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71.
Koushik N. Kumar K. Kishore Ramkumar Geetha Subrahmanyam K. V. Kishore Kumar G. Hocking W. K. He Maosheng Latteck Ralph 《Climate Dynamics》2020,54(9):4059-4074
Climate Dynamics - In the present communication, characteristics of mean winds and planetary waves in the mesosphere lower thermosphere (MLT) region during sudden stratospheric warming (SSW) events... 相似文献
72.
Morphodynamic assessment of side channel systems using a simple one‐dimensional bifurcation model and a comparison with aerial images 下载免费PDF全文
R. Pepijn van Denderen Ralph M. J. Schielen Astrid Blom Suzanne J. M. H. Hulscher Maarten G. Kleinhans 《地球表面变化过程与地形》2018,43(6):1169-1182
Side channel construction is a common intervention applied to increase a river's conveyance capacity and to increase its ecological value. Past modelling efforts suggest two mechanisms affecting the morphodynamic change of a side channel: (1) a difference in channel slope between the side channel and the main channel and (2) bend flow just upstream of the bifurcation. The objective of this paper was to assess the conditions under which side channels generally aggrade or degrade and to assess the characteristic timescales of the associated morphological change. We use a one‐dimensional bifurcation model to predict the development of side channel systems and the characteristic timescale for a wide range of conditions. We then compare these results to multitemporal aerial images of four side channel systems. We consider the following mechanisms at the bifurcation to be important for side channel development: sediment diversion due to the bifurcation angle, sediment diversion due to the transverse bed slope, partitioning of suspended load, mixed sediment processes such as sorting at the bifurcation, bank erosion, deposition due to vegetation, and floodplain sedimentation. There are limitations to using a one‐dimensional numerical model as it can only account for these mechanisms in a parametrized manner, but the model reproduces general behaviour of the natural side channels until floodplain‐forming processes become important. The main result is a set of stability diagrams with key model parameters that can be used to assess the development of a side channel system and the associated timescale, which will aid in the future design and maintenance of side channel systems. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. 相似文献
73.
Longting Mo Shujun Ye Jichun Wu Ralph G. StahlJr. Nancy R. Grosso Jennifer C. Wang 《Environmental Earth Sciences》2017,76(2):92
Delineation of contaminant sources is vital for successful groundwater and soil remediation. With reliable source information, remediation time and cost can be dramatically reduced. An optimal contamination source search strategy incorporating Monte Carlo method, Kalman filtering and fuzzy set theory was applied to a contaminated site in Nanjing to define suspected multiple DNAPL source locations. Using the available nine sample data, the algorithm identified sources #1 and #4 as true sources, and sources #5 and #6 as false ones. The algorithm results for sources #2 and #3 were inconclusive. Three numerical experiments based on specific site conditions were then designed and conducted to determine the influencing factors on the algorithm’s convergence in sources #2 and #3. The numerical experiments tested the effect of multiple sources, the effect of the sources location in relation to the groundwater flow direction and the effect of a low permeability field on the convergence of the algorithm. Based on the numerical experiments and an understanding of the manufacturing site operations, sources #2 and #3 are likely to be true sources. Their moderate weights have been stabilized due to the existence of multiple true sources and the scarcity of informative sampling data, caused by the low permeability field. The moderate weight value of source #3 also includes a contribution from an overlapping plume caused by the sources’ parallel-to-flow layout pattern. It can be concluded that the algorithm works best for high permeability sites where potential source locations are scattered and source location patterns are orthogonal to the groundwater flow. 相似文献
74.
P.I. Macreadie M.E. Baird S.M. Trevathan-Tackett A.W.D. Larkum P.J. Ralph 《Marine pollution bulletin》2014
Seagrasses are among the planet’s most effective natural ecosystems for sequestering (capturing and storing) carbon (C); but if degraded, they could leak stored C into the atmosphere and accelerate global warming. Quantifying and modelling the C sequestration capacity is therefore critical for successfully managing seagrass ecosystems to maintain their substantial abatement potential. At present, there is no mechanism to support carbon financing linked to seagrass. For seagrasses to be recognised by the IPCC and the voluntary C market, standard stock assessment methodologies and inventories of seagrass C stocks are required. Developing accurate C budgets for seagrass meadows is indeed complex; we discuss these complexities, and, in addition, we review techniques and methodologies that will aid development of C budgets. We also consider a simple process-based data assimilation model for predicting how seagrasses will respond to future change, accompanied by a practical list of research priorities. 相似文献
75.
Brian J. Anderson James A. Slavin Haje Korth Scott A. Boardsen Thomas H. Zurbuchen Jim M. Raines George Gloeckler Ralph L. McNutt Jr. Sean C. Solomon 《Planetary and Space Science》2011,59(15):2037-2050
Magnetic field and plasma data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft on the outbound portions of the first (M1) and second (M2) flybys of Mercury reveal a region of depressed magnetic field magnitude and enhanced proton fluxes adjacent to but within the magnetopause, which we denote as a dayside boundary layer. The layer was present during both encounters despite the contrasting dayside magnetic reconnection, which was minimal during M1 and strong during M2. The overall width of the layer is estimated to be between 1000 and 1400 km, spanning most of the distance from the dayside planetary surface to the magnetopause in the mid-morning. During both flybys the magnetic pressure decrease was ∼1.6 nPa, and the width of the inner edge was comparable to proton gyro-kinetic scales. The maximum variance in the magnetic field across the inner edge was aligned with the magnetic field vector, and the magnetic field direction did not change markedly, indicating that the change in field intensity was consistent with an outward plasma-pressure gradient perpendicular to the magnetic field. Proton pressures in the layer inferred from reduced distribution observations were 0.4 nPa during M1 and 1.0 nPa during M2, indicating either that the proton pressure estimates are low or that heavy ions contribute substantially to the boundary-layer plasma pressure. If the layer is formed by protons drifting westward from the cusp, there should be a strong morning–afternoon asymmetry that is independent of the interplanetary magnetic field (IMF) direction. Conversely, if heavy ions play a major role, the layer should be strong in the morning (afternoon) for northward (southward) IMF. Future MESSENGER observations from orbit about Mercury should distinguish between these two possibilities. 相似文献
76.
Ralph S. Lewis Mary L. Digiacomo-Cohen Nancy Friedrich Neff Richard Hyde 《Marine Georesources & Geotechnology》2013,31(2-3)
The Connecticut effort in year 9 of the Continental Margins Program concentrated on a 13.6-mi2 area south of the Housatonic River. This area was chosen for a side-scan sonar survey because it lies on and adjacent to the bathymetric expressions of two fairly large, subcropping, potential sources of coarse material. Previous seismic work in the area indicated that outcrops or subcrops of these potential source deposits could be delineated using their bathymetric expression. Owing to the limited resolution of the seismic data, a correlation between bottom type and underlying source deposits could not be made with the seismics alone. Results from the November 1993 side-scan survey show that although the source deposits have discernible bathymetric expressions, they are not cropping out as much as expected. As a result, bottom type is not necessarily determined by subcropping deposits in this particular area. Year 10 work was concentrated in Fishers Island Sound, where three areas of potential interest for near-shore gravel resources had previously been identified. These areas were surveyed, during the spring of 1996, using the RoxAnn Seabed Classification System. A small video camera and a Van Veen grab sampler were used to calibrate and verify the RoxAnn data. Although previous sampling had indicated the presence of gravel or gravelly sediment in all of the survey areas, the RoxAnn results showed less gravel than anticipated. Vibrant eelgrass beds and other habitat indicators were detected in all of the survey areas. Given the variable sediment results, and the high habitat potential of the areas surveyed, the likelihood of developing a sand and gravel supply from the near shore of Fishers Island Sound appears quite low. 相似文献
77.
A new parameter estimation algorithm is described for identifying the stiffness properties of torsionally coupled shear buildings from their linear response due to ambient excitations or during low-amplitude forced-vibration tests. The algorithm is based on the time-domain equations of motion, and yields estimates of the stiffness properties using a measure of the equilibrium of forces acting on each floor over a time interval. The banded structure of the stiffness matrix — a property intrinsic to torsion-shear buildings — is exploited to decompose the initial inverse problem into several problems of reduced size. This decomposition allows the identification of lateral and torsional stiffnesses of individual stories, independent of the others. The algorithm utilizes vibration data where input excitation is known/measured, which is typical for forced-vibration tests and earthquakes. If the ambient vibrations of the structure are adequately uncorrelated to the (unknown) external forces that induce such vibrations, then the algorithm can also be modified for output-only system identification. The proposed algorithm is verified — and its various attributes are investigated — using simulation data from the ‘Analytical Phase I’ of the IASC (International Association for Structural Control)-ASCE (American Society of Civil Engineers) benchmark studies. The companion article is devoted to the algorithm's application to experimental data, using data from the ‘Experimental Phase’ of the same benchmark studies. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
78.
Hartmut H. Hellmer Monika Rhein Günther Heinemann Janna Abalichin Wafa Abouchami Oliver Baars Ulrich Cubasch Klaus Dethloff Lars Ebner Eberhard Fahrbach Martin Frank Gereon Gollan Richard J. Greatbatch Jens Grieger Vladimir M. Gryanik Micha Gryschka Judith Hauck Mario Hoppema Oliver Huhn Torsten Kanzow Boris P. Koch Gert König-Langlo Ulrike Langematz Gregor C. Leckebusch Christof Lüpkes Stephan Paul Annette Rinke Bjoern Rost Michiel Rutgers van der Loeff Michael Schröder Gunther Seckmeyer Torben Stichel Volker Strass Ralph Timmermann Scarlett Trimborn Uwe Ulbrich Celia Venchiarutti Ulrike Wacker Sascha Willmes Dieter Wolf-Gladrow 《Ocean Dynamics》2016,66(11):1379-1413
79.
Joel W. Harrison Ralph E. H. Smith 《Aquatic Sciences - Research Across Boundaries》2013,75(3):399-411
Phytoplankton from Lake Ontario and six small Canadian lakes (Dorset Lakes) were supplemented with nitrogen (N) and phosphorus (P) to determine how nutrients affect Photosystem II (PSII) variable fluorescence and photoinhibition in natural freshwater communities. Susceptibility of PSII to photoinhibition by photosynthetically active radiation (PAR) and ultraviolet radiation (UVR), as well as recovery potential, was quantified using changes in variable fluorescence and compared between N- and P-supplemented (Nu+) and non-supplemented (Nu?) Lake Ontario phytoplankton. Nu+ communities exhibited slightly higher variable fluorescence than Nu? when dark-adapted (Fv:Fm) or under constant illumination (Fq′:Fm′). Rates of relative electron transport (rETR) were greater for Nu+ than Nu? phytoplankton, with higher non-photochemical quenching (NPQ) by Nu? samples. The initial slope of the rETR-irradiance curve (α) did not differ significantly between nutrient treatments, but the saturation irradiance (EK) was significantly higher for Nu+ samples than for Nu? samples. Nutrient supplementation increased rates of PAR- and UVR-dependent damage but also recovery, so that net PSII photoinhibition was equally severe as in the absence of added N and P. Additions of N, P, and N + P did not significantly alter Fv:Fm of Dorset Lakes phytoplankton. Compared to the range of variable fluorescence observable over the diel cycle of photoinhibition and recovery in Lake Ontario, the effects of supplemental nutrients observed in this study were minor. 相似文献
80.
We utilise a global finite-element sea ice–ocean model (FESOM), focused on the Antarctic marginal seas, to analyse projections of ice shelf basal melting in a warmer climate. Ice shelf–ocean interaction is described using a three-equation system with a diagnostic computation of temperature and salinity at the ice–ocean interface. A tetrahedral mesh with a minimumhorizontal resolution of 4 km and hybrid vertical coordinates is used. Ice shelf draft, cavity geometry, and global ocean bathymetry have been derived from the RTopo-1 data set. The model is forced with the atmospheric output from two climate models: (1) the Hadley Centre Climate Model (HadCM3) and (2) Max Planck Institute’s ECHAM5/MPI-OM coupled climate model. Results from experiments forced with their twentieth century output are used to evaluate the modelled present-day ocean state. Sea ice coverage is largely realistic in both simulations; modelled ice shelf basal melt rates compare well with observations in both cases, but are consistently smaller for ECHAM5/MPI-OM. Projections for future ice shelf basal melting are computed using atmospheric output for the Intergovernmental Panel on Climate Change (IPCC) scenarios E1 and A1B. In simulations forced with ECHAM5 data, trends in ice shelf basal melting are small. In contrast, decreasing convection along the Antarctic coast in HadCM3 scenarios leads to a decreasing salinity on the continental shelf and to intrusions of warm deep water of open ocean origin. In the case of the Filchner–Ronne Ice Shelf (FRIS), this water reaches deep into the cavity, so that basal melting increases by a factor of 4 to 6 compared to the present value of about 90 Gt/year. By the middle of the twenty-second century, FRIS becomes the dominant contributor to total ice shelf basal mass loss in these simulations. Our results indicate that the surface freshwater fluxes on the continental shelves may be crucial for the future of especially the large cold water ice shelves in the Southern Ocean. 相似文献