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251.
Mauri J. Valtonen Kimmo A. Innanen Leena Tähtinen 《Astrophysics and Space Science》1984,107(2):209-222
Numerical simulations have been performed in order to study the gravitational interaction between the outlying population of the Galaxy and the Large Magellanic Cloud (LMC). The following effects have been noted: (1) halo objects are partially removed from the orbital distance of the LMC such that a local minimum is created in the radial density profile; (2) a large number of halo objects go into escape orbits. This possibility makes the determination of the mass of the Galaxy by virial theorem arguments unreliable; (3) the orbit of the LMC decays faster than what one would expect on the basis of the classical dynamical friction calculation. The excess friction derives from three-body effects, which are not accounted for in the classical dynamical friction. Finally, the fates of globular clusters and dwarf galaxies near the Galaxy and the LMC are discussed. 相似文献
252.
K. A. Hämeen-Anttila 《Earth, Moon, and Planets》1984,31(3):271-299
The statistical theory of partially elastic collisions and gravitational encounters is generalized for non-identical particles having different masses, internal densities, radii, elastic parameters or other properties. In those cases which are numerically studied, the differences in elasticity do not produce significant effects: essentially the same distribution of orbits is found if the mean value of the restitution coefficients is used for all the particles. Differences in the internal density lead to the equipartition of energy in the random motion, although this approximation is rather rough and affords for the lightest particles an overestimated energy. Mass differences which correspond to a dispersion of radii produce a complicated behaviour. The equipartition of the energy is occasionally possible, but if the mass differences are large, the velocity dispersion can be inversely proportional to the radius. Other distributions also occur. 相似文献
253.
Ph. Drobinski S. Bastin J. Dusek G. Zängl P. H. Flamant 《Meteorology and Atmospheric Physics》2006,92(3-4):285-306
Summary This paper investigates the characteristics of channelled airflow in the vicinity of a junction of three idealized valleys
(one valley carrying the incoming flow and two tributaries carrying the outflow), using a two-dimensional single-layer shallow
water model. Particular attention is given to the flow splitting occurring at the junction. Nondimensionalized, the model
depends on the valley geometry, the Reynolds number, which is related to the eddy viscosity, and on the difference of the
hydrostatic pressure imposed at the exit of the tributaries. At the spatial scale considered in this study, the Rossby number
relating the inertial and Coriolis forces is always larger than 1, implying that the effect of earth rotation can be neglected
to a first approximation.
The analysis of the flow structure within the three valleys as well as the calculation of the split ratio (fraction of the
air flow diverted into one of the two downstream valleys with respect to the total mass flux in the upstream valley) show
that (i) the flow pattern depends strongly on the Reynolds number while the split ratio is comparatively insensitive; (ii)
the valley geometry and the difference between the upstream and downstream hydrostatic pressures affect the flow pattern,
the location of the split point and the split ratio; (iii) the relative contribution of flow deflection by the sidewalls and
the blocking/splitting mechanism differs between the settings of a “Y-shape” valley and a “T-shape” valley.
Quantitative comparison of the present results with numerical simulations of realistic cases and with observations collected
in the region of the Rhine and Seez valleys (Switzerland) (“Y-shape” valley) and in the region of the Inn and Wipp valleys
(Austria) (“T-shape” valley) during the Mesoscale Alpine Programme (MAP) field experiment shows good agreement provided that
the normalized valley depth NΔH/Uu significantly exceeds 1, i.e., when “flow around” is expected. A structural disagreement between the idealized simulations
and the observed wind field is found only when NΔH/Uu ≃ 1, that is, in the “flow over” regime. This shows that the dimensionless valley depth
is indeed a good indicator for flow splitting, implying that the stratification is a key player in reality. 相似文献
254.
Use of biliary PAH metabolites as a biomarker of pollution in fish from the Baltic Sea 总被引:2,自引:0,他引:2
Vuorinen PJ Keinänen M Vuontisjärvi H Barsiene J Broeg K Förlin L Gercken J Kopecka J Köhler A Parkkonen J Pempkowiak J Schiedek D 《Marine pollution bulletin》2006,53(8-9):479-487
During field campaigns of the BEEP project (Biological Effects of Environmental Pollution in Marine Coastal Ecosystems) in 2001-2002, metabolites of polycyclic aromatic hydrocarbons (PAHs) were determined in bile samples from three fish species, flounder (Platichthys flesus), perch (Perca fluviatilis) and eelpout (Zoarces viviparus), from four separate areas in the Baltic Sea. Two determination methods were applied: fixed wavelength fluorescence (FF) for pyrene-type metabolites and high-performance liquid chromatography with fluorescence detection (HPLC). There was a good correlation between the FF method and 1-OH pyrene determined by HPLC. Normalisation of the FF data for absorbance at 380 nm or bile protein concentrations greatly increased variance in one third and decreased it in two thirds of the cases and resulted in a loss of significant differences (protein normalisation) between the sampling stations, but normalisation of the HPLC data had little effect on the results. The biliary PAH metabolite content was usually higher in males than in females. In perch and eelpout the biliary PAH contents were at similar levels, whereas in flounder the levels were lower. The sampling areas arranged in decreasing order of biliary PAH contents were: Wismar Bay > Gulf of Gdansk > Lithuanian coast > Kvadofj?rden (reference area). It is concluded that FF with un-normalised data is a reliable and simple method for monitoring purposes and only one sex of a selected species should be used. 相似文献
255.
Sh. K. Baltybaev O. A. Levchenkov L. K. Levsky O. Eklund T. Kilpeläinen 《Petrology》2006,14(3):247-261
The Ladoga, Russia, and adjacent Sulkava, southeastern Finland, metamorphic complexes are the two largest “granulite” provinces of the Svecofennian domain. In this area, the domain is composed of outer and inner zones. Sulkava is situated in the inner zone, which principally can be compared to the accretionary arc complex of Southern Finland. Ladoga is situated in the outer zone, which is correlated with the accretionary arc complexes of central and Western Finland. The complexes contain different metamorphic assemblages, which are caused by the different composition of the sedimentary protoliths: the rocks of the Sulkava metamorphic complex are higher in Al and K than those of the Ladoga Complex. Pb-Pb step leaching dating was used to determine the age of prograde sillimanite from both complexes. The dates thus obtained constrain metamorphic peaks for the Sulkava and Ladoga complexes at 1799 ± 19 Ma and 1878 ± 7 Ma, respectively, which is consistent with the U-Pb monazite ages of gneisses from both of the complexes. The differences in the ages of the metamorphic minerals from these complexes reflect the Early Svecofennian (1.89–1.86 Ga) and Late Svecofennian (1.83–1.79 Ga) metamorphic stages in the Fennoscandian Svecofennides. 相似文献
256.
Due to the climate change, vegetation of tundra ecosystems is predicted to shift toward shrub and tree dominance, and this change may influence bryophytes. To estimate how changes in growing environment and the dominance of vascular plants influence bryophyte abundance, we compared the relationship of occurrence of bryophytes among other plant types in a five-year experiment of warming (T), fertilization (F) and T + F in two vegetation types, heath and meadow, in a subarctic–alpine ecosystem. We compared individual leaf area among shrub species to confirm that deciduous shrubs might cause severe shading effect. Effects of neighboring functional types on the performance of Hylocomium splendens was also analyzed.Results show that F and T + F treatments significantly influenced bryophyte abundance negatively. Under natural conditions, bryophytes in the heath site were negatively related to the abundance of shrubs and lichens and the relationship between lichens and bryophytes strengthened after the experimental period. After five years of experimental treatments in the meadow, a positive abundance relationship emerged between bryophytes and deciduous shrubs, evergreen shrubs and forbs. This relationship was not found in the heath site. Our study therefore shows that the abundance relationships between bryophytes and plants in two vegetation types within the same area can be different. Deciduous shrubs had larger leaf area than evergreen shrubs but did not show any shading effect on H. splendens. 相似文献
257.
Gabriella Sch?ll-Barna Attila Demény Gábor Serlegi Szilvia Fábián Pál Sümegi István Fórizs Bernadett Bajnóczi 《Journal of Paleolimnology》2012,47(1):87-100
Geochemical records of bivalve shells have been increasingly studied in the last decade to obtain information on climate conditions.
In this paper we present stable isotope compositions of living and prehistoric shells of freshwater mussels (Unionidae) and
their relationships with climate conditions in a shallow lake environment of Lake Balaton, West-Central Hungary. Physical
conditions and stable oxygen isotope compositions of lake water samples were monitored where living bivalves were collected.
Comparisons between seasonal variations in ambient temperature, water composition and within-shell isotopic variations indicate
that the shells of Unio pictorum do reflect local changes at high resolution and thus can be used to study past conditions. Additionally, shells covering
the last two decades were gathered at several locations along the lake in order to determine spatial and temporal variations
in the shells’ isotopic compositions as a function of weather conditions. As an application, prehistoric shells collected
in archaeological excavations were analysed in order to study past environmental variations. Climate variations during the
Late Copper Age (5460–4870 cal. yr BP) have been assumed on the basis of geomorphological and archaeozoological observations
at the site Balatonkeresztúr-Réti-dűlő (south of Lake Balaton), that suggested increasing humidity as a cause of changes in
settlement location and domestic livestock husbandry. Stable carbon and oxygen isotope compositions of prehistoric bivalve
shells were analyzed from excavations representing five archaeological subphases (Boleráz subphase, 5460–5310 cal. yr BP;
two transitional subphases around 5310 cal. yr BP; Early Classic subphase, 5310–5060 cal. yr BP; Late Classic subphase, 5040–4870 cal.
yr BP). The analyses revealed significant negative C and O isotope shifts in the transitional subphases relative to the earlier
and later subphases. The isotopic variations indicate that the local climate became relatively wet and possibly cold around
5310 cal. yr BP, then it returned to drier (and likely warmer) conditions during the Classic subphases. This interpretation
is in agreement with previous studies on climate changes related to the “5.3 ky event” in the European continental area and
the North Atlantic Region, indicating an Atlantic influence in the Carpathian Basin. 相似文献
258.
Elevation gradients of European climate change in the regional climate model COSMO-CLM 总被引:1,自引:0,他引:1
A transient climate scenario experiment of the regional climate model COSMO-CLM is analyzed to assess the elevation dependency
of 21st century European climate change. A focus is put on near-surface conditions. Model evaluation reveals that COSMO-CLM
is able to approximately reproduce the observed altitudinal variation of 2 m temperature and precipitation in most regions
and most seasons. The analysis of climate change signals suggests that 21st century climate change might considerably depend
on elevation. Over most parts of Europe and in most seasons, near-surface warming significantly increases with elevation.
This is consistent with the simulated changes of the free-tropospheric air temperature, but can only be fully explained by
taking into account regional-scale processes involving the land surface. In winter and spring, the anomalous high-elevation
warming is typically connected to a decrease in the number of snow days and the snow-albedo feedback. Further factors are
changes in cloud cover and soil moisture and the proximity of low-elevation regions to the sea. The amplified warming at high
elevations becomes apparent during the first half of the 21st century and results in a general decrease of near-surface lapse
rates. It does not imply an early detection potential of large-scale temperature changes. For precipitation, only few consistent
signals arise. In many regions precipitation changes show a pronounced elevation dependency but the details strongly depend
on the season and the region under consideration. There is a tendency towards a larger relative decrease of summer precipitation
at low elevations, but there are exceptions to this as well. 相似文献
259.
Climate projections from multi-model ensembles are commonly represented by the multi-model mean (MMM) climate change. As an alternative, various subjectively formulated schemes for performance-based weighting of models have been proposed. Here, a more objective framework for model weighting is developed. A key ingredient of this scheme is a calibration step quantifying the relationship between intermodel similarity in observable climate and intermodel similarity in simulated climate change. Models that simulate the observable climate better are only given higher weight where and when such an intermodel relationship is found, and the difference in weight between better and worse performing models increases with the strength of this relationship. The method is applied to projections of temperature change from the Third Coupled Model Intercomparison Project. First, cross-validation is used to estimate the potential of the method to improve the accuracy of climate change estimates and to search for suitable predictor variables. The decrease in cross-validation error allowed by the weighting is relatively modest but not negligible, and it could potentially be increased if better predictor variables were found. Second, observations are used to weight the models, to study the differences between the weighted mean and MMM estimates of twenty-first century temperature change and the sensitivity of these differences to the predictor variables and observational data sets used. 相似文献
260.
We derive a conceptual model of the flow over heterogeneous terrain consisting of patches with contrasting Bowen ratios. Upward
moving eddies are assumed to carry heterogeneous properties, whereas downward moving eddies carry homogeneous properties.
This results in a decorrelation of temperature and humidity as the contrast between the patches increases. We show that this
model is able to reproduce the relationship developed by Lamaud and Irvine (Boundary-Layer Meteorol. 120:87–109, 2006). Some
details differ from their expression but are in accordance with data obtained over African savannah. We extend the conceptual
model to a combination of any scalars, not necessarily linked through the surface energy balance (as is the case for temperature
and humidity). To this end we introduce a new parameter that describes the surface heterogeneity in surface fluxes. The results
of the current model can be used to predict the discrepancy between similarity relationships for different scalars over heterogeneous
terrain. 相似文献