Three-dimensional benchmark for variable-density flow and transport simulation: matching semi-analytic stability modes for steady unstable convection in an inclined porous box     

Clifford I. Voss  Craig T. Simmons  Neville I. Robinson 《Hydrogeology Journal》2010,18(1):5-23

This benchmark for three-dimensional (3D) numerical simulators of variable-density groundwater flow and solute or energy transport consists of matching simulation results with the semi-analytical solution for the transition from one steady-state convective mode to another in a porous box. Previous experimental and analytical studies of natural convective flow in an inclined porous layer have shown that there are a variety of convective modes possible depending on system parameters, geometry and inclination. In particular, there is a well-defined transition from the helicoidal mode consisting of downslope longitudinal rolls superimposed upon an upslope unicellular roll to a mode consisting of purely an upslope unicellular roll. Three-dimensional benchmarks for variable-density simulators are currently (2009) lacking and comparison of simulation results with this transition locus provides an unambiguous means to test the ability of such simulators to represent steady-state unstable 3D variable-density physics.  相似文献   

Editors’ Message: The 2013 Editors’ Choice articles,a new editor,and the 2013 ‘Coolest Paper’ award     

Clifford Voss  Matthew Currell  Tom Gleeson 《Hydrogeology Journal》2014,22(2):293-294

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Characterization of the mesospheric sodium layer at La Palma     

L. Michaille  J.B. Clifford  J.C. Dainty †  T. Gregory  J.C. Quartel  F.C. Reavell  R.W. Wilson  N.J. Wooder 《Monthly notices of the Royal Astronomical Society》2001,328(4):993-1000

We present the results of a mesospheric sodium monitoring programme at La Palma carried out through five campaigns of one week each, from 1999 September to 2000 August. The yearly averaged parameters of the layer (the sodium column density and the width) are given. We show that the short time-scale dynamics of the layer are governed by the sporadic layers with an average frequency of one event per night. The influence of the short time-scale dynamics of the layer on an adaptive optics system working on the William Herschel Telescope is quantified. It appears that it is a small effect in terms of defocus error. Finally, we present data obtained during the Perseid meteor shower and show that the dynamics of the sodium layer undergoes a transition with the meteoric activity.  相似文献   

An environmental appraisal of the severn barrage: edited by T.L. Shaw Department of Civil Engineering,University of Bristol, 1977, 2nd ed, 155 pages, £3.00     

Clifford S. Russell 《Marine Policy》1978,2(1):72-74

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Composition of rhodizite     

Clifford Frondel  Jun Ito 《Mineralogy and Petrology》1965,10(1-4):409-412

Summary A chemical analysis of rhodizite from Manjaka, Madagascar, establishes the new formula CsAl₄Be₄B₁₁(OH)₄O₂₅. Space group P43m; a₀ 7.317±0.001 Å; density 3.44±0.01 (meas.), 3.47 (calc.); Z=1. The index of refraction, 1.693±0.001 (Na), and the unit cell dimension are identical within the limits state for material from lithia-pegmatites at Manjaka, Antsongombato, Antandrokomby and Ambalalehifotsy, in Madagascar. Hardness 81/2.

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Zusammenfassung Eine chemische Analyse von Rhodizit an Material von Manjaka, Madagaskar, liefert die neue Formel CsAl₄Be₄B₁₁(OH)₄O₂₅. Raumgruppe P43m, a₀=7,317±0,001 Å; Dichte 3,44±0,01 (exp.), 3,47 (ber.); Z=1. Der Brechungsindex, 1,693±0,001 (Na), und die Gitterkonstante sind innerhalb der angegebenen Grenzen für Material von den Lithium-Pegmatiten bei Manjaka, Antsongombato, Antandrokomby und Ambalalehifotsy auf Madagaskar gleich. Härte 81/2.

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Dedicated to ProfessorF. Machatschki on the occasion of his 70th birthday.  相似文献   

Interstellar colonization: A new parameter for the Drake equation?     

Clifford Walters  Raymond A. Hoover  R.K. Kotra 《Icarus》1980,41(2):193-197

M.H. Hart (1975, Quart. J. Roy. Astron. Soc.16, 128–135) has argued that the absence of evidence for extraterrestrial visits to Earth indicates that there are no other advanced civilizations in our galaxy capable of interstellar colonization. If so, the possible success of any SETI program must be questioned. The authors propose that limited interstellar colonization may occur and an attempt is made to show the effects of such a journey on the Drake equation.  相似文献   

Sedimentary processes,stratigraphic sequences and middens: the link between archaeology and geoheritage—a case study from the Quaternary of the Broome region,Western Australia     

P. Clifford 《Australian Journal of Earth Sciences》2019,66(6):955-972

The cliffed and active dune coastal region of Broome provides an excellent record of Pleistocene and Holocene stratigraphy of desert environments interfacing with the Indian Ocean. The Mesozoic Broome Sandstone is the basal stratigraphic unit in the area and is overlain by Pleistocene red desert quartz sand (Mowanjum Sand). Modern coastal processes of waves, wind and tide have resulted in distinctive sedimentary bodies (stratigraphic units) clearly linked to the sedimentary environment. The Mowanjum Sand, reworked by coastal winds, generates the landward-ingressing orange quartzose Churchill Sand, or reworked by waves and abraded to white sand with the addition of carbonate grains that form the beaches (Cable Beach Sand) and with eolian action, coastal dunes or inland-ingressing white dunes (Shoonta Hill Sand). These sedimentary bodies and stratigraphic units form a template with which to locate and interpret archaeological middens and Indigenous occupation over the past 5000?years in a context of coastal occupation, coastal stability, mean sea-level changes, climate changes, and availability of marine food and freshwater. Shell middens and stone artefacts form definitive layers or horizons in relation to the stratigraphy, in places in situ, and elsewhere reworked as sheets and plumes; understanding their inter-relationships has enabled the unravelling of the archaeological history and relating Indigenous occupation to biofacies and lithofacies. The array of sedimentary, biofacies and stratigraphic units are of national geoheritage significance in their own right. The addition of archaeological deposits as stratigraphic units provides a link between geoheritage and archaeology, where the archaeological materials are viewed as part of the complex stratigraphic story, part of the coastal history, and part of the geoheritage story.  相似文献   

Controls on deep critical zone architecture: a historical review and four testable hypotheses   总被引：1，自引：0，他引：1       下载免费PDF全文

Clifford S. Riebe  W. Jesse Hahm  Susan L. Brantley 《地球表面变化过程与地形》2017,42(1):128-156

The base of Earth's critical zone (CZ) is commonly shielded from study by many meters of overlying rock and regolith. Though deep CZ processes may seem far removed from the surface, they are vital in shaping it, preparing rock for infusion into the biosphere and breaking Earth materials down for transport across landscapes. This special issue highlights outstanding challenges and recent advances of deep CZ research in a series of articles that we introduce here in the context of relevant literature dating back to the 1500s. Building on several contributions to the special issue, we highlight four exciting new hypotheses about factors that drive deep CZ weathering and thus influence the evolution of life‐sustaining CZ architecture. These hypotheses have emerged from recently developed process‐based models of subsurface phenomena including: fracturing related to subsurface stress fields; weathering related to drainage of bedrock under hydraulic head gradients; rock damage from frost cracking due to subsurface temperature gradients; and mineral reactions with reactive fluids in subsurface chemical potential gradients. The models predict distinct patterns of subsurface weathering and CZ thickness that can be compared with observations from drilling, sampling and geophysical imaging. We synthesize the four hypotheses into an overarching conceptual model of fracturing and weathering that occurs as Earth materials are exhumed to the surface across subsurface gradients in stress, hydraulic head, temperature, and chemical potential. We conclude with a call for a coordinated measurement campaign designed to comprehensively test the four hypotheses across a range of climatic, tectonic and geologic conditions. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Downvalley fining of hillslope sediment in an alpine catchment: implications for downstream fining of sediment flux in mountain rivers     

Leonard S. Sklar  Clifford S. Riebe  Jennifer Genetti  Shirin Leclere  Claire E. Lukens 《地球表面变化过程与地形》2020,45(8):1828-1845

The size distributions of sediment delivered from hillslopes to rivers profoundly influence river morphodynamics, including river incision into bedrock and the quality of aquatic habitat. Yet little is known about the factors that influence size distributions of sediment produced by weathering on hillslopes. We present results of a field study of hillslope sediment size distributions at Inyo Creek, a steep catchment in granitic bedrock of the Sierra Nevada, USA. Particles sampled near the base of hillslopes, adjacent to the trunk stream, show a pronounced decrease in sediment size with decreasing sample elevation across all but the coarsest size classes. Measured size distributions become increasingly bimodal with decreasing elevation, exhibiting a coarse, bouldery mode that does not change with elevation and a more abundant finer mode that shifts from cobbles at the highest elevations to gravel at mid elevations and finally to sand at low elevations. We interpret these altitudinal variations in hillslope sediment size to reflect changes in physical, chemical, and biological weathering that can be explained by the catchment's strong altitudinal gradients in topography, climate, and vegetation cover. Because elevation and travel distance to the outlet are closely coupled, the altitudinal trends in sediment size produce a systematic decrease in sediment size along hillslopes parallel to the trunk stream. We refer to this phenomenon as ‘downvalley fining.’ Forward modeling shows that downvalley fining of hillslope sediment is necessary for downstream fining of the long-term average flux of coarse sediment in mountain landscapes where hillslopes and channels are coupled and long-term net sediment deposition is negligible. The model also shows that abrasion plays a secondary role in downstream fining of coarse sediment flux but plays a dominant role in partitioning between the bedload and suspended load. Patterns observed at Inyo Creek may be widespread in mountain ranges around the world. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

Strong slope-aspect control of regolith thickness by bedrock foliation     

Jordan D. Leone  W. Steven Holbrook  Clifford S. Riebe  Jon Chorover  Ty P.A. Ferré  Bradley J. Carr  Russell P. Callahan 《地球表面变化过程与地形》2020,45(12):2998-3010

The porous near-surface layer of the Earth's crust – the critical zone – constitutes a vital reservoir of water for ecosystems, provides baseflow to streams, guides recharge to deep aquifers, filters contaminants from groundwater, and regulates the long-term evolution of landscapes. Recent work suggests that the controls on regolith thickness include climate, tectonics, lithology, and vegetation. However, the relative paucity of observations of regolith structure and properties at landscape scales means that theoretical models of critical zone structure are incompletely tested. Here we present seismic refraction and electrical resistivity surveys that thoroughly characterize subsurface structure in a small catchment in the Santa Catalina Mountains, Arizona, USA, where slope-aspect effects on regolith structure are expected based on differences in vegetation. Our results show a stark contrast in physical properties and inferred regolith thickness on opposing slopes, but in the opposite sense of that expected from environmental models and observed vegetation patterns. Although vegetation (as expressed by normalized difference vegetation index [NDVI]) is denser on the north-facing slope, regolith on the south-facing slope is four times thicker (as indicated by lower seismic velocities and resistivities). This contrast cannot be explained by variations in topographic stress or conventional hillslope morphology models. Instead, regolith thickness appears to be controlled by metamorphic foliation: regolith is thicker where foliation dips into the topography, and thinner where foliation is nearly parallel to the surface. We hypothesize that, in this catchment, hydraulic conductivity and infiltration capacity control weathering: infiltration is hindered and regolith is thin where foliation is parallel to the surface topography, whereas water infiltrates deeper and regolith is thicker where foliation intersects topography at a substantial angle. These results suggest that bedrock foliation, and perhaps by extension sedimentary layering, can control regolith thickness and must be accounted for in models of critical zone development. © 2020 John Wiley & Sons, Ltd.  相似文献