%. The PELCOM land cover database was applied successfully in various environmental and climate studies. The use of AVHRR satellite data has, however, its limitations for monitoring purposes due to the fine scale on which thematic land cover occur. Medium resolution satellite imagery (e.g. MODIS and MERIS) will play a crucial role in the near future in monitoring the European landscape.
[Show abstract][Hide abstract] ABSTRACT: The PELCOM (Pan-European Land Cover Monitoring) project aimed at
developing a consistent methodology to derive land cover information on
a European scale from Earth Observation data for environmental
monitoring. A major achievement was the establishment of a 1-km
Pan-European Land Cover Database that has been applied in various
environmental and climate studies. For this purpose, multi-spectral and
multi-temporal NOAA-AVHRR satellite imagery and ancillary data have been
integrated. Validation has shown a good accuracy
[Show abstract][Hide abstract] ABSTRACT: A macroscale hydrological model including an improved representation of the surface processes is validated at a regional scale using the data set collected during the Hapex-Mobilhy experiment (1986). Using observed atmospheric forcing with a resolution of 5 km and realistic fields of vegetation and soil types in a large area, the hydrological model correctly simulates daily streamflows for two contrasting climatological years in 1986 and 1987, provided that a sub-grid scale parameterization of surface runoff is included in the surface scheme. The comparison with point observations of evaporation and soil water content shows that the surface scheme satisfactorily reproduces the annual water budget for very diverse land uses. It is shown that, although the annual rainfall was significantly lower in 1987, the annual evaporation was higher than in 1986 because precipitation fell mainly in summer. The reduction of precipitation contributed to a comparable reduction of total runoff, and therefore a drastic decrease in the river flows as confirmed by the observations. The second part of the study addresses the issue of modelling the surface water budget at a large grid scale. Once validated, the macroscale hydrological model is used as a reference to test aggregation methods. Aggregated fluxes of evaporation, drainage and surface runoff are found to be close to the reference fluxes provided that sub-grid scale surface runoff and interception losses are taken into account in the aggregation procedure.
Journal of Hydrology 04/1999; 217(1-2-217):97-118. DOI:10.1016/S0022-1694(99)00020-7 · 3.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: As a first step towards coupling atmospheric and hydrological models, this article describes the implementation of the Interface Soil Biosphere Atmosphere (ISBA) surface scheme within a macroscale hydrological model at the regional scale. The introduction of the diurnal cycle in the hydrological model allows a coupling with the atmosphere through the energy balance and water budget computations. The coupled model is used in a forced mode, i.e. atmospheric forcing is imposed on the surface scheme without any retroaction of the surface exchanges on the atmosphere. The initial version of ISBA was modified to account for sub-grid runoff. Existing classifications of soil and vegetation at high spatial resolution (1 km) were used in conjunction with satellite information to monitor the monthly evolution of the vegetation. A dense surface network facilitated the interpolation of atmospheric fields with a time step of 3 h. The two years considered show large differences of annual precipitation and potential evaporation, which have an impact on the regime of the river flows. A one-dimensional study presents the sensitivity of the partitioning of precipitation into evaporation and runoff to the sub-grid runoff and drainage parameterizations, soil depth, and vegetation cover.
Journal of Hydrology 04/1999; 217(1-2-217):75-96. DOI:10.1016/S0022-1694(99)00019-0 · 3.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: HAPEX-Sahel was an international experiment designed to provide the field data needed to model the climate of the Sahel and its dependence on land surface conditions. The design of the experiment was based on the study of a 1° square experimental domain in which there were three observational supersites. At each of these supersites detailed hydro-meteorological studies were made at subsites for each of the three principal vegetation types: millet, fallow savannah and tiger bush. Remote sensing from satellite and aircraft was used to scale up from the local to the regional scale. Hydrological monitoring, from 1991 to 1993, was combined with an 8-week intensive observation period that covered the end of the wet season and the beginning of the dry season in 1992. The structure and content of the HAPEX-Sahel Special Issue are described and an introduction is given to the HAPEX-Sahel information system where the data are stored.
Journal of Hydrology 02/1997; 188(1). DOI:10.1016/S0022-1694(96)03308-2 · 3.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The boundary layer development over the Central sites of HAPEX-Sahel has been simulated for 4 individual days. The study is divided into two parts. In the first part a surface scheme is calibrated for the dominant land cover of the area, the fallow savanna. This calibration is performed for a period of 54 days encompassing the transition from wet to dry. A comparison is then made with another more detailed surface scheme. The model, in spite of its drastic simplification of reality, is able to reproduce the overall water balance for the period. In terms of relative contribution of soil evaporation and vegetation transpiration it does not depart significantly from the more complex scheme.In the second part, the surface scheme is coupled to a one-dimensional version of the ARPEGE model (operational forecast model at Meteo-France). The importance of the direct heating of the boundary layer by aerosols is stressed. With minor modifications of calibration parameters from the first phase, boundary layer profiles of temperature and also mixing ratio are reproduced with a good accuracy.
Journal of Hydrology 02/1997; 189:1017-1039. DOI:10.1016/S0022-1694(96)03181-2 · 3.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper summarizes the results of HAPEX-Sahel presented in this Special Issue to produce a broad picture of the role land surface processes play in determining Sahelian climate. It highlights key achievements in the field of rainfall analysis, surface hydrology, surface energy and carbon balance, and large-scale meteorology and remote sensing. It discusses further research needed to understand the role of the land surface in the Sahel and makes some suggestions as to how to approach this problem through further modelling and data analysis, making use of the increased understanding produced by HAPEX-Sahel.
Journal of Hydrology 02/1997; 189(1). DOI:10.1016/S0022-1694(96)03183-6 · 3.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Au cours de l'expérience Hapex-Sahel un réseau de 12 stations automatiques météorologiques a été installé. Quelques comparaisons sont présentées avec les mesures faites par l'Orstom. La variabilité des paramètres météorologiques est ensuite étudiée pour l'ensemble de la période intensive, ce qui permet de réaliser un zonage du degré carré. La période des 25 et 26 septembre est ensuite examinée. Les cycles diurnes des paramètres météorologiques présentent des différences significatives et informatives d'un site à l'autre. (Résumé d'auteur)
[Show abstract][Hide abstract] ABSTRACT: A methodology has been developed for estimating regional evapotranspiration using remote sensing data. The transfer of sensible and latent heat is described by an aerodynamic resistance and a surface resistance. The aerodynamic resistance is formulated on the basis of the Monin-Obukhov similarity theory and the surface resistance is determined from the energy balance equation using remotely sensed surface temperature. The regional evapotranspiration is then calculated with a one-layer resistance model. The overall surface transfer characteristics at regional scale are characterized by an effective roughness length and this parameter was determined from local roughness length with the aid of remote sensing measurements.The model was validated by using the data obtained during the HAPEX-MOBILHY (Hydrologic Atmospheric Pilot Experiment and Modelisation du Bilan Hydrique) experiment. At local scale with homogeneous surface conditions, good agreement (r = 0.90, RMSE = 50 W m−2) was obtained between model estimates and the measurements at ground stations. The regional evapotranspiration from the agricultural part of the HAPEX-MOBILHY was also calculated with the one-layer resistance model by combining the NOAA-AVHRR with ground-based meteorological data. The calculated regional evapotranspiration was compared with the area averages of the evapotranspiration measured by the ground-based stations. The two techniques agreed fairly well and the average difference was only 28 W m−2.
[Show abstract][Hide abstract] ABSTRACT: Hapex Sahel (Hydrologic Atmospheric Pilot Experiment in the Sahel) was an international program focused on the soil-plant-atmosphere energy, water and carbon balance in the west African Sahel. It was intended to improve our understanding of the interaction between the Sahel and the general atmospheric circulation, both at present and in the future, providing a base line for studies of climate change. It was carried out in a 1° × 1° area of west Niger over a 3–4 year period with an 8-week intensive observation period from August to October 1992. HAPEX-Sahel was funded by a wide range of agencies in seven participating countries. Over 170 scientists visited and worked in the field. An interdisciplinary approach was adopted with contributed studies in hydrology and soil moisture, surface fluxes and vegetation, remote sensing science, and meteorology and mesoscale modeling. Detailed field measurements were concentrated at 3 “supersites” and 3 ancillary sites. Four aircraft were used for remote sensing and flux measurement. Observations from space were acquired from nine sensors on seven different satellite platforms. Conditions in 1992 turned out to be average for the last decade with good gradients of precipitation and a variety of vegetation productivities between the study sites. An information system has been established to provide a data base to disseminate the measurements. An active program of meetings, workshops, and interdisciplinary studies is now in progress.
[Show abstract][Hide abstract] ABSTRACT: HAPEX-Sahel was a land-surface-atmosphere interaction experiment, carried out in the Sahelian region of West Africa in Niger in 1992. This publication served as a reference for the execution of the experiment. It summarizes scientific objectives and the experimental strategy, and it provides descriptions of the measurement sites and of the contributions of individual investigating teams. Operational details as well as the database strategy are given.
[Show abstract][Hide abstract] ABSTRACT: The HAPEX-Sahel experiment was carried out in Niger, West Africa, in 1992 and 1992, with an intensive observation period from August to October 1992. It was aimed at improving the parametrization of interactions between land surface and atmosphere on the GCM grid box scale. Remote sensing was combined with hydrological and meteorological modelling to develop aggregation techniques that can be used in large-scale estimates of the hydrological and meteorological behaviour of large areas in the Sahel. The experimental strategy is described and some preliminary results are presented.
[Show abstract][Hide abstract] ABSTRACT: The Hydrologic Atmospheric Pilot EXperiment in the Sahel (HAPEX-Sahel) was carried out in Niger, West Africa, during 1991 - 1992, with an intensive observation period (IOP) in August - October 1992. It aims at improving the parameterization of land surface atmosphere interactions at the Global Circulation Model (GCM) gridbox scale. The experiment combines remote sensing and ground based measurements with hydrological and meteorological modelling to develop aggregation techniques for use in large scale estimates of the hydrological and meteorological behaviour of large areas in the Sahel. The experimental strategy consisted of a period of intensive measurements during the transition period of the rainy to the dry season, backed up by a series of long term measurements in a 1^ by 1^ square in Niger. Three “supersites” were instrumented with a variety of hydrological and (micro) meteorological equipment to provide detailed information on the surface energy exchange at the local scale. Boundary layer measurements and aircraft measurements were used to provide information at scales of 100 - 500 km2. All relevant remote sensing images were obtained for this period. This programme of measurements is now being analyzed and an extensive modelling programme is under way to aggregate the information at all scales up to the GCM grid box scale. The experimental strategy and some preliminary results of the IOP are described.
[Show abstract][Hide abstract] ABSTRACT: The main thrust of the HAPEX-MOBILHY experiment was towards investigating techniques involved in integrating the turbulent surface fluxes measured at local sites to a larger scale approaching that used in general circulation models.Some aspects of the field data collected at various times and spatial scales are presented. Annual cycle of the soil moisture at many sites is discussed in relation with outputs of a large scale hydrological model. At shorter time scales, the spatial variability of surface energy partition is examined with regard to spatial contrasts in albedo, surface roughness and plant properties related to the two main vegetation classes found in the HAPEX square: A pine forest and the nearby agricultural area.Finally, examples of daily spatial integration with an atmospheric mesoscale model including a comprehensive treatment of land surface processes are presented.
Surveys in Geophysics 01/1991; 12(1):31-61. DOI:10.1007/BF01903411 · 3.45 Impact Factor