Seasonal Variations in Drag Coefficient over a Sastrugi-Covered Snowfield in Coastal East Antarctica     

Charles Amory  Hubert Gallée  Florence Naaim-Bouvet  Vincent Favier  Etienne Vignon  Ghislain Picard  Alexandre Trouvilliez  Luc Piard  Christophe Genthon  Hervé Bellot 《Boundary-Layer Meteorology》2017,164(1):107-133

The surface of windy Antarctic snowfields is subject to drifting snow, which leads to the formation of sastrugi. In turn, sastrugi contribute to the drag exerted by the snow surface on the atmosphere and hence influence drifting snow. Although the surface drag over rough sastrugi fields has been estimated for individual locations in Antarctica, its variation over time and with respect to drifting snow has received little attention. Using year-round data from a meteorological mast, seasonal variations in the neutral drag coefficient at a height of 10 m \((C_{{ DN}10})\) in coastal Adelie Land are presented and discussed in light of the formation and behaviour of sastrugi based on observed aeolian erosion patterns. The measurements revealed high \(C_{{ DN}10} \) values \((\ge \) 2 \(\times \) 10\(^{-3})\) and limited drifting snow (35% of the time) in summer (December–February) versus lower \(C_{{ DN}10} \) values \((\approx \) 1.5 \(\times \) \(10^{-3})\) associated with more frequent drifting snow (70% of the time) in winter (March–November). Without the seasonal distinction, there was no clear dependence of \(C_{{ DN}10} \) on friction velocity or wind direction, but observations revealed a general increase in \(C_{{ DN}10} \) with rising air temperature. The main hypothesis defended here is that higher temperatures increase snow cohesion and the development of sastrugi just after snow deposition while inhibiting the sastrugi streamlining process by raising the erosion threshold. This increases the contribution of the sastrugi form drag to the total surface drag in summer when winds are lighter and more variable. The analysis also showed that, in the absence of erosion, single snowfall events can reduce \(C_{{ DN}10} \) to \(1\,\times \,10^{-3}\) due to the burying of pre-existing microrelief under newly deposited snow. The results suggest that polar atmospheric models should account for spatial and temporal variations in snow surface roughness through a dynamic representation of the sastrugi form drag.  相似文献   

Cluster Analysis of Atmospheric Dynamics and Pollution Transport in a Coastal Area     

Anton Sokolov  Egor Dmitriev  Elena Maksimovich  Hervé Delbarre  Patrick Augustin  Cyril Gengembre  Marc Fourmentin  Nadine Locoge 《Boundary-Layer Meteorology》2016,161(2):237-264

  相似文献   

Micro- to mesoscale evolution of beaches in response to climatic shift: observation and conceptual modelling (Brittany,France)     

Regnauld  Hervé  Binois  Sandrine  Fouqué  Christophe  Lemasson  Loic 《Geologie en Mijnbouw》1998,77(3-4):323-332

In the framework of the European project 'Climate change and coastal evolution in Europe', the microscale (< 1 year) morphological evolution of beaches in Brittany (W France) has been monitored for some years and linked to immediate forcings, the first one being tides (with a 13-m tidal range), and the second storm intensity. For the same sites, a mesoscale (decades) evolution was reconstructed from air photographs and historical data. The evolution is forced first of all by human activities, and secondly by climatic shift: wind direction changes and average wind velocity evolution. These results were used to build and calibrate a model of coastal behaviour. The model is based on a simple sediment cell (input – transit – output) and simulates the movements of volumes of sands under successive morphodynamic processes. Each of these processes is modelled (in terms of volume exchanges) and parameterized by the relevant atmospheric element. Thus, the model is not deterministic but simulates different probabilities for atmospheric events and morphological response. The model runs for one year and produces realistic sediment accumulation values and coastal retreat rates. When run for half a century, results are also quite comparable to the field data. Storm frequency changes appear to be an important element for the evolution, but the main and first forcing is the availability of sediment.  相似文献   

Contexte lithostructural, âges 40K40Ar et géochimie du volcanisme calco-alcalin tertiaire de Cap-d'Ail dans le tunnel ferroviaire de Monaco     

Jean-Pierre Ivaldi  Hervé Bellon  Pierre Guardia  Christian Mangan  Carla Müller  Jean-Louis Perez  Serge Terramorsi 《Comptes Rendus Geoscience》2003,335(4):411-421

Lithostratigraphic and structural framework, chronology and geochemistry of the Tertiary calc-alkaline volcanism of Cap-d'Ail (French Maritime Alps) are specified according to the new data collected during the earthworks of the recent railway tunnel built between Cap-d'Ail and Monaco. Two different magmatic events were dated: the first one, to Palaeogene, the other one, to Neogene. To cite this article: J.-P. Ivaldi et al., C. R. Geoscience 335 (2003).  相似文献   

Geodynamic control on Messinian clay sedimentation in the Central Mediterranean Sea     

Hervé Chamley 《Geo-Marine Letters》1989,9(3):179-184

Study of clay sedimentation in the Tyrrhenian area during late Miocene time allows to distinguish continental, synsedimentary, and diagenetic influences on the composition of sedimentary rocks, according to the geographic location. In south Sicily smectite-rich assemblages were mainly eroded from soils developed under subarid conditions. On the eastern margin of Sardinia abundant micaillite was supplied from exposed areas tectonically rejuvenated during the opening of the Tyrrhenian Sea. In the westernmost part of the Tyrrhenian Basin evaporative environments combined to strong geothermal gradient allowed the formation of sediments rich in euhedral chlorite.  相似文献   

Can bottom-up procedures improve the performance of stream classifications?     

Ton Snelder  José Barquín Ortiz  Doug Booker  Nicolas Lamouroux  Hervé Pella  Ude Shankar 《Aquatic Sciences - Research Across Boundaries》2012,74(1):45-59

Top-down methods for defining stream classifications are based on a conceptual model or expert-defined rules, whereas bottom-up methods use biological training data and statistical modelling. We compared the performance of six classification methods for explaining the taxonomic composition of invertebrate and fish assemblages recorded at 327 and 511 sites, respectively, distributed throughout France. Classification 1 and 2 were top-down classifications; The European Water Framework System A (WFDa,) and the French Hydro-ecoregions (HER 2). Four bottom-up classification procedures of increasing complexity were defined based on 11 variables that included watershed characteristics describing climate, topography, and geology, and site characteristics including elevation, bed slope and temperature. Classification 3 was defined using matrix correlation (MC) to select a combination of variable categories that produced the best discrimination of the observed taxonomic composition. Classification 4 and 5 were defined by clustering the sites based on their taxonomic data and then using linear discriminant analysis (LDA) and Random forests (RF) to discriminate the clusters based on the environmental variables. Classification 6 was defined using generalized dissimilarity modelling (GDM). Our hypothesis was that the bottom-up classifications would perform better because they flexibly accommodate complex relationships between compositional and environmental variation. We tested the classifications using the classification strength statistic (CS). The RF-based classification fitted the taxonomic patterns better than GDM or LDA and these latter classifications generally fitted better than the MC, WFDa or HER classifications. Cross validation analysis showed that differences in predictive CS (i.e. the CS statistics produced from sites not used in defining the classifications) were often significant. However, these differences were generally small. Gains in predictive performance of classifications appear to be small relative to the increase in complexity in the manner in which environmental variables are combined to define classes.  相似文献   

Bedload tracing in a high‐sediment‐load mountain stream     

Frédéric Liébault  Hervé Bellot  Margot Chapuis  Sébastien Klotz  Michaël Deschâtres 《地球表面变化过程与地形》2012,37(4):385-399

This paper reports a radiofrequency identification (RFID) tracing experiment implemented in a high‐sediment‐load mountain stream typical of alpine gravel‐bed torrents. The study site is the Bouinenc Torrent, a tributary to the Bléone River in southeast France that drains a 38·9‐km² degraded catchment. In spring 2008, we deployed 451 tracers with b‐axis ranging from 23 to 520 mm. Tracers were seeded along eight cross‐sections located in the upstream part of the lowest 2·3 km of the stream. Three tracer inventories were implemented in July 2008, 2009 and 2010. Recovery rates calculated for mobile tracers declined from 78% in 2008 to 45% in 2009 and 25% in 2010. Observations of tracer displacement revealed very high sediment dispersion, with frontrunners having travelled more than 2 km only three months after their deployment. The declining recovery rate over time was interpreted as resulting from rapid dispersion rather than deep burial. We evaluated that 64% of the tracers deployed in the active channel were exported from the 2·3‐km study reach three years after the onset of the tracing experiment. Travel distances were characterized by right‐skewed and heavy‐tailed distributions, correctly fitted by a power‐law function. This supports the idea that in gravel‐bed rivers with abundant sediment supply relative to transport capacity, bedload transport can be viewed as a superdiffusive sediment dispersion process. It is also shown that tracers initially deployed in the low‐flow channel were characterized by a 15‐ to 30‐fold increase of mobility compared to tracers deployed in gravel bars. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

40Ar/39Ar temporal framework for the Alleret maar lacustrine sequence (French Massif-Central): Volcanological and paleoclimatic implications     

Sébastien Nomade  Agnès Gauthier  Hervé Guillou  Jean-François Pastre 《Quaternary Geochronology》2010,5(1):20-27

The Alleret maar (Massif Central, France) is part of the few Western European early middle Pleistocene lacustrine sequences. In the AL3 core several new ash layers were recovered in the 10 first meters of the sedimentary filling. We obtained three ⁴⁰Ar/³⁹Ar ages, which range from 683 ± 5 ka (MSWD: 1.2, n = 17) to 722 ± 6 ka (MSWD: 3.2, n = 18). All the studied ash layers belong to the Super-Besse eruptive cycle of the Sancy volcano. Based on the chronostratigraphy that we have derived we estimate that the age of the main eruption could correspond to the Sancy volcano caldera formation at 725 ka close to the end of MIS 18 and that the Super-Besse explosive episode duration lasted only about 40 ka. The time framework we build evidences that the Alleret lacustrine sequence represents a time interval of probably 180 ka spanning from MIS 18 to MIS 14. This sequence offers the first well constrained comparison between terrestrial environmental history and that preserved in marine sediments during the Mid-Pleistocene Revolution.  相似文献   

Regional hydromorphological characterization with continuous and automated remote sensing analysis based on VHR imagery and low‐resolution LiDAR data          下载免费PDF全文

Luca Demarchi  Simone Bizzi  Hervé Piégay 《地球表面变化过程与地形》2017,42(3):531-551

Recent developments in remote sensing (RS) technologies lead the way in characterizing river morphology at regional scales and inferring potential channel responses to human pressures. In this paper, a unique regional database of continuous hydromorphological variables (HyMo DB) based on areal and topographic data has been generated from RS analysis. Key riverscape units with specific geomorphic meaning have been automatically mapped for 1700 km² of river floodplains from simultaneous very‐high‐resolution (VHR) near‐infrared aerial imagery and low‐resolution LiDAR‐derived products. A multi‐level, geographical object‐based architecture (GEOBIA) was employed to integrate both spectral and topographic information and generate a regional classifier able to automatically map heterogeneous fluvial patterns in different geographical and topographical contexts of the Piedmont Region (Italy). This HyMo‐generated DB offers a unique set of tools for hydromorphologists and can be exploited for different purposes. For the first time, topographic information can be exploited regionally per riverscape unit class, allowing for quantitative analysis of their regional spatial and statistical variability. In this manner, river types can be automatically characterized and classified using objective and repeatable hydromorphological variables. We discuss the potential of quantifying functional links between riverscape units and their driving processes, a valuable source of information to start assessing and highlighting the entity of potential channel adjustments at the regional scale to human pressures. The HyMo DB can also be integrated with historical, field‐based information to better comprehend current fluvial changes at a local scale. In view of future RS acquisitions, the present approach will result in a suitable procedure for quantitative, objective and continuous monitoring of river evolutions over large scales. This type of hydromorphological characterization will allow regional trends and patterns to be highlighted through time and river management strategies to thus be implemented at both regional and local scales. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Wall-slab joint parameter identification of a reinforced concrete structure using possibly corrupted modal data     

Hugo Oliveira  François Louf  Estelle Hervé-Secourgeon  Fabrice Gatuingt 《国际地质力学数值与分析法杂志》2020,44(1):19-39

In numerical models, the connections among component members are crucial for the prediction of structural behaviour under different types of solicitations. In reinforced structures, the connections are often assumed rigid, what may not be realistic in many practical cases. As alternative, a semi-rigid behaviour depending on a set of independent parameters can be proposed. In this case, a new difficulty arises, which is finding the appropriate values for those parameters. The present study proposes a numerical strategy for identification of the connection parameters based on the constitutive relation error (CRE). To include all available information, an augmented version (Modified CRE) is implemented. The parameters search is iterative and require large amount of system response analysis. To increase the computational efficiency, a reduced order model is adopted. The proposed approach shows low-sensitivity to limited lack of information and also to support condition variability, both of them verified numerically. In this work, experimental tests for a real 1:4 scale structure is utilized for finding the parameters corresponding to the first three modal shapes. A good agreement between numerical predictions and observations is verified, what highlights the accuracy and stability of the proposed numerical approach. The present study may also find applications in the domain of design of experiments.  相似文献