Suivi des flux d'énergie, d'eau et de carbone à la surface : apport de la télédétection et de la modélisation du rayonnement solaire absorbé par la végétation

It is known that a global 4% increase of land surface albedo (also called reflectivity) may result approximately in a decrease of 0.7°C in the Earth’s equilibrium temperature. Nowadays the surface properties (including albedo) are changing under climatic and human pressure. At the same time, there is a debate that divides the scientific community about the potential trends (increase or decrease) affecting the surface incoming solar radiation since mid-1980 (resulting of a decrease or increase of aerosol concentration in the atmosphere, respectively). The Earth is a complex system driven at the surface level by three cycles (energy, water, and carbon). These cycles are not insensitive to changes of surface reflectivity, incoming radiation, or aerosol properties. For example, some argue that the increase of diffuse radiation during the last decades would have led to an exceed of carbon uptake by the Earth’s vegetation of 9.3%. The main issue raised here is to assess the added value of the knowledge in absorbed solar radiation by the surface (combination of incoming solar radiation with surface albedo) and, especially, by the vegetation for the monitoring of energy, water and carbon fluxes.In this work, I have used satellite observations and modeled the radiative transfer theory in order to make dynamic mapping of solar radiation absorbed by the surface and through the vertical dimension of the vegetation. First, I quantified each uncertainty source affecting incoming solar radiation, surface albedo and the way radiation is split between horizontal and vertical heterogeneity. In a second step, I measured the added value of using this absorbed radiation mapping of the surface by satellite to estimate the energy and water fluxes at the surface. The resulting improved scores of weather forecast models in the short-range time scale suggested potential feedbacks at the climatic time scale over sensible areas such as the Sahel region. Another significant outcome is that the developments proposed to better characterize the vertical heterogeneity within the canopy led to an improvement of 15% of annual global terrestrial gross primary production (GPP). Moreover, this study has led to measure the impact of the lack of knowledge of spatial and temporal variability of aerosol properties (concentration and type). I have shown that the tracking of temporal changes of directional properties of reflectance allows me to retrieve to the amount of aerosols in the atmosphere as precisely as other widely used methods but with a higher frequency (5 times more) by using data from geostationary satellite. Finally, this study addresses some possibilities to better track temporal changes of properties of reflectivity of surface and aerosol of atmosphere, and to access to a better monitoring of biogeochemical cycles of the terrestrial biosphere.Au niveau global, il a été estimé qu’une augmentation de 4% de l’albédo (ou réflectivité) de la surface provoquerait une diminution de 0,7° de la température d’équilibre de la Terre. Or les propriétés des surfaces (dont l’albédo) changent sous la pression climatique et l’action de l’homme. Parallèlement à ce changement des propriétés de surface un débat divise la communauté scientifique sur une éventuelle diminution ou augmentation du rayonnement incident à la surface depuis le milieu des années 1980 (conséquence d’une augmentation ou diminution de la concentration d’aérosols dans l’atmosphère). La Terre est un système complexe piloté en sa surface par 3 cycles (énergie, eau et carbone). Ces cycles ne sont pas insensibles à ces changements de propriété de réflectivité de surface, de rayonnement solaire incident ou de concentration en aérosols. Certains avancent ainsi qu’une augmentation du rayonnement diffus durant les dernières décennies aurait déjà entraîné un excédent de captation de carbone par la végétation de 9.3%. La problématique ici soulevée est d’évaluer l’apport de la connaissance du flux solaire absorbé par la surface (combinaison du rayonnement solaire et de l’albédo de surface) et plus particulièrement par sa partie végétative pour le suivi des flux d’énergie, d’eau et de carbone. Dans ce travail, j’ai fait appel à l’observation satellitaire et à la modélisation du transfert radiatif pour cartographier la dynamique du rayonnement solaire absorbé par la surface et sur la verticale de la végétation. Dans un premier temps, chacune des sources d’incertitudes sur le rayonnement incident, sur l’albédo de surface mais aussi sur la répartition du rayonnement entre les hétérogénéités horizontales et verticales à la surface furent quantifiées. Puis tout en discutant l’effet de ces incertitudes, j’ai mesuré l’apport de l’utilisation de cette cartographie par satellite du rayonnement solaire absorbé pour estimer les flux d’énergie et d’eau en surface ; ce qui améliora les scores de prévision du temps à court terme et permis également de suggérer des rétroactions à l’échelle climatique sur des zones sensibles tel le Sahel. Aussi une correction de biais de 15% sur l’estimation de la production primaire brute de carbone à l’échelle planétaire démontra l’importance des développements réalisés afin de caractériser les hétérogénéités verticales dans le couvert. Finalement, ce travail m’a conduit à chiffrer l’impact de la méconnaissance des variabilités spatiales et temporelles des propriétés des aérosols (concentration et type). J’ai montré que le suivi au cours du temps des propriétés de directionalité de la réflectivité de surface (tel abordé dans la première partie de mon étude) pouvait aussi permettre de remonter à la quantité d’aérosol dans l’atmosphère. L’utilisation d’observations issues de satellite géostationnaire permet d’estimer la concentration en aérosol avec la même qualité mais avec une fréquence de détection plus élevée (x5 environ) que les méthodes classiques. Enfin, ce travail dresse des pistes pour améliorer la détection des changements des propriétés de réflectivité de surface et d’aérosols de l’atmosphère, et atteindre un suivi encore meilleur des cycles biogéochimiques de la biosphère terrestre

Daily estimates of the tropospheric aerosol optical thickness over land surface from MSG geostationary observations

The paper presents an innovative method to derive aerosol optical thickness (AOT) on a continental scale, using MSG observation. The approach consists in taking into account the high temporal resolution of the observing system, in order to discriminate between surface and aerosol effects. A suitably extended semi-empirical BRDF model is applied, combined with a recursive scheme. The method is not instrument-specific (can be adapted to instruments onboard polar satellites) and was tested with MSG/SEVIRI data over mid-latitude and African regions. The aerosol optical thickness estimates are compared to AERONET ground measurements and to the corresponding MODIS product over land. The method appears very promising for tracking anthropogenic emissions in the troposphere and also for estimating dust events over bright surfaces. The high spatial and temporal resolution of the estimate is appropriate to investigate the dependence of AOT on the density of urbanization and potentially on motor vehicle traffic. Finally, this study suggests that this approach is appropriate for multi-sensor data fusion, for the simultaneous retrieval of surface albedo and aerosol optical thickness, and to generate these products in near-real time with a very high generation frequency

Land surface albedo from MSG/SEVIRI: retrieval method, validation, and application for weather forecast

The European Meteorological Satellite Organization (EUMETSAT) maintains a number of decentralized processing centers dedicated to different scientific themes. The Portuguese Meteorological Institute hosts the Satellite Application Facility on Land Surface Analysis (LSA-SAF). The primary objective of the LSA-SAF is to provide added-value products for the meteorological and environmental science communities with main applications in the fields of climate modeling, environmental management, natural hazards management, and climate change detection. Since 2005 data from Meteosat Second Generation satellite are routinely processed in near real time by the LSA-SAF operational system in Lisbon. Presently, the delivered operational products comprise land surface albedo and temperature, shortwave and long-wave downwelling radiation fluxes, vegetation parameters and snow cover. After more than ten years (1999-2010) of research, development, and progressive operational activities, a summary of the surface albedo product characteristics and performances is presented. The relevance of LSA-SAF albedo product is analyzed through a weather forecast model (ALADIN) in order to account for the inter-annual spatial and temporal variability. Results clearly show a positive impact on the 12-hour forecast of 2m temperatures

Comparison of two methods for aerosol optical depth retrieval over North Africa from MSG/SEVIRI data

A comparison between the algorithm for Land Aerosol property and Bidirectional reflectance Inversion by Time Series technique (LABITS) and a daily estimation of aerosol optical depth (AOD) algorithm (AERUS-GEO) over land surface using MSG/SEVIRI data over North Africa is presented. To obtain indications about the quantitative performance of two AOD retrieval methods mentioned above, daily SEVIRI AOD values is considered with respect to those measured from the global aerosol-monitoring Aerosol Robotic Network (AERONET) data. The correlation coefficient (R2) between retrieved SEVIRI AOD at 650 nm from the AERUS-GEO algorithm and the AERONET Level 2.0 daily average AOD at 675 nm is 0.80 and root mean square error (RMSE) is 0.044, and R2 between retrieved AOD from the LABITS algorithm and AERONET AOD is 0.80 and RMSE is 0.037

Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE

This study was funded by the WSL-internal COSMIC project (5233.00148.001.01), the ETHZ (Laboratory of Ion Beam Physics), the Swiss National Science Foundation (SNF Grant 200021L_157187/1), and as the Czech Republic Grant Agency project no. 17-22102s.Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale. Moreover, it is unknown if atmospheric radiocarbon enrichment events of cosmogenic origin leave spatiotemporally consistent fingerprints. Here we measure the 14C content in 484 individual tree rings formed in the periods 770–780 and 990–1000 CE. Distinct 14C excursions starting in the boreal summer of 774 and the boreal spring of 993 ensure the precise dating of 44 tree-ring records from five continents. We also identify a meridional decline of 11-year mean atmospheric radiocarbon concentrations across both hemispheres. Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation. To improve understanding of the return frequency and intensity of past cosmic events, which is particularly important for assessing the potential threat of space weather on our society, further annually resolved 14C measurements are needed.Publisher PDFPeer reviewe

Monitoring energy, water, and carbon fluxes at the surface : using remote sensing techniques and modelling of solar radiation absorbed by the vegetation

Au niveau global, il a été estimé qu’une augmentation de 4% de l’albédo (ou réflectivité) de la surface provoquerait une diminution de 0,7° de la température d’équilibre de la Terre. Or les propriétés des surfaces (dont l’albédo) changent sous la pression climatique et l’action de l’homme. Parallèlement à ce changement des propriétés de surface un débat divise la communauté scientifique sur une éventuelle diminution ou augmentation du rayonnement incident à la surface depuis le milieu des années 1980 (conséquence d’une augmentation ou diminution de la concentration d’aérosols dans l’atmosphère). La Terre est un système complexe piloté en sa surface par 3 cycles (énergie, eau et carbone). Ces cycles ne sont pas insensibles à ces changements de propriété de réflectivité de surface, de rayonnement solaire incident ou de concentration en aérosols. Certains avancent ainsi qu’une augmentation du rayonnement diffus durant les dernières décennies aurait déjà entraîné un excédent de captation de carbone par la végétation de 9.3%. La problématique ici soulevée est d’évaluer l’apport de la connaissance du flux solaire absorbé par la surface (combinaison du rayonnement solaire et de l’albédo de surface) et plus particulièrement par sa partie végétative pour le suivi des flux d’énergie, d’eau et de carbone. Dans ce travail, j’ai fait appel à l’observation satellitaire et à la modélisation du transfert radiatif pour cartographier la dynamique du rayonnement solaire absorbé par la surface et sur la verticale de la végétation. Dans un premier temps, chacune des sources d’incertitudes sur le rayonnement incident, sur l’albédo de surface mais aussi sur la répartition du rayonnement entre les hétérogénéités horizontales et verticales à la surface furent quantifiées. Puis tout en discutant l’effet de ces incertitudes, j’ai mesuré l’apport de l’utilisation de cette cartographie par satellite du rayonnement solaire absorbé pour estimer les flux d’énergie et d’eau en surface ; ce qui améliora les scores de prévision du temps à court terme et permis également de suggérer des rétroactions à l’échelle climatique sur des zones sensibles tel le Sahel. Aussi une correction de biais de 15% sur l’estimation de la production primaire brute de carbone à l’échelle planétaire démontra l’importance des développements réalisés afin de caractériser les hétérogénéités verticales dans le couvert. Finalement, ce travail m’a conduit à chiffrer l’impact de la méconnaissance des variabilités spatiales et temporelles des propriétés des aérosols (concentration et type). J’ai montré que le suivi au cours du temps des propriétés de directionalité de la réflectivité de surface (tel abordé dans la première partie de mon étude) pouvait aussi permettre de remonter à la quantité d’aérosol dans l’atmosphère. L’utilisation d’observations issues de satellite géostationnaire permet d’estimer la concentration en aérosol avec la même qualité mais avec une fréquence de détection plus élevée (x5 environ) que les méthodes classiques. Enfin, ce travail dresse des pistes pour améliorer la détection des changements des propriétés de réflectivité de surface et d’aérosols de l’atmosphère, et atteindre un suivi encore meilleur des cycles biogéochimiques de la biosphère terrestre.It is known that a global 4% increase of land surface albedo (also called reflectivity) may result approximately in a decrease of 0.7°C in the Earth’s equilibrium temperature. Nowadays the surface properties (including albedo) are changing under climatic and human pressure. At the same time, there is a debate that divides the scientific community about the potential trends (increase or decrease) affecting the surface incoming solar radiation since mid-1980 (resulting of a decrease or increase of aerosol concentration in the atmosphere, respectively). The Earth is a complex system driven at the surface level by three cycles (energy, water, and carbon). These cycles are not insensitive to changes of surface reflectivity, incoming radiation, or aerosol properties. For example, some argue that the increase of diffuse radiation during the last decades would have led to an exceed of carbon uptake by the Earth’s vegetation of 9.3%. The main issue raised here is to assess the added value of the knowledge in absorbed solar radiation by the surface (combination of incoming solar radiation with surface albedo) and, especially, by the vegetation for the monitoring of energy, water and carbon fluxes.In this work, I have used satellite observations and modeled the radiative transfer theory in order to make dynamic mapping of solar radiation absorbed by the surface and through the vertical dimension of the vegetation. First, I quantified each uncertainty source affecting incoming solar radiation, surface albedo and the way radiation is split between horizontal and vertical heterogeneity. In a second step, I measured the added value of using this absorbed radiation mapping of the surface by satellite to estimate the energy and water fluxes at the surface. The resulting improved scores of weather forecast models in the short-range time scale suggested potential feedbacks at the climatic time scale over sensible areas such as the Sahel region. Another significant outcome is that the developments proposed to better characterize the vertical heterogeneity within the canopy led to an improvement of 15% of annual global terrestrial gross primary production (GPP). Moreover, this study has led to measure the impact of the lack of knowledge of spatial and temporal variability of aerosol properties (concentration and type). I have shown that the tracking of temporal changes of directional properties of reflectance allows me to retrieve to the amount of aerosols in the atmosphere as precisely as other widely used methods but with a higher frequency (5 times more) by using data from geostationary satellite. Finally, this study addresses some possibilities to better track temporal changes of properties of reflectivity of surface and aerosol of atmosphere, and to access to a better monitoring of biogeochemical cycles of the terrestrial biosphere

Daily estimates of aerosol optical thickness over land surface based on a directional and temporal analysis of SEVIRI MSG visible observations

International audienceThis paper presents an innovative method for obtaining a daily estimate of a qualitycontrolled aerosol optical thickness (AOT) of a vertical column of the atmosphere over the continents. Because properties of land surface are more stationary than the atmosphere, the temporal dimension is exploited for simultaneous retrieval of the surface and aerosol bidirectional reflectance distribution function (BRDF) coming from a kerneldriven reflectance model. Off-zenith geometry of illumination enhances the forward scattering peak of the aerosol, which improves the retrieval of AOT from the aerosol BRDF. The solution is obtained through an unconstrained linear inversion procedure and perpetuated in time using a Kalman filter. On the basis of numerical experiments using the 6S atmospheric code, the validity of the BRDF model is demonstrated. The application is carried out with data from the Spinning Enhanced Visible and Infra Red Imager (SEVIRI) instrument on board the geostationary Meteosat Second Generation (MSG) satellite from June 2005 to August 2007 for midlatitude regions and from March 2006 to June 2006 over desert sites. The satellite-derived SEVIRI AOT compares favorably with Aerosol Robotic Network (AERONET) measurements for a number of contrasted stations and also similar Moderate Resolution Imaging Spectroradiometer (MODIS) products, within 20% of relative accuracy. The method appears competitive for tracking anthropogenic aerosol emissions in the troposphere and shows a potential for the challenging estimate of dust events over bright targets. Moreover, a high-frequency distribution of AOT provides hints as to the variability of pollutants according to town density and, potentially, motor vehicle traffic. The outcomes of the present study are expected to promote a monitoring of the global distributions of natural and anthropogenic sources and sinks of aerosol, which are receiving increased attention because of their climatic implications

How will climate change affect the vegetation cycle over France? A generic modeling approach

The implementation of adaptation strategies of agriculture and forestry to climate change is conditioned by the knowledge of the impacts of climate change on the vegetation cycle and of the associated uncertainties. Using the same generic Land Surface Model (LSM) to simulate the response of various vegetation types is more straightforward than using several specialized crop and forestry models, as model implementation differences are difficult to assess. The objective of this study is to investigate the potential of a LSM to address this issue. Using the SURFEX (“Surface Externalisée”) modeling platform, we produced and analyzed 150-yr (1950–2100) simulations of the biomass of four vegetation types (rainfed straw cereals, rainfed grasslands, broadleaf and needleleaf forests) and of the soil water content associated to each of these vegetation types over France. Statistical methods were used to quantify the impact of climate change on simulated phenological dates. The duration of soil moisture stress periods increases everywhere in France, especially for grasslands with, on average, an increase of 9 days per year in near-future (NF) conditions and 36 days per year in distant-future (DF) conditions. For all the vegetation types, leaf onset and the annual maximum LAI occur earlier. For straw cereals in the Languedoc-Provence-Corsica area, NF leaf onset occurs 18 days earlier and 37 days earlier in DF conditions, on average. On the other hand, local discrepancies are simulated for the senescence period (e.g. earlier in western and southern France for broadleaf forests, slightly later in mountainous areas of eastern France) for both NF and DF. Changes in phenological dates are more uncertain in DF than in NF conditions in relation to differences in climate models, especially for forests. Finally, it is shown that while changes in leaf onset are mainly driven by air temperature, longer soil moisture stress periods trigger earlier leaf senescence over most of France. This shows that developing in situ soil moisture networks could help monitoring the long-term impacts of climate change

What is the potential of cropland albedo management in the fight against global warming? A case study based on the use of cover crops

International audienceLand cover management in agricultural areas is a powerful tool that could play a role in the mitigation of climate change and the counterbalance of global warming. First, we attempted to quantify the radiative forcing that would increase the surface albedo of croplands in Europe following the inclusion of cover crops during the fallow period. This is possible since the albedo of bare soil in many areas of Europe is lower than the albedo of vegetation. By using satellite data, we demonstrated that the introduction of cover crops into the crop rotation during the fallow period would increase the albedo over 4.17% of Europe's surface. According to our study, the effect resulting from this increase in the albedo of the croplands would be equivalent to a mitigation of 3.16 MtCO(2)-eq.year(-1) over a 100 year time horizon. This is equivalent to a mitigation potential per surface unit (m(2)) of introduced cover crop over Europe of 15.91 gCO(2)-eq.year(-1). m(-2). This value, obtained at the European scale, is consistent with previous estimates. We show that this mitigation potential could be increased by 27% if the cover crop is maintained for a longer period than 3 months and reduced by 28% in the case of no irrigation. In the second part of this work, based on recent studies estimating the impact of cover crops on soil carbon sequestration and the use of fertilizer, we added the albedo effect to those estimates, and we argued that, by considering areas favourable to their introduction, cover crops in Europe could mitigate human-induced agricultural greenhouse gas emissions by up to 7% per year, using 2011 as a reference. The impact of the albedo change per year would be between 10% and 13% of this total impact. The countries showing the greatest mitigation potentials are France, Bulgaria, Romania, and Germany