Dynamic processes observed at a gas hydrate outcropping on the continental slope of the Gulf of Mexico   总被引：1，自引：2，他引：1  

Michael F. Vardaro  Ian R. MacDonald  Leslie C. Bender  Norman L. Guinasso Jr 《Geo-Marine Letters》2006,26(1):6-15

A deep-sea time-lapse camera and several temperature probes were deployed on the Gulf of Mexico continental shelf at a biological community associated with a gas hydrate outcropping to study topographic and hydrologic changes over time. The deployment site, Bush Hill (GC-185), is located at 27°47.5′ N and 91°15.0′ W at depths of ∼540 m. The digital camera recorded one still image every 6 h for July-October in 2001, every 2 h for the month of June 2002, and every 6 h for the month of July 2002. Temperature probes were in place at the site for the entire experimental period. The data recovered provide a record of processes that occur at gas hydrate mounds. Sediment resuspension over the mound causes significant variation in luminosity of the time-lapse photographs. A marked diurnal pattern can be seen in the temperature and luminosity records. No major change in shape or size of the gas hydrate outcrop at this site was observed during this study. Stable topography of the gas hydrate mound, combined with high bacterial activity and sediment turnover, appears to focus biological activity in the mound area. Frequency and recurrence of sediment resuspension indicate that short-term change in the depth and distribution of surface sediments is a feature of the benthos at the site. Because the sediment interface is a critical environment for hydrocarbon oxidation and chemosynthesis, short-term variability and heterogeneity may be important characteristics of these settings.  相似文献   

Effects of Roof-Edge Roughness on Air Temperature and Pollutant Concentration in Urban Canyons     

Amir A. Aliabadi  E. Scott Krayenhoff  Negin Nazarian  Lup Wai Chew  Peter R. Armstrong  Afshin Afshari  Leslie K. Norford 《Boundary-Layer Meteorology》2017,164(2):249-279

The influence of roof-edge roughness elements on airflow, heat transfer, and street-level pollutant transport inside and above a two-dimensional urban canyon is analyzed using an urban energy balance model coupled to a large-eddy simulation model. Simulations are performed for cold (early morning) and hot (mid afternoon) periods during the hottest month of the year (August) for the climate of Abu Dhabi, United Arab Emirates. The analysis suggests that early in the morning, and when the tallest roughness elements are implemented, the temperature above the street level increases on average by 0.5 K, while the pollutant concentration decreases by 2% of the street-level concentration. For the same conditions in mid afternoon, the temperature decreases conservatively by 1 K, while the pollutant concentration increases by 7% of the street-level concentration. As a passive or active architectural solution, the roof roughness element shows promise for improving thermal comfort and air quality in the canyon for specific times, but this should be further verified experimentally. The results also warrant a closer look at the effects of mid-range roughness elements in the urban morphology on atmospheric dynamics so as to improve parametrizations in mesoscale modelling.  相似文献   

Isotopic studies of leaf water. Part 2: Between-age isotopic variations in pine needles     

Yong Shu  Eric S. Posmentier  Matthew P. Ayres  Leslie J. Sonder 《Geochimica et cosmochimica acta》2008,72(21):5189-5200

We studied between-age oxygen and hydrogen isotopic variations of two pine species, red pine (Pinus resinosa Ait) and white pine (Pinus strobus L.), with particular attention paid to variations associated with needle maturity. The 2D model by Shu et al. (this volume) was applied to simulate oxygen isotopic ratios of the pine needles; calculated results were compared with data collected from trees in the Hanover, NH area. The calculations indicate that the mechanism most responsible for age-dependent isotopic variations is the variation in transpiration rate along the needle. Because stomata mature progressively from needle tip to needle base, young needles transpire most of their water near the tip. Mature needles, on the other hand, transpire water more evenly along their lengths. We cannot successfully simulate hydrogen isotopic variations, and it remains to be investigated what mechanism or physical process is not included in the model that affects hydrogen isotopic compositions in leaf water.  相似文献   

A continuum‐discrete model using Darcy's law: formulation and verification          下载免费PDF全文

Majid Goodarzi  Chung Yee Kwok  Leslie George Tham 《国际地质力学数值与分析法杂志》2015,39(3):327-342

This paper presents a numerical scheme for fluid‐particle coupled discrete element method (DEM), which is based on poro‐elasticity. The motion of the particles is resolved by means of DEM. While within the proposition of Darcian regime, the fluid is assumed as a continuum phase on a Eulerian mesh, and the continuity equation on the fluid mesh for a compressible fluid is solved using the FEM. Analytical solutions of traditional soil mechanics examples, such as the isotropic compression and one‐dimensional upward seepage flow, were used to validate the proposed algorithm quantitatively. The numerical results showed very good agreement with the analytical solutions, which show the correctness of this algorithm. Sensitivity studies on the effect of some influential factors of the coupling scheme such as pore fluid bulk modulus, volumetric strain calculation, and fluid mesh size were performed to display the accuracy, efficiency, and robustness of the numerical algorithm. It is revealed that the pore fluid bulk modulus is a critical parameter that can affect the accuracy of the results. Because of the iterative coupling scheme of these algorithms, high value of fluid bulk modulus can result in instability and consequently reduction in the maximum possible time‐step. Furthermore, the increase of the fluid mesh size reduces the accuracy of the calculated pore pressure. This study enhances our current understanding of the capacity of fluid‐particle coupled DEM to simulate the mechanical behavior of saturated granular materials. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Geoarchaeology of the Boca Negra Wash Area,Albuquerque Basin,New Mexico,USA     

Vance T. Holliday  Bruce B. Huckell  James H. Mayer  Steven L. Forman  Leslie D. McFadden 《Geoarchaeology》2006,21(8):756-802

Dozens of Paleoindian sites, including the Boca Negra Wash (BNW) Folsom site (LA 124474), are scattered across a basalt plateau (the West Mesa) on the western side of the Albuquerque Basin, and adjacent uplands. The BNW site, like many others in the area, is located near a small (˜60 × 90 m) playa basin that formed in a depression on the basalt surface and was subsequently covered by an eolian sand sheet (Unit 1) dated by OSL to ˜23,000 yr B.P. Most of the basin fill is ˜2 m of playa mud (Units 2 and 3) dating ˜13,970 ¹⁴C yr B.P. (17,160–16,140 cal yr B.P.) at the sand–mud interface to ˜2810 ¹⁴C yr B.P. (˜2960–2860 cal yr B.P.) at the top. C/N ratios suggest that the BNW playa basin probably held water more often during the Folsom occupation; stable carbon isotope values indicate C3 vegetation was more common as well, but C4 grasses became dominant in the Holocene. Cores extracted from four playa basins nearby revealed a similar stratigraphy and geochronology, documenting presence of wetlands on playa floors during the Paleoindian occupation of the area. © 2006 Wiley Periodicals, Inc.  相似文献   

Fe K-edge XAFS spectra of phyllosilicates of varying crystallinity     

Leslie L. Baker  Daniel G. Strawn 《Physics and Chemistry of Minerals》2012,39(8):675-684

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The space environment of Mercury at the times of the second and third MESSENGER flybys     

Daniel N. Baker  Dusan Odstrcil  Brian J. Anderson  C. Nick Arge  Mehdi Benna  George Gloeckler  Haje Korth  Leslie R. Mayer  Jim M. Raines  David Schriver  James A. Slavin  Sean C. Solomon  Pavel M. Trávní?ek  Thomas H. Zurbuchen 《Planetary and Space Science》2011,59(15):2066-2074

The second and third flybys of Mercury by the MESSENGER spacecraft occurred, respectively, on 6 October 2008 and on 29 September 2009. In order to provide contextual information about the solar wind properties and the interplanetary magnetic field (IMF) near the planet at those times, we have used an empirical modeling technique combined with a numerical physics-based solar wind model. The Wang–Sheeley–Arge (WSA) method uses solar photospheric magnetic field observations (from Earth-based instruments) in order to estimate the inner heliospheric radial flow speed and radial magnetic field out to 21.5 solar radii from the Sun. This information is then used as input to the global numerical magnetohydrodynamic model, ENLIL, which calculates solar wind velocity, density, temperature, and magnetic field strength and polarity throughout the inner heliosphere. WSA-ENLIL calculations are presented for the several-week period encompassing the second and third flybys. This information, in conjunction with available MESSENGER data, aid in understanding the Mercury flyby observations and provide a basis for global magnetospheric modeling. We find that during both flybys, the solar wind conditions were very quiescent and would have provided only modest dynamic driving forces for Mercury's magnetospheric system.  相似文献   

Lessons learned and need for instrumented liquefaction sites     

T. Leslie Youd  Jamieson H. Steidl  Robert L. Nigbor 《Soil Dynamics and Earthquake Engineering》2004,24(9-10):639

Instrumented sites provide essential information for understanding and modeling of ground response and ground deformation. For example, significant new lessons were learned from responses at the Wildlife Liquefaction Array (WLA) including: (1) soil softening led to lengthening of period of transmitted ground motions; (2) soil softening also led to attenuation of short-period spectral accelerations (<0.7 s); (3) amplification of long period motions (>0.7 s) occurred due to liquefaction-induced ground oscillation; and (4) ground oscillation led to a continued rise of pore water pressures after strong ground shaking ceased. A new and expanded instrumented site is being developed 70 m downstream from the old WLA site as part of the NSF Network for Earthquake Engineering Simulation (NEES). The new site has more accelerometers, piezometers and ground deformation measurement devices and the data will be streamed to the NEES-grid in near real time.  相似文献   

The Cambridge guide to the solar system,by Kenneth R. Lang     

Leslie F. Bleamaster 《Meteoritics & planetary science》2004,39(1):159-160

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A pragmatic assessment of government support for organic agriculture in Ireland     

Duram LA 《Irish geography : bulletin of the Geographical Society of Ireland》2010,43(3):249-263

Drawing on a pragmatic approach, this paper provides an analysis of government support for organic farming in Ireland. Varying levels of encouragement and programmes are provided to farmers in their conversion from conventional to organic production, and in their maintenance of organic production. As support policies vary across regions and are linked to European Union legislation, it is challenging to document the many types of support in place. This paper investigates relevant technical, financial, and policy support available to organic farmers in Ireland. As an exploratory study, it develops an assessment of Ireland within eight key categories of organic agricultural support: policy, leadership, technical support, financial support, research, education and information, marketing and promotion, and future outlook. Information and data from the Irish Department of Agriculture, Fisheries and Food (DAFF), the Irish Agriculture and Food Development Authority (Teagasc), and other government and semi-governmental agencies were utilized to assess the level of support in each category. This assessment provides key findings which will allow policymakers, organizations and citizens to better understand the current situation and set a path for the future development of organic farming in Ireland.  相似文献