Localized stirring in a field of salt-fingers     

M. G. Wells  R. W. Griffiths 《Dynamics of Atmospheres and Oceans》2002,35(4)

In a series of laboratory experiments, a partially mixed patch was produced in thick linear concentration gradients favorable to salt-finger convection. Salt-fingers, which give rise to an up-gradient flux of buoyancy, can reduce and invert the density gradient in the initial imposed patch. This leads to overturning convection within the patch if (a) the ratio of ambient T and S gradients, R_ρ=αT_z/βS_z, is near one; (b) the initial imposed turbulence results in a nearly well-mixed patch; and (c) the patch thickness is large enough that convective eddies are able to transport T and S faster than salt-fingers. Once overturning occurs, subsequent turbulent entrainment can lead to growth of the patch thickness. Experimental results for one-dimensional patches (layers) agree well with the theoretical prediction. This thickening is in contrast to the collapse that a partially mixed three-dimensional patch would experience due to lateral intrusion in a wide tank.  相似文献   

The geology and petrology of the Hannington Trachyphonolite formation,Kenya Rift Valley     

Paul S. Griffiths  Ian L. Gibson 《Lithos》1980,13(1):43-53

Field, petrographical and geochemical studies of a group of late Pleistocene, alkaline and mildly peralkaline trachytic and trachyphonolitic lavas from the northern Kenya Rift have been undertaken. A large number of flows were erupted from widely dispersed centres to form an extensive volcanic shield within the floor of the rift. The major element composition of most rocks was substantially modified during crystallisation, but other data show that differentiation within the suite was the result of protracted feldspar fractionation of a trachytic magma with intially very low abundances of residual trace elements.  相似文献   

A technique for the molecular genetic analysis of Daphnia resting eggs from sub-recent lake sediments     

Vicky A. Reid  Gary R. Carvalho  D. Glen George  Huw I. Griffiths 《Journal of Paleolimnology》2002,27(4):481-486

The Polymerase Chain Reaction (PCR) was employed to facilitategenetic analyses of Daphnia ephippial egg banks fromsediment sequences from lakes Windermere and Esthwaite in the English LakeDistrict, UK. We present a robust DNA extraction technique for resting eggs andan account of genetic characterisation of dated resting eggs from sedimentcores using both mitochondrial and nuclear DNA markers. We also providepreliminary results from genetic analysis of egg banks preserved in shortsediment cores. PCR was employed to facilitategenetic analyses of Daphnia ephippial egg banks collected from both lakes. Thelong-term environmental data available from lake records can provide a backdropagainst which molecular genetic-environmental interactions over time can beexamined, providing a technique for direct assessments of population responseto environmental change in paleolimnological studies of biodiversityhistory.  相似文献   

Severe thunderstorms in New South Wales: Climatology and means of assessing the impact of climate change     

D. J. Griffiths  J. R. Colquhoun  K. L. Batt  T. R. Casinader 《Climatic change》1993,25(3-4):369-388

A climatology of severe thunderstorms in the Australian State of New South Wales is described, using the data base of more than 1000 severe thunderstorm events held in the New South Wales Regional Office of the Bureau of Meteorology. Previously only a tornado climatology was described. Severe thunderstorms are the second most costly weather phenomenon in Australia (after tropical cyclones), on the basis of insurance payouts for major events since June 1967. Two thirds of this cost occurs in New South Wales.Severe thunderstorms show marked diurnal and seasonal distributions with maxima in the warmer months and during the late evening. Distributions are similar to those in the United States. The peak months for large and giant hail, tornadoes, strong winds and flash floods differ, being respectively November, December, January and February. While these temporal distributions are reasonably well defined, knowledge of the historical and spatial distributions is limited. A means of deriving a spatial distribution is given, which is based on the frequency near Sydney and the relative frequency at a time when there was a more uniform population distribution over the State.The climatology of severe thunderstorms is not an adequate basis for assessing the impacts of a future climate change. However, there are possible means of doing this utilising numerical climate models, and these are suggested.Author for correspondence.  相似文献   

Viscous entrainment by sinking plumes     

R.W. Griffiths  J.S. Turner 《Earth and Planetary Science Letters》1988,90(4):467-477

Geochemical models invoking several distinct reservoirs in the mantle, with different time histories, raise important questions about the exchange of mass between them. If two of these reservoirs are the upper and lower mantle, above and below about 700 km, then sinking of cold slabs through this level is one of a number of possible ways in which mixing can occur. In addition, if slabs do penetrate the transition zone, surrounding upper layer material will be dragged downwards. We have examined the interaction of very viscous plumes, or slabs, with density and viscosity interfaces in a series of laboratory experiments using fluids of different viscosities and densities and have documented several mechanisms which can lead to significant entrainment and mixing. If a slab remains planar as it passes through a density interface, a boundary layer of lighter fluid is pulled into the lower layer and we predict the consequent mass flux. When a near-vertical slab becomes unstable to folding (as it does if it has a sufficient viscosity contrast with its surroundings and its length is greater than about five times its thickness), there is another more efficient entrainment mechanism: upper layer fluid is trapped between the folds in the slab. The effective entrainment increases as the density difference between the upper and lower layers decreases. An increase in viscosity with depth also leads to buckling instability and folding of the surrounding material into the slab material. On the other hand, when there is substantial density difference between the layers a dense slab can cease to sink through the interface but spread out along the interface because it is unstable and incorporates enough upper layer fluid between its folds to become neutrally buoyant. The range of slab behaviour occurring in the mantle is not known but we draw attention to the various possibilities and to the implications for mass flux between layers.  相似文献   

Estuarine clay mineral distribution: Modern analogue for ancient sandstone reservoir quality prediction     

Joshua Griffiths  Richard H. Worden  Luke J. Wooldridge  James E. P. Utley  Robert A. Duller  Rhiannon L. Edge 《Sedimentology》2019,66(6):2011-2047

The spatial distribution of clay minerals in sandstones, which may both enhance or degrade reservoir quality, is poorly understood. To address this, clay mineral distribution patterns and host‐sediment properties (grain size, sorting, clay fraction abundance and bioturbation intensity) have, for the first time, been determined and mapped at an unprecedentedly high‐resolution in a modern estuarine setting (Ravenglass Estuary, UK). Results show that the estuary sediment is dominated by illite with subordinate chlorite and kaolinite, although the rivers supply sediment with less illite and significantly more chlorite than found in the estuary. Fluvial‐supplied sediment has been locally diluted by sediment derived from glaciogenic drift deposits on the margins of the estuary. Detailed clay mineral maps and statistical analyses reveal that the estuary has a heterogeneous distribution of illite, chlorite and kaolinite. Chlorite is relatively most abundant on the northern foreshore and backshore and is concentrated in coarse‐grained inner estuary dunes and tidal bars. Illite is relatively most abundant (as well as being most crystalline and most Fe–Mg‐rich) in fine‐grained inner estuary and central basin mud and mixed flats. Kaolinite has the highest abundance in fluvial sediment and is relatively homogenous in tidally‐influenced environments. Clay mineral distribution patterns in the Ravenglass Estuary have been strongly influenced by sediment supply (residence time) and subsequently modified by hydrodynamic processes. There is no relationship between macro‐faunal bioturbation intensity and the abundance of chlorite, illite or kaolinite. Based on this modern‐analogue study, outer estuarine sediments are likely to be heavily quartz cemented in deeply‐buried (burial temperatures exceeding 80 to 100°C) sandstone reservoirs due to a paucity of clay grade material (<0·5%) to form complete grain coats. In contrast, chlorite‐enriched tidal bars and dunes in the inner estuary, with their well‐developed detrital clay coats, are likely to have quartz cement inhibiting authigenic clay coats in deeply‐buried sandstones.  相似文献   

Modelling the impact of sea-level rise on urban flood probability in SE China     

James Andrew Griffiths  Fangfang Zhu  Faith Ka Shun Chan  David Laurence Higgitt 《地学前缘(英文版)》2019,10(2):363-372

Urban drainage systems in coastal cities in SE China are characterized by often complex canal and sluicegate systems that are designed to safely drain pluvial flooding whilst preventing tidal inundation.However, the risk of coastal flooding in the region is expected to increase over the next 50-100 years, as urban areas continue to expand and sea-levels are expected to rise. To assess the impact of projected sealevel rise on this type of urban drainage system, a one-dimensional model and decision support tool was developed. The model indicated that although sea-level rise represents a significant challenge, flood probability will continue to be most influenced by rainfall. Events that are significant enough to cause flooding will most likely be minimally impacted by changes to the tidal frame. However, it was found that a sea-level rise of up to 1.2 m by 2010 would result in increased drainage times and higher volumes of over-topping when flooding occurs.  相似文献   

A method for generating virtual soil profiles with complex,multi-layer stratigraphy     

M. P. Crisp  M. B. Jaksa  Y. L. Kuo  G. A. Fenton  D. V. Griffiths 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2019,13(2):154-163

This paper presents a framework for generating multi-layer, unconditional soil profiles with complex stratigraphy, which simulates the effects of natural erosion and sedimentation processes. The stratigraphy can have varying degrees of randomness and can include features such as lenses, as well as sloped and undulating layers. The method generates the soil comprising the layers using local average subdivision (LAS), and a random noise component that is added to the layer boundaries. The layers are created by generating coordinates of key points in the simulated ground profile, which are then interpolated with a customised, 2D, linear interpolation algorithm. The resulting simulations facilitate more accurate probabilistic modelling of geotechnical engineering systems because they provide more realistic geologies, such as those usually encountered in the ground. Fortran code implementing this framework is included as supplementary material.  相似文献   

Land degradation and vegetation distribution in Chott El Beida wetland,Algeria     

M. Khaznadar  I.N. Vogiatzakis  G.H. Griffiths 《Journal of Arid Environments》2009,73(3):369-377

The aim of this study is to explore the environmental factors that determine plant community distribution in northeast Algeria. This paper provides a quantitative analysis of the vegetation–environment relationships for a study site in the Chott El Beida wetland, a RAMSAR site in Setif, Algeria. Sixty vegetation plots were sampled and analysed using TWINSPAN and Detrended Correspondence Analysis (DCA) in order to identify the principal vegetation communities and determine the environmental gradients associated with these. 127 species belonging to 41 families and 114 genera were recorded. Six of the recorded species were endemic representing 4.7% of the total species. The richest families were Compositae, Gramineae, Cruciferae and Chenopodiaceae. Therophytes and hemicryptophytes were the most frequent life forms. The Mediterranean floristic element is dominant and is represented by 39 species. The samples were classified into four main community types. The principal DCA axes represent gradients of soil salinity, moisture and anthropogenic pressure. The use of classification in combination with ordination techniques resulted in a good discrimination between plant communities and a greater understanding of controlling environmental factors. The methodology adopted can be employed for improving baseline information on plant community ecology and distribution in often critically endangered Mediterranean wetland areas.  相似文献   

Visualizing geomorphology: improving communication of data and concepts through engagement with the arts          下载免费PDF全文

Stephen Tooth  Heather A. Viles  Ant Dickinson  Simon J. Dixon  Anna Falcini  Hywel M. Griffiths  Harriet Hawkins  Jessica Lloyd‐Jones  Julian Ruddock  Varyl R. Thorndycraft  Brian Whalley 《地球表面变化过程与地形》2016,41(12):1793-1796

  相似文献