Literatura académica sobre el tema "Schéma de neige"

Cet article présente les développements récents au Centre européen pour les prévisions météorologiques à moyen terme (CEPMMT) concernant la représentation des surfaces continentales pour la modélisation globale en prévision du temps à diverses échelles (de quelques jours à la saison), ainsi que pour les réanalyses de l'atmosphère. Les représentations de l'hydrologie, de la végétation, de la neige et des lacs ont été améliorées en s'appuyant sur des mesures in situ et des observations de télédétection spatiale. Les progrès récents en vue d'une initialisation réaliste des différentes composantes sont décrits. Plusieurs défis posés par la modélisation du système Terre sont finalement exposés. This paper describes recent developments undertaken at the European Centre for Medium Range Weather Forecasts (ECMWF) regarding the description of land surfaces at global scale for numerical weather prediction at various forecast ranges (from few days to several months) including atmospheric reanalyses. As a result, physical processes describing hydrology, vegetation snow and lakes have been revised and improved using in-situ measurements and space observations. The importance of a realistic initialisation of the various components of the land surface is underlined, including recent progress in this area. Finally, a number of challenges raised by the Earth System Modelling are presented.

Abstract Despite their potential influences on surface water and climate, groundwater processes are generally not represented in climate models. Here, a simple groundwater scheme including two-dimensional flow dynamics and accounting for groundwater–river exchanges is introduced into the global Total Runoff Integrated Pathways (TRIP) river routing model coupled to the Météo-France climate model. This original scheme is tested in offline mode over France at high () and low (0.5°) resolution against a dense network of river discharge and water table observations over the 1970–2010 period, and is compared to the fine-tuned Système d’Analyze Fournissant des Renseignements Atmosphériques à la Neige (SAFRAN)–Interactions between Soil, Biosphere, and Atmosphere (ISBA) coupled hydrometeorological model (MODCOU). In addition, the simulated terrestrial water storage (TWS) variations are compared to the TWS estimates from the Gravity Recovery and Climate Experiment (GRACE) satellite mission. The aquifer basins over France are defined using the World-wide Hydrogeological Mapping and Assessment Programme (WHYMAP) groundwater resources map, a simplified French lithological map, and the International Geological Map of Europe (IGME). TRIP is forced by daily runoff and drainage data derived from a preexisting simulation of the ISBA land surface scheme driven by the high-resolution SAFRAN meteorological analysis. Four simulations are carried out with or without groundwater at both resolutions. Results show that the groundwater scheme allows TRIP to better capture the spatiotemporal variability of the observed river discharges and piezometric heads. Summer base flows are particularly improved over the main rivers of France. Decreasing the horizontal resolution has a limited impact on the simulated discharges, while it slightly degrades the simulation of water table variations.

Zusammenfassung. Das kognitives Modell der Depression postuliert, dass Patienten mit Depression – und Menschen mit erhöhtem Depressionsrisiko – zu stärkerer Aufmerksamkeit auf negative Informationen und bevorzugte Erinnerung negativer Ereignisse neigen ( Beck, 2008 ). Die resultierende negative Interpretation der eigenen Lebenserfahrungen und Erwartung negativer Konsequenzen (Katastrophisieren) führt dann zu einer Schleife von negativer Stimmung, Pessimismus und Anhedonie. In diesem Modell ist also das dysfunktionale kognitive Schema, welches wiederum durch eine Kombination von genetischen und Entwicklungsfaktoren verursacht wird, ein Kernmechanismus des klinischen Syndroms und der zentrale Ansatzpunkt für psychologische Interventionen. Dieser Artikel diskutiert die experimentelle Evidenz für solche dysfunktionalen Schemata, besonders im Bereich der Übergewichtung negativer Informationen bei Prozessen der Aufmerksamkeit und des Gedächtnisses. Die computationale Entscheidungstheorie kann erklären, wie aus solchen Übergewichtungen negativer Informationen (und Untergewichtung positiver Informationen) sowohl Verhaltenssymptome der Depression (psychomotorische Verlangsamung) als auch pessimistische Grundeinstellungen entstehen können. Ein solcher „negative bias“ kann der gesunden Emotionsregulierung im Wege stehen und damit eine Vulnerabilität für Depression begründen. Die zunehmenden Erkenntnissen über die kognitiven Prozesse der Depression können für die Entwicklung von Früherkennungsverfahren klinisch relevant werden und neue Therapieansätze, sowohl in der computergestützen kognitiven Intervention (z.B. „cognitive bias modification“) als auch in der Selbstregulierung des Gehirns (Neurofeedback), begründen.

Abstract The Saint-Gilles breccias, on the western flank of Piton des Neiges volcano, are clearly identified as debris avalanche deposits. A petrographic, textural and structural analysis of the breccias and inter-bedded autochthonous lava flows enables us to distinguish at least four successive flank slides. The oldest deposit sampled the hydrothermally-altered inner parts of the volcano, and has a large volume. Failure was favored by the presence of a deep intensely-weathered layer. The younger deposits are from superficial sources, as their products are rarely hydrothermalized and are more vesicular. The breccia formation, and especially the progressive breaking up occurring during the debris avalanche displacement, indicates the existence of high speed transport. In the Cap La Houssaye coastal area, abrasion and striation of the underlying lava formation, as well as the packing features observed in the breccia, are considered to be deceleration structures. Introduction Huge landslides of volcano flanks, whether or not initiated by magmatic intrusions, have been recognized as catastrophic events since the 1980 Mount St Helens eruption. On oceanic shield volcanoes, the contribution of failure to the edifice-building process was proposed by Moore [1964] and suggested elsewhere for Hawaii [Lipman et al., 1985 ; Moore et al., 1989], Reunion island [Lénat et al., 1989], Etna [McGuire et al., 1991], and Canarias [Carracedo, 1994, 1996 ; Marty et al., 1996]. This contribution is particularly obvious in island volcanoes showing a U-shaped caldera open to the ocean. Several mechanisms inherent to the causes of failure have been proposed, such as dyke intrusion [McGuire et al., 1990 ; Iverson, 1995 ; Voight and Elsworth, 1997], caldera collapse [Marty et al., 1997], or volcanic spreading [Borgia et al., 1992 ; van Wyk de Vries and Francis, 1997]. Invariably, other factors have been proposed as favorable to volcanic destabilization, such as the probable occurrence of deep low-cohesion layers due to the existence of pyroclastic or hyaloclastic layers [Duffield et al., 1982 ; Siebert, 1984] or an old basement. Gravity spreading models are now frequently proposed to explain the destruction of volcanic edifices [Borgia et al., 1992 ; Merle and Borgia, 1996 ; van Wyk de Vries and Borgia, 1996 ; van Wyk de Vries and Francis, 1997], most of them taking into account basal or intra-volcanic weakness zones. We propose that in such a scenario, density heterogeneity should be an important factor governing the slow evolution of the volcanic pile. Clague and Denlinger [1994] proposed a olivine-rich ductile basal layer that influences the stability of volcano flanks. On Reunion island, a large volcanic landslide has been proposed to explain the peculiar morphology of Piton de la Fournaise-Grand Brûlé [Vincent and Kieffer, 1978]. Bathymetric surveys [Bachèlery and Montagionni, 1983 ; Lénat et al., 1989, 1990 ; Cochonnat et al., 1990 ; Lénat and Labazuy, 1990 ; Labazuy, 1991 ; Bachèlery, 1995 ; Ollier et al., 1998] have confirmed the offshore occurrence of debris avalanche deposits. Similar deposits are also known to exist along the western, northern and southwestern submarine flanks of the Piton des Neiges volcano. Unlike other deposits showing inland prolongation, “Saint-Gilles breccias” displays a well-preserved and non-weathered texture and structure. Because of striking analogies between the “Saint-Gilles breccias” and, for example, the Cantal stratovolcano debris avalanche deposits [Cantagrel, 1995], we conclude that these formations are the products of repeated avalanches during the Piton des Neiges basaltic period [Bachèlery et al., 1996]. We propose an interpretation of their origin, emplacement mechanism and their role in the evolutionary process of the western flank of Piton des Neiges. Volcano-structural setting Mechanical instability of oceanic volcanic edifices generates huge flank landslides, with lateral and mainly submarine transport of sub-aerial materials. These landslides participate in the building of the lower submarine slopes of the volcano. Geophysical surveys have detected low cohesion materials in most offshore Reunion island areas [Malengrau et al., 1999 ; de Voogd et al., 1999 ; Lénat et al., 2001] showing that these materials have largely contributed to the construction of offshore Reunion Island. Such deposits are also found in the inner part (“Cirques”) of Piton des Neiges [Maillot, 1999]. On the other hand, electric and electromagnetic soundings have revealed a deep extending conductor within the Piton de la Fournaise volcanic pile [Courteaud et al., 1997 ; Lenat et al., 2000]. Interpretations about the nature and origin of this conductor depend on its location. In the central caldera zone, as revealed by SP positive anomalies [Malengrau et al., 1994 ; Zlotnicki et al., 1994], the hydrothermal and magmatic complex is probably responsible for the observed low resistivities. Along the flanks, such a hypothesis may not be realistic. Courteaud [1996] suggests the occurrence of a deep argilized layer of volcano-detritic origin. In any case, the hydrothermal complex with high fluid pressures and secondary minerals appears as a potential weak zone that may contribute to the volcano’s instability [Lopez and Williams, 1993 ; Frank, 1995]. Chronology and stratigraphy Extent of the debris avalanche deposits The various breccias found at the western end of Reunion island, on the Piton des Neiges volcano flank, cover a 16 km2 area between Cap Marianne and Saint-Gilles (fig. 1). They are overlain upwards (> 250 to 300 m) by trachyandesitic (mugearite) lava flows of Piton des Neiges differentiated series [Billard, 1974]. Some restricted breccia outcrops in deep valleys from Bernica to the north up to l’Hermitage to the south indicate the existence of larger extension of the debris avalanche deposits. Furthermore, breccias with similar “Saint-Gilles” facies appear down the Maïdo cliff to Mafate “Cirque” at an altitude 1300 m, beneath 600 m of mugearite and some olivine basalt flows. Unpublished electromagnetic data (CSAMT soundings) confirm the inland continuity of the “Saint-Gilles breccias” up to the Maïdo along the Piton des Neiges western flank, hidden by mugearitic flows. Available bathymetric surveys offshore Saint Paul – Saint Gilles areas show the obvious underwater prolongation of “Saint-Gilles breccias” : a shallow depth (< 100 m) plateau followed by a slope with hummocky surface down to 2 500 m depth [Bachèlery et al., 1996 and fig. 2]. From this data, the total surface of “Saint-Gilles” debris avalanche deposits is estimated as more than 500 km2. Chronology A coastal cliff, from Ravine Bernica to Boucan Canot, provides the best outcrop of the northern part of “Saint-Gilles breccias”, with a clear inter-bedding of breccia units and lava formations (photo 1and fig. 3). – The lower breccia unit (Br I), of unknown thickness, has a remarkable friable aspect and a grayish color. – The first autochthonous lava formation (L1) consists in thin pahoehoe olivine basalt flows filling large valleys dug into “Br I”. The top of this formation is striated by the overlying “Br II” unit (photo 2). – Breccia unit “Br II” is interbedded between L1 and L2 olivine basalts. More compact and massive, “Br II” is characterized by a reddish matrix and dark blocks, with many curved fracture surfaces. – On “Br II” or directly on L1, picritic basalt flows L2 are found, filling narrow valleys. – Breccia unit “Br III” lies on “Br II” with a striking sheared contact plane visible along the main road (photo 3). It is a typical debris avalanche deposit with large imbricate blocks within a fine-grained beige matrix. – Once again, basaltic flows of lava formation L3 fill a valley dug into “Br III” near Petite Anse river. – Breccia unit “Br IV” rests on L3 at Petite Anse, but its contact with “Br III” elsewhere is not clear. The facies of this unit is very similar to the “Br III”. All the breccia units are covered by basaltic and trachyandesitic flows from the end of the Piton des Neiges basaltic series, and differentiated series. In the Saint-Gilles river, two formations are superposed : picritic basalts (L4) have flowed on the “Br IV” breccia unit, latter aphyric trachy-andesitic (mugearite) flows (L6) overlapped L4 and the breccia landforms, reaching in places the coastal area. To the north, at Plateau Caillou, plagioclase-phyric basalt flows (L5) are found between mugearite and breccias. Elsewhere on Piton des Neiges, such flows are symptomatic of the transition from the basaltic series to the differentiated series [Billard, 1974]. The occurrence of autochthonous basaltic formations L1 to L3, inter-bedded with “Saint-Gilles breccias”, enables us to distinguish at least four superposed breccia units. Although the emplacement age of the lower “Br I” is not known precisely, it is overlain and therefore older than Cap Marianne pahoehoe lavas (L1) dated at 0.452 Ma [Mc Dougall, 1971]. On the other hand, the upper breccia units are younger than the pahoehoe olivine basalt at Cap la Houssaye dated at 0,435 Ma but older than L5 plagioclasic basalts dated at 0.35 Ma. Geological description of the “breccia sequence” In the synthetic lithologic log (fig. 4) of the Saint-Gilles area, autochthonous lava formations are clearly broken into four separate breccia units. Lava formations. – L1 formation consists of numerous thin pahoehoe olivine-rich to aphyric basaltic flows. Both L2 and L3 formations are characterized by a few thicker (decametric) olivine (frequently picritic) basalt flows. Breccia units. – All breccia units display common characteristics such as the universal association of two facies (photo 4) : (i) a matrix – sandy to silty – facies containing a non-sorted mixture of non-stratified heterogeneous materials ranging from granular size to blocky elements, (ii) coherent large blocks and large pieces (‘block’ facies) of various lithology such as lava flow, scorias, pyroclastics or other breccias ; blocks displaying frequent “jigsaw” features. The lower breccia unit “Br 1” (fig. 4) has a more compact but very heterogeneous aspect, with a chaotic distribution of blocks in a less-developed matrix. This unit is characterized by a deep hydrothermal alteration with a lot of zeolites, chlorite, clays, calcite and oxides. The upper breccia units, “Br II” to “Br IV” (fig. 4) are less heterogeneous than “Br I” because their matrix facies are more voluminous and because the matrix clearly separates the bigger blocks. In both facies, a great diversity of fresh lithologic types such as picritic basalt, olivine-phyric basalt, plagioclase-phyric basalt and aphyric more or less vesicular basalts, gabbro, dunite are found, with no or only few slightly zeolitised blocks. Plurimetric to metric blocks are severely fractured, disintegrated into millimetric to decimetric angular pieces. The frequent polygenic aspect is due to block juxtaposition or imbrication. The abundant matrix is composed of crushed rocks and mineral elements, fine-grained (< mm), showing frequent fluidity and bedding marks (photo 5). The very heterogeneous composition of the matrix is confirmed at a microscopic scale. On the contrary, cores of blocks appear as jigsaw-puzzle-like monolithologic pieces of various basaltic rocks. At their edges, disintegration leads to progressive mixing with neighboring blocks that feed the matrix. Discussion Originality of “Saint-Gilles breccias” “Saint-Gilles breccias” constitute one of the few cases [see also Cantagrel et al., 1999] of debris avalanche deposit outcroppings on the sub-aerial part of an oceanic shield volcano. The main part of the deposit is suspected to be offshore. Their hummocky surface in delineating parallel ridges can be compared to the one described offshore the Grand Brûlé area, east of Piton de la Fournaise [Bachèlery et al., 1996]. “Saint-Gilles breccias” were deposited after several Piton des Neiges flank slide events that were separated by basaltic flows. Repeated debris avalanches have also been proposed to explain Piton de la Fournaise offshore deposits [Lenat et al., 1990 ; Labazuy, 1991]. The occurrence of autochthonous interbedded lava formations is essential to interpret the thick piling up of slide material along Reunion volcano flanks as deposits of repeated avalanches at the same place, instead of as being the products of a single huge event. Many structural and textural features noticed in the upper breccia units reveal crucial information on the emplacement mechanism of debris avalanches. For instance, brecciated blocks are typical of progressive break-up during transport processes. Blocks can simply be fractured, or they can be so severely disintegrated that stretching and mixing with other blocks and matrix formation are observed. The observation of such phenomena implies the existence of numerous percussive events between rocks, as well as internal vibrations in the debris avalanche and therefore the existence of high-speed transport. Lava formations L1 underlying upper breccia units are truncated and strongly striated in a seaward direction (photo 2), parallel to the breccia morphological ridges. In the same way, internal contact surfaces between upper breccia units are shear planes underlain by cataclastic layers and lenses (photo 3). Such structures are interpreted as due to drastic deceleration effects of avalanches reaching a topographic leveling out in the coastal area. This concords with the occurrence of sub-vertical contact areas between the blocks and the matrix. These injections of matrix between the blocks are generated bottom-up from the shear plane at the moment of the sudden deceleration of the avalanche. Other fracture planes that are in accordance with the morphology of ridges, are found in “Br III” unit (see fig. 5). They are interpreted as the result of packing effects. Origin of flank failures Although the source area of breccia formations has not yet been clearly identified, it has to be in the central part of Piton des Neiges as seen in the western cliff of “cirque de Mafate”. Furthermore, “Br I” deeply weathered materials evidently come from the hydrothermalized core of the volcano. Though the “Br I” thickness is not known, the volume involved may be considerable and a part of this volume must constitute the main body of Saint-Gilles offshore deposits. The upper breccias units “Br II” to “Br IV” display very similar textures and lithologies, with dominant non-altered basaltic rocks from the “Phase II” building stage of Piton des Neiges [Billard, 1974]. These units are very thin in the coastal area of Cap La Houssaye (see fig. 2) despite a proximal facies (meaning a deposit in the transport zone nearer than the main deposit zone). They obviously originate from shallow flank slides of restricted extent. We suggest that the upper Saint-Gilles deposits are due to repeated events that produced thin high-speed debris avalanches. Emplacement modalities The morphology of “Saint-Gilles breccias”, or submarine deposits offshore Grand Brûlé (east of Piton de la Fournaise volcano), are typical of sliding movements along shallow depth shear planes (several hundred meters up to two kilometers) within the volcanic pile. But several levels of decollement are suggested by seismic refraction and reflection profiles offshore La Reunion, the deepest corresponding to the top of the preexisting oceanic sediments [de Voogt et al., 1999]. Until now, in Reunion Island, only shallow failures affecting the upper parts of volcanic edifices, with deposits on the lower slopes, have been positively identified. Conditions that trigger giant flank landslides affecting oceanic shields remain poorly understood but we can reasonably speculate that weak hydrothermally-altered layers in the inner part of the volcano favor these gravity-driven processes related to repeated dike injections. The “Saint-Gilles breccia” sequence is considered as a multiphase lateral collapse structure whose first event (“Br I”) was apparently the most voluminous. The corresponding deposit displays frequent hydrothermally-altered material symptomatic of originating from the Piton des Neiges core. Within Piton des Neiges, the low cohesive weathered layer is quite extensive [Nativel, 1978 ; Rançon, 1982] possibly reaching down the volcano flanks [Courteaud et al., 1997]. The interpretative scheme that we propose (fig. 6) in our evaluation of the conditions for the emplacement of Saint-Gilles sequence, takes into account the existence of such a mechanical discontinuity within the volcanic pile. We propose that the massive landslide failure of the west flank of Piton des Neiges volcano that produced the “Br I” breccia, provided efficient channels for younger Piton des Neiges lavas to reach the western and southwestern coastline. Morphological features, as well as radiometric data [Mc Dougall, 1971 ; Gillot and Nativel, 1982] and magnetic surveys [Lénat et al., 2001], yield evidence for preferential accumulation of lava during the last 0.5 m.y. (corresponding mainly to the differentiated series) in this part of the volcano. The relative asymmetry of Piton des Neiges was acquired by rift migration in response to the first huge landslide that produced the “Br I” unit of “Saint-Gilles breccia”, in the manner described by Lipman et al. [1990] for Mauna Loa volcano in Hawaii. The later repetition of flank collapses is consistent with similar structures on other oceanic islands. Since the first lateral collapse, the Piton des Neiges edifice was probably characterized by the existence of an asymmetrical steeper western flank where the old zeolite-rich “Br I” deposits possibly act as a detachment surface for later successive landslides which may have occurred recurrently over a short time interval.

Introduction: * The proposed TWIST model aims to achieve a secure MANET by detecting and mitigating packet dropping attack using finite state machine based IDS model. * To determine the trust values of the nodes using context-aware trust calculation * To select the trustworthy nodes as watchdog nodes for performing intrusion detection on the network * To detect and isolate the packet dropping attackers from routing activities, the scheme uses FSM based IDS for differen-tiating the packet dropping attacks from genuine nodes in the MANET. Method: In this methodology, instead of launching an intrusion detection system (IDS) in all nodes, an FSM based IDS is placed in the trustworthy watchdog nodes for detecting packet dropping attacker nodes in the network. The proposed FSM based intrusion detection scheme has three steps. The three main steps in the proposed scheme are context- aware trust calculation, watchdog node selection, and FSM based intrusion detection. In the first process, the trust calculation for each node is based on specific parameters that are different for malicious nodes and normal nodes. The second step is the watchdog node selection based on context-aware trust value calculation for ensuring that the trust-worthy network monitors are used for detecting attacker nodes in the network. The final process is FSM based intrusion detection, where the nodes acquire each state based on their behavior during the data routing. Based on the node behavior, the state transition occurs, and the nodes that drop data packets exceeding the defined threshold are moved to the malicious state and restricted to involve in further routing and services in the network Result: The performance of the proposed (TWIST) mechanism is assessed using the Network Simulator 2 (NS2). The proposed TWIST model is implemented by modifying the Ad-Hoc On-Demand Distance Vector (AODV) protocol files in NS2. Moreover, the proposed scheme is compared with Detection and Defense against Packet Drop attack in the MANET (DDPD) scheme. A performance analysis is done for the proposed TWIST model using performance metrics such as detection accuracy, false-positive rate, and overhead and the performance result is compared with that of the DDPD scheme. After the compare result we have analyzed that the proposed TWIST model exhibits better performance in terms of detection accuracy, false positive rate, energy consumption, and overhead compared to the existing DDPD scheme. Conclusion: In the TWIST model, an efficient packet dropping detection scheme based on the FSM model is proposed that efficiently detects the packet dropping attackers in the MANET. The trust is evaluated for each node in the network, and the nodes with the highest trust value are selected as watchdog nodes. The trust calculation based on parameters such as residual energy, the interaction between nodes and the neighbor count is considered for determining watchdog node selec-tion. Thus, the malicious nodes that drop data packets during data forwarding cannot be selected as watchdog nodes. The FSM based intrusion detection is applied in the watchdog nodes for detecting attackers accurately by monitoring the neigh-bor nodes for malicious behavior. The performance analysis is performed between the proposed TWIST mechanism and existing DDPD scheme. The proposed TWIST model exhibits better performance in terms of detection accuracy, false positive rate, energy consumption, and overhead compared to the existing DDPD scheme Discussion: This work may extend the conventional trust measurement of MANET routing, which adopts only routing behavior observation to cope with malicious activity. In addition, performance evaluation of proposed work under packet dropping attack has not been performed for varying the mobility of nodes in terms of speed. Furthermore, various perfor-mance metric parameters like route discovery latency and malicious discovery ratio which can be added for evaluate the performance of protocol in presence of malicious nodes. This may be considered in future work for extension of protocol for better and efficient results. Furthermore, In the future, the scheme will focus on providing proactive detection of packet dropping attacker nodes in MANET using a suitable and efficient statistical method.

Résumé : Le bilan d’énergie de la Terre est largement influencé par la variation de l’albédo de surface (fraction de l'énergie solaire réfléchie par une surface). Ces variations sont modifiées par la présence, l’épaisseur et les propriétés physiques de la neige. Le réchauffement climatique observé a un impact significatif sur l'évolution du couvert nival, ce qui influence grandement l'albédo de surface, et en retour modifie le climat. Malgré l’importance de l’albédo de surface, plusieurs modèles calculent l’albédo de manière empirique, ce qui peut entraîner des biais significatifs entre les simulations et les observations selon les surfaces étudiées. Le schéma de surface canadien, Canadian Land Surface Scheme, CLASS (utilisé au Canada dans les modèles climatiques Global Climate Model et Modèle Régional Canadien du Climat), modélise l’évolution spatiale et temporelle des propriétés de la neige, dont l'albédo. L’albédo de CLASS est calculé selon la hauteur et l’âge (métamorphisme) de la neige au sol, et selon l’accumulation de la neige sur la canopée. Les objectifs de ce travail sont d’analyser le comportement de l’albédo (simulé et mesuré) et d’améliorer le paramétrage de l’albédo de surface pendant l’hiver sur des régions à l’est du Canada. Plus précisément, le comportement de l’albédo a été étudié par l’analyse de la sensibilité de CLASS 3.6 aux paramètres prescrits (paramètres qui sont utilisés dans les calculs du modèle dont les valeurs sont fixes et définies empiriquement). En plus de l’analyse des variations temporelles de l’albédo en fonction des conditions météorologiques pour les terrains de végétation basse (noté "gazon") et de conifères. Aussi, l’amélioration du paramétrage a été tentée en optimisant (pour le gazon et les conifères) ou en modifiant (pour le gazon) les calculs considérant les paramètres prescrits dont l’albédo de CLASS est sensible. En premier lieu, nous avons montré que la sensibilité de l’albédo de CLASS en terrain de gazon dépend grandement du seuil du taux de précipitation nécessaire pour que l’albédo soit actualisé (à sa valeur maximale) dans le modèle. Faire varier ce seuil entraîne que les simulations quotidiennes d’albédo de surface enneigées vont s’étaler en majorité entre 0.62 à 0.8 (supérieur à l’étalement normalement simulé). Le modèle est aussi sensible à la valeur d’actualisation de l’albédo dont la variation entraîne que l’albédo enneigé quotidien peut s’étaler de jusqu’à 0.48 à 0.9. En milieu forestier (conifères), le modèle est peu sensible aux paramètres prescrits étudiés. La comparaison entre les albédos simulés et les mesures au sol montrent une sous-estimation du modèle de -0.032 (4.3 %) à SIRENE (gazon au sud du Québec), de -0.027 (3.4 %) à Goose-Bay (gazon en site arctique) et de -0.075 (27.1 %) à la Baie-James (forêt boréale). Lorsque comparée avec les données MODIS (MODerate resolution Imaging Spectroradiometer) la sous-estimation du modèle à la Baie-James est de -0.011 (5.2 %). On montre que la valeur de l'albédo mesurée lors des précipitations de neige à Goose Bay est en moyenne supérieure à la valeur d'actualisation de l'albédo dans le modèle (0.896 par rapport 0.84), ce qui peut expliquer la sous-estimation. En forêt, un des problèmes provient de la faible valeur de l'albédo de la végétation enneigée (ajout de 0.17 dans le visible), tandis que l’albédo de surface mesuré peut être augmenté de 0.37 (par rapport à la végétation sans neige). Aussi, l’albédo de la neige sur la canopée ne diminue pas avec le temps contrairement à ce qui est observé. En second lieu, nous avons tenté d’améliorer le paramétrage, en optimisant des paramètres prescrits (aucune amélioration significative n’est obtenue) et en modifiant la valeur d'actualisation de l’albédo de la neige en zone de gazon. Cette valeur, normalement fixe, a été rendue variable selon la température et le taux de précipitations. Les résultats démontrent que les modifications n’apportent pas d'améliorations significatives de la RMSE (Root Mean Square Error) entre les simulations et les mesures d’albédo. Les modifications sont toutefois pertinentes pour ajouter de la variabilité aux fortes valeurs d’albédo simulées ainsi que pour améliorer la compréhension du comportement des simulations d’albédo. Aussi, la méthodologie peut être reproduite pour d’autres études qui veulent étudier la représentativité et améliorer les simulations d’un modèle.
Abstract : The surface energy balance of northern regions is closely linked to surface albedo (fraction of solar radiation reflected by a surface) variations. These variations are strongly influenced by the presence, depth and physical properties of the snowpack. Climate change affects significantly snow cover evolution, and decreases surface albedo and snow albedo with positive feedback to climate. Despite the importance of the albedo, many models empirically compute it, which can induce significant biases with albedo observations depending on studied surfaces. The Canadian Land Surface Scheme, CLASS (used in Canada into the Canadian Regional Climate Model, and the Global Climate Model), simulates the spatial and temporal evolution of snow state variables including the albedo. The albedo is computed according to the depth of snow on the ground as well as the accumulation of snow in trees. The albedo seasonal evolution for snow on ground is estimated in CLASS from an empirical aging expression with time and temperature and a “refresh” based on a threshold of snowfall depth. The seasonal evolution of snow on canopy is estimated from an interception expression with trees type and snowfall density and an empirical expression for unloading rate with time. The objectives of this project are to analyse albedo behavior (simulated and measured) and to improve CLASS simulations in winter for Eastern Canada. To do so, sensitivity test were performed on prescribed parameters (parameters that are used in CLASS computation, their values are fixed, and determined empirically). Also, albedo evolution with time and meteorological conditions were analysed for grass and coniferous terrain. Finally, we tried to improve simulations by optimizing sensitive prescribed parameters for grass and coniferous terrain, and by modifying the refresh albedo value for grass terrain. First, we analysed albedo evolution and modelling biases. Grass terrain showed strong sensitivity to the precipitation rate threshold (for the albedo to refresh to its maximum value), and to the value of the albedo refresh. Both are affected by input data of precipitation rate and phase. The modification of precipitation threshold rate generates daily surface albedo to vary mainly (75 % of data in winter) between 0.62 and 0.8, which is a greater fluctuation than for a normal simulation over winter. The modification of the albedo refresh value generates surface albedo to vary mainly (75 %) between 0.66 and 0.79, but with extreme values, 25 % of data, from 0.48 to 0.9. Coniferous areas showed small sensitivity to studied prescribed parameters. Also, comparisons were made between simulated and measured mean albedo during winter. CLASS underestimates the albedo by -0.032 (4.3 %) at SIRENE (grass in Southern Quebec), by -0.027 (3.4 %) at Goose Bay (grass in arctic site) and by -0.075 (27.1 %) at James Bay (boreal forest) (or -0.011 (5.2 %) compared to MODIS (MODerate resolution Imaging Spectroradiometer) data). A modelling issue in grass terrain is the small and steady maximum albedo value (0.84) compared to measured data in arctic condition (0.896 with variation of an order of 0.09 at Goose Bay, or 0.826 at SIRENE with warmer temperatures). In forested areas, a modelling issue is the small albedo increase (+0.17 in the visible range, +0.04 in NIR) for the part of the vegetation that is covered by snow (total surface albedo gets to a maximum of 0.22) compared to events of high surface albedo (0.4). Another bias comes from the albedo value of the snow trapped on canopy which does not decrease with time in opposition to observed surface albedo which is lower at the end of winter and which suggests snow metamorphism occurred. Secondly, we tried to improve simulations by optimizing prescribed parameters and by modifying the albedo’s maximum value computation. Optimisations were made on sensitive prescribed parameters or on those that seemed unsuited. No significant RMSE (Root Mean Square Error) improvements were obtained from optimisations in both grass and coniferous area. Improvements of albedo simulations were tried by adjusting the maximum value (normally fixed) with temperature and precipitation rate, in grass terrain. Results show that these modifications did not significantly improved simulations’ RMSE. Nevertheless, the latter modification improved the correlation between simulated and measured albedo. These statistics were made with the whole dataset which can reduce the impact of modifications (they were mainly affecting albedo during a precipitation event), but it allows to overview the new model performance. Modifications also added variability to maximum values (closer to observed albedo) and they increased our knowledge on surface albedo behavior (simulated and measured). The methodology is also replicable for other studies that would aim to analyse and improve simulations of a surface model.