The albedo of snow for partially cloudy skies     

B. J. Choudhury  A. T. C. Chang 《Boundary-Layer Meteorology》1981,20(3):371-389

The albedo of snow for different cloudiness conditions is an important parameter in the Earth's radiation budget analysis and in the study of snowpack's thermal conditions. In this study an efficient approximate method is derived to calculate the incident spectral solar flux and snow-cover albedo in terms of different atmospheric, cloud, and snow parameters. The global flux under partially cloudy skies is expressed in terms of the clear sky flux and a coefficient which models the effect of scattering and absorption by cloud patches and multiple reflections between the cloud base and snowcover. The direct and the diffuse components of the clear sky flux are obtained using the spectral flux outside the atmosphere and the spectral transmission coefficients for absorption and scattering by molecules and aerosols.The spectral snow reflectance model considers both specular surface reflection and volumetric multiple scattering. The surface reflection is calculated by using a crystal-shape-dependent bidirectional reflectance distribution function; the volumetric multiple scattering is calculated by using a crystal-size-dependent approximate solution in the radiative transfer equation. The input parameters to the model are atmospheric precipitable water, ozone content, turbidity, cloud optical thickness, the size and shape of ice crystals of snow and surface pressure. The model yields spectral and integrated solar flux and snow reflectance as a function of solar elevation and fractional cloudcover.The model is illustrated using representative parameters for the Antarctic coastal regions. The albedo for a clear sky depends inversely on the solar elevation. At high elevations the albedo depends primarily upon the grain size; at low elevation the albedo depends on grain size and shape. The gradient of the albedo-elevation curve increases as the grains become larger and faceted. The albedo for a densely overcast sky is a few percent higher than the clear-sky albedo at high elevations. A simple relationship between grain size and the overcast albedo is obtained. For a set of grain size and shape, the albedo as a function of solar elevation and fractional cloud cover is tabulated.  相似文献   

Rayleigh-waves on cylindrical surfaces of visco-elastic material of general linear type in presence of a magnetic field     

Ajoy Roy Choudhury 《Pure and Applied Geophysics》1970,80(1):137-146

Summary The propagation of disturbances in cylindrical surfaces of visco-elastic material of general linear type (i) cylinder surrounded by a vacuum (ii) and on an infinite body of same material having a cylindrical cavity, when the surface is subjected to an initially static magnetic field, making use of the electro-magnetic equations of Maxwell, equations of elasticity and the stress-strain relations of the material.  相似文献   

Lattice dynamics and Raman spectroscopy of protoenstatite Mg2Si2O6     

Subrata Ghose  Narayani Choudhury  S. L. Chaplot  C. Pal Chowdhury  S. K. Sharma 《Physics and Chemistry of Minerals》1994,20(7):469-477

Enstatites (Mg₂Si₂O₆) are important rock forming silicates of the pyroxene group whose structures are characterised by double MgO₆ octahedral bands and single silicate chains. Orthoenstatite transforms to protoenstatite above 1273 K with a doubling of the a axis and a rearrangement of the silicate chains with respect to the Mg₂₊ ions. Lattice dynamical calculations based on a rigid-ion model in the quasi-harmonic approximation provide theoretical estimates of elastic constants, long wavelength phonon modes, phonon dispersion relations, total and partial density of states and inelastic neutron scattering cross-sections of protoenstatite. The computed elastic constants are in good agreement with experimental data. The computed density of states of a chain silicate such as protoenstatite is distinct from that of olivines (forsterite, Mg₂SiO₄ and fayalite, Fe₂-SiO₄) with isolated silicate tetrahedra. The band gaps in the density of states in forsterite are largely due to the separation in the frequency ranges of the external and internal vibrations of the isolated silicate group, whereas in protoenstatite these gaps are filled by the vibrations of the bridging oxygens of the silicate chain. The computed density of states is used to calculate the specific heat, the mean square atomic displacements and temperature factors. Validity of these calculations are supported by Raman scattering measurements. Polarised and unpolarised Raman spectra are obtained from small single crystals of protoenstatite (Li,Sc)_0.6Mg_1.4Si₂O₆ stable at room temperature using the 488 nm or 514.5 nm lines of an Ar⁺ ion laser and a micro-Raman spectrometer with backscattering geometry. The Raman spectra were analysed and interpreted based on the lattice dynamical model. The experimental Raman frequencies and mode assignments (based on polarised single crystal spectra) are in good agreement with those obtained from lattice dynamical calculations.  相似文献   

Implications for planning of hydroelectric projects in Northeast India: an analysis of the impacts of the Tipaimukh project     

Choudhury  Nazrana Begam  Dey Choudhury  Soma Roy 《GeoJournal》2021,86(4):1999-2019

In recent years, while developed countries are decommissioning hydroelectric projects (HEPs)/dams at a higher rate than constructing new ones, developing economies are doing the opposite. Although HEPs are regarded green, renewable and cheap, recent studies reveal that they do more harm than good. The impacts of HEPs are multiple, including socio-economic, environmental and geopolitical. India’s Northeastern (NE) region is a part of the Ganges–Brahmaputra–Meghna river basin, and has immense hydropower potential. A large number of HEPs are planned or under construction in the region, mainly in the hilly states of Arunachal Pradesh, Manipur, Meghalaya, Sikkim and Mizoram. One such mega project is the Tipaimukh multipurpose HEP, to be constructed on the Barak river in the state of Manipur. The project is highly controversial, and is being protested against by the locals, academicians, researchers, social activists, NGOs, as well as Bangladesh. Further, a serious cost–benefit analysis, judicious environmental impact assessment and democratic public hearings have been questioned. The area submergence of the project is disproportionately high compared to other such projects of NE India, and the project site and the adjoining areas form a part of Indo-Burma global biodiversity hotspot. The present study details the impacts of the project from multidisciplinary perspectives, categorized into bio-physical, socio-economic and geopolitical impacts, including impacts on the downstream riparian nation—Bangladesh. Based on the study, we have provided recommendations, if ever the construction of project is actualized. Since there is lack of any such analysis, the present study is of immense significance on the decision making of the project, and paves a guideline for impact analysis of the upcoming large number of projects of the region.

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Numerical investigation of the stability of landslide-affected slopes in Kerala,India, under extreme rainfall event     

Das  Tanmoy  Rao  Vansittee Dilli  Choudhury  Deepankar 《Natural Hazards》2022,114(1):751-785

Natural Hazards - This study examines the stability and failure mechanisms of two landslide-affected slopes in Kerala, India, after an extreme rainfall event. The landslide was triggered by an...  相似文献   

Seismic passive earth resistance using modified pseudo-dynamic method     

Anindya Pain  Deepankar Choudhury  S. K. Bhattacharyya 《地震工程与工程振动(英文版)》2017,16(2):263-274

In earthquake prone areas, understanding of the seismic passive earth resistance is very important for the design of different geotechnical earth retaining structures. In this study, the limit equilibrium method is used for estimation of critical seismic passive earth resistance for an inclined wall supporting horizontal cohesionless backfill. A composite failure surface is considered in the present analysis. Seismic forces are computed assuming the backfill soil as a viscoelastic material overlying a rigid stratum and the rigid stratum is subjected to a harmonic shaking. The present method satisfies the boundary conditions. The amplification of acceleration depends on the properties of the backfill soil and on the characteristics of the input motion. The acceleration distribution along the depth of the backfill is found to be nonlinear in nature. The present study shows that the horizontal and vertical acceleration distribution in the backfill soil is not always in-phase for the critical value of the seismic passive earth pressure coefficient. The effect of different parameters on the seismic passive earth pressure is studied in detail. A comparison of the present method with other theories is also presented, which shows the merits of the present study.  相似文献   

Statistical analysis of dynamic fibrils observed from NST/BBSO observations     

Thambaje Gopalan Priya  Jiang-Tao Su  Jie Chen  Yuan-Yong Deng  Debi Prasad Choudhury 《天文和天体物理学研究(英文版)》2018,(2)

We present the results obtained from the analysis of dynamic fibrils in NOAA active region(AR) 12132, using high resolution Hα observations from the New Solar Telescope operating at Big Bear Solar Observatory. The dynamic fibrils are seen to be moving up and down, and most of these dynamic fibrils are periodic and have a jet-like appearance. We found from our observations that the fibrils follow almost perfect parabolic paths in many cases. A statistical analysis on the properties of the parabolic paths showing an analysis on deceleration, maximum velocity, duration and kinetic energy of these fibrils is presented here. We found the average maximum velocity to be around 15 kms~(-1) and mean deceleration to be around 100 ms~(-2). The observed deceleration appears to be a fraction of gravity of the Sun and is not compatible with the path of ballistic motion due to gravity of the Sun. We found a positive correlation between deceleration and maximum velocity. This correlation is consistent with simulations done earlier on magnetoacoustic shock waves propagating upward.  相似文献   

Seismicity and reservoir induced crustal motion study around the Tehri Dam, India     

Swapnamita Choudhury  Param K. Gautam  Ajay Paul 《Acta Geophysica》2013,61(4):923-934

The Tehri Dam is located in a seismotectonically active region in the Indian Himalayan belt. This 260.5 m high dam has a live water storage of 2615 × 10⁶ m³ and is capable of generating crustal deformation corresponding to water fluctuation. Filling of the reservoir started in October 2005. Seismic data around the dam between 2000 and 2010 shows that seismicity is corresponding to drawdown levels of the reservoir rather than to higher water levels. GPS data at twelve local benchmarks were collected from 2006 to 2008 during filling and drawdown reservoir levels. The velocity vectors show ground motion to be between ～0.69–1.50 mm in the different filling-drawdown cycles with reference to the permanent station at Ghuttu. The motion appears to be inwards into the reservoir when the reservoir is filled and outwards when the reservoir is drained. This ground motion corresponds to elastic deformation and rebound due to effect of the oscillating water levels.  相似文献   

Seismic yield accelerations of MSW landfills by pseudo-dynamic approach   总被引：2，自引：2，他引：0  

Deepankar Choudhury  Purnanand Savoikar 《Natural Hazards》2011,56(1):275-297

Estimation of seismic yield acceleration for the municipal solid waste landfills is the basic step for design to mitigate the hazard of earthquake on landfills. For most of the landfills constructed in the seismic zones, influence of the vertical component of seismic acceleration is often neglected assuming that it is not in phase with the horizontal seismic acceleration and hence does not affect the design parameters largely. Conventional pseudo-static analysis using limit equilibrium method does not consider the variation of seismic inertial forces with time and also effect of vertically propagating shear and primary waves, which can have serious effect on the design values. To overcome the limitations of existing methodology, in the present study, pseudo-dynamic approach, which takes into account the effects of time, primary and secondary waves, frequency of excitation, is applied for the first time for dynamic analysis of landfills for three different typical practical geometries of landfills. Chosen sets of landfills resemble some of the existing landfill geometries. Design equations and charts are developed for factor of safety against base sliding and seismic yield acceleration for these three typical landfill geometries. The results are compared with the similar results obtained by using conventional pseudo-static analysis, and the influence of vertical seismic acceleration on factor of safety against base sliding and yield acceleration is reported.  相似文献   

Equivalent-Linear Seismic Ground Response Analysis of Some Typical Sites in Mumbai   总被引：4，自引：0，他引：4  

V. S. Phanikanth  Deepankar Choudhury  G. Rami Reddy 《Geotechnical and Geological Engineering》2011,29(6):1109-1126

Any earthquake event is associated with a rupture mechanism at the source, propagation of seismic waves through underlying rock and finally these waves travel through the soil layers to the particular site of interest. The bedrock motion is significantly modified at the ground surface due to the presence of local soil layers above the bedrock beneath the site of interest. The estimation of the amplifications in ground response due to the local soil sites is a complex problem to the designers and the problem is more important for mega cities like Mumbai in India, where huge population may get affected due to devastations of earthquake. In the present study, the effect of local soil sites in modifying ground response is studied by performing one dimensional equivalent-linear ground response analysis for some of the typical Mumbai soil sites. Field borelog data of some typical sites in Mumbai city viz. Mangalwadi site, Walkeswar site, BJ Marg near Pandhari Chawl site are considered in this study. The ground responses are observed for range of input motions and the results are presented in terms of surface acceleration time history, ratio of shear stress to vertical effective stress versus time, acceleration response spectrum, Fourier amplitude ratio versus frequency etc. The typical amplifications of ground accelerations considering four strong ground motions with wide variation of low to high MHA, frequency contents and durations are obtained. Results show that MHA, bracketed duration, frequency content have significant effects on the amplification of seismic accelerations for typical 2001 Bhuj motion. The peak ground acceleration amplification factors are found to be about 2.50 for Mangalwadi site, 2.60 for Walkeswar site and 3.45 for BJ Marg site using 2001 Bhuj input motion. The response spectrum along various soil layers are obtained which will be useful for designers for earthquake resistant design of geotechnical structures in Mumbai for similar sites in the absence of site specific data.  相似文献