A conservative, unconditionally stable, second-order three-point differencing scheme for the diffusion–convection equation     

Christoph Clauser  Stefan Kiesner 《Geophysical Journal International》1987,91(3):557-568

A three-point differencing scheme for the diffusion–convection equation is presented that offers all the advantages of both the central and the one-sided ('upwind') differencing scheme without suffering from their drawbacks. Specifically, the scheme is conservative, unconditionally stable, and second-order-accurate in space. It is free of oscillations and over- or undershoots, simple to code, and requires essentially no more computing time than the one-sided scheme. Although known for a relatively long time in numerical mathematics, the scheme apparently has not received sufficient attention from modellers of hydrothermal systems or contaminant transport in the geosciences. In order to fill this gap a comparison is made between this scheme and the widely used one-sided scheme for the transient diffusion–convection equation in different time discretizations. The results are discussed taking into account other approaches towards minimizing numerical diffusion.  相似文献   

Fast Object Detection for Use Onboard Satellites     

Martin Bange  Stefan Jordan  Michael Biermann  Thomas Kämpke  Ralf-Dieter Scholz 《Experimental Astronomy》2002,13(2):101-117

We propose an object detection algorithm which is efficient and fast enough to be used in (almost) real time with the limited computer capacities onboard satellites. For stars below the saturation limit of the CCD detectors it is based on a four neighbourhood local maximum criterion in order to find the centre of a stellar image. For saturated stars it is based on the assumption that the image is increasing monotonically towards the centre in the unsaturated part of the image. The algorithm also calculates approximate stellar magnitudes and efficiently rejects most of the cosmics which would otherwise lead to a large number of false detections. The quality of the algorithm was evaluated with the help of a large set of simulated data for the DIVA satellite mission; different assumptions were made for the noise level, and the presence of cosmics or for a variable sky background.We could show that our algorithm fulfills the requirements for DIVA; only in the case of simulated images which included the bright galaxy M31 some fainter stars could not be detected in the galaxy's vicinity. Since stellar images contain large areas without any stars, we propose an additional block-skipping algorithm which can be coded on special-purpose hardware. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Fabric development in a fragment of tethyan oceanic lithosphere from the piemonte ophiolite nappe of the western Alps, Valtournanche, Italy     

W.Stefan Vogler 《Journal of Structural Geology》1987,9(8)

South of the Matterhorn the Valtournanche cuts through Alpine serpentinites, metagabbros, meta-pillowbasalts and metasediments—dismembered remnants of the Jurassic Tethyan oceanic crust, reassembled in the Piemonte ophiolite nappe. This study deals with a serpentinized ultramafic to mafic layered complex stemming from a spreading ridge environment. Cumulus fabrics of various kinds can be read through antigorite pseudomorphs, still allowing the detailed reconstruction of deep oceanic crust. Relics of igneous and metamorphic olivine prove crustal conditions during deformation. Fracturing of cumulus olivine was succeeded by plastic flow that activated low-temperature slip systems. Concomitant recrystallization produced metaperidotite only along shear zones, which are ascribed to subduction of the oceanic crust. At the turning point from subduction to obduction a static metamorphic event resulted in recovery and grain growth of recrystallized olivine. Afterwards serpentinization of the complex took place still under static conditions. Deformation of the serpentinite led to a sequence of four phases, involving non-penetrative cleavage formation, stretching and folding. This deformation is structurally related to obduction of the complex although partly accompanied by subduction zone metamorphism. Final movements of the ophiolites were due to cataclastic thrusting forming subnappe boundaries.  相似文献   

Paleoclimatic estimates from water and energy budgets of East African Lakes   总被引：1，自引：0，他引：1  

Stefan Hastenrath  John E. Kutzbach 《Quaternary Research》1983,19(2):141-153

The Turkana, Nakuru-Elmenteita, and Naivasha basins in the Rift Valley of East Africa experienced high water levels during the period of 10,000–7000 yr B.P. Analyses of the modern hydrologic and energy budgets for these basins, along with that of Lake Victoria, were used to infer the amount of precipitation that would have been required to maintain the enlarged paleolakes of the early Holocene. Precipitation must have been at least 150–300 mm/yr (15 to 35%) above the modern average. The precipitation estimates were fairly consistent among the various basins, but no quantitative estimate was made for the additional precipitation required to account for overflow from the Rift Valley lake basins. Discharge from the Lake Victoria basin around 1880 A.D. was considerably above the more recent average, and the increased discharge into the White Nile for that period might have been similar to that of the early Holocene. A sensitivity analysis showed that temperature changes were probably not too important for changing the hydrologic-energy budget; changes of albedo, Bowen ratio, and cloudiness were likely to have been of greater importance.  相似文献   

A simple model of seasonal open ocean convection     

Till Kuhlbrodt  Sven Titz  Ulrike Feudel  Stefan Rahmstorf 《Ocean Dynamics》2001,52(1):36-49

 Aspects of open ocean deep convection variability are explored with a two-box model. In order to place the model in a region of parameter space relevant to the real ocean, it is fitted to observational data from the Labrador Sea. A systematic fit to OWS Bravo data allows us to determine the model parameters and to locate the position of the Labrador Sea on a stability diagram. The model suggests that the Labrador Sea is in a bistable regime where winter convection can be either “on” or “off ”, with both these possibilities being stable climate states. When shifting the surface buoyancy forcing slightly to warmer or fresher conditions, the only steady solution is one without winter convection.  We then introduce short-term variability by adding a noise term to the surface temperature forcing, turning the box model into a stochastic climate model. The surface forcing anomalies generated in this way induce jumps between the two model states. These state transitions occur on the interannual to decadal time scale. Changing the average surface forcing towards more buoyant conditions lowers the frequency of convection. However, convection becomes more frequent with stronger variability in the surface forcing. As part of the natural variability, there is a non-negligible probability for decadal interruptions of convection. The results highlight the role of surface forcing variability for the persistence of convection in the ocean.  相似文献   

Turf exfoliation in the high Drakensberg, Southern Africa     

Stefan W. Grab 《Geografiska Annaler: Series A, Physical Geography》2002,84(1):39-50

Limited research attention has focussed on turf exfoliation as a denudation process in mountain environments. This paper examines some characteristics of turf exfoliation forms identified within particular valley zones in the Drakensberg alpine belt. Morphological and sedimentological data are presented for turf exfoliated sites investigated in the Mashai Valley of eastern Lesotho. It is found that a variety of processes, including needle ice action, biological activity, fluvial processes and deflation, operating synergistically, are responsible for contemporary turf exfoliation in the high Drakensberg. It is apparent that the strong seasonality from mild, wet summers to cold, dry winters has helped induce the annual cycle of dominating processes.  相似文献   

Circulation and teleconnection mechanisms of Northeast Brazil droughts     

Stefan Hastenrath   《Progress in Oceanography》2006,70(2-4):407

The Northern Nordeste of Brazil has its short rainy season narrowly concentrated around March–April, when the interhemispheric southward gradient of sea surface temperature (SST) is weakest and the Intertropical Convergence Zone (ITCZ), which is the main rainbearing system for the Nordeste, reaches its southernmost position in the course of the year. The recurrent Secas (droughts) have a severe socio-economic impact in this semi-arid region. In drought years, the pre-season (October–January) rainfall is scarce, the interhemispheric SST gradient weakened and the basin-wide southerly (northerly) wind component enhanced (reduced), all manifestations of an anomalously far northward ITCZ position. Apart from this ensemble of Atlantic indicators, the Secas also tend to be preceded by anomalously warm equatorial Pacific waters in January. During El Niño years, an upper-tropospheric wave train extends from the equatorial eastern Pacific to the northern tropical Atlantic, affecting the patterns of upper-tropospheric topography and divergence, and hence of vertical motion over the Atlantic. The altered vertical motion leads to a weaker meridional pressure gradient on the equatorward flank of the North Atlantic subtropical high, and thus weaker North Atlantic tradewinds. The concomitant reduction of evaporation and wind stirring allows for warmer surface waters in the tropical North Atlantic and thus steeper interhemispheric meridional thermal gradient. Consequently, the ITCZ stays anomalously far North and the Nordeste rainy season becomes deficient.  相似文献   

Rates of oxygen transfer from air bubbles to aqueous NaCl solutions at various temperatures     

Stefan A Zieminski  Robert M Hume III  Robert Durham 《Marine Chemistry》1976,4(4):333-346

Experiments have been conducted to investigate the effects of temperature on the interfacial surface area and on the rate of oxygen transfer from air bubbles dispersed in aqueous NaCl solutions. Tests were also conducted to estimate the effects of salt concentration on the size of the bubbles. In addition to NaCl solutions, seawater was used in some tests. The temperature effects were investigated at 5, 10, 15, 20, 25, and 30°C. The results showed a pronounced effect of the salt on the size of the bubbles, which first decreased sharply with increasing concentration, but showed no further drop when the concentration was increased beyond 0.6 M. Both in seawater and in the 0.6 M solution, the mass transfer rate, K_LA, increased almost linearly when temperature was increased within the range from 5 to 25°C. The salt solution, as well as the seawater, showed an increase of K_LA of 60–70% over that in pure water at the same temperatures. This effect was the result of increased surface area of bubbles because of decreased coalescence. The increase in surface area was strongly temperature dependent, especially between 15 and 20°C. Contrary to this behavior the surface area in pure water showed, practically, no temperature dependence. The results are explained and discussed on the basis of ion-water interactions.  相似文献   

Life under pressure: insights from electronic data-storage tags into cod swimbladder function     

van der Kooij  Jeroen; Righton  David; Strand  Espen; Michalsen  Kathrine; Thorsteinsson  Vilhjalmur; Svedang  Henrik; Neat  Francis C.; Neuenfeldt  Stefan 《ICES Journal of Marine Science》2007,64(7):1293-1301

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Lithium isotope composition of ordinary and carbonaceous chondrites, and differentiated planetary bodies: Bulk solar system and solar reservoirs     

Hans-Michael Seitz  Gerhard P. Brey  Jutta Zipfel  Ulrich Ott  Stefan Weyer  Soodabeh Durali  Stephan Weinbruch 《Earth and Planetary Science Letters》2007,260(3-4):582-596

In order to better constrain the Li isotope composition of the bulk solar system and Li isotope fractionation during accretion and parent body processes, Li isotope compositions and concentrations were determined on a number of meteorite falls and finds. This is the first comprehensive study that systematically investigates a representative set of samples from carbonaceous chondrites (CI, CM2, CO3, CV3, CK4 and one ungrouped member), enstatite chondrites (EH, EL), ordinary chondrites (H, L, LL), and achondrites (one eucrite, diogenites, one pallasite, and a silicate inclusion from a IAB iron).

Carbonaceous chondrites have an average isotope composition of δ⁷Li = + 3.2‰ ± 1.9 (2σ) which agrees with the average composition of relatively pristine olivines (representative for the bulk composition) from the Earth primitive upper mantle (PUM). This is lighter than the average δ⁷Li of the basaltic differentiates of the Earth, Moon and Mars and the achondrites. It is an important observation, however, that the lighter end of the isotopic range of the differentiates always coincides with the averages of the mantle olivines and the carbonaceous chondrites. From this we conclude that the bulk of the inner solar system consists mostly of material from carbonaceous chondrites and that the variation seen in the differentiates is due to planetary body processes. Ordinary chondrites are significantly lighter than carbonaceous chondrites. No significant differences in δ⁷Li exist between enstatite chondrites (n = 3) and carbonaceous or ordinary chondrites. The difference between carbonaceous and ordinary chondrites and the variability within the chondrites could indicate the existence of distinct Li isotope reservoirs in the early solar nebula.  相似文献