Recent changes in the spatio-temporal characteristics of monsoon intraseasonal oscillations

Theoretical and Applied Climatology - Monsoon intraseasonal oscillations (MISOs) define a significant proportion of intraseasonal variability of the Indian summer monsoon. In the backdrop of...     

Structural set up in northern part of Kumaun Himalaya

The Main Central Thrust (MCT) and the Main Boundary Thrust (MBT) are the two major thrusts in Kumaun, the MCT forming the boundary between highly sheared, deformed and mylonitized rocks of the Great Himalayan Central Crystallines and the Lesser Himalayan metasedimentaries. While in the Central Crystallines four-folding episodes are observed of which two are of the Precambrian age, the Lesser Himalayan rocks show only two phases of folding. MCT has its own distinctive structural history and the crystalline mass comprises an integral part of peninsular India.     

Geochemistry of Mansar Lake sediments, Jammu, India: Implication for source-area weathering, provenance, and tectonic setting

The sediment geochemistry, including REE, of surface and core samples from Mansar Lake, along with mineralogical investigations, have been carried out in order to understand the provenance, source area weathering, hydrolic sorting and tectonic setting of the basin. The geochemical signatures preserved in these sediments have been exploited as proxies in order to delineate these different parameters.The major element log values (Fe₂O₃/K₂O) vs (SiO₂/Al₂O₃) and (Na₂O/K₂O) vs (SiO₂/Al₂O₃) demarcate a lithology remarkably similar to that exposed in the catchment area. The chondrite normalized REE patterns of lake samples are similar to Post Archaean Australian Shale (PAAS) with LREE enrichment, a negative Eu anomaly and almost flat HREE pattern similar to a felsic and/or cratonic sedimentary source. However, the La–Th–Sc plot of samples fall in a mixed sedimentary domain, close to Upper Continental Crust (UCC) and PAAS, suggesting sedimentary source rocks for the Mansar detritus. It also indicates that these elements remained immobile during weathering and transportation. The mineralogical characteristic, REEs, and high field strength elements (HFSE), together with the high percentage of metamorphic rock fragments in the Siwalik sandstone, support a metamorphic source for lower Siwalik sediments. A very weak positive correlation between Zr and SiO₂, poor negative correlation with Al₂O₃, negative correlation of (La/Yb)_N and (Gd/Yb)_N ratios with SiO₂ and positive correlation with Al₂O₃, suggest that Zr does not dominantly control the REE distribution in Mansar sediments. The petrographic character and textural immaturity indicate a short distance transport for the detritus. The distribution of elements in core samples reflect fractionation. The higher Zr/Th and Zr/Yb ratios in coarse sediments and PAAS compared to finer grained detritus indicate sedimentary sorting. Plots of the geochemical data on tectonic discrimination diagrams suggest that the sediments derived from the lower Siwalik were originated within a cratonic interior and later deposited along a passive margin basinal setting. It therefore reveals lower Siwalik depositional history.     

Differential Rotation in Self Gravitating Filaments

The effect of differential rotation on the equilibria of self-gravitating filaments has been examined for isothermal and logatropic equations of state (EOS). Parametric regions for existence of solutions and upper limits for (the ratio of the rotational kinetic energy to the gravitational potential energy) have been worked out for the different laws of rotation. Effect of magnetic field has also been discussed.     

Robe’s restricted problem of 2+2 bodies when the bigger primary is a Roche ellipsoid and the smaller primary is an oblate body

In this problem, one of the primaries of mass m ₁ is a Roche ellipsoid filled with a homogeneous incompressible fluid of density ρ ₁. The smaller primary of mass m ₂ is an oblate body outside the Ellipsoid. The third and the fourth bodies (of mass m ₃ and m ₄ respectively) are small solid spheres of density ρ ₃ and ρ ₄ respectively inside the Ellipsoid, with the assumption that the mass and the radius of the third and the fourth body are infinitesimal. We assume that m ₂ is describing a circle around m ₁. The masses m ₃ and m ₄ mutually attract each other, do not influence the motions of m ₁ and m ₂ but are influenced by them. We have extended the Robe’s restricted three-body problem to 2+2 body problem under the assumption that the fluid body assumes the shape of the Roche ellipsoid (Chandrashekhar in Ellipsoidal figures of equilibrium, Chap. 8, Dover, New York, 1987). We have taken into consideration all the three components of the pressure field in deriving the expression for the buoyancy force viz (i) due to the own gravitational field of the fluid (ii) that originating in the attraction of m ₂ (iii) that arising from the centrifugal force. In this paper, equilibrium solutions of m ₃ and m ₄ and their linear stability are analyzed. We have proved that there exist only six equilibrium solutions of the system, provided they lie within the Roche ellipsoid. In a system where the primaries are considered as Earth-Moon and m ₃,m ₄ as submarines, the equilibrium solutions of m ₃ and m ₄ respectively when the displacement is given in the direction of x ₁-axis or x ₂-axis are unstable.     

The preparation and characterization of vanadium-substituted goethite: The importance of temperature

Despite a close geo-chemical association between vanadium (V) and iron (Fe) in natural environments, there is little research on the substitution of V in goethite. To assess the effect of temperature on V-substitution in goethite, a series of V-substituted goethite were prepared under varying synthesis temperatures, and analysed using wet chemical and multi-spectroscopic techniques. Vanadium substitution was inversely related to synthesis temperatures and was hindered by the oxidation of V³⁺ to V^4+/5+ as indicated by X-ray absorption near-edge spectroscopy. The presence of V (V⁵⁺ > V⁴⁺ > V³⁺) at high temperature hindered the nucleation of goethite and crystal growth along particular faces resulting in large-sized and twinned crystals as shown by transmission electron microscopy. The large-sized goethite crystals released more Fe (mmoles) per unit surface area during proton-promoted dissolution than the smaller-sized crystals, which could be due to distorted V^4+/5+ local coordination environments in the mineral structure. The dissolution studies showed a heterogeneous distribution of V and/or crystal defects in goethite crystals. The results show that low synthesis temperatures preserved the oxidation state of V³⁺, which has ionic radius and hydrolytic properties similar to Fe³⁺, and hence resulted in as much as 13.3 mol per cent substitution. The structural stability of the goethite decreased upon V-substitution in order; V³⁺ > V⁴⁺ > V⁵⁺. This research provides important information about the interaction between temperature, V incorporation, and crystal structure properties of goethite for V sequestration and other potentially toxic metal cations.     

Reflection and refraction of SH‐waves at a corrugated interface between two monoclinic elastic half‐spaces

A problem of reflection and transmission of shear wave incident upon a corrugated interface between two monoclinic solid half‐spaces have been investigated. Rayleigh's method of approximation is used to investigate the reflection and transmission coefficients for first and second approximation of the corrugation. For a special interface, closed‐form expressions of these coefficients for the first order approximation of the corrugation are obtained. It is found that these coefficients are functions of amplitude of corrugation, elastic parameters of the media, frequency of the incident wave and angle of incidence. The numerical computations are performed for a specific model and the results obtained are presented graphically. It is found that the reflection and transmission coefficients are strongly influenced by the corrugation and the elastic properties of the media. Results of some earlier workers in this field have been reduced as particular case from the present formulation. Copyright © 2004 John Wiley & Sons, Ltd.     

Nature of magmatism and sedimentation at a Columbia active margin: Insights from combined U–Pb and Lu–Hf isotope data of detrital zircons from NW India

Detrital zircons from a Palaeoproterozoic quartzite, deposited between 1.85 and 1.82 Ga in the northern Aravalli orogen of NW India, show a distinctive age peak of ca. 1.85 Ga and variable, but largely subchondritic εHf_{1.85 Ga} between ? 1.3 and ? 21.0 corresponding to hafnium model ages of 2.5 to 3.6 Ga. These data indicate an important period of reworking of ancient (Eo- to Neoarchaean), strongly heterogeneous continental crust at this time. Prevalence of ca. 1.85 Ga subduction-related granitoids, almost identical U–Pb age spectra and εHf_t of detrital zircons in ca. 1.85 Ga metasedimentary rocks in the Aravalli orogen and the inner Lesser Himalaya indicate similar sediment provenances and thus a geological connection between these two terranes during late Palaeoproterozoic. All together, the data constrain a rapid succession of sedimentation, metamorphism and subduction-related magmatic activity and support the interpretation of an active geodynamic realm along the entire north Indian margin at ca. 1.85 Ga. Comparison of detrital zircon data in conjunction with published paleomagnetic data from north India and other crustal blocks of the Columbia supercontinent, additionally, suggest a close affinity of north India with Madagascar, the Cathaysia block of South China and South Korea during Columbia times.     

Silting patterns in the reservoirs of small- and medium-sized earthen check dams in humid subtropical monsoon regions

Silting of reservoirs is a ubiquitous process whenever water is impounded. Despite substantial work on the rates of silting, the spatial pattern of silting in reservoirs is not clearly understood. While it is anticipated that the variability of silting increases with decreasing reservoir size, not much is known about siltation in subtropical humid regions affected by monsoon rainfall. This paper presents the initial results of geomorphic analysis of six earthen check dams in the Shiwalik foothills of the Himalayas (India) in areas that are inhabited by high proportions of disempowered populations. These check dams include three small-sized dams (Dhamala-II, Rel Majra and Sukhomajri-II) and three medium-sized dams (Bunga-I, Parachh-II and Siswan). Field data were collected from each reservoir. The methods used included spatial interpolations of bed depth and silt thickness in each reservoir, Structure from Motion photogrammetry to extract multiple channel cross-sections from photographic scans along tributary mouths, texture analysis of bed and bank materials, and visual observations of hill slopes around the check dams. Based on this study, silt tended to concentrate either in the middle portion of the reservoir or near the dam; however, silt accumulation did not always occur along the dam or in the deepest portion of the reservoirs. Areas located downstream from these check dams are heavily used for various human activities and the channel network has almost been eliminated. While these earthen check dams may be cost-effective tools for water conservation in economically marginalized areas, if abandoned, these structures may pose a physical hazard to downstream communities. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Quality Assessment of Atmospheric Motion Vectors Over the Indian Ocean

Because conventional observations over the oceans are not available, especially during tropical cyclones, multi-spectral atmospheric motion vectors (AMVs) estimated from geostationary satellites are routinely assimilated in the numerical weather prediction models at different operational centres across the globe. The derived AMVs are generally validated with radiosonde observations available over land at synoptic hours; however, over the ocean there is a limited scope to assess the quality of AMVs. Over ocean, AMVs can be validated with radiosonde data available from opportunistic ships or using dropsonde data available from aircrafts. In this study, the accuracy of the AMVs derived from the geostationary satellites Kalpana-1 and Meteosat-7 is evaluated over the oceanic region. Radiosonde data available from a ship cruise held in the Bay of Bengal during the period 09 July–08 August 2012 and from the Cal/Val site situated at Kavaratti Island (72.62°E, 10.57°N) in the southern Indian Ocean are used to assess the AMV accuracy. In this study, 83 radiosonde profiles are used to validate the Kalpana-1 AMVs, to allow a better understanding of AMV errors over the Indian Ocean. The RMSVD of Kalpana-1 AMVs for the high-, mid- and low-levels are found to be 7.9, 9.4 and 5.3 m s^?1, respectively, while the corresponding RMSVD for Meteosat-7 AMVs are 9.1, 5.5 and 3.7 m s^?1. A similar accuracy is observed when the AMVs are validated against the NCEP analyses collocated with the nearest radiosonde locations. The high RMSVD and bias for Kalpana-1 AMVs at the mid-level and Meteosat-7 AMVs at the high-level are associated with the limitation of satellite winds to resolve the upper-level easterly jet in conjunction with errors in the height assignment. This study could help the numerical modellers to assign appropriate observation error over this region during the assimilation of AMVs into the NWP models.