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81.
An analysis of the meteorological data collected by the research vessel ORV Sagarkanya for the mean latent and sensible heat
fluxes over the Arabian Sea has indicated appreciable changes between active and weak phases of the southwest monsoon of 1986.
We suggest that: (a) the presence of a core of low level winds associated with the Somali jet and its southward shift during
the season, along with (b) a ridge in surface pressure over the central Arabian Sea could be responsible for the deficit in
monsoon rainfall along the west coast of India in 1986. 相似文献
82.
83.
R. P. Dhir S. K. Tandon B. K. Sareen R. Ramesh T. K. G. Rao A. J. Kailath N. Sharma 《Journal of Earth System Science》2004,113(3):473-515
The calcretes in the Thar desert occur in a variety of settings, including the piedmonts, sheetwash aggraded plains; and this
study adds calcretes in regolith and colluvio-alluvial plains to the group of settings in which calcretes occur in the region.
Field logs, morphological details and analytical data such as petrographic, cathodoluminescence and geochemical characteristics
are described along with a discussion on their implications. Sand dunes and sandy plains dating to < 20 ka have weakly developed
calcretes. The better-developed calcrete horizons occur in piedmonts, interdunes or in areas that have sufficient groundwater.
Deep sections in the region show phases of calcrete development in aeolian sand aggradation at ∼ 150, ∼ 100, ∼ 60 and 27–14
ka. The extensive sheetwash plains have mature calcretes and date to mid-Pleistocene. Our studies indicate that these calcretes
represent a hybrid process, where carbonate enrichment of the originally calcareous host occurred due to periodically raised
groundwaters, and its differentiation into nodules occurred under subaerial environment i.e., after recession of groundwater.
Deep sections also show a stack of discrete calcretes that developed in individual aggradation episodes with hiatuses as indicated
by ESR dating results. Nodules display a multiplicity of carbonate precipi tation events and internal reorganization of calcitic
groundmass. The process is accompanied by degradation and transformation of unstable minerals, particularly clays and with
a neosynthesis of palygorskite.
The ancient calcretes are dated from the beginning of the Quaternary to ∼ 600 ka and show more evolved morphologies marked
by brecciation, dissolution, laminar growth on brecciated surfaces, pisolites and several generations of re-cementation. Mica/chlorite
schists and such other rocks are particularly vulnerable to replacement by carbonate. In an extreme case, replacement of quartzose
sandstone was observed also. The presence of stretches of alluvio-colluvial plains in an area presently devoid of drainage
bespeaks of occasional high-energy fluvial regime, under a semi-arid climate. The mid-Pleistocene period saw a shift towards
more arid climate and this facilitated sheetwash aggradation. Finally, during the late Pleistocene, aggradation of aeolian
sands indicated a progressively drier climate. However, this does not find its reflection in stable isotope data. The amount
of carbonate in the form of calcretes is substantial. The present studies indicate that aeolian dust or rainwater are minor
contributors to the carbonate budget. A more important source was provided by the pre-existing calcretes in the sheetwash
aggraded plains and detrital carbonate in the aeolian sediments. The original source of carbonate in the region, however,
remains unresolved and will need further investigations. Electron spin resonance protocols for the dating of calcretes were
developed as a part of this study and the results accorded well with geological reasoning 相似文献
84.
Ramesh Narayan 《Astronomy& Geophysics》2003,44(6):6.22-6.26
The George Darwin Lecture presented at the Royal Astronomical Society, London, 13 December 2002, by Ramesh Narayan . 相似文献
85.
86.
N. Vedachalam S. Ramesh P. Udaya Prasanth G. A. Ramadass 《Marine Georesources & Geotechnology》2018,36(8):966-973
Natural gas hydrates is considered as a strategic unconventional clean hydrocarbon resource in the energy sector. Understanding the behavior of the rising methane gas bubbles during production leaks from the deep marine gas hydrate reservoirs well head is essential for environmental impact studies and to design environmental monitoring systems. Numerical model for quantitatively characterizing the vertical dissolution pattern of the wellhead released methane gas bubbles is analyzed for three potential gas hydrate locations in India. Simulation results indicate that the methane bubbles with diameter of 10?mm can transport methane gas till 650, 800, and 750?m from the seabed in the Krishna–Godavari(KG), Mahanadi and Andaman basins respectively. Results brought out that potential well head damage during methane hydrate production at 1050?m water depth could release up to 28?m3 of methane gas, in which 50% of the molar mass shall get dissolved within 40?m of water column from the seafloor. 相似文献
87.
R.?ThayakaranEmail author N.?I.?Ramesh 《Stochastic Environmental Research and Risk Assessment (SERRA)》2017,31(3):705-724
Stochastic rainfall models are widely used in hydrological studies because they provide a framework not only for deriving information about the characteristics of rainfall but also for generating precipitation inputs to simulation models whenever data are not available. A stochastic point process model based on a class of doubly stochastic Poisson processes is proposed to analyse fine-scale point rainfall time series. In this model, rain cells arrive according to a doubly stochastic Poisson process whose arrival rate is determined by a finite-state Markov chain. Each rain cell has a random lifetime. During the lifetime of each rain cell, instantaneous random depths of rainfall bursts (pulses) occur according to a Poisson process. The covariance structure of the point process of pulse occurrences is studied. Moment properties of the time series of accumulated rainfall in discrete time intervals are derived to model 5-min rainfall data, over a period of 69 years, from Germany. Second-moment as well as third-moment properties of the rainfall are considered. The results show that the proposed model is capable of reproducing rainfall properties well at various sub-hourly resolutions. Incorporation of third-order moment properties in estimation showed a clear improvement in fitting. A good fit to the extremes is found at larger resolutions, both at 12-h and 24-h levels, despite underestimation at 5-min aggregation. The proportion of dry intervals is studied by comparing the proportion of time intervals, from the observed and simulated data, with rainfall depth below small thresholds. A good agreement was found at 5-min aggregation and for larger aggregation levels a closer fit was obtained when the threshold was increased. A simulation study is presented to assess the performance of the estimation method. 相似文献
88.
Alexander Tchekhovskoy Jonathan C. McKinney Ramesh Narayan 《Monthly notices of the Royal Astronomical Society》2007,379(2):469-497
Active galactic nuclei, X-ray binaries, pulsars and gamma-ray bursts are all believed to be powered by compact objects surrounded by relativistic plasma flows driving phenomena such as accretion, winds and jets. These flows are often accurately modelled by the relativistic magnetohydrodynamic (MHD) approximation. Time-dependent numerical MHD simulations have proven to be especially insightful, but one regime that remains difficult to simulate is when the energy scales (kinetic, thermal, magnetic) within the plasma become disparate. We develop a numerical scheme that significantly improves the accuracy and robustness of the solution in this regime. We use a modified form of the weighted essentially non-oscillatory (WENO) method to construct a finite-volume general relativistic hydrodynamics code called wham that converges at fifth order. We avoid (1) field-by-field decomposition by adaptively reducing down to two-point stencils near discontinuities for a more accurate treatment of shocks and (2) excessive reduction to low-order stencils, as in the standard WENO formalism, by maintaining high-order accuracy in smooth monotonic flows. Our scheme performs the proper surface integral of the fluxes, converts cell-averaged conserved quantities to point-conserved quantities before performing the reconstruction step, and correctly averages all source terms. We demonstrate that the scheme is robust in strong shocks, very accurate in smooth flows and maintains accuracy even when the energy scales in the flow are highly disparate. 相似文献
89.
Recent research has shown evidence of strong coupling between the atmosphere and lithosphere in coastal regions, associating abnormal atmospheric phenomena to the occurrence of strong earthquakes. Surface latent heat flux (SLHF), total column water vapor (CWV), relative humidity (RH) and total ozone column (TOC), analyzed over the epicentral region of the Denali fault earthquake of November 3, 2002, exhibit anomalous behavior that could be related to the earthquake preparatory process and its occurrence. The complementary nature of the parameters provides strong support that the anomalous values were driven by lithospheric processes, rather than other atmospheric phenomena. Due to the wide availability of remote sensing observations of atmospheric parameters, the detection of anomalies can be used to mitigate the earthquake risks. 相似文献
90.
The maximum entropy method (MEM) of image reconstructtion is discussed in the context of incomplete Fourier information (as
in aperture synthesis). Several current viewpoints on the conceptual foundation of the method are analysed and found to be
unsatisfactory. It is concluded that the MEM is a form of model-fitting, the model being a non-linear transform of a band-limited
function. A whole family of ’entropies’ can be constructed to give reconstructions which (a) are individually unique, (b)
have sharpened peaks and (c) have flattened baselines. The widely discussed 1nB and - B1nB forms of the entropy are particular
cases and lead to Lorentzian and Gaussian shaped peaks respectively. However, they hardly exhaust the possibilities-for example,
B1/2 is equally good.
The two essential features of peak sharpening and baseline flattening are shown to depend on a parameter which can be controlled
by adding a suitable constant to the zero spacing correlation ρ00. This process, called FLOATing, effectively tames much of the unphysical behaviour noted in earlier studies of the MEM. A
numerical scheme for obtaining the MEM reconstruction is described. This incorporates the FLOAT feature and uses the fast
Fourier transform (FFT), requiring about a hundred FFTs for convergence. Using a model brightness distribution, the MEM reconstructions
obtained for different entropies and different values of the resolution parameter are compared. The results substantiate the
theoretically deduced properties of the MEM.
To allow for noise in the data, the least-squares approach has been widely used. It is shown that this method is biased since
it leads to deterministic residuals which do not have a Gaussian distribution. It is suggested that fitting the noisy data
exactly has the advantage of being unbiased even though the noise appears in the final map. A comparison of the strengths
and weaknesses of the MEM and CLEAN suggests that the MEM already has a useful role to play in image reconstruction. 相似文献