Grid cell distortion and MODFLOW's integrated finite-difference numerical solution   总被引：1，自引：0，他引：1  

Romero DM  Silver SE 《Ground water》2006,44(6):797-802

The ground water flow model MODFLOW inherently implements a nongeneralized integrated finite-difference (IFD) numerical scheme. The IFD numerical scheme allows for construction of finite-difference model grids with curvilinear (piecewise linear) rows. The resulting grid comprises model cells in the shape of trapezoids and is distorted in comparison to a traditional MODFLOW finite-difference grid. A version of MODFLOW-88 (herein referred to as MODFLOW IFD) with the code adapted to make the one-dimensional DELR and DELC arrays two dimensional, so that equivalent conductance between distorted grid cells can be calculated, is described. MODFLOW IFD is used to inspect the sensitivity of the numerical head and velocity solutions to the level of distortion in trapezoidal grid cells within a converging radial flow domain. A test problem designed for the analysis implements a grid oriented such that flow is parallel to columns with converging widths. The sensitivity analysis demonstrates MODFLOW IFD's capacity to numerically derive a head solution and resulting intercell volumetric flow when the internal calculation of equivalent conductance accounts for the distortion of the grid cells. The sensitivity of the velocity solution to grid cell distortion indicates criteria for distorted grid design. In the radial flow test problem described, the numerical head solution is not sensitive to grid cell distortion. The accuracy of the velocity solution is sensitive to cell distortion with error <1% if the angle between the nonparallel sides of trapezoidal cells is <12.5 degrees. The error of the velocity solution is related to the degree to which the spatial discretization of a curve is approximated with piecewise linear segments. Curvilinear finite-difference grid construction adds versatility to spatial discretization of the flow domain. MODFLOW-88's inherent IFD numerical scheme and the test problem results imply that more recent versions of MODFLOW 2000, with minor modifications, have the potential to make use of a curvilinear grid.  相似文献   

Effects of observed and experimental climate change on terrestrial ecosystems in northern Canada: results from the Canadian IPY program   总被引：1，自引：0，他引：1  

Gregory H. R. Henry  Karen A. Harper  Wenjun Chen  Julie R. Deslippe  Robert F. Grant  Peter M. Lafleur  Esther Lévesque  Steven D. Siciliano  Suzanne W. Simard 《Climatic change》2012,115(1):207-234

Tundra and taiga ecosystems comprise nearly 40?% of the terrestrial landscapes of Canada. These permafrost ecosystems have supported humans for more than 4500?years, and are currently home to ca. 115,000 people, the majority of whom are First Nations, Inuit and Métis. The responses of these ecosystems to the regional warming over the past 30?C50?years were the focus of four Canadian IPY projects. Northern residents and researchers reported changes in climate and weather patterns and noted shifts in vegetation and other environmental variables. In forest-tundra areas tree growth and reproductive effort correlated with temperature, but seedling establishment was often hindered by other factors resulting in site-specific responses. Increased shrub cover has occurred in sites across the Arctic at the plot and landscape scale, and this was supported by results from experimental warming. Experimental warming increased vegetation cover and nutrient availability in most tundra soils; however, resistance to warming was also found. Soil microbial diversity in tundra was no different than in other biomes, although there were shifts in mycorrhizal diversity in warming experiments. All sites measured were sinks for carbon during the growing season, with expected seasonal and latitudinal patterns. Modeled responses of a mesic tundra system to climate change showed that the sink status will likely continue for the next 50?C100?years, after which these tundra systems will likely become a net source of carbon dioxide to the atmosphere. These IPY studies were the first comprehensive assessment of the state and change in Canadian northern terrestrial ecosystems and showed that the inherent variability in these systems is reflected in their site-specific responses to changes in climate. They also showed the importance of using local traditional knowledge and science, and provided extensive data sets, sites and researchers needed to study and manage the inevitable changes in the Canadian North.  相似文献   

A proposal for a new scenario framework to support research and assessment in different climate research communities   总被引：1，自引：0，他引：1  

Detlef P. van Vuuren  Keywan Riahi  Richard Moss  Jae Edmonds  Allison Thomson  Nebojsa Nakicenovic  Tom Kram  Frans Berkhout  Rob Swart  Anthony Janetos  Steven K. Rose  Nigel Arnell 《Global Environmental Change》2012,22(1):21-35

In this paper, we propose a scenario framework that could provide a scenario “thread” through the different climate research communities (climate change – vulnerability, impact, and adaptation - and mitigation) in order to support assessment of mitigation and adaptation strategies and climate impacts. The scenario framework is organized around a matrix with two main axes: radiative forcing levels and socio-economic conditions. The radiative forcing levels (and the associated climate signal) are described by the new Representative Concentration Pathways. The second axis, socio-economic developments comprises elements that affect the capacity for mitigation and adaptation, as well as the exposure to climate impacts. The proposed scenarios derived from this framework are limited in number, allow for comparison across various mitigation and adaptation levels, address a range of vulnerability characteristics, provide information across climate forcing and vulnerability states and span a full century time scale. Assessments based on the proposed scenario framework would strengthen cooperation between integrated-assessment modelers, climate modelers and vulnerability, impact and adaptation researchers, and most importantly, facilitate the development of more consistent and comparable research within and across these research communities.  相似文献   

季节性流感的多元激发理论及其包含气象和社会行为因素的数学模型   总被引：4，自引：0，他引：4  

ZHOU Steven Zhixiang 《热带气象学报》2009,25(6):706-716

为了解流感的季节性和爆发的生物力学机制,在收集、分析、计算全球各地流感监测资料的基础上,建立了流感传播的多元激发理论及相应的数值模式,并运用该模式对全球各大洲不同气候带的季节性流感和周际流感进行了数值模拟和预报尝试。模式近似解的数学表达式以及模式输出与实况高度吻合的模拟结果证明：流感的发病率呈高度自相关,同时受生物、气象环境和社会-行为因素支配;流感的爆发是各种因素协同作用的结果。动力共振、太阳辐射、降水、露点温度变化、行为和干预效应以及人群对优势流行毒株的免疫力是支配流感季节性和每周活动变化的关键因素。具体表现为,对于所有气候区,流感发病率都是降水日数的指数函数,日照时数的1/4次方的负指数函数。在温带和寒带地区,季节性流感的发病率是露点温度的负指数函数。在热带地区,则是露点与其年平均值绝对偏差的指数函数。研究提示：早期干预(例如适时和广泛接种疫苗、迅速隔离病例、停学、遏制早期病例总数等)是控制和预防季节性流感的关键;适量的日光照射或在雨季和日照短的季节补充维生素D、增加户外活动、发布高危天气预警、设定适中的室内露点温度是值得高度重视的预防策略。模式的模拟结果在很大程度上揭示了流感季节性的机理和证明了著名医学家和诺贝尔奖获得者RobeRt EdgaR Hope-Simpson在上世纪提出的流感季节性成因的猜想。模拟结果还表明,模式对每周流感发病率具有一定的预报能力。该理论和模型很可能适用于当前的甲型流感大流行和其它传染性疾病,可被进一步开发应用。  相似文献   

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   

Preface: Workshop on matter,astrophysics, gravitation,ions and cosmology—MAGIC23     

César A. Zen Vasconcellos  Dimiter Hadjimichef  Magno Machado  Benno Bodmann  Marcelo Netz-Marzola  Geovane Naysinger  Mariana Vargas Magaña  Peter O. Hess  Horst Stöcker  Steven Gullberg  Remo Ruffini 《Astronomische Nachrichten》2024,345(2-3):e240029

We outline our experience in organizing the first edition of the Workshop on Matter, Astrophysics, Gravitation, Ions and Cosmology, held in virtual and in-person format, denominated MAGIC23, held from 6 to 10 March, 2023, in Praia do Rosa, Santa Catarina, Brazil. The event aimed to bring together leading academic scientists, professors, students, and research scholars for exchanging experiences and discuss the most recent innovations, trends, practical challenges, and experimental and theoretical solutions adopted in the investigation fields within the scope of the meeting. The workshop offered to the participants a platform for scientific and academic projects, partnerships, and presentation of high-quality research contributions describing original and unpublished results on topics related to matter, astrophysics, gravitation, ions, and cosmology.  相似文献   

The Ushnus in the astronomy of the Inca culture     

Milton Rojas Gamarra  Mônica Baptista Pereira Estrázulas  Steven R. Gullberg  César A. Zen Vasconcellos 《Astronomische Nachrichten》2024,345(2-3):e230181

Inferences are made about the relationship that existed between the Ushnus, pyramid-shaped, terraced structures used by the Incas in the most important ceremonies of the Tawantinsuyo, and Inka Astronomy. We draw attention to Ayni, Kawsaypacha, Duality, and Tinkuy principles, multidimensional codes of conduct and wisdom that are at the root of the Andean cosmovision and on their perception of the world and the Cosmos. These principles, examined as postulates, allow to elaborate axiomatic propositions to identify the Ushnus with ancient Astronomy practices. In a complementary statement, starting from a bi-conditional proposition, we may infer through reciprocal corollaries that the Inka earliest roots to a holistic learning and educational ambient in the Tawantinsuyo was not elitist, instead it was based on a epistemological construct that differs from the corresponding Western educational ambients. An epistemological and cognitive approach allows to identify an ancient elaborate process of knowledge construction, based on the four fundamental principles, corresponding to different levels of assimilation and comprehension. As a complementary aspect, we identify some of the most preserved Ushnus of the Inka “Empire.” Then we complement this contribution with a broader interpretation for the Ushnus.  相似文献   

A comparison of local simulations and reduced models of MRI-induced turbulence     

Pierre Lesaffre  Steven A. Balbus  Henrik Latter 《Monthly notices of the Royal Astronomical Society》2009,396(2):779-787

We run mean-field shearing-box numerical simulations with a temperature-dependent resistivity and compare them to a reduced dynamical model. Our simulations reveal the co-existence of two quasi-steady states, a 'quiet' state and an 'active' turbulent state, confirming the predictions of the reduced model. The initial conditions determine on which state the simulation ultimately settles. The active state is strongly influenced by the geometry of the computational box and the thermal properties of the gas. Cubic domains support permanent channel flows, bar-shaped domains exhibit eruptive behaviour, and horizontal slabs give rise to infrequent channels. Meanwhile, longer cooling time-scales lead to higher saturation amplitudes.  相似文献   

Annual cycles in the interstellar scintillation time-scales of PKS B1519–273 and PKS B1622–253     

Steven J. B. Carter  Simon P. Ellingsen  Jean-Pierre Macquart  James E. J. Lovell 《Monthly notices of the Royal Astronomical Society》2009,396(3):1222-1230

We have used the University of Tasmania's 30-m radio telescope at Ceduna in South Australia to regularly monitor the flux density of a number of southern blazars. We report the detection of an annual cycle in the variability time-scale of the centimetre radio emission of PKS B1622−253. Observations of PKS B1519−273 over a period of nearly 2 yr confirm the presence of an annual cycle in the variability time-scale in that source. These observations prove that interstellar scintillation is the principal cause of inter-day variability at radio wavelengths in these sources. The best-fitting annual cycle model for both sources implies a high degree of anisotropy in the scattering screen and that it has a large velocity offset with respect to the local standard of rest. This is consistent with a greater screen distance for these 'slow' intra-day variability (IDV) sources than for rapid scintillators such as PKS B0405−385 or J1819+3845.  相似文献   

Oxidation of the Kaapvaal lithospheric mantle driven by metasomatism   总被引：2，自引：1，他引：1  

Steven Creighton  Thomas Stachel  Sergei Matveev  Heidi Höfer  Catherine McCammon  Robert W. Luth 《Contributions to Mineralogy and Petrology》2009,157(4):491-504

The oxidation state, reflected in the oxygen fugacity (fO₂), of the subcratonic lithospheric mantle is laterally and vertically heterogeneous. In the garnet stability field, the Kaapvaal lithospheric mantle becomes progressively more reducing with increasing depth from Δlog fO₂ FMQ-2 at 110 km to FMQ-4 at 210 km. Oxidation accompanying metasomatism has obscured this crystal-chemical controlled depth-fO₂ trend in the mantle beneath Kimberley, South Africa. Chondrite normalized REE patterns for garnets, preserve evidence of a range in metasomatic enrichment from mild metasomatism in harzburgites to extensive metasomatism by LREE-enriched fluids and melts with fairly unfractionated LREE/HREE ratios in phlogopite-bearing lherzolites. The metasomatized xenoliths record redox conditions extending up to Δlog fO₂ = FMQ, sufficiently oxidized that magnesite would be the stable host of carbon in the most metasomatized samples. The most oxidized lherzolites, those in or near the carbonate stability field, have the greatest modal abundance of phlogopite and clinopyroxene. Clinopyroxene is modally less abundant or absent in the most reduced peridotite samples. The infiltration of metasomatic fluids/melts into diamondiferous lithospheric mantle beneath the Kaapvaal craton converted reduced, anhydrous harzburgite into variably oxidized phlogopite-bearing lherzolite. Locally, portions of the lithospheric mantle were metasomatized and oxidized to an extent that conversion of diamond into carbonate should have occurred. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献