Magnetic Helicity in fast dynamos     

Andrew Gilbert 《地球物理与天体物理流体动力学》2013,107(2):135-151

The behaviour of magnetic helicity in kinematic dynamos at large magnetic Reynolds number is considered. Hughes, et al . [ Phys. Lett. A 223 , 167-172 (1996)] observe that the relative helicity tends to zero in the limit of large magnetic Reynolds number. This paper gives upper bounds on the helicity, by relating the helicity spectrum to the energy spectrum. These bounds are confirmed by numerical simulation and the distribution of helicity over scales is considered. Although it is found that the total helicity becomes small in the limit of high conductivity, there can remain significant, but cancelling, helicity at large and small scales of the field. This is illustrated by considering the evolution of helicity in the stretch-twist-fold dynamo picture.  相似文献   

The second generation of post-socialist change: Gorky Park and public space in Moscow     

Alexander Kalyukin  Andrew Byerley 《Urban geography》2013,34(5):674-695

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Comparison of dew point temperature estimation methods in Southwestern Georgia     

Marcus D. Williams  Scott L. Goodrick  Andrew Grundstein  Marshall Shepherd 《自然地理学》2013,34(4):255-267

Recent upward trends in acres irrigated have been linked to increasing near-surface moisture. Unfortunately, stations with dew point data for monitoring near-surface moisture are sparse. Thus, models that estimate dew points from more readily observed data sources are useful. Daily average dew temperatures were estimated and evaluated at 14 stations in Southwest Georgia using linear regression models and artificial neural networks (ANN). Estimation methods were drawn from simple and readily available meteorological observations, therefore only temperature and precipitation were considered as input variables. In total, three linear regression models and 27 ANN were analyzed. The two methods were evaluated using root mean square error (RMSE), mean absolute error (MAE), and other model evaluation techniques to assess the skill of the estimation methods. Both methods produced adequate estimates of daily averaged dew point temperatures, with the ANN displaying the best overall skill. The optimal performance of both models was during the warm season. Both methods had higher error associated with colder dew points, potentially due to the lack of observed values at those ranges. On average, the ANN reduced RMSE by 6.86% and MAE by 8.30% when compared to the best performing linear regression model.  相似文献   

TEMPORAL AND SPATIAL ASPECTS OF BLOWING DUST IN THE MOJAVE AND COLORADO DESERTS OF SOUTHERN CALIFORNIA, 1973–1994     

Andrew J. Bach  Anthony J. Brazel  Nicholas Lancaster 《自然地理学》2013,34(4):329-353

Although dust storms rarely occur in southern California's deserts, blowing dust often reduces visibility, and large spatial and temporal variability in the frequency of blowing dust occurs throughout the region. On average only 1.3 dust storms occur in the study area each year. The annual average number of dust events (visibility <11 km) is 18.0, with the Coachella Valley being dustiest region, averaging 37.8 dust events each year. Mean annual frequencies of dust events for 1973–1994 are mapped, showing a core of activity centered over the Imperial/Coachella Valley region, with fewer dust events around the periphery of the study area. Most stations show a coherent temporal pattern of dust frequency during the period 1973–1994, with the mid-1970s experiencing the most dust. Blowing dust generally was absent from all stations during 1979–1983, 1987–1989, and 1992–1994. The mid-1980s were moderately dusty and 1990–1991 saw a return to very dusty conditions, possibly resulting from below-normal precipitation and increased anthropogenic disturbances. Dust events in the Mojave Desert characteristically occur during the winter to spring months (February-May), associated with dry frontal activity, and are largely absent during the dry summer months. The Colorado Desert experiences a similar seasonal distribution of dust events, but has more summer events, usually associated with convective thunderstorms. Frequencies of blowing dust have weak, but statistically significant, correlations with mean annual and antecedent precipitation, suggesting that complex processes control dust emission. [Key words: blowing dust, dust storms, Mojave Desert, Colorado Desert, wind erosion.]  相似文献   

An application-driven approach to terrain model construction     

Amii R. Darnell  Andrew A. Lovett  Jenni Barclay  Richard A. Herd 《International journal of geographical information science》2013,27(8):1171-1191

Terrain is a surface phenomenon that is measured, modelled, and mapped. However, it is continuously variable and must be simulated by points or mathematical equations that are inherently approximations. The error induced by digitally represented terrain can propagate to surface derivatives and geographical information science (GIS) applications where topography is considered. This can lead to uncertainty in model predictions and the use of data that are unfit for the application to which they are intended. This article outlines the problem of uncertainty in terrain representation and demonstrates the consequences for volcanic mudflow modelling. The response of a simple least-cost single flow algorithm to input parameters was investigated in order to assess output variation from the different sources of input variation. Elevation error was modelled with a probability density function (PDF) and propagated through stochastic simulation (Monte Carlo). Such combined uncertainty and sensitivity analyses enabled a qualitative judgement of the relative significance of elevation error on the flow model prediction. Different methods for terrain model construction were considered and show that supplementing global positioning system (GPS) measurements with information from field notes and reconnaissance photographs greatly improved the model performance and reduced the uncertainty. It is concluded that in terms of validity of model results, there is no substitute for constructing an elevation model that is informed by the terrain.  相似文献   

Geosocial gauge: a system prototype for knowledge discovery from social media     

Arie Croitoru  Andrew Crooks  Jacek Radzikowski  Anthony Stefanidis 《International journal of geographical information science》2013,27(12):2483-2508

The remarkable success of online social media sites marks a shift in the way people connect and share information. Much of this information now contains some form of geographical content because of the proliferation of location-aware devices, thus fostering the emergence of geosocial media – a new type of user-generated geospatial information. Through geosocial media we are able, for the first time, to observe human activities in scales and resolutions that were so far unavailable. Furthermore, the wide spectrum of social media data and service types provides a multitude of perspectives on real-world activities and happenings, thus opening new frontiers in geosocial knowledge discovery. However, gleaning knowledge from geosocial media is a challenging task, as they tend to be unstructured and thematically diverse. To address these challenges, this article presents a system prototype for harvesting, processing, modeling, and integrating heterogeneous social media feeds towards the generation of geosocial knowledge. Our article addresses primarily two key components of this system prototype: a novel data model for heterogeneous social media feeds and a corresponding general system architecture. We present these key components and demonstrate their implementation in our system prototype, GeoSocial Gauge.  相似文献   

Crafting Regional Cultural Production: emergence,crisis and consolidation in the Gold Coast surfboard industry   总被引：1，自引：0，他引：1  

Andrew Warren 《The Australian geographer》2013,44(4):365-381

Surfboard-making is concentrated in regions with vibrant surfing subcultures, suitable waves and sufficient expertise in crafting boards, by hand, to suit prevailing coastal conditions. This article charts the rise of the Gold Coast as Australia's most concentrated cluster of surfboard-making, from its origins as do-it-yourself craft in backyards and sheds, to professional, export-oriented industry. Out of highly informal, subcultural origins, a regional cluster emerged in the 1960s, fuelled by the growth of surfing, tourism and suburban development, and by the needs of surfers for customised boards that suit their body size, surfing style and local wave conditions. What transpired was a vernacular form of coastal creative industry combining craft skills, subcultural knowledge and design flair. Nevertheless, a mix of factors, including automation, increased competition from cheap imports, increasingly aggressive retail trade, and mistrust among board-makers, has threatened viability. Some workshops overextended into export production, marketing and automation, and consequently suffered from exposure to risk (as well as a commensurate decline in regional tourism). Others survived, and in some ways turned full circle: those small workshops continuing to service local markets, within their means, were most able to ride out the industry's turbulent times. Small-scale cultural production persists despite the volatility of the larger brands, geared instead around loyal customers who want hand-crafted boards made by shapers who they know and trust.  相似文献   

Heterogeneous redox conditions, arsenic mobility, and groundwater flow in a fractured-rock aquifer near a waste repository site in New Hampshire, USA     

Philip T. Harte  Joseph D. Ayotte  Andrew Hoffman  Kinga M. Révész  Marcel Belaval  Steven Lamb  J. K. B?hlke 《Hydrogeology Journal》2012,20(6):1189-1201

Anthropogenic sources of carbon from landfill or waste leachate can promote reductive dissolution of in situ arsenic (As) and enhance the mobility of As in groundwater. Groundwater from residential-supply wells in a fractured crystalline-rock aquifer adjacent to a Superfund site in Raymond, New Hampshire, USA, showed evidence of locally enhanced As mobilization in relatively reducing (mixed oxic-anoxic to anoxic) conditions as determined by redox classification and other lines of evidence. Redox classification was determined from geochemical indicators based on threshold concentrations of dissolved oxygen (DO), nitrate (NO ₃ ^– ), iron (Fe²⁺), manganese (Mn²⁺), and sulfate (SO ₄ ^2– ). Redox conditions were evaluated also based on methane (CH₄), excess nitrogen gas (N₂) from denitrification, the oxidation state of dissolved As speciation (As(III) and As(V)), and several stable isotope ratios. Samples from the residential-supply wells primarily exhibit mixed redox conditions, as most have long open boreholes (typically 50–100?m) that receive water from multiple discrete fractures with contrasting groundwater chemistry and redox conditions. The methods employed in this study can be used at other sites to gauge redox conditions and the potential for As mobilization in complex fractured crystalline-rock aquifers where multiple lines of evidence are likely needed to understand As occurrence, mobility, and transport.  相似文献   

Mineralogical, stable isotope, and fluid inclusion studies of spatially related porphyry Cu and epithermal Au-Te mineralization, Fakos Peninsula, Limnos Island, Greece     

Andrew P. Fornadel  Panagiotis Ch. Voudouris  Paul G. Spry  Vasilios Melfos 《Mineralogy and Petrology》2012,105(1-2):85-111

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Evaluation of coastal inundation hazard for present and future climates   总被引：2，自引：1，他引：1  

Andrew J. Condon  Y. Peter Sheng 《Natural Hazards》2012,62(2):345-373

Coastal inundation from hurricane storm surges causes catastrophic damage to lives and property, as evidenced by recent hurricanes including Katrina and Wilma in 2005 and Ike in 2008. Changes in hurricane activity and sea level due to a warming climate, together with growing coastal population, are expected to increase the potential for loss of property and lives. Current inundation hazard maps: Base Flood Elevation maps and Maximum of Maximums are computationally expensive to create in order to fully represent the hurricane climatology, and do not account for climate change. This paper evaluates the coastal inundation hazard in Southwest Florida for present and future climates, using a high resolution storm surge modeling system, CH3D-SSMS, and an optimal storm ensemble with multivariate interpolation, while accounting for climate change. Storm surges associated with the optimal storms are simulated with CH3D-SSMS and the results are used to obtain the response to any storm via interpolation, allowing accurate representation of the hurricane climatology and efficient generation of hazard maps. Incorporating the impact of anticipated climate change on hurricane and sea level, the inundation maps for future climate scenarios are made and affected people and property estimated. The future climate scenarios produce little change to coastal inundation, due likely to the reduction in hurricane frequency, except when extreme sea level rise is included. Calculated coastal inundation due to sea level rise without using a coastal surge model is also determined and shown to significantly overestimate the inundation due to neglect of land dissipation.  相似文献