Laboratory Study of Topographic Effects on the Near-surface Tornado Flow Field     

Alireza Razavi  Partha P. Sarkar 《Boundary-Layer Meteorology》2018,168(2):189-212

To study topographic effects on the near-surface tornado flow field, the Iowa State University tornado simulator was used to simulate a translating tornado passing over three different two-dimensional topographies: a ridge, an escarpment and a valley. The effect of the translation speed on maximum horizontal wind speeds is observed for translation speeds of 0.15 and 0.50 \(\hbox {m}\,\hbox {s}^{-1}\), with the lower value resulting in a larger maximum horizontal wind speed. The tornado translation over the three topographies with respect to flat terrain is assessed for changes in: (a) the maximum horizontal wind speeds in terms of the flow-amplification factor; (b) the maximum aerodynamic drag in terms of the tornado speed-up ratio; (c) the maximum duration of exposure at any location to high wind speeds of a specific range in terms of the exposure amplification factor. Results show that both the maximum wind amplification factor of 14%, as well as the maximum speed-up ratio of 14%, occur on the ridge. For all topographies, the increase in aerodynamic drag is observed to be maximized for low-rise buildings, which illustrates the importance of the vertical profiles of the horizontal wind speed near the ground. The maximum exposure amplification factors, estimated for the range of wind speeds corresponding to the EF2 (50–60 \(\hbox {m}\,\hbox {s}^{-1}\)) and EF3 (61–75 \(\hbox {m}\,\hbox {s}^{-1})\) scales, are 86 and 110% for the ridge, 4 and 60% for the escarpment and ? 6 and 47% for the valley, respectively.  相似文献   

Shear waves in elastic medium with void pores welded between vertically inhomogeneous and anisotropic magnetoelastic semi-infinite media     

Shishir Gupta  Mostaid Ahmed  Abhijit Pramanik 《Acta Geophysica》2017,65(1):139-149

The paper intends to study the propagation of horizontally polarized shear waves in an elastic medium with void pores constrained between a vertically inhomogeneous and an anisotropic magnetoelastic semi-infinite media. Elasto-dynamical equations of elastic medium with void pores and magnetoelastic solid have been employed to investigate the shear wave propagation in the proposed three-layered earth model. Method of separation of variables has been incorporated to deduce the dispersion relation. All possible special cases have been envisaged and they fairly comply with the corresponding results for classical cases. The role of inhomogeneity parameter, thickness of layer, angle with which the wave crosses the magnetic field and anisotropic magnetoelastic coupling parameter for three different materials has been elucidated and represented by graphs using MATHEMATICA.  相似文献   

Studies on Schumann Resonance Phenomena and Some Recent Advancements     

Abhijit Ghosh  Biswas  Debasish  Hazra  Pranab  Guha  Gautam  De  S. S. 《Geomagnetism and Aeronomy》2019,59(8):980-994

Geomagnetism and Aeronomy - Discrete spectra of frequencies at 8, 14, 20, 26, … Hz are generated by electromagnetic emission from lightning sources and can be regarded as excitation of AC...  相似文献   

Assessment of village-wise groundwater draft for irrigation: a field-based study in hard-rock aquifers of central India     

R. K. Ray  T. H. Syed  Dipankar Saha  B. C. Sarkar  A. K. Patre 《Hydrogeology Journal》2017,25(8):2513-2525

Extracted groundwater, 90% of which is used for irrigated agriculture, is central to the socio-economic development of India. A lack of regulation or implementation of regulations, alongside unrecorded extraction, often leads to over exploitation of large-scale common-pool resources like groundwater. Inevitably, management of groundwater extraction (draft) for irrigation is critical for sustainability of aquifers and the society at large. However, existing assessments of groundwater draft, which are mostly available at large spatial scales, are inadequate for managing groundwater resources that are primarily exploited by stakeholders at much finer scales. This study presents an estimate, projection and analysis of fine-scale groundwater draft in the Seonath-Kharun interfluve of central India. Using field surveys of instantaneous discharge from irrigation wells and boreholes, annual groundwater draft for irrigation in this area is estimated to be 212 × 10⁶ m³, most of which (89%) is withdrawn during non-monsoon season. However, the density of wells/boreholes, and consequent extraction of groundwater, is controlled by the existing hydrogeological conditions. Based on trends in the number of abstraction structures (1982–2011), groundwater draft for the year 2020 is projected to be approximately 307 × 10⁶ m³; hence, groundwater draft for irrigation in the study area is predicted to increase by ～44% within a span of 8 years. Central to the work presented here is the approach for estimation and prediction of groundwater draft at finer scales, which can be extended to critical groundwater zones of the country.  相似文献   

Assessing redox properties of standard humic substances     

S. Rakshit  D. Sarkar 《International Journal of Environmental Science and Technology》2017,14(7):1497-1504

Redox properties of humic substances (HS) control important biogeochemical processes. Thus, accurate estimation of redox properties of HS is essential. However, there is no general consensus regarding the best available measurement method of HS redox properties. In this study, we compared several common HS redox property measurement methods using anthraquinone-2,6-disulfonate (AQDS) as model compound, and standard Elliot soil humic acid (1S102H, ESHA), reference Pahokee peat (1R103H, PPHA), and Suwannee River natural organic matter (1R101N, SRNOM) as representative HS. We found that the H₂/Pd reduction method followed by incubation with ferric citrate (FeCit) reagent was incomplete, and the H₂/Pd reduction method followed by incubation with potassium ferricyanide (K₃Fe(CN)₆) was insensitive. Stannous chloride (SnCl₂) reduction followed by titration of excess stannous (Sn²⁺) by potassium dichromate (K₂Cr₂O₇) was found to be most accurate. These findings will help in future investigations on detailed characterizations of functional groups of HS responsible for oxidation/reduction reactions.  相似文献   

Chemical evolution in the high arsenic groundwater of the Huhhot basin (Inner Mongolia,PR China) and its difference from the western Bengal basin (India)     

Abhijit Mukherjee  Prosun Bhattacharya  Fei Shi  Alan E. Fryar  Arun B. Mukherjee  Zheng M. Xie  Gunnar Jacks  Jochen Bundschuh 《Applied Geochemistry》2009

Elevated As concentrations in groundwater of the Huhhot basin (HB), Inner Mongolia, China, and the western Bengal basin (WBB), India, have been known for decades. However, few studies have been performed to comprehend the processes controlling overall groundwater chemistry in the HB. In this study, the controls on solute chemistry in the HB have been interpreted and compared with the well-studied WBB, which has a very different climate, physiography, lithology, and aquifer characteristics than the HB. In general, there are marked differences in solute chemistry between HB and WBB groundwaters. Stable isotopic signatures indicate meteoric recharge in the HB in a colder climate, distant from the source of moisture, in comparison to the warm, humid WBB. The major-ion composition of the moderately reducing HB groundwater is dominated by a mixed-ion (Ca–Na–HCO₃–Cl) hydrochemical facies with an evolutionary trend along the regional hydraulic gradient. Molar ratios and thermodynamic calculations show that HB groundwater has not been affected by cation exchange, but is dominated by weathering of feldspars (allitization) and equilibrium with gibbsite and anorthite. Mineral weathering and mobilization of As could occur as recharging water flows through fractured, argillaceous, metamorphic or volcanic rocks in the adjoining mountain-front areas, and deposits solutes near the center of the basin. In contrast, WBB groundwater is Ca–HCO₃-dominated, indicative of calcite weathering, with some cation exchange and silicate weathering (monosiallitization).  相似文献   

Mercury enrichments in core sediments in Hugli–Matla–Bidyadhari estuarine complex,north-eastern part of the Bay of Bengal and their ecotoxicological significance     

M. Chatterjee  J. Canário  S. K. Sarkar  V. Branco  A. K. Bhattacharya  K. K. Satpathy 《Environmental Geology》2009,57(5):1125-1134

Mercury concentrations (Hg_T) in fine-grained fraction (<63 μm) of core sediments of the Hugli–Matla–Bidyadhari estuarine complex, India were analyzed. Results revealed a wide range of spatial variations (<4–93 ng g⁻¹ dry weight) with a definite enhancement level at the lower stretch of the estuarine complex infested with mangrove plants, which might act as a sink to Hg_T. An elevated concentration of Hg was encountered in surface/subsurface layer of the core in majority of the cases resulting from physical, biogenic and postdepositional diagenetic processes that remobilized and resuspended the metal from deeper sediments. A strong positive correlation was observed between the Hg and clay fraction content of the sediments, while the correlations of Hg with Al, Fe and Mn were poor. Based on the index of geoaccumulation (I _geo) and effects range-low (ER-L) value, it is considered that the sediments are less polluted and thus there is less chance of ecotoxicological risk to organisms living in sediments.  相似文献   

Determination of Ocean Wave Period from Altimeter Data Using Wave-Age Concept     

Jignesh Kshatriya  Abhijit Sarkar  Raj Kumar 《Marine Geodesy》2013,36(1):71-79

This study makes use of the concept of wave age in estimating ocean wave period from space borne altimeter measurements of backscattering coefficient and significant wave height. Introduction of wave age allowed better accounting of the difference between swells and wind waves. Using two years (1998 and 1999) data of TOPEX/Poseidon altimeter and ocean data buoy observations in the Indian Ocean, coefficients were generated for wave period, which were subsequently tested against data for the years 2000 and 2001. The results showed the wave period accuracy to be of the order of 0.6 sec (against 1.3 sec obtained with the semiempirical approach, reported earlier).  相似文献   

Inter-Comparison of Numerical Model Generated Surface Winds with QuikSCAT Winds over the Indian Ocean     

Sanjib K. Deb  Suchandra A. Bhowmick  Raj Kumar  Abhijit Sarkar 《Marine Geodesy》2013,36(4):391-408

The accurate surface wind in the equatorial Indian Ocean is crucial for modeling ocean circulation over this region. In this study, the surface wind analysis generated at the European Center for Medium Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP) are compared with NASA QuikSCAT satellite derived Level2B (swath level) and Level3 (gridded) surface winds for the year 2005. It is observed that the ECMWF winds exhibit speed bias of 1.5 m/s with respect to QuikSCAT Level3 in the southern equatorial Indian Ocean. The NCEP winds are found to exhibit speed bias (1.0–1.5 m/s) in the southern equatorial Indian Ocean specifically during January–February 2005. The biases are also observed in the analysis when compared with Level2B product as well; however, it is less in comparison to Level3 products. The amplitude of daily variations of both ECMWF and NCEP wind speed in Bay of Bengal and parts of the Arabian Sea is about 80% of that in QuikSCAT, while in the equatorial Indian Ocean it is about 60% of that of QuikSCAT.  相似文献   

Substrate control on formation and maturation of glauconites in the Middle Eocene Harudi Formation, western Kutch, India     

Santanu Banerjee  Shovan Lal ChattorajP.K. Saraswati  Somnath DasguptaUrbashi Sarkar 《Marine and Petroleum Geology》2012,30(1):144-160

Formation and maturation of glauconites within the early Middle Eocene Harudi Formation, built up in a lagoon to shelf transition in western Kutch, India is addressed taking recourse to a combined sedimentological, micropalaeontological and mineralogical investigation. The glauconite is authigenic and its occurrence coincides with maximum flooding. The glauconite is more matured within the fecal pellets and is less matured within the intra-particle pores of bioclasts namely, foraminifera, ostracoda, gastropoda and bryozoa. SEM-EDS, XRD and geochemical studies clearly document that the infilling within the intra-particle pores of bioclasts belong to nascent to slightly evolved glauconite (<5 wt% K₂O), while the pellet belongs to slightly evolved to evolved glauconite (5-7 wt% K₂O). Based on chemical characteristics and SEM investigations the glauconitization process can be best explained by the ‘verdissement theory’. The glauconite possibly formed as initial authigenic glauconitic smectite precipitates; while the pellet matured subsequently by addition of K, the process was hindered in case of infillings. The maturation process also involved addition of Si and release of Al, with or without release of Mg. The glauconite maturation was facilitated in case of fecal pellets because of higher porosity, inter-granular nature of pores and availability of necessary elements in semi-confined micro-environments. In contrast, the maturation was aborted at an early stage in case of infillings because of the minute, closed nature of the pores and prevalent alkaline conditions not allowing dissolution of calcitic tests. A low negative cerium anomaly suggests glauconite formation in sub-oxic micro-environments, created by decay of organic matters within the fecal pellets and bioclasts. A clear case of substratum control on glauconitization becomes apparent.  相似文献