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61.
Matthew K. MacDonald Bruce J. Davison Muluneh A. Mekonnen Alain Pietroniro 《水文科学杂志》2013,58(16):2860-2871
ABSTRACTThe low density of meteorological stations in parts of Canada necessitates using numerical weather prediction (NWP)/assimilation output for hydrological modelling. In this study, comparisons are made of simulated land surface variables when using field observations versus NWP output as forcing for two well-instrumented sites: the mountainous and forested Marmot Creek Basin (MCRB) in the Canadian Rocky Mountains, and a prairie cropland/grassland site (Kenaston). The Canadian Land Surface Scheme 3.6 (CLASS) was used for modelling. The Global Environmental Multiscale (GEM) model with Canadian Precipitation Analysis (CaPA) was also used as forcing. There was good agreement between observed meteorology and GEM/CaPA, though some deficiencies in GEM/CaPA were identified: the effects of sub-grid topography on incoming radiation and wind speed were not accounted for at MCRB, and CaPA did not capture some convective rainfall events at Kenaston. CLASS simulations using both sets of forcing showed difficulties in simulating snow depth, soil moisture and evapotranspiration; certain difficulties were linked to GEM/CaPA deficiencies and/or CLASS. Both sets of forcing tended to overestimate the duration of snow cover at MCRB, but during different years. With GEM/CaPA as forcing, CLASS overestimated the duration of frozen soils. The GEM/CaPA precipitation difficulties at Kenaston degraded soil moisture simulations.
EDITOR A. Castellarin; ASSOCIATE EDITOR E. Volpi 相似文献
62.
Collisions between planetesimals at speeds of several kilometres per second were common during the early evolution of our Solar System. However, the collateral effects of these collisions are not well understood. In this paper, we quantify the efficiency of heating during high-velocity collisions between planetesimals using hydrocode modelling. We conducted a series of simulations to test the effect on shock heating of the initial porosity and temperature of the planetesimals, the relative velocity of the collision and the relative size of the two colliding bodies. Our results show that while heating is minor in collisions between non-porous planetesimals at impact velocities below 10 km s−1, in agreement with previous work, much higher temperatures are reached in collisions between porous planetesimals. For example, collisions between nearly equal-sized, porous planetesimals can melt all, or nearly all, of the mass of the bodies at collision velocities below 7 km s−1. For collisions of small bodies into larger ones, such as those with an impactor-to-target mass ratio below 0.1, significant localised heating occurs in the target body. At impact velocities as low as 5 km s−1, the mass of melt will be nearly double the mass of the impactor, and the mass of material shock heated by 100 K will be nearly 10 times the mass of the impactor. We present a first-order estimate of the cumulative effects of impact heating on a porous planetesimal parent body by simulating the impact of a population of small bodies until a disruptive event occurs. Before disruption, impact heating is volumetrically minor and highly localised; in no case was more than about 3% of the parent body heated by more than 100 K. However, heating during the final disruptive collision can be significant; in about 10% of cases, almost all of the parent body is heated to 700 K (from an initial temperature of ∼300 K) and more than a tenth of the parent body mass is melted. Hence, energetic collisions between planetesimals could have had important effects on the thermal evolution of primitive materials in the early Solar System. 相似文献
63.
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65.
Dominik C. HEZEL Sara S. RUSSELL Aidan J. ROSS Anton T. KEARSLEY 《Meteoritics & planetary science》2008,43(11):1879-1894
Abstract— Modal abundances of Ca,Al‐rich inclusions (CAIs) are poorly known and reported data scatter across large ranges. CAIs are Poisson distributed, and if only small areas (<1000 mm2) are studied, the data are probably not representative of the true CAI modal abundances, explaining their reported large scatter in a single chondrite group. We combine reported CAI modal abundances and our own set, and present a complete list of CAI modal abundances in carbonaceous chondrites. This includes (in area%): CV: 2.98, CM: 1.21, Acfer 094: 1.12, CO: 0.99, CK/CV (Ningqiang and Dar al Gani [DaG] 055): 0.77, CK: 0.2, CR: 0.12 and CB: 0.1. CAIs are Poisson distributed and if only small areas are studied, the data are probably not representative of the true CAI modal abundances, Carbonaceous chondrites have excess bulk Al concentrations when compared to the CI‐chondritic value. We find a correlation between this excess and CAI modal abundances and conclude that the excess Al was delivered by CAIs. The excess Al is only a minor fraction (usually ?10 rel%, but 25 rel% in case of CVs) of the bulk chondrite Al and cannot have contributed much 26Al to heat the chondrite parent body. Ordinary, enstatite, R and K chondrites have an Al deficit relative to CI chondrites and only very low CAI modal abundances, if any are present at all. Carbonaceous chondrites also had an initial Al deficit if the contribution of Al delivered by CAIs is subtracted. Therefore all chondrites probably lost a refractory rich high‐T component. Only minor amounts of CAIs are present in the matrix or have been present in the chondrule precursor aggregates. Most CAI size distributions contain more than one size population, indicating that CAIs from within a single meteorite group had different origins. 相似文献
66.
Anthony Stockdale William Davison Hao Zhang John Hamilton-Taylor 《Aquatic Geochemistry》2010,16(4):575-585
Formation and dissolution of authigenic Fe and Mn (oxyhydr)oxides influence cycling of trace metals in oxic/suboxic surface
sediments. We used the diffusive gradients in thin films technique (DGT) to estimate the association of cobalt with iron and
manganese oxides. We compared Co, Fe and Mn maxima measured by DGT in the pore waters of fresh and aged marine sediment cores
and estimated the Co/Fe and Co/Mn ratios in the metal oxides. A Mn maximum was not visible in DGT concentration profiles of
freshly collected sediment cores, but after ageing the sediment, we observed a distinct Mn peak, presumably due to broadening
of the depth range over which the various electron acceptors occur. Estimated Co/Mn ratios from both experiments are within
the range of literature values for marine sediments, but the value from the aged experiment is at the lower end of the range.
This is attributed to stimulation of sulphate reduction and precipitation of cobalt sulphides. The good correlation between
Co and Fe maxima in the fresh sediments is attributed to the similarity of their reactions with sulphide rather than Co being
released during authigenic Fe oxide reduction. 相似文献
67.
68.
The technique of Diffusional Gradients in Thin-films (DGT) can be used in situ to obtain high resolution profiles of trace-metals in sediment pore waters. Substances sampled by DGT continuously diffuse through a diffusion layer comprising a hydrogel prior to being immobilized by binding to a resin layer. DGT therefore measures a time averaged flux from the pore water to the resin. Interpretation of this flux as pore water concentration is problematic for two reasons. Firstly, the pore water concentration adjacent to the sampler may become depleted by the DGT induced flux. Secondly, if there are steep vertical chemical gradients in the pore waters, they may relax by diffusion along the gradient within the gel layer. The extent of relaxation depends on the diffusion coefficient, gradient steepness, and diffusion layer thickness. Two dimensional (2D) numerical models of DGT deployments in horizontally uniform sediments were used to investigate to what extent DGT measured profiles accurately reproduced (a) the shape of pore water concentration profiles, and (b) the magnitude of pore water concentrations. A method is developed which translates high resolution DGT measured flux profiles into reliable estimates of pore water concentrations. Linear relationships are given which estimate the minimum DGT measured peak width (as a function of diffusion layer thickness) that ensures accurate reproduction of the shape and the magnitude of peaks in pore water concentrations. Peaks in DGT profiles obtained from assemblies with diffusion layer thicknesses of 0.3 mm (0.5 mm) should be at least 1.2 mm (1.8 mm) wide for their shape to reflect accurately their true shape in the pore water, and at least 1.7 mm (2.7 mm) wide to ensure the peak concentration is accurately estimated. 相似文献
69.
ABSTRACTTraditionally, hydrological models are only calibrated to reproduce streamflow regime without considering other hydrological state variables, such as soil moisture and evapotranspiration. Limited studies have been performed on constraining the model parameters, despite the fact that the presence of a large number of parameters may provide large degree of freedom, resulting in equifinality and poor model performance. In this study, a multi-objective optimization approach is adopted, and both streamflow and soil moisture data are calibrated simultaneously for an experimental study basin in the Saskatchewan Prairies in western Canada. The results of this study show that the multi-objective calibration improves model fidelity compared to the single objective calibration. Moreover, the study demonstrates that single objective calibration performed against only streamflow can fairly mimic the streamflow hydrograph but does not yield realistic estimation of other fluxes such as evapotranspiration and soil moisture (especially in deeper soil layers). 相似文献
70.
The rate of oxidation of ferrous iron in a seasonally anoxic lake was measured on 39 occasions with respect to both depth and time. Sample disturbance was minimal as only oxygen had to be introduced to initiate the reaction. The data were consistent with the simple rate law for homogeneous chemical kinetics previously established for synthetic solutions. The rate constant for the oxidation reaction in lake water was indistinguishable from that measured in synthetic samples. It did not appear to be influenced by changes in the microbial populations or by changes in any particulate or soluble components in the water, including iron and manganese. Analysis of the errors inherent in the kinetic measurements showed that the estimation of pH was the major source of inaccuracy and that values of the rate constant determined by different workers could easily differ by a factor of six.The present data, together with a comprehensive survey of the literature, are used to suggest a ‘universal’ rate constant of ca. 2 × 1013 M?2 atm?1 min?1 (range 1.5–3 × 1013) in the rate law for natural freshwaters in the pH range 6.5–7.4. Discrepancies in the effects of ionic strength and interfering substances reported in the literature are highlighted. Generally substances have only been found to interfere at concentrations which far exceed those in most natural waters. 相似文献