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311.
S. A. Curtis P. E. Clark J. R. Marshall J. A. Nuth F. A. Minetto C. I. Calle 《Earth, Moon, and Planets》2010,107(2-4):147-155
Work presented here addresses the issue of grain accretion, an essential yet poorly understood process in planetary system formation, linking the dynamically modeled steps of temperature-dependent condensation of gases after proto-sun gravitational collapse to coalescence of kilometer-size planetesimals into planets. The mechanism for grain accretion has proven difficult to model dynamically. Here, we attempt to test the thesis that the accretion process is electrostatically-driven by non-uniform charging of grains in a low discharge/weak field environment equivalent to periodic conditions in protoplanetary nebulae during solar discharge events such as flares. We simulate in the laboratory the behavior of grains in relationship to surfaces in such an environment. The nature of the observed disaggregation, repulsion, and acceleration of grains away from initial surfaces, and their reaggregation as coatings on surrounding oppositely charged surfaces, provide an empirical experimental basis for an electrostatically-driven model for grain behavior and accretion. Similar weak discharge processes in the protoplanetary disk solar nebula could give rise to increased grain acceleration and collisional compression induced surface coating, necessary conditions for increased accretion. The frequency, timing, and level of energetic output of the proto-sun would influence the effectiveness of such processes in developing stable aggregates, and the nature of the solar system that would result. 相似文献
312.
Scott Andrew Wooldridge Terence J. Done Colette R. Thomas Iain I. Gordon Paul A. Marshall Roger N. Jones 《Climatic change》2012,112(3-4):945-961
The threats of wide-scale coral bleaching and reef demise associated with anthropogenic (global) climate change are widely known. Less well considered is the contributing role of conditions local to the reef, in particular reef water quality, in co-determining the physiological tolerance of corals to increasing sea temperatures and declining pH. Here, the modelled benefit of reduced exposure to dissolved inorganic nitrogen (DIN) in terrestrial runoff, which raises the thermal tolerance of coastal coral communities on the central Great Barrier Reef (Australia), is considered alongside alternative future warming scenarios. The simulations highlight that an 80% reduction in DIN ‘buys’ an additional ~50–60?years of reef-building capacity for No Mitigation (‘business-as-usual’) bleaching projections. Moreover, the integrated management benefits provided by: (i) local reductions of ~50% in DIN contained in river loads, and (ii) global stabilisation of atmospheric CO2 below 450?ppm can help ensure the persistence of hard-coral-dominated reefscapes beyond 2100. The simulations reinforce the message that beyond the global imperative to mitigate future atmospheric CO2 emissions there still remains the need for effective local management actions that enhance the resistance and resilience of coral reef communities to the impacts of climate change. 相似文献
313.
Gilles Orban de Xivry Phil Marshall 《Monthly notices of the Royal Astronomical Society》2009,399(1):2-20
We present an investigation of the relationships between the radio properties of a giant radio galaxy MRC B0319−454 and the surrounding galaxy distribution with the aim of examining the influence of intergalactic gas and gravity associated with the large-scale structure on the evolution in the radio morphology. Our new radio continuum observations of the radio source, with high surface brightness sensitivity, images the asymmetries in the megaparsec-scale radio structure in total intensity and polarization. We compare these with the three-dimensional galaxy distribution derived from galaxy redshift surveys. Galaxy density gradients are observed along and perpendicular to the radio axis: the large-scale structure is consistent with a model wherein the galaxies trace the ambient intergalactic gas and the evolution of the radio structures are ram-pressure limited by this associated gas. Additionally, we have modelled the off-axis evolution of the south-west radio lobe as deflection of a buoyant jet backflow by a transverse gravitational field: the model is plausible if entrainment is small. The case study presented here is a demonstration that giant radio galaxies may be useful probes of the warm-hot intergalactic medium believed to be associated with moderately over dense galaxy distributions. 相似文献
314.
Plio‐Pleistocene speleothems from australopithecine‐bearing caves of South Africa have the potential to yield paleoenvironmental and geochronological information using isotope geochemistry. Prior to such studies it is important to assess the preservation of geochemical signals within the calcitic and aragonitic speleothems, given the tendency of aragonitic speleothems to recrystallize to calcite. This study documents the geochemical suitability of speleothems from the principal hominin‐bearing deposits of South Africa. We use petrography, together with stable isotope and trace element analysis, to identify the occurrence of primary aragonite, primary calcite, and secondary calcite. This study highlights the presence of diagenetic alteration at many of the sites, often observed as interbedded primary and secondary fabrics. Trace element and stable isotopic values distinguish primary calcite from secondary calcite and offer insights into geochemical aspects of the past cave environment. δ13C values of the primary and secondary calcites range from +6 to −9‰ and δ18O values range from −4 to −6‰. The data are thus typical of meteoric calcites with highly variable δ13C and relatively invariant δ18O. High carbon isotope values in these deposits are associated with the effects of recrystallization and rapid outgassing of CO2 during precipitation. Mg/Ca and Sr/Ca ratios differ between primary and secondary calcite speleothems, aiding their identification. Carbon and oxygen isotope values in primary calcite reflect the proportion of C3 and C4 vegetation in the local environment and the oxygen isotope composition of rainfall. Primary calcite speleothems preserve the pristine geochemical signals vital for ongoing paleoenvironmental and geochronological research. © 2009 Wiley Periodicals, Inc. 相似文献
315.
Tropical cyclone track and intensity prediction: The generation and assimilation of high-density, satellite-derived data 总被引:1,自引:0,他引:1
J. F. Le Marshall L. M. Leslie R. F. Abbey Jr. L. Qi 《Meteorology and Atmospheric Physics》2002,80(1-4):43-57
Summary The impact of recent scientific and technological advances in tropical cyclone track, intensity and structure modeling is
discussed. Since the early 1990s, developments have occurred in remote sensing, data assimilation procedures, numerical models
and high performance computing. In particular, there is now quasi-continuous high spatial and temporal resolution data coverage
over the previously data-sparse oceans where tropical cyclones spend most of their life cycles. There has been a rapid development
of data assimilation methodologies capable of using these data to initialize high-resolution prediction models. Model developments
have reached a stage of maturity where the representation of many of the physical processes necessary for improved tropical
cyclone track and intensity prediction are now included. Finally, available computer power has reached the teraflop range.
Most operational centers have high performance computers capable of tropical cyclone modeling at resolutions necessary for
skillful track and intensity simulations.
This article focuses on combining all of the above developments in a tropical cyclone data analysis and prediction system.
The system has produced statistically significant reductions in the mean forecast error statistics for tropical cyclone track
predictions and resulted in far more realistic simulations of tropical cyclone intensity and structure. A large number of
tropical cyclones have been modeled, with emphasis on those classified as being “difficult” storms to predict accurately.
These difficult storms are most responsible for rapidly growing forecast errors. Our results are illustrated by case studies
of such tropical cyclones.
Received October 9, 2001 Revised December 28, 2001 相似文献
316.
317.
318.
Restoration of the upper Strawberry River included bank stabilization techniques because it was assumed that excessive bank erosion was degrading spawning habitat for Bonneville cutthroat trout (BCT). Using a long‐term aerial photograph record, the historical range of variability in bank erosion rates and channel geometry was determined, and this information was used to assess present‐day conditions and the rationale for restoration. Relative to historical variability, the channel planform was relatively stable and bank erosion rates were the lowest recorded in the post‐disturbance era. Although a historical loss of riparian vegetation coincided with a shift to a wider and more sinuous channel, lateral migration rates declined and the channel narrowed as riparian cover increased in the decades before restoration, indicating a process of natural recovery. Furthermore, it was found that the percentage of fine sediment in the streambed before restoration was insufficient to affect BCT spawning success. Together these results suggest that bank erosion and fine sediment did not affect the quality of spawning habitat or the abundance of BCT on the upper Strawberry River. The results highlight how a historical analysis can be used to identify the sources of habitat degradation and inform the selection of restoration goals and strategies. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
319.
Assessment of satellite-based rainfall estimates in urban areas in different geographic and climatic regions 总被引:1,自引:0,他引:1
This paper presents an assessment of the 3B42 research version rainfall product from NASA’s Tropical Rainfall Measuring Mission
Multi-satellite Precipitation Analysis (TMPA). The study provides new results of TMPA data accuracy in urban areas and highlights
trends associated with the climatological indicators of temperature and relative humidity in cities. Ten years (1998-2007)
of TMPA data were analyzed for three case study cities in the United States (Houston, Atlanta, and Las Vegas) and one in Korea
(Cheongju), representing semi-arid to humid climates. At each location, an urbanized river basin and non-urbanized river basin
were selected and comparisons between TMPA and rain gage observations were made for recorded storm events in the study period,
the largest storm events by total depth, and selected hurricanes and topical storms. The results indicate TMPA data match
well with rain gage observations at all locations. TMPA is slightly underestimated for semi-arid regions and overestimated
for humid regions. The relative magnitude of TMPA rain event accumulation compared to rain gage accumulation is noted to be
smaller for urbanized watersheds and high intensity events. The correlation of TMPA accuracy with temperature and relative
humidity and the analysis of accuracy by season indicate TMPA is more accurate for convective rainfall events. This suggests
a possible linkage between the observed urban-modified temperatures, hypothesized enhanced convection, and improved TMPA accuracy
in urban areas. 相似文献
320.
Katherine C. Filippino Margaret R. Mulholland Peter W. Bernhardt George E. Boneillo Ryan E. Morse Matthew Semcheski Harold Marshall Nancy G. Love Quinn Roberts Deborah A. Bronk 《Estuaries and Coasts》2011,34(2):269-280
Total maximum daily loads for nitrogen (N) are currently being established for the Chesapeake Bay watershed. While we know
inorganic N is bioavailable in the environment and therefore its input contributes to cultural eutrophication, the bioavailability
of organic N is unclear. Using bioassay experiments, we examined the impact of effluent-derived organic nitrogen (EON) from
wastewater treatment plants on natural water samples collected along an estuarine/salinity gradient within the lower Chesapeake
Bay watershed. All of the inorganic N and between 31% and 96% of the EON was removed during biotic bioassays within the first
2 days. Further, there was substantial abiotic reactivity of effluent N when it was added to natural water samples. Results
demonstrate that organic and inorganic N in effluent is removed to support the growth of microbial communities. These are
the first results aimed at assessing the reactivity of EON in natural waters along an estuarine/salinity gradient. 相似文献