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461.
Johannes Bruns 《第四纪科学杂志》1989,4(3):267-272
Tectonic and soil mechanics investigations of a strongly plastic clay (Miocene ‘Mica Clay’) led to the detection of strain phenomena induced in connection with the formation of Elsterian buried channels. A microjoint system with randomly distributed fissures is thought to result from early Elsterian dessiccation and frost crack formation. It occurs frequently in the upper part of the clay horizon. A younger joint system, which is encountered throughout the entire clay body, comprises macrojoints arranged in three joint sets. The evidence strongly suggests that they were formed by gravitational and kinematic loading of the clay body by overriding ice. The macrojoints display various secondary alterations such as slickensides, escarpments, shear zones, erosive and pressure zones, the formation of which is seen in context with the channel incision. Investigations into the genesis of these alterations indicate a strong horizontally-orientated stress induction lateral to the channels, which resulted in stiffness modulus anisotropy. It has also caused a horizontal compression of the clay body, which at that time was already consolidated through ice overburdening. Horizontal compression of the clay led to evorsion of pore water eroding clay particles from the joint walls. Lower water content of the clay, in combination with increased dry unit weights, are found adjacent to the channels today. 相似文献
462.
Johannes Feddema Keith Oleson Gordon Bonan Linda Mearns Warren Washington Gerald Meehl Douglas Nychka 《Climate Dynamics》2005,25(6):581-609
This study assesses the sensitivity of the fully coupled NCAR-DOE PCM to three different representations of present-day land
cover, based on IPCC SRES land cover information. We conclude that there is significant model sensitivity to current land
cover characterization, with an observed average global temperature range of 0.21 K between the simulations. Much larger contrasts
(up to 5 K) are found on the regional scale; however, these changes are largely offsetting on the global scale. These results
show that significant biases can be introduced when outside data sources are used to conduct anthropogenic land cover change
experiments in GCMs that have been calibrated to their own representation of present-day land cover. We conclude that hybrid
systems that combine the natural vegetation from the native GCM datasets combined with human land cover information from other
sources are best for simulating such impacts. We also performed a prehuman simulation, which had a 0.39 K ~higher average
global temperature and, perhaps of greater importance, temperature changes regionally of about 2 K. In this study, the larger
regional changes coincide with large-scale agricultural areas. The initial cooling from energy balance changes appear to create
feedbacks that intensify mid-latitude circulation features and weaken the summer monsoon circulation over Asia, leading to
further cooling. From these results, we conclude that land cover change plays a significant role in anthropogenically forced
climate change. Because these changes coincide with regions of the highest human population this climate impact could have
a disproportionate impact on human systems. Therefore, it is important that land cover change be included in past and future
climate change simulations. 相似文献
463.
A unified global height reference system as a basis for IGGOS 总被引:1,自引:0,他引:1
The definition of a global height reference system is based on a mean sea surface, gravity field parameters, and a three-dimensional terrestrial reference frame (TRF). Tide gauge records, satellite altimetry, gravity measurements on Earth and from space, TRF coordinates, and spirit levelling have to be combined for the realization of the vertical reference frame. Observations and parameters have to be consistent with respect to the used standards, conventions and models. They have to provide globally unified reference surfaces (geoid or quasigeoid, respectively, and mean sea surface). The continental reference systems of Europe (EUREF, ECGN) and South America (SIRGAS) are considering these requirements in their strategies. They are presented here, and slightly different definitions and realizations for a globally unified height reference system are discussed. 相似文献
464.
Ivo Thonon Johannes R. Roberti Hans Middelkoop Marcel van der Perk Peter A. Burrough 《地球表面变化过程与地形》2005,30(10):1327-1343
Due to a lack of data on settling velocities (ws) and grain size distributions (GSDs) in ?oodplain environments, sedimentation models often use calibrated rather than measured parameters. Since the characteristics of suspended matter differ from those of deposited sediment, it is impossible to derive the ws and GSD from the latter. Therefore, one needs to measure in situ suspended sediment concentrations (SSCs), settling velocities, effective grain sizes and sedimentation ?uxes. For this purpose we used the LISST‐ST, a laser particle sizer combined with a settling tube. In 2002 (twice) and 2004, we located the LISST‐ST with an optical backscatter sensor and sediment traps in two ?oodplains in The Netherlands: one along the unembanked IJssel River, another along the embanked Waal River. Measurements revealed that the SSC in the ?oodplains varied in relation to the SSC in the river channel. Smaller ?ocs dominated the SSC, while larger ?ocs dominated the potential sedimentation ?uxes. The in situ GSD in the IJssel ?oodplain was signi?cantly coarser than in the Waal ?oodplain, while the dispersed median grain sizes were equal for both ?oodplains. Therefore, the dispersed median grain size was two to ?ve times smaller than the effective one. The in situ grain size exhibited a signi?cant positive relationship with ws, although the ws for the largest ?ocs showed high variability. Consequently, the variability in sedimentation ?uxes was also large. In the actual sedimentation ?uxes, and hence in sedimentation models, in situ grain sizes up to about 20 µm can be neglected. In ?oodplain sedimentation models the relation between settling velocity and in situ grain size can be used instead of Stokes's law, which is only valid for dispersed grain sizes. These models should also use adequate data on ?ow conditions as input, since these strongly in?uence the suspended sediment characteristics. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
465.
A previously developed two-dimensional numerical model is further developed for simulating the transport of dissolved contaminants originating from dissolution of a coal tar pool in a stratified, saturated porous medium. The model is used to simulate contaminant transport resulting from a rectangular-shaped coal-tar-pool dissolution experiment conducted in a large-scale experimental aquifer. The experimental porous medium consists of two sand strata, a high-hydraulic-conductivity upper stratum and a low-hydraulic-conductivity bottom stratum. The experiment was conducted to a time of 354 days and the groundwater velocity was changed several times within this duration. Model simulations show good agreement against observed contaminant concentrations, and simulations show that dissolved solute below the pool migrated deeper into the bottom stratum as compared to the upper stratum. Furthermore, simulations also suggest that contaminant concentrations in the lower stratum never reached quasi steady-state during the experimental time frame. 相似文献
466.
Volker Tesmer Johannes Boehm Robert Heinkelmann Harald Schuh 《Journal of Geodesy》2007,81(6-8):409-421
This paper compares estimated terrestrial reference frames (TRF) and celestial reference frames (CRF) as well as position time-series in terms of systematic differences, scale, annual signals and station position repeatabilities using four different tropospheric mapping functions (MF): The NMF (Niell Mapping Function) and the recently developed GMF (Global Mapping Function) consist of easy-to-handle stand-alone formulae, whereas the IMF (Isobaric Mapping Function) and the VMF1 (Vienna Mapping Function 1) are determined from numerical weather models. All computations were performed at the Deutsches Geodätisches Forschungsinstitut (DGFI) using the OCCAM 6.1 and DOGS-CS software packages for Very Long Baseline Interferometry (VLBI) data from 1984 until 2005. While it turned out that CRF estimates only slightly depend on the MF used, showing small systematic effects up to 0.025 mas, some station heights of the computed TRF change by up to 13 mm. The best agreement was achieved for the VMF1 and GMF results concerning the TRFs, and for the VMF1 and IMF results concerning scale variations and position time-series. The amplitudes of the annual periodical signals in the time-series of estimated heights differ by up to 5 mm. The best precision in terms of station height repeatability is found for the VMF1, which is 5–7% better than for the other MFs. 相似文献
467.
Whilst earth surface landforms evolve largely as a result of the activity of certain living organisms, the biologically driven geomorphic changes have evolutionary consequences on organisms and ecological communities. Recognition of complex biotic–abiotic feedbacks may lead to deep changes in our understanding of geomorphology. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
468.
A new efficient parameterization scheme for solar short-wave radiative heating, as a component of the net radiative effects in the atmosphere, is tested in a three-dimensional mesoscale model. This model is designed with moist convective processes in mind, so that the radiative parameterization (solar plus thermal infrared) are interactive with the cloud field. Previous work by the authors with only an infrared scheme has demonstrated that cloud-radiation interactions are characterized by strong cloud-top cooling, leading to upper cloud-layer destabilization. The effects of including solar heating are to modulate the strength of the strong infrared cooling, thereby leading to weaker interactions between clouds, radiation, and mesoscale fields. The present study shows that even on the mesoscale and for relatively short time-spans, radiative processes in the presence of clouds are not negligible.As a further step, a simple fractional cloud cover parameterization is introduced and the model response is compared with results omitting this parameterization. 相似文献
469.
Johannes H. Schroeder Ph.D. 《International Journal of Earth Sciences》1972,61(2):708-730
A variety of precipitative synsedimentary CaCO3 cements lines or fills cavities and surrounds sediment particles within Recent algal cup reefs which dot the rim of the Bermuda platform. These include aragonite needle, spherulitic, and lath cements as well as calcite micrite, palisade, scale, and blocky cements, listed according to respective relative abundances. No distribution pattern of cements is apparent; various combinations of three or four cements are found in almost every sample or thin section. From the occurrence of various cements, factors determining composition and fabric are deduced:
- Morphology, composition, and coating of the substrate.
- Direct or indirect influence of organisms such as algae, pelecypods, or crustaceans.
- Micro-environment, specifically size and permeability, rate of sea-water circulation through the micro-environment, substrates and organisms within the microenvironment.