Simulating a white dwarf dominated Galactic halo     

Chris B. Brook  Daisuke Kawata  Brad K. Gibson 《Monthly notices of the Royal Astronomical Society》2003,343(3):913-923

  相似文献   

N-body simulations of the Magellanic stream   总被引：1，自引：0，他引：1  

Tim W. Connors  Daisuke Kawata  Brad K. Gibson 《Monthly notices of the Royal Astronomical Society》2006,371(1):108-120

A suite of high-resolution N -body simulations of the Magellanic Clouds–Milky Way system are presented and compared directly with newly available data from the H  i Parkes All-Sky Survey (HIPASS). We show that the interaction between Small Magellanic Clouds (SMC) and Large Magellanic Clouds results in both a spatial and kinematical bifurcation of both the stream and the leading arm. The spatial bifurcation of the stream is readily apparent in the HIPASS data, and the kinematical bifurcation is also tentatively identified. This bifurcation provides strong support for the tidal disruption origin for the Magellanic stream. A fiducial model for the Magellanic Clouds (MCs) is presented upon completion of an extensive parameter survey of the potential orbital configurations of the MCs and the viable initial boundary conditions for the disc of the SMC. The impact of the choice of these critical parameters upon the final configurations of the stream and leading arm is detailed.  相似文献   

A conceptual model of avalanche hazard     

Grant Statham  Pascal Haegeli  Ethan Greene  Karl Birkeland  Clair Israelson  Bruce Tremper  Chris Stethem  Bruce McMahon  Brad White  John Kelly 《Natural Hazards》2018,90(2):663-691

This conceptual model of avalanche hazard identifies the key components of avalanche hazard and structures them into a systematic, consistent workflow for hazard and risk assessments. The method is applicable to all types of avalanche forecasting operations, and the underlying principles can be applied at any scale in space or time. The concept of an avalanche problem is introduced, describing how different types of avalanche problems directly influence the assessment and management of the risk. Four sequential questions are shown to structure the assessment of avalanche hazard, namely: (1) What type of avalanche problem(s) exists? (2) Where are these problems located in the terrain? (3) How likely is it that an avalanche will occur? and (4) How big will the avalanche be? Our objective was to develop an underpinning for qualitative hazard and risk assessments and address this knowledge gap in the avalanche forecasting literature. We used judgmental decomposition to elicit the avalanche forecasting process from forecasters and then described it within a risk-based framework that is consistent with other natural hazards disciplines.  相似文献   

Circulation of the Venus upper mesosphere/lower thermosphere: Doppler wind measurements from 2001–2009 inferior conjunction,sub-millimeter CO absorption line observations     

R. Todd Clancy  Brad J. Sandor  Gerald Moriarty-Schieven 《Icarus》2012,217(2):794-812

Sub-millimeter ¹²CO (346 GHz) and ¹³CO (330 GHz) line absorptions, formed within the mesospheric to lower thermospheric altitude (70–120 km) region of the Venus atmosphere, have been mapped across the nightside disk of Venus during 2001–2009 inferior conjunctions, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of these thermal line absorptions supports temperature and CO mixing profile retrievals, as described in a companion paper (Clancy et al., 2012). Here, we consider the analysis of the sharp line absorption cores of these CO spectra in terms of accurate Doppler wind profile measurements at 95–115 km altitudes versus local time (～8 pm–4 am) and latitude (～60N–60S). These Doppler wind measurements support determinations of the nightside zonal and subsolar-to-antisolar (SSAS) circulation components over a variety of timescales. The average behavior fitted from 21 retrieved maps of ¹²CO Doppler winds (obtained over hourly, daily, weekly, and interannual intervals) indicates stronger average zonal (85 m/s retrograde) versus SSAS (65 m/s) circulation at the 1 μbar pressure (108–110 km altitude) level. However, the absolute and relative magnitudes of these circulation components exhibit extreme variability over daily to weekly timescales. Furthermore, the individual Doppler wind measurements within each nightside mapping observation generally show significant deviations (20–50 m/s, averaged over 5000 km horizontal scales) from the simple zonal/SSAS solution, with distinct local time and latitudinal characters that are also time variable. These large scale residual circulations contribute 30–70% of the observed nightside Doppler winds at any given time, and may be most responsible for global variations in nightside lower thermospheric trace composition and temperatures, as coincidentally retrieved CO abundance and temperature distributions do not correlate with solution retrograde zonal and SSAS winds (see companion paper, Clancy et al., 2012). Limited comparisons of these nightside submillimeter results with dayside infrared Doppler wind measurements suggest distinct dayside versus nightside circulations, in terms of zonal winds in particular. Combined ¹²CO and ¹³CO Doppler wind mapping observations obtained since 2004 indicate that the average zonal and SSAS wind components increase by 50–100% between altitudes of 100 and 115 km. If gravity waves originating from the cloud levels are responsible for the extension of zonal winds into the thermosphere (Alexander, M.J. [1992]. Geophys. Res. Lett. 19, 2207–2210), such waves deposit substantial momentum (i.e., break) in the lower nightside thermosphere.  相似文献   

Celebrating fifty years of NASA and Earth Observations from Space. Part III: Urban Remote Sensing and Sustainable Urban Systems     

Kamlesh Lulla  M. Duane Nellis  Brad Rundquist 《国际地球制图》2013,28(3):177-178

  相似文献   

Celebrating Geocarto International's Reach     

Kamlesh Lulla  M. Duane Nellis  Brad Rundquist 《国际地球制图》2013,28(1):1-2

  相似文献   

South Korea reaches a major milestone in earth observations     

Kamlesh Lulla  M. Duane Nellis  Brad Rundquist 《国际地球制图》2013,28(4)

  相似文献   

Comparative Evaluation of Single‐Well LNAPL Tracer Testing at Five Sites          下载免费PDF全文

Andrew Pennington  Jonathon Smith  Brad Koons  Craig E. Divine 《Ground Water Monitoring & Remediation》2016,36(2):45-58

Light nonaqueous phase liquid (LNAPL) tracer testing is a technique used to directly measure LNAPL flow in situ and evaluate LNAPL mobility and recoverability. The test method consists of adding a fluorescent oil‐soluble tracer to LNAPL within a well, isolating small volumes of LNAPL with known tracer concentrations for use as in‐well calibration standards, and measuring the rate of tracer concentration decline in the well over time. The test measures LNAPL flux through the well, which is directly related to LNAPL mobility and recoverability in the surrounding formation. Test results for a total of 29 wells at five sites are presented. Results from LNAPL tracer testing were comparable to results obtained through other methods, and the method offers a time‐averaged result measured over a relatively long period, in ambient conditions, and reflects the influences of heterogeneity and hydraulic changes. In some cases, tracer concentration decline followed unexpected patterns, and these data have led to a better understanding of test assumptions, mechanisms influencing tracer distribution, and options to improve test execution and data interpretation. Method improvements developed over the course of the field studies included refinement of pre‐test screening of LNAPL fluorescence and improvements to measurement equipment. Overall, the field studies confirmed the technical validity and usefulness of the LNAPL tracing technique to support LNAPL mobility and recoverability assessments.  相似文献   

An Injectable Apatite Permeable Reactive Barrier for In Situ 90Sr Immobilization     

Vince R. Vermeul  Jim E. Szecsody  Brad G. Fritz  Mark D. Williams  Robert C. Moore  Jonathan S. Fruchter 《Ground Water Monitoring & Remediation》2014,34(2):28-41

An injectable permeable reactive barrier (PRB) technology was developed to sequester ⁹⁰Sr in groundwater through the in situ formation of calcium‐phosphate mineral phases, specifically apatite that incorporates ⁹⁰Sr into the chemical structure. This injectable barrier technology extends the PRB concept to sites where groundwater contaminants are too deep or where site conditions otherwise preclude the application of more traditional trench‐emplaced barriers. An integrated, multiscale development and testing approach was used that included laboratory bench‐scale experiments, an initial pilot‐scale field test, and the emplacement and evaluation of a 300‐feet‐long treatability‐test‐scale PRB. The apatite amendment formulation uses two separate precursor solutions, one containing a Ca‐citrate complex and the other a Na‐phosphate solution, to form apatite precipitate in situ. Citrate is needed to keep calcium in solution long enough to achieve a more uniform and areally extensive distribution of precipitate formation. In the summer of 2008, the apatite PRB technology was applied as a 91‐m‐long (300 feet) PRB on the downgradient edge of a ⁹⁰Sr plume beneath the Hanford Site in Washington State. The technology was deployed to reduce ⁹⁰Sr flux discharging to the Columbia River. Performance assessment monitoring data collected to date indicate that the barrier is meeting treatment objectives (i.e., 90% reduction in ⁹⁰Sr concentration). The average reduction in ⁹⁰Sr concentrations at four downgradient compliance monitoring locations was 95% relative to the high end of the baseline range approximately 1 year after treatment, and continues to meet remedial objectives more than 4 years after treatment.  相似文献   

Dynamic interactions of life and its landscape: feedbacks at the interface of geomorphology and ecology     

Liam Reinhardt  Douglas Jerolmack  Brad J. Cardinale  Veerle Vanacker  Justin Wright 《地球表面变化过程与地形》2010,35(1):78-101

There appears to be no single axis of causality between life and its landscape, but rather, each exerts a simultaneous influence on the other over a wide range of temporal and spatial scales. These influences occur through feedbacks of differing strength and importance with co‐evolution representing the tightest coupling between biological and geomorphological systems. The ongoing failure to incorporate these dynamic bio‐physical interactions with human activity in landscape studies limits our ability to predict the response of landscapes to human disturbance and climate change. This limitation is a direct result of the poor communication between the ecological and geomorphological communities and consequent paucity of interdisciplinary research. Recognition of this failure led to the organization of the Meeting of Young Researchers in Earth Science (MYRES) III, titled ‘Dynamic Interactions of Life and its Landscape’. This paper synthesizes and expands upon key issues and findings from that meeting, to help chart a course for future collaboration among Earth surface scientists and ecologists: it represents the consensus view of a competitively selected group of 77 early‐career researchers. Two broad themes that serve to focus and motivate future research are identified: (1) co‐evolution of landforms and biological communities; and (2) humans as modifiers of the landscape (through direct and indirect actions). Also outlined are the state of the art in analytical, experimental and modelling techniques in ecological and geomorphological research, and novel new research avenues that combine these techniques are suggested. It is hoped that this paper will serve as an interdisciplinary reference for geomorphologists and ecologists looking to learn more about the other field. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献