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31.
We develop and analyze a mixed finite element method for the solution of an elliptic system modeling a porous medium with large cavities, called vugs. It consists of a second-order elliptic (i.e., Darcy) equation on part of the domain coupled to a Stokes equation on the rest of the domain, and a slip boundary condition (due to Beavers–Joseph–Saffman) on the interface between them. The tangential velocity is not continuous on the interface. We consider a 2-D vuggy porous medium with many small cavities throughout its extent, so the interface is not isolated. We use a certain conforming Stokes element on rectangles, slightly modified near the interface to account for the tangential discontinuity. This gives a mixed finite element method for the entire Darcy–Stokes system with a regular sparsity pattern that is easy to implement, independent of the vug geometry, as long as it aligns with the grid. We prove optimal global first-order L 2 convergence of the velocity and pressure, as well as the velocity gradient in the Stokes domain. Numerical results verify these rates of convergence and even suggest somewhat better convergence in certain situations. Finally, we present a lower dimensional space that uses Raviart–Thomas elements in the Darcy domain and uses our new modified elements near the interface in transition to the Stokes elements.  相似文献   
32.
Lake Winnipeg, the seventh largest lake in North America, is located at the boundary between the Interior Plains and the Canadian Shield in Manitoba, Canada. Seismic profiles were obtained in Lake Winnipeg on two geoscientific cruises in 1994 and 1996. These data indicate the morphology of the bedrock surface. In most cases, a clear distinction between low relief Paleozoic carbonate rock and high relief Precambrian rock can be made. In northern Lake Winnipeg, the eastern limit of Paleozoic rock is clearly demarcated 30 km west of the previous estimate of its position. In southern Lake Winnipeg, all or most of the Paleozoic sequence terminates at a prominent buried escarpment in the centre of the lake. This indicates that Paleozoic rock on the eastern shore, known from drilling and outcrops, is an outlier. Major moraines are apparent as abrupt, large ridges having a chaotic internal reflection pattern. These include the Pearson Reef Moraine, the George Island Moraine and the offshore extension of The Pas Moraine. Little evidence for extensive or thick till was observed. Instead, fine-grained sediments deposited in glacial Lake Agassiz rest directly on bedrock over most of the lake basin. Hence an episode of erosion to bedrock was associated with glaciation and/or deglaciation. The Agassiz Sequence sediments are well-stratified, drape underlying relief and in some areas are over 100 m thick. In places, stratification in these sediments is disrupted, perhaps by dewatering. Evidence of erosion of Agassiz Sequence sediments by recent currents was observed. The contact between the Agassiz Sequence and the overlying Winnipeg Sequence sediments is a marked angular unconformity. The Agassiz Unconformity indicates up to 10 m of erosion in places. The low-relief character of this unconformity precludes subaerial erosion and the lack of till, moraines, or extensive deformation precludes glacial erosion. Waves appear to be the most likely erosional agent, either in waning Lake Agassiz or early Lake Winnipeg time. Winnipeg Sequence sediments, in places very thin, mantle most of the lakefloor. These sediments were deposited in the present Lake Winnipeg and are faintly stratified to massive and reach about 10 m in thickness in deep water. On the surface of the Winnipeg Sequence, vigorous, episodic currents are thought to contribute to the construction of flow-transverse sand waves as much as 6 m high in a deep, narrow constriction in the lake.  相似文献   
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The occurrence of pronounced climate reversals during the last glacial termination has long been recognised in palaeoclimate records from both hemispheres and from high to low latitudes. Accurate constraint of both the timing and magnitude of events, such as the Younger Dryas and Antarctic Cold Reversal, is vital in order to test different hypotheses for the causes and propagation of abrupt climate change. However, in contrast to higher‐latitude regions, well‐dated records from the Tropics are rare and the structure of late‐glacial tropical climate remains uncertain. As a step toward addressing this problem, we present an in situ cosmogenic 3He surface exposure chronology from Nevado Coropuna, southern Peru, documenting a significant fluctuation of the ice margin during the late‐glacial period. Ten tightly clustered ages from a pair of moraines located halfway between the modern glacier and the Last Glacial Maximum terminus range from 11.9 to 13.9 ka and give an arithmetic mean age of 12.8 ± 0.7 ka (1σ). These data constitute direct evidence for a readvance, or prolonged stillstand, of glaciers in the arid Andes of southwestern Peru. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   
35.
First measurements of SO2 and SO in the Venus mesosphere (70-100 km) are reported. This altitude range is distinctly above the ∼60-70 km range to which nadir-sounding IR and UV investigations are sensitive. Since July 2004, use of ground-based sub-mm spectroscopy has yielded multiple discoveries. Abundance of each molecule varies strongly on many timescales over the entire sub-Earth Venus hemisphere. Diurnal behavior is evident, with more SO2, and less SO, at night than during the day. Non-diurnal variability is also present, with measured SO2 and SO abundances each changing by up to 2× or more between observations conducted on different dates, but at fixed phase, hence identical sub-Earth Venus local times. Change as large and rapid as a 5σ doubling of SO on a one-week timescale is seen. The sum of SO2 and SO abundances varies by an order of magnitude or more, indicating at least one additional sulfur reservoir must be present, and that it must function as both a sink and source for these molecules. The ratio SO2/SO varies by nearly two orders of magnitude, with both diurnal and non-diurnal components. In contrast to the strong time dependence of molecular abundances, their altitude distributions are temporally invariant, with far more SO2 and SO at 85-100 km than at 70-85 km. The observed increase of SO2 mixing ratio with altitude requires that the primary SO2 source be upper mesospheric photochemistry, contrary to atmospheric models which assert upward transport as the only source of above-cloud SO2. Abundance of upper mesospheric aerosol, with assumption that it is composed primarily of sulfuric acid, is at least sufficient to provide the maximum gas phase (SO + SO2) sulfur reported in this study. Sulfate aerosol is thus a plausible source of upper mesospheric SO2.  相似文献   
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SPASE—for Space Physics Archive Search and Extract—is a group with a charter to promote collaboration and sharing of data for the Space Plasma Physics community. A major activity is the definition of the SPASE Data Model which defines the metadata necessary to describe resources in the broader heliophysics data environment. The SPASE Data Model is primarily a controlled vocabulary with hierarchical relationships and with the ability to form associations between described resources. It is the result of many years of effort by an international collaboration (see http://www.spase-group.org) to unify and improve on existing Space and Solar Physics data models. The genesis of the SPASE group can be traced to 1998 when a small group of individuals saw a need for a data model. Today SPASE has a large international participation from many of the major space research organizations. The design of the data model is based on a set of principles derived from evaluation of the existing heliophysics data environment. The development guidelines for the data model are consistent with ISO-2788 (expanded in ANSI/NISO Z39.19) and the administration for the data model is comparable to that described in the ISO standards ISO-11179 and ISO-20943. Since the release of version 1.0 of the data model in 2005, the model has undergone a series of evolutions. SPASE released version 2.0 of its data model in April 2009. This version presents a significant change from the previous release. It includes the capability to describe a wider range of data products and to describe expert annotations which can be associated with a resource. Additional improvements include an enhanced capability to describe resource associations and a more unified approach to describing data products. Version 2.0 of the SPASE Data Model provides a solid foundation for continued integration of worldwide research activities and the open sharing of data.  相似文献   
38.
Use of public participation geographic information systems (PPGIS) studies that collect local knowledge in a spatial format is increasing as a tool in natural resources management. Qualitative PPGIS studies have been conducted as individual interviews, as workshops, and in focus groups. As the number of qualitative PPGIS studies increases, so does the need to understand their quality. Saturation, the point when the researcher determines that the collection of additional data will provide minimal new information as it relates to a particular issue, directly reflects on the validity of the study. While the concept of saturation is well established, it is still inconsistently assessed and reported. Furthermore, how saturation applies to qualitatively collected spatial data has not been addressed. A method is presented to assess spatial saturation of qualitative PPGIS data from 19 focus groups that were conducted to investigate important places for recreation, livelihoods, and the environment in the Florida Panhandle.  相似文献   
39.
Given its high dependence on rainfed agriculture and its comparatively low adaptive capacity, Africa is frequently invoked as especially vulnerable to climate change. Within Africa, there is likely to be considerable variation in vulnerability to climate change both between and within countries. This paper seeks to advance the agenda of identifying the hot spots of what we term “climate security” vulnerability, areas where the confluence of vulnerabilities could put large numbers of people at risk of death from climate-related hazards. This article blends the expertise of social scientists and climate scientists. It builds on a model of composite vulnerability that incorporates four “baskets” or processes that are thought to contribute to vulnerability including: (1) physical exposure, (2) population density, (3) household and community resilience, and (4) governance and political violence. Whereas previous iterations of the model relied on historical physical exposure data of natural hazards, this paper uses results from regional model simulations of African climate in the late 20th century and mid-21st century to develop measures of extreme weather events—dry days, heat wave events, and heavy rainfall days—coupled with an indicator of low-lying coastal elevation. For the late 20th century, this mapping process reveals the most vulnerable areas are concentrated in Chad, the Democratic Republic of the Congo, Niger, Somalia, Sudan, and South Sudan, with pockets in Burkina Faso, Ethiopia, Guinea, Mauritania, and Sierra Leone. The mid 21st century projection shows more extensive vulnerability throughout the Sahel, including Burkina Faso, Chad, Mali, northern Nigeria, Niger, and across Sudan.  相似文献   
40.
This paper discusses the role and relevance of the shared socioeconomic pathways (SSPs) and the new scenarios that combine SSPs with representative concentration pathways (RCPs) for climate change impacts, adaptation, and vulnerability (IAV) research. It first provides an overview of uses of social–environmental scenarios in IAV studies and identifies the main shortcomings of earlier such scenarios. Second, the paper elaborates on two aspects of the SSPs and new scenarios that would improve their usefulness for IAV studies compared to earlier scenario sets: (i) enhancing their applicability while retaining coherence across spatial scales, and (ii) adding indicators of importance for projecting vulnerability. The paper therefore presents an agenda for future research, recommending that SSPs incorporate not only the standard variables of population and gross domestic product, but also indicators such as income distribution, spatial population, human health and governance.  相似文献   
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