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排序方式: 共有740条查询结果,搜索用时 15 毫秒
91.
This study attempts to analyse paleoceanographic changes in the Central Indian Ocean (Deep Sea Drilling Project Site 237), linked to monsoon variability as well as deep-sea circulation during the Plio-Pleistocene. We used factor and cluster analyses of census data of the 34 most dominant species of benthic foraminifera that enabled us to identify five biofacies: Astrononion umbilicatulum–Uvigerina proboscidea (Au–Up), Pullenia bulloides–Bulimina striata (Pb–Bs), Globocassidulina tumida–Nuttallides umbonifera (Gt–Nu), Gyroidinoides nitidula–Cibicides wuellerstorfi (Gn–Cw) and Cassidulina carinata–Cassidulina laevigata (Cc–Cl) biofacies. Knowledge of the environmental preferences of modern deep-sea benthic foraminifera helped to interpret the results of factor and cluster analyses in combination with oxygen and carbon isotope values. The biofacies indicative of high surface productivity, resulting from a stronger South Equatorial Current (Au–Up and Pb–Bs biofacies), dominate the early Pliocene interval (5.6–4.5 Ma) of global warmth. An intense Indo-Pacific ‘biogenic bloom’ and strong Oxygen Minimum Zone extended to intermediate depths (1000–2000 m) over large parts of the Indian Ocean in the early Pliocene. Since 4.5 Ma, the food supply in the Central Indian Ocean dropped and fluctuated while deep waters were corrosive (biofacies Gt–Nu, Gn–Cw). The Pleistocene interval is characterized by an intermediate flux of organic matter (Cc–Cl biofacies). 相似文献
92.
Namrata Das Saroj Sundar Baral Parth Sarathi Mahapatra Trupti Das Gautam Roy Chaudhury Surendra Nath Das 《Theoretical and Applied Climatology》2012,110(1-2):55-63
Precipitation samples collected during 2005–2009 from a rural forest station of Bhubaneswar were analyzed for their chemical composition. The samples were collected through a wet-only (WO) collector and two bulk (B1 and B2) collectors. The ions were evenly balanced indicating good data quality. The overall pH of rainwater was slightly acidic and ~47% of all rain events during the period were acidic (pH?<?5.6). Multilinear regression analysis showed relation between the free acidity (H+) and other components in rainwater. Enrichment factors (EF) of the major components with respect to their sources such as marine and crustal were calculated. Maximum EF was observed for NO 3 ? for both marine and crustal sources for all the three collectors. Source apportionments were also carried for the ions. Trend analysis showed continuous increase in most of the ions over years during the study period driven by anthropogenic emissions. Statistical/factorial analysis established correlation among different ions. 相似文献
93.
Sukanta Kumar Das Sanjib Kumar Deb C. M. Kishtawal P. K. Pal 《Theoretical and Applied Climatology》2012,109(1-2):81-94
Seasonal prediction of Indian Summer Monsoon (ISM) has been attempted for the current year 2011 using Community Atmosphere Model (CAM) developed at the National Centre for Atmospheric Research (NCAR). First, 30?years of model climatology starting from 1981 to 2010 has been generated to capture the variability of ISM over the Indian region using 30 seasonal simulations. The simulated model climatology has been validated with different sets of observed climatology, and it was observed that the simulated climatological rainfall is affected by model bias. Subsequently, a bias correction procedure using the Tropical Rainfall Measuring Mission (TRMM) 3B43 rainfall has been proposed. The bias-corrected rainfall climatology shows both spatial and temporal variability of ISM satisfactorily. Further, four sets of 10-member ensemble simulations of ISM 2009 and 2010 have been performed in hindcast mode using observed sea surface temperature (SST) and persistence of April SST anomaly, and it has been found that the bias-corrected model rainfall captures the seasonal variability of ISM reasonably well with some discrepancies in these two contrasting monsoon years. With this positive background, the seasonal prediction of ISM 2011 has been carried out in forecast mode with the assumption of persistence of May SST anomaly from June through September 2011. The model assessment shows an 11% deficiency in All-India Rainfall (AIR) of ISM 2011. In particular, the monthly accumulated rains are predicted to be 101% (17.6?cm), 86% (24.3?cm), 83% (21.0?cm) and 95% (15.5?cm) of normal AIR for the months of June, July, August and September, respectively. 相似文献
94.
In this paper, characterization of cirrus clouds are made by using data from ground based polarization lidar and radiosonde measurements over Chung-Li (24.58°N, 121.10°E), Taiwan for a period of 1999–2006. During this period, the occurrence of cirrus clouds is about 37% of the total measurement nights over the lidar site. Analysis of the measurement gives the statistical characteristics about the macrophysical properties such as occurrence height, ambient temperature, and its geometrical thickness while the microphysical properties are interpreted in terms of extinction coefficient, optical depth, effective lidar ratio and depolarization ratio. The effective lidar ratio has been retrieved by using the simulation technique of backscattered lidar signals. The effect of multiple scattering has been taken into the account by a model calculation. Summer (Jun–Aug) shows the maximum appearances of cirrus due to its formation mechanism. It is shown that tropopause cirrus clouds may occur with a probability of about 24%. These clouds are usually optically thin and having laminar in structure with some cases resembling the characteristics similar to that of polar stratospheric clouds (PSCs). The radiative properties of the cirrus clouds are also discussed in detail by the empirical equations with results show a positive feedback on any climate change. 相似文献
95.
In this paper, we have considered a model of our universe containing five components as its constituents. Then, we have done
here the statefinder diagnostics for this model. This model can successfully explain the accelerated expansion of the universe
given that it satisfies a certain condition. Here we have considered the modified Chaplygin gas as the dynamically changing
part of the dark energy component of our universe. Chaplygin gas provides early deceleration and late time acceleration of
the universe. The graphical representation of statefinder parameters shows that the total evolution of the universe starts
from radiation era to phantom model. 相似文献
96.
We examine the behaviour of accretion flow around a rotating black hole in presence of cooling. We obtain global flow solutions
for various accretion parameters that govern the accreting flow. We show that standing isothermal shock wave may develop in
such an advective accretion flow in presence of cooling. This shocked solution has observational consequences as it successfully
provides the possible explanations of energy spectra as well as generation of outflows/jets of various galactic and extra-galactic
black hole candidates. We study the properties of isothermal shock wave and find that it strongly depends on the cooling efficiency.
We identify the region in the parameter space spanned by the specific energy and specific angular momentum of the flow for
standing isothermal shock as a function of cooling efficiencies and find that parameter space gradually shrinks with the increase
of cooling rates. Our results imply that accretion flow ceases to contain isothermal shocks when cooling is beyond its critical
value. 相似文献
97.
Jayanta K. Bhattacharjee Atri Bhattacharya Tapas K. Das Arnab K. Ray 《Monthly notices of the Royal Astronomical Society》2009,398(2):841-852
In a novel approach to studying viscous accretion flows, viscosity has been introduced as a perturbative effect, involving a first-order correction in the α-viscosity parameter. This method reduces the problem of solving a second-order non-linear differential equation (Navier–Stokes equation) to that of an effective first-order equation. Viscosity breaks down the invariance of the equilibrium conditions for stationary inflow and outflow solutions, and distinguishes accretion from wind. Under a dynamical systems classification, the only feasible critical points of this 'quasi-viscous' flow are saddle points and spirals. On large spatial scales of the disc, where a linearized and radially propagating time-dependent perturbation is known to cause a secular instability, the velocity evolution equation of the quasi-viscous flow has been transformed to bear a formal closeness with Schrödinger's equation with a repulsive potential. Compatible with the transport of angular momentum to the outer regions of the disc, a viscosity-limited length-scale has been defined for the full spatial extent over which the accretion process would be viable. 相似文献
98.
R. K. Das D. P. K. Banerjee N. M. Ashok 《Monthly notices of the Royal Astronomical Society》2009,398(1):375-384
We present near-infrared (1–2.5 μm) spectroscopic and photometric results of Nova V2615 Ophiuchi which was discovered in outburst in 2007 March. Our observations span a period of ∼80 d starting from 2007 March 28 when the nova was at its maximum light. The evolution of the spectra is shown from the initial P Cygni phase to an emission-line phase and finally to a dust formation stage. The characteristics of the JHK spectra are very similar to those observed in a nova outburst occurring on a carbon–oxygen white dwarf. We analyse an observed line at 2.088 μm and suggest that it could be due to Fe ii excited by Lyman α fluorescence. The highlight of the observations is the detection of the first overtone bands of carbon monoxide (CO) in the 2.29–2.40 μm region. The CO bands are modelled to estimate the temperature and mass of the emitting CO gas and also to place limits on the 12 C/13 C ratio. The CO bands are recorded over several epochs, thereby allowing a rare opportunity to study the evolution from a phase of constant strength through a stage when the CO is destroyed fairly rapidly. We compare the observed time-scales involved in the evolution of the CO emission and find a good agreement with model predictions that investigate the chemistry in a nova outflow during the early stages. 相似文献
99.
M. K. Das Pankaj Narang S. Mahajan M. Yuasa 《Journal of Astrophysics and Astronomy》2009,30(3-4):177-185
We have investigated the out of plane equilibrium points of a passive micron size particle and their stability in the field of radiating binary stellar systems Krüger-60, RW-Monocerotis within the framework of photo-gravitational circular restricted three-body problem. We find that the out of plane equilibrium points (L i , i = 6, 7, 8, 9) may exist for range of β 1 (ratio of radiation to gravitational force of the massive component) values for these binary systems in the presence of Poynting-Robertson drag (hereafter PR-drag). In the absence of PR-drag, we find that the motion of a particle near the equilibrium points L 6,7 is stable in both the binary systems for a specific range of β 1 values. The PR-drag is shown to cause instability of the various out of plane equilibrium points in these binary systems. 相似文献
100.
In high-Mg, Al metapelites, monophase sapphirine corona occur around spinel–corundum aggregates in monomineralic cordierite
layers, and bi-phase orthopyroxene–sillimanite aggregates replace locally warped sapphirine in polygonized cordierite aggregates.
P–T phase topologies computed (Perple_X software) using compositions of cordierite-rich layers that host the reaction textures
did not match the assemblages for the discontinuous reactions spinel + corundum + cordierite → sapphirine and sapphirine + cordierite → orthopyroxene + sillimanite.
Instead, the reaction assemblages were reproduced using P–T pseudosection analysis for micro-domain reaction volumes estimated from compositions of product phases in the volume proportion
they occur. The results are consistent with known phase relations deduced using Schreinmakers P–T grids. Apparently, the compositions of cordierite-rich layers that hosted the reaction textures were inadequate chemical
proxies for determining P–T–X relations of phase-boundary controlled reactions influenced by compositions of the nearest-neighbor minerals in the proportion
they react (effective composition), and not in the proportion they existed in the layer/bulk rock. In other words, P–T–X phase topologies and reconstructed P–T paths in dry and aluminous rocks may be best understood by thermodynamic modeling of reactions using effective reaction volume
compositions rather than the bulk composition of the rock or the mineralogical layer that host the reaction textures. 相似文献