Application of Dynamic Subgrid-scale Models for Large-eddy Simulation of the Daytime Convective Boundary Layer over Heterogeneous Surfaces   总被引：1，自引：0，他引：1  

Hsin-Yuan Huang  Bjorn Stevens  Steven A. Margulis 《Boundary-Layer Meteorology》2008,126(3):327-348

The sensitivity of large-eddy simulation (LES) to the representation of subgrid-scale (SGS) processes is explored for the case of the convective boundary layer (CBL) developing over surfaces with varying degrees of spatial heterogeneity. Three representations of SGS processes are explored: the traditional constant Smagorinsky–Lilly model and two other dynamic models with Lagrangian averaging approaches to calculate the Smagorinsky coefficient (C _S ) and SGS Prandtl number (Pr). With initial data based roughly on the observed meteorology, simulations of daytime CBL growth are performed over surfaces with characteristics (i.e. fluxes and roughness) ranging from homogeneous, to striped heterogeneity, to a realistic representation of heterogeneity as derived from a recent field study. In both idealized tests and the realistic case, SGS sensitivities are mostly manifest near the surface and entrainment zone. However, unlike simulations over complex domains or under neutral or stable conditions, these differences for the CBL simulation, where large eddies dominate, are not significant enough to distinguish the performance of the different SGS models, irrespective of surface heterogeneity.  相似文献   

Development of the east coast fishing industry : Assessment of Canadian government policies     

Janet E. Forrest 《Marine Policy》1981,5(4):294-301

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Testing the evolutionary link between submillimetre galaxies and quasars: CO observations of QSOs at z∼ 2     

K. E. K. Coppin  A. M. Swinbank  R. Neri  P. Cox  D. M. Alexander  Ian Smail  M. J. Page  J. A. Stevens  K. K. Knudsen  R. J. Ivison  A. Beelen  F. Bertoldi  A. Omont 《Monthly notices of the Royal Astronomical Society》2008,389(1):45-62

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Near‐surface hydraulic conductivity of northern hemisphere glaciers          下载免费PDF全文

Ian T. Stevens  Tristram D.L. Irvine‐Fynn  Philip R. Porter  Joseph M. Cook  Arwyn Edwards  Martin Smart  Brian J. Moorman  Andy J. Hodson  Andrew C. Mitchell 《水文研究》2018,32(7):850-865

The hydrology of near‐surface glacier ice remains a neglected aspect of glacier hydrology despite its role in modulating meltwater delivery to downstream environments. To elucidate the hydrological characteristics of this near‐surface glacial weathering crust, we describe the design and operation of a capacitance‐based piezometer that enables rapid, economical deployment across multiple sites and provides an accurate, high‐resolution record of near‐surface water‐level fluctuations. Piezometers were employed at 10 northern hemisphere glaciers, and through the application of standard bail–recharge techniques, we derive hydraulic conductivity (K) values from 0.003 to 3.519 m day^?1, with a mean of 0.185 ± 0.019 m day^?1. These results are comparable to those obtained in other discrete studies of glacier near‐surface ice, and for firn, and indicate that the weathering crust represents a hydrologically inefficient aquifer. Hydraulic conductivity correlated positively with water table height but negatively with altitude and cumulative short‐wave radiation since the last synoptic period of either negative air temperatures or turbulent energy flux dominance. The large range of K observed suggests complex interactions between meteorological influences and differences arising from variability in ice structure and crystallography. Our data demonstrate a greater complexity of near‐surface ice hydrology than hitherto appreciated and support the notion that the weathering crust can regulate the supraglacial discharge response to melt production. The conductivities reported here, coupled with typical supraglacial channel spacing, suggest that meltwater can be retained within the weathering crust for at least several days. Not only does this have implications for the accuracy of predictive meltwater run‐off models, but we also argue for biogeochemical processes and transfers that are strongly conditioned by water residence time and the efficacy of the cascade of sediments, impurities, microbes, and nutrients to downstream ecosystems. Because continued atmospheric warming will incur rising snowline elevations and glacier thinning, the supraglacial hydrological system may assume greater importance in many mountainous regions, and consequently, detailing weathering crust hydraulics represents a research priority because the flow path it represents remains poorly constrained.  相似文献   

The processes that control leucosome compositions in metasedimentary granulites: perspectives from the Southern Marginal Zone migmatites,Limpopo Belt,South Africa     

J. Taylor  G. Nicoli  G. Stevens  D. Frei  J.‐F. Moyen 《Journal of Metamorphic Geology》2014,32(7):713-742

Anatexis of metapelitic rocks at the Bandelierkop Quarry (BQ) locality in the Southern Marginal Zone of the Limpopo Belt occurred via muscovite and biotite breakdown reactions which, in order of increasing temperature, can be modelled as: (1) Muscovite + quartz + plagioclase = sillimanite + melt; (2) Biotite + sillimanite + quartz + plagioclase = garnet + melt; (3) Biotite + quartz + plagioclase = orthopyroxene ± cordierite ± garnet + melt. Reactions 1 and 2 produced stromatic leucosomes, which underwent solid‐state deformation before the formation of undeformed nebulitic leucosomes by reaction 3. The zircon U–Pb ages for both leucosomes are within error identical. Thus, the melt or magma formed by the first two reactions segregated and formed mechanically solid stromatic veins whilst temperature was increasing. As might be predicted from the deformational history and sequence of melting reactions, the compositions of the stromatic leucosomes depart markedly from those of melts from metapelitic sources. Despite having similar Si contents to melts, the leucosomes are strongly K‐depleted, have Ca:Na ratios similar to the residua from which their magmas segregated and are characterized by a strong positive Eu anomaly, whilst the associated residua has no pronounced Eu anomaly. In addition, within the leucosomes and their wall rocks, peritectic garnet and orthopyroxene are very well preserved. This collective evidence suggests that melt loss from the stromatic leucosome structures whilst the rocks were still undergoing heating is the dominant process that shaped the chemistry of these leucosomes and produced solid leucosomes. Two alternative scenarios are evaluated as generalized petrogenetic models for producing Si‐rich, yet markedly K‐depleted and Ca‐enriched leucosomes from metapelitic sources. The first process involves the mechanical concentration of entrained peritectic plagioclase and garnet in the leucosomes. In this scenario, the volume of quartz in the leucosome must reflect the remaining melt fraction with resultant positive correlation between Si and K in the leucosomes. No such correlation exists in the BQ leucosomes and in similar leucosomes from elsewhere. Consequently, we suggest disequilibrium congruent melting of plagioclase in the source and consequential crystallization of peritectic plagioclase in the melt transfer and accumulation structures rather than at the sites of biotite melting. This induces co‐precipitation of quartz in the structures by increasing SiO₂ content of the melt. This process is characterized by an absence of plagioclase‐induced fractionation of Eu on melting, and the formation of Eu‐enriched, quartz + plagioclase + garnet leucosomes. From these findings, we argue that melt leaves the source rapidly and that the leucosomes form incrementally as melt or magma leaving the source dumps its disequilibrium Ca load, as well as quartz and entrained ferromagnesian peritectic minerals, in sites of magma accumulation and escape. This is consistent with evidence from S‐type granites suggesting rapid magma transfer from source to high level plutons. These findings also suggest that leucosomes of this type should be regarded as constituting part of the residuum from partial melting.  相似文献   

Computer Science and Programming Courses in Geography Departments in the United States     

Forrest J. Bowlick  Daniel W. Goldberg  Sarah Witham Bednarz 《The Professional geographer》2017,69(1):138-150

Geographic information systems (GIS) are fundamental information technologies. The capabilities and applications of GIS continue to rapidly expand, requiring practitioners to have new skills and competencies, especially in computer science. There is little research, however, about how best to prepare the next generation of GIScientists with adequate computer science skills. This article explores how U.S. geography departments are introducing and developing computer science and programming skills in their geography and GIS degree programs. We review the degree requirements in fifty-five geography departments and discover that forty-four of them offer some kind of GIS programming course. Of the 210 separate degree options identified, however, only 22 require one of these courses for a degree. There is little consistency or emphasis on computer science and programming skills in geography or GIS degrees, despite the immense importance of these components in geography and GIS careers. We propose future research along distinct investigative tracks to build a research-based understanding of the educational interactions among GIS, computer science, programming, and geography.  相似文献   

Simultaneous X-ray and IR variability in the quasar 3C 273     

Ian M  Hardy  Anthony Lawson  rew Newsam  Alan Marscher  Ian Robson  Jason Stevens 《Monthly notices of the Royal Astronomical Society》1999,310(2):571-576

From a combination of high-quality X-ray observations from the NASA Rossi X-ray Timing Explorer ( RXTE ) and IR observations from the UK Infrared Telescope (UKIRT) we show that the medium-energy X-ray (3–20 keV) and near-IR fluxes in the quasar 3C 273 are highly correlated. It is widely believed that the X-ray emission in quasars like 3C 273 arises from Compton scattering of low-energy seed photons, and our observations provide the first reliable detection of correlated variations in 3C 273 between the X-ray band and any lower energy band. For a realistic electron distribution we demonstrate that it is probable that each decade of the seed-photon distribution from the mm to IR wavebands contributes roughly equally to the medium-energy X-ray flux. However, the expected mm variations are too small to be detected above the noise, probably explaining the lack of success of previous searches for a correlation between X-ray and mm variations. In addition, we show that the IR leads the X-rays by  0.75±0.25 d  . These observations rule out the 'External Compton' emission process for the production of the X-rays.  相似文献   

A possible active galactic nucleus in M82?     

K. A. Wills  A. Pedlar  T. W. B. Muxlow  I. R. Stevens 《New Astronomy Reviews》1999,43(8-10)

MERLIN and VLA observations of the galaxy M82 have detected a jet-like feature emanating from one of the compact sources, 44.01+59.6. The proximity of this source to the dynamical centre of M82 led us to suspect that it could be a weak active nucleus rather than an SNR. We imaged this source using the EVN at 15 mas and although it shows a compact shell-like structure which could be consistent with an SNR, we note that the EVN image shows a bright region within the source which is elongated along the jet direction. There are distinct similarities between this source and the Sgr A complex at the centre of our own Galaxy which contains the compact AGN, Sgr A^*.  相似文献   

The metamorphic and alteration history of West Rand Group shales from distal portions of the Witwatersrand Basin   总被引：1，自引：0，他引：1  

G. Stevens  R. F. Preston 《Mineralogy and Petrology》1999,66(1-3):123-147

Summary The maximum metamorphic grade in West Rand Group shales from three boreholes to the east and south of the Vredefort Dome is lower greenschist facies. In most samples the peak metamorphic assemblage consists of quartz + muscovite + chlorite, with the phyllosilicates defining a tectonic cleavage. In many of the samples more coarsely crystalline alteration zones that clearly crosscut both bedding and the tectonic cleavage overprint this assemblage. The alteration zones are localized around minor faults and shears where fluid ingress has occurred. Muscovite was not observed in these zones and the cleaved matrix assemblages has been replaced by assemblages consisting of pyrite + quartz ± chlorite ± stilpnomelane ± calcite ± carbonaceous matter ± Mn-garnet ± epidote ± chalcopyrite. In some of the alteration zones stilpnomelane is a very common product and appears to arise through the breakdown of chlorite + muscovite. K and Fe metasomatism are indicated during this reaction, as well as the obvious mobility of S, Cu and C. Assays on small samples indicate gold introduction into the shales during the alteration event. Chlorite thermometry was used to estimate the equilibration temperature of chlorite from the cleaved matrix and the alteration zones. Temperatures close to 300°C are indicated for both chlorite varieties, and for all three boreholes. These results, and the petrographic data, imply that fluid influx post-dated the formation of the tectonic cleavage, but occurred close to the peak of metamorphism. Comparison between the chlorite thermometry data from this study with those from similar studies in the goldfields indicates very similar peak metamorphic grades in both areas, suggesting that these metamorphic conditions may be typical of almost the entire basin. The one exception is obviously the amphibolite facies metamorphism exposed in the northwestern sector of the collar of the Vredefort Dome. Several recent studies have suggested that this metamorphism is representative of a widespread zone of higher grade metamorphism within the central portions of the basin. The results of this study indicate that this is unlikely to be the case and suggest that these amphibolite facies rocks are probably unique to the northern portions of the dome.

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Die Metamorphose- und Alterationsgeschichte von Peliten der West Rand Group im distalen Bereich des Witwatersrand Beckens

Zusammenfassung Dieser Artikel dokumentiert die Metamorphose und Alteration einiger Tonschiefer der West Rand Group, welche in drei Bohrkernen östlich und südlich des Vredefort Domes aufgeschlossen sind. In allen untersuchten Proben entspricht der maximale Metamorphosegrad der unteren Grünschiefer Fazies. Die maximalen metamorphen Bedingungen sind häufig durch die Mineralparagenese Quarz + Muscovit + Chlorit charakterisiert, wobei die Schichtsilikate eine ausgeprägte Schieferung definieren. Viele der Proben zeichnen sich durch gröber kristalline Bereiche intensiver Alteration aus, die sowohl die Schieferung als auch die Schichtung deutlich schneiden und in denen die metamorphe Mineralparagenese überprägt ist. Diese Alterationszonen befinden sich im Bereich von kleineren Störungen und Scherzonen, entlang derer Fluide eingedrungen sind. Muscovit fehlt in diesen Bereichen und die Matrixminerale sind durch die Paragenese Pyrit ± Quarz ± Chlorit ± Stilpnomelan ± Kalzit ± Kohlenstoff-haltiges Material ± Mn-Granat ± Epidot ± Kupferkies verdrängt worden. In einigen Alterationszonen findet man häufig Stilpnomelan, welcher möglicherweise auf Kosten von Muscovit und Chlorit gewachsen ist. Versuche, diese Reaktion mit Hilfe von gemessenen Mineralzusammensetzungen unter Annahme eines isochemischen Systems auszugleichen, scheiterten. Dies ist wahrscheinlich auf die Mobilität von K und Fe, sowie S, Cu and C zurückzuführen. Untersuchungen an kleinen Proben deuten darauf hin, daß die Alteration mit einer Goldanreicherung verbunden war. Die Abschätzung der Gleichgewichtstemperaturen von Chlorit der geschieferten Matrix sowie der Alterationszone mit Hilfe von Chlorit-Thermometrie ergab Temperaturen um 300°C für beide Chlorit-Varietäten aus allen drei Bohrkernen, Diese thermometrischen und petrographischen Daten sprechen dafür, daß die Fluidinfiltration nach Anlegung der Schieferung nahe dem Metamorphosehöhepunkt erfolgte. Ein Vergleich der Daten aus der Chlorit-Thermometrie dieser Studie mit den Ergebnissen ähnlicher Studien in den Goldfeldern ergibt sehr ähnliche metamporphe Bedingungen, was möglicherweise dafür spricht, daß diese Bedingungen typisch für das ganze Becken sind. Die einzige Ausnahme bildet offensichtlich die amphibolitfazielle Metamorphose in den nordwestlichen Randbereichen des Vredefort Domes. Einige neuere Studien spekulieren, daß diese Metamorphose eventuell für eine breite Zone höhergradiger Metamorphose im zentralen Teil des Beckens repräsentativ ist. Unsere Resultate deuten jedoch eher darauf hin, daß diese amphibolitfaziellen Gesteine auf den nördlichen Teil des Domes beschränkt sind.

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A regime shift in sediment export from a coastal watershed during a record wet winter,California: Implications for landscape response to hydroclimatic extremes          下载免费PDF全文

Amy E. East  Andrew W. Stevens  Andrew C. Ritchie  Patrick L. Barnard  Pamela Campbell‐Swarzenski  Brian D. Collins  Christopher H. Conaway 《地球表面变化过程与地形》2018,43(12):2562-2577

Small, steep watersheds are prolific sediment sources from which sediment flux is highly sensitive to climatic changes. Storm intensity and frequency are widely expected to increase during the 21st century, and so assessing the response of small, steep watersheds to extreme rainfall is essential to understanding landscape response to climate change. During record winter rainfall in 2016–2017, the San Lorenzo River, coastal California, had nine flow peaks representing 2–10‐year flood magnitudes. By the third flood, fluvial suspended sediment showed a regime shift to greater and coarser sediment supply, coincident with numerous landslides in the watershed. Even with no singular catastrophic flood, these flows exported more than half as much sediment as had a 100‐year flood 35 years earlier, substantially enlarging the nearshore delta. Annual sediment load in 2017 was an order of magnitude greater than during an average‐rainfall year, and 500‐fold greater than in a recent drought. These anomalous sediment inputs are critical to the coastal littoral system, delivering enough sediment, sometimes over only a few days, to maintain beaches for several years. Future projections of megadroughts punctuated by major atmospheric‐river storm activity suggest that interannual sediment‐yield variations will become more extreme than today in the western USA, with potential consequences for coastal management, ecosystems, and water‐storage capacity. The occurrence of two years with major sediment export over the past 35 years that were not associated with extremes of the El Niño Southern Oscillation or Pacific Decadal Oscillation suggests caution in interpreting climatic signals from marine sedimentary deposits derived from small, steep, coastal watersheds, to avoid misinterpreting the frequencies of those cycles. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.  相似文献