Rapid fluctuations in the position,size, and brightness of intense solar metre-wave radio sources     

R. A. Duncan 《Solar physics》1985,97(1):173-182

A comparison of quiescent type I solar radio sources with concurrent intense impulsive type III, V, and type II sources shows that whereas the type I sources are usually small and stable the type III, V, and II sources are usually large and unstable. We conclude that the large size and variability of the type III, V, and II radio sources cannot be attributed to instrumental error or ionospheric refraction but must instead reflect the size and variability of the coronal structures on which they arise.  相似文献   

Victimisation beyond the metropolis: an Australian case study     

C J Duncan 《Area》1997,29(2):119-128

Summary Victimisation surveys have revealed that high crime rate areas not only have a high rate of victim prevalence, but that a small proportion of the victims may experience more than half of the crime reported. This paper examines how the prevalence and concentration of victimisation influence the incidence rate in a low crime rate area.  相似文献   

Mean flux estimation and cospectra of airborne carbon dioxide and water vapour eddy flux measurements in the planetary surface layer     

Antoine Saucier  Mike R. Duncan  G. L. Austin 《Boundary-Layer Meteorology》1991,55(3):227-254

CO₂ and H₂O eddy-flux measurements derived from airborne vertical wind and concentration data are examined to determine the accuracy of the eddy-flux estimate in moderately unstable conditions within the surface layer. Integral scale estimation is used to determine the minimum length of a sample required to achieve a given accuracy. Cospectral analysis is used to examine the scales responsible for the transport of water vapour and carbon dioxide in the surface layer. Cospectra are found to broaden as the sampling altitude increases, a well known result. At low altitudes, the cospectrum is found to become negative for large scales. A significant fraction of the flux is carried by this range of scales, which suggests that scales larger than the boundary-layer height may play an important role in the transport at low altitudes.  相似文献   

Monte Vulture volcano (Basilicata,Italy): an analysis of morphology and volcaniclastic facies     

J E Guest  A M Duncan  D K Chester 《Bulletin of Volcanology》1991,53(3):228-229

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Complex coastal change in response to autogenic basin infilling: An example from a sub‐tropical Holocene strandplain          下载免费PDF全文

Christopher J. Hein  Duncan M. FitzGerald  Luis H. P. de Souza  Ioannis Y. Georgiou  Ilya V. Buynevich  Antonio H. da F. Klein  João Thadeu de Menezes  William J. Cleary  Thelma L. Scolaro 《Sedimentology》2016,63(6):1362-1395

Thick bay‐fill sequences that often culminate in strandplain development serve as important sedimentary archives of land–ocean interaction, although distinguishing between internal and external forcings is an ongoing challenge. This study employs sediment cores, ground‐penetrating radar surveys, radiocarbon dates, palaeogeographic reconstructions and hydrodynamic modelling to explore the role of autogenic processes – notably a reduction in wave energy in response to coastal embayment infilling – in coastal evolution and shoreline morphodynamics. Following a regional 2 to 4 m highstand at ca 5·8 ka, the 75 km² Tijucas Strandplain in southern Brazil built from fluvial sediments deposited into a semi‐enclosed bay. Holocene regressive deposits are underlain by fluvial sands and a Pleistocene transgressive–regressive sequence, and backed by a highstand barrier‐island. The strandplain is immediately underlain by 5 to 16 m of seaward‐thickening, fluvially derived, Holocene‐age, basin‐fill mud. Several trends are observed from the landward (oldest) to the seaward (youngest) sections of the strandplain: (i) the upper shoreface and foreshore become finer and thinner and shift from sand‐dominated to mud‐dominated; (ii) beachface slopes decrease from >11° to ca 7°; and (iii) progradation rates increase from 0·4 to 1·8 m yr^?1. Hydrodynamic modelling demonstrates a correlation between progressive shoaling of Tijucas Bay driven by sea‐level fall and sediment infilling and a decrease in onshore wave‐energy transport from 18 to 4 kW m^?1. The combination of allogenic (sediment supply, falling relative sea‐level and geology) and autogenic (decrease in wave energy due to bay shoaling) processes drove the development of a regressive system with characteristics that are rare, if not unique, in the Holocene and rock records. These findings demonstrate the complexities in architecture styles of highstand and regressive systems tracts. Furthermore, this article highlights the diverse internal and external processes and feedbacks responsible for the development of these intricate marginal marine sedimentary systems.  相似文献   

Benthic foraminifera of the Holocene transgressive west-central Florida inner shelf: paleoenvironmental implications     

Tessa M. Hill  Gregg R. Brooks  David S. Duncan  Franco S. Medioli   《Marine Geology》2003,200(1-4):263-272

The sedimentology, stratigraphic position, and benthic foraminiferal biostratigraphy of early- to mid-Holocene deposits from the west-central Florida shelf suggest that barrier islands developed along this coast as early as 8.3 ka, in an environment that was more arid than today. Predominant foraminifera of three paralic sedimentary facies deposited between 5.3 and 8.3 ka include miliolids, Elphidium spp., and Ammonia spp., all of which are common in back-barrier environments. Foraminiferal assemblages also suggest that early back-barrier sediments were deposited in a hypersaline environment, similar to that of the arid Laguna Madre of the western Gulf of Mexico. Modern back-barrier foraminifera in the Tampa Bay region are indicative of the humid subtropical climate of today. Thus, the climate of west-central Florida at approximately 8 ka was more arid than today, which is consistent with recent studies showing that climate in the Gulf of Mexico was dryer and cooler during this time period.  相似文献   

CENTRAL OTAGO. A Symposium to mark the Centenary of the 'Golden Decade' of the 1860s in Central Otago. Edited by R. G. Lister and R. P. Hargrcaves     

C. Duncan 《New Zealand geographer》1966,22(1):107-108

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Burrow Structure and Burrowing Activity of the Funnel-feeding Enteropneust Balanoglossus aurantiacus in Bogue Sound, North Carolina, USA     

P. Bruce  Duncan 《Marine Ecology》1987,8(1):75-95

Abstract. This study of the enteropneust, Balanoglossus aurantiacus , in Bogue Sound, NC, provides information on the general structure and variability in geometry of its burrow, B. aurantiacus employs a funnel at the anterior end of its burrow to cave in surface sediments. It ingests, processes, then deposits these sediments back on the surface as fecal castings. Measurements made on resin casts of burrows and on photographs of surface features (burrow openings and funnels) revealed a new and consistent pattern of burrow construction. This pattern distinguishes B. aurantiacus burrows from those reported for other funnel feeders and enteropneusts (except, perhaps, for B. clavigerus). Laboratory and field observations revealed that, within its burrow, B. aurantiacus is elongated, can double-back on itself, and can use anterior burrow openings for subsequent fecal castings. Tests for significant changes in location of surface features gave estimates of how frequently individuals in aquaria and field populations abandon old burrow locations and construct new burrows. Field observations showed that B. aurantiacus will reopen a blocked burrow opening.  相似文献   

Winter processes on northern salt marshes: Evaluating the impact of in-situ peat compaction due to ice loading, Wells, ME     

Brittina A. Argow  Duncan M. FitzGerald 《Estuarine, Coastal and Shelf Science》2006,69(3-4):360

New England salt marshes are dominated by the supratidal high marsh grass, Spartina patens. This grass forms a nearly planar surface, which makes it highly vulnerable to the predicted regime of accelerated sea-level rise (SLR). If the high marsh cannot keep pace with rising sea level it will be transformed to intertidal environments, leading to unusually rapid coastal evolution. Winter processes such as ice-loading of surface peat may degrade the marsh surface. Large volumes of snow and ice compress peat, resulting in shallow compaction and a net loss of elevation in some areas.On the Webhannet marsh in Maine, a simulated ice compaction experiment indicates that thick ice can compress marsh peat (46 cm simulated ‘ice’ produced 6.9 ± 0.3 mm of compaction; 24 cm ‘ice’ produced 3.0 ± 0.8 mm). Experimental data suggest that ice thicknesses greater than 10 cm depress the marsh surface by 2 mm for each cm of total ice thickness. However, surface elevations rebounded to near-control levels within 2 weeks of the removal of simulated ‘ice’ from the surface of the marsh. Normal winter ice accumulations on New England marshes, therefore, do not appear to be sufficient to permanently compact marsh surface peat and lead to loss in marsh surface elevation.  相似文献   

Have applications of continuous rainfall–runoff simulation realized the vision for process‐based flood frequency analysis?          下载免费PDF全文

Rob Lamb  Duncan Faulkner  Paul Wass  David Cameron 《水文研究》2016,30(14):2463-2481

Keith Beven was amongst the first to propose and demonstrate a combination of conceptual rainfall–runoff modelling and stochastically generated rainfall data in what is known as the ‘continuous simulation’ approach for flood frequency analysis. The motivations included the potential to establish better links with physical processes and to avoid restrictive assumptions inherent in existing methods applied in design flood studies. Subsequently, attempts have been made to establish continuous simulation as a routine method for flood frequency analysis, particularly in the UK. The approach has not been adopted universally, but numerous studies have benefitted from applications of continuous simulation methods. This paper asks whether industry has yet realized the vision of the pioneering research by Beven and others. It reviews the generic methodology and illustrates applications of the original vision for a more physically realistic approach to flood frequency analysis through a set of practical case studies, highlighting why continuous simulation was useful and appropriate in each case. The case studies illustrate how continuous simulation has helped to offer users of flood frequency analysis more confidence about model results by avoiding (or exposing) bad assumptions relating to catchment heterogeneity, inappropriateness of assumptions made in (UK) industry‐standard design event flood estimation methods, and the representation of engineered or natural dynamic controls on flood flows. By implementing the vision for physically realistic analysis of flood frequency through continuous simulation, each of these examples illustrates how more relevant and improved information was provided for flood risk decision‐making than would have been possible using standard methods. They further demonstrate that integrating engineered infrastructure into flood frequency analysis and assessment of environmental change are also significant motivations for adopting the continuous simulation approach in practice. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献