Volume 122, Issue 4
August 2013, pages 899-1171
pp 899-933 August 2013
Meenakshi Chatterjee D Shankar G K Sen P Sanyal D Sundar G S Michael Abhisek Chatterjee P Amol Debabrata Mukherjee K Suprit A Mukherjee V Vijith Siddhartha Chatterjee Anwesha Basu Madhumita Das Saranya Chakraborti Aravind Kalla Surja Kanta Misra Soumya Mukhopadhyay Gopal Mandal Kankan Sarkar
Situated in the eastern coastal state of West Bengal, the Sundarbans Estuarine System (SES) is India’s largest monsoonal, macro-tidal delta-front estuarine system. It comprises the southernmost part of the Indian portion of the Ganga–Brahmaputra delta bordering the Bay of Bengal. The Sundarbans Estuarine Programme (SEP), conducted during 18–21 March 2011 (the Equinoctial Spring Phase), was the first comprehensive observational programme undertaken for the systematic monitoring of the tides within the SES. The 30 observation stations, spread over more than 3600 km2, covered the seven inner estuaries of the SES (the Saptamukhi, Thakuran, Matla, Bidya, Gomdi, Harinbhanga, and Raimangal) and represented a wide range of estuarine and environmental conditions. At all stations, tidal water levels (every 15 minutes), salinity, water and air temperatures (hourly) were measured over the six tidal cycles. We report the observed spatio-temporal variations of the tidal water level. The predominantly semi-diurnal tides were observed to amplify northwards along each estuary, with the highest amplification observed at Canning, situated about 98 km north of the seaface on the Matla. The first definite sign of decay of the tide was observed only at Sahebkhali on the Raimangal, 108 km north of the seaface. The degree and rates of amplification of the tide over the various estuarine stretches were not uniform and followed a complex pattern. A least-squares harmonic analysis of the data performed with eight constituent bands showed that the amplitude of the semi-diurnal band was an order of magnitude higher than that of the other bands and it doubled from mouth to head. The diurnal band showed no such amplification, but the amplitude of the 6-hourly and 4-hourly bands increased headward by a factor of over 4. Tide curves for several stations displayed a tendency for the formation of double peaks at both high water (HW) and low water (LW). One reason for these double-peaks was the HW/LW stands of the tide observed at these stations. During a stand, the water level changes imperceptibly around high tide and low tide. The existence of a stand at most locations is a key new finding of the SEP. We present an objective criterion for identifying if a stand occurs at a station and show that the water level changed imperceptibly over durations ranging from 30 minutes to 2 hours during the tidal stands in the SES. The tidal duration asymmetry observed at all stations was modified by the stand. Flow-dominant asymmetry was observed at most locations, with ebb-dominant asymmetry being observed at a few locations over some tidal cycles. The tidal asymmetry and stand have implications for human activity in the Sundarbans. The longer persistence of the high water level around high tide implies that a storm surge is more likely to coincide with the high tide, leading to a greater chance of destruction. Since the stands are associated with an amplification of the 4-hourly and 6-hourly constituents, storm surges that have a similar period are also likely to amplify more during their passage through the SES.
pp 935-946 August 2013
Till now low-level winds were retrieved using Kalpana-1 infrared (IR) images only. In this paper, an attempt has been made to retrieve low-level cloud motion vectors using Kalpana-1 visible (VIS) images at every half an hour. The VIS channel provides better detection of low level clouds, which remain obscure in thermal IR images due to poor thermal contrast. The tracers are taken to be 15 × 15 pixel templates and hence each wind corresponds to about 120km × 120km at sub-satellite point. Multiplet based wind retrieval technique is followed for VIS wind derivation. However, for height assignment of VIS winds, collocated IR image is used. Due to better contrast between cloud and ocean surface, the low level atmospheric flow is captured better as compared to IR winds. The validation of the derived VIS winds is done with Global Forecast System (GFS) model winds and Oceansat-II scatterometer (OSCAT) winds.
pp 947-956 August 2013
Comparison of reflectivity data of radars onboard CloudSat and TRMM is performed using coincident overpasses. The contoured frequency by altitude diagrams (CFADs) are constructed for two cases: (a) only include collocated vertical profiles that are most likely to be raining and (b) include all collocated profiles along with cloudy pixels falling within a distance of about 50 km from the centre point of coincidence. Our analysis shows that for both cases, CloudSat underestimates the radar reflectivity by about 10 dBZ compared to that of TRMM radar below 15 km altitude. The difference is well outside the uncertainty value of ∼2 dBZ of each radar. Further, CloudSat reflectivity shows a decreasing trend while that of TRMM radar an increasing trend below 4 km height. Basically W-band radar that CloudSat flies suffers strong attenuation in precipitating clouds and its reflectivity value rarely exceeds 20 dBZ though its technical specification indicates the upper measurement limit to be 40 dBZ. TRMM radar, on the other hand, cannot measure values below 17 dBZ. In fact combining data from these two radars seems to give a better overall spatial structure of convective clouds.
pp 957-966 August 2013
Indian National Satellite (INSAT) 3A was launched in the year 2003 with communication and remote sensing payloads. The later payloads contain very high resolution radiometer (VHRR) and charge coupled devices (CCD) camera. In this paper, post-launch calibration of INSAT 3A CCD is discussed. A cross radiometric calibration was carried out with well calibrated advanced wide field of view sensor (AWiFS) of Indian Remote Sensing Satellite (IRS P6). Three concurrent scenes of December, January and February were used in this study. Calibration was carried out under different land cover classes such as snow, vegetation, forest, water and cloud. Regression analysis suggests correlation coefficient of 0.95, 0.92 and 0.60 for Red, NIR and SWIR channels with slope values 1.839, 1.589 and 2.232, respectively. New calibration coefficients were used to estimate at-sensor radiance and reflectance in all the three channels. Dynamic range of reflectances was found to be improved by using new calibration coefficients. Normalized difference snow index and vegetation index (NDSI and NDVI) have shown an improvement with new coefficients and were found closer to represent in situ data of different land covers and cloud.
pp 967-977 August 2013
Using JTWC (Joint Typhoon Warning Center) best track analysis data for the Indian Ocean cyclones, we developed an empirical equation for prediction of maximum surface wind speed of tropical cyclones during first 6–12 hours of landfall along the coastline of Indian subcontinent. A non-linear data fitting approach, the Genetic Algorithm, has been used to develop the above empirical equation using data for 74 tropical cyclones that made landfall on the coasts of India, Bangladesh and Myanmar during the period 1978–2011. For an out of sample validation test, the mean absolute error of the prediction was found to be 5.2 kt, and a correlation of 0.97. Our analysis indicates that time-integration of land area intercepted by cyclones during the landfall is a better predictor of post-landfall intensity compared to post-landfall time span. This approach also helps to tackle the complexity of coastline geometry of Indian subcontinent area.
pp 979-990 August 2013
Accurate prediction of movement and intensity of tropical cyclone is still most challenging problem in numerical weather prediction. The positive progress in this field can be achieved by providing network of observations in the storm region and best representation of atmospheric physical processes in the model. In the present study later part was attempted to investigate the sensitivity of movement and intensity of the severe cyclonic storm AILA to different physical processes in the Weather Research and Forecasting model. Three sets of experiments were conducted for convection, microphysics (MP) and planetary boundary layer (PBL) processes. Model-simulated fields like minimum central surface pressure, maximum surface wind, track and vector displacement error are considered to test the sensitivity. The results indicate that the movement of the system is more sensitive to the cumulus physics and the intensity of the cyclone is sensitive to both PBL and cumulus physics. The combination of Betts Miller Janjic (BMJ) for convection, Yonsei University (YSU) for PBL and Purdue Lin (LIN) for microphysics is found to perform better than other combination schemes. The horizontal and vertical features of the system along with its special features like complete northward movement of the system throughout the travel period and the consistent cyclonic storm intensity until 15 hrs after the landfall could be well simulated by the model.
pp 991-1004 August 2013
The purpose of this study is to address prediction of the start date and the duration of breaks in the summer monsoon rains using multi-model superensemble. The availability of datasets from the ‘observing system research and predictability experiment (THORPEX)’ initiated a forecast data archive, called THORPEX interactive grand global ensemble (TIGGE), makes it possible to use forecasts from a suite of individual ensemble prediction systems (member models) and to construct multi-model superensemble forecasts that are designed to remove the collective bias errors of the suite of models. Precipitation datasets are important for this study, we have used high resolution daily gridded rainfall dataset of India Meteorological Department (IMD), in addition to rainfall estimates from tropical rainfall microwave mission (TRMM) satellite and the CPC morphing technique (CMORPH). The scientific approach of this study entails the use of a multi-model superensemble for forecast and to verify against the rainfall information during a training phase, as well as during a forecast phase. We examine the results of forecasts out to day-10 and ask how well do forecast strings of day-1 through day-10 handle the prediction of the onset and duration of the breaks in the summer monsoon rains. Our results confirm that it is possible to predict the onset of a dry spell, around week in advance from the use of the multi-model superensemble and a suite of TIGGE models.We also examine trajectories of the parcels arriving in India in such forecasts from member models and from the multi-model superensemble to validate the arrival of descending dry desert air from the Arabian region during the dry spells and its mode of transition from wet spell. Some phenological features such as a shift in the latitude of the tropical easterly jet and changes in its intensity during break periods are additional observed features that are validated from the history of multi-model superensemble forecasts. Invariably this multi-model superensemble performs better than any single model in proving the better forecasts during our experiment period.
pp 1005-1011 August 2013
It is now well known that there is a strong association of the extremes of the Indian summer monsoon rainfall (ISMR) with the El Ni˜no and southern oscillation (ENSO) and the Equatorial Indian Ocean Oscillation (EQUINOO), later being an east–west oscillation in convection anomaly over the equatorial Indian Ocean. So far, the index used for EQUINOO is EQWIN, which is based on the surface zonal wind over the central equatorial Indian Ocean. Since the most important attribute of EQUINOO is the oscillation in convection/precipitation, we believe that the indices based on convection or precipitation would be more appropriate. Continuous and reliable data on outgoing longwave radiation (OLR), and satellite derived precipitation are now available from 1979 onwards. Hence, in this paper, we introduce new indices for EQUINOO, based on the difference in the anomaly of OLR/precipitation between eastern and western parts of the equatorial Indian Ocean. We show that the strong association of extremes of the Indian summer monsoon with ENSO and EQUINOO is also seen when the new indices are used to represent EQUINOO.
pp 1013-1021 August 2013
Electromagnetic waves from lightning activity, commonly known as atmospherics or sferics serve as an effective tool for studying the lower ionosphere as well as thunderstorm activity. It is also useful for locating lightning strokes regionally and globally. In this paper, we present the analysis of the Integrated Field Intensity of Sferics (IFIS) at six discrete VLF frequencies for 30 lightning-associated overhead thunderstorms in Tripura, within the period from August 2009 to October 2010. An ingeniously developed well calibrated GPS locked software VLF receiver, located at the Department of Physics, Tripura University (23.5°N, 91.25°E), is used for the present study. Two distinct types of variations of IFIS, (i) single peak and (ii) dual peak are found characterizing each thunderstorm and their occurrence show nearly inverse character. The spectral character of IFIS rise rate, fall rate and rate of enhancement for each type is studied searching for suitable frequencies in the VLF range to forecast a thunderstorm. It is concluded that VLF sferics from 3–10 kHz are the most effective in terms nowcasting an incoming thunderstorm well before 3–4 hours of its peak occurrence, when there may not be any visual indication of the thunderstorm.
pp 1023-1033 August 2013
Trace fossils that record animal and plant activity are described for the first time from the Middle Siwalik, Neogene deposits of Darjeeling Himalaya. Sedimentary facies association attests to a channel–interchannel floodplain fluviatile setting. The intimate association of the burrows with phytoliths, rhizoliths, leaf compressions and coal lenses suggest that the tracemakers dominated a floodplain habitat. Point bar deposits host a low diversity Planolites-Naktodemasis-Macanopsis-Cylindricum equilibrium ichnocoenosis in the heterolithic fine sandstone-siltstone-shale facies that alternates with dense, monospecific colonization of Planolites as opportunistic pioneers relocating under stressed condition. Interlayered floodplain deposits in the fluvial successions preserve enigmatic large diameter, vertical tubes within thin to thick-bedded, dark silty shale facies. These tubes bear mixed characters assignable to both crayfish burrows and large-diameter rhizoliths. Further work on these tubes is necessary to make more accurate interpretations of those structures. Shallow to moderate burrow depths; intermittent, short-lived colonization events and preservation of rhizoliths and rhizohalos under fluctuating moisture content indicate short-term fluctuations of a relatively high water table (close to the paleosurface) in an imperfectly drained proximal floodplain setting. Ichnotaxa distribution and their inferred ethology provide significant faunal data that may put constraints on the reconstruction of Middle Siwalik depositional environment.
pp 1035-1044 August 2013
This study is the first detailed investigation of the platinum-group elements (PGE) at the eastern margin of the Deccan volcanic province of India. One of the PGE, osmium, is not included largely because of analytical problems. The study is focused on mafic volcanics and dykes from four areas including Amarkantak, Umaria, Shahdol and Chirimiri. The first two localities represent two lava piles of about 170 and 400 m thickness respectively. In Umaria, 16 flows have been demarcated based on petrography and field studies. The Shahdol samples are basal lava formations overlying Gondwana sediments (Carboniferous) and the Chirimiri samples are dykes. In this study, the western Deccan province is defined as the Western Ghats plus Kutch. On average, the PGE are ∼20% higher in Amarkantak than Umaria and the flows are ∼13% higher in PGE than the dykes. A Zr vs. Pd scattergram found a strong positive correlation for these two elements except for one Umaria sample which indicated severe Pd loss. A comparison of west and east parts of the Deccan volcanic province using primitive mantle normalization showed that higher values prevailed in the western province suite in the Ni-Ir-Ru-Pt region. In contrast, eastern province values dominated in the Pd-Au-Cu region at the ‘Cu’ end of the profiles. A strong dominance of Pd in the eastern Deccan was also of interest. A number of factors, for example, percentage partial melting of the source rock and the temperature and pressure of partial melting strongly influence the character of these profiles. The observed PGE profile characteristics probably result in part from a long distance of subsurface transport of Deccan magma from the western to eastern regions.
pp 1045-1053 August 2013
Reflection of plane waves is studied at a free surface of a perfectly conducting transversely isotropic elastic solid half-space with initial stress. The governing equations are solved to obtain the velocity equation which indicates the existence of two quasi planar waves in the medium. Reflection coefficients and energy ratios for reflected qP and qSV waves are derived and computed numerically for a particular material. Effects of the initial stress and magnetic field are shown graphically on these reflection coefficients and energy ratios.
pp 1055-1063 August 2013
The solution of the static deformation of a homogeneous, isotropic, perfectly elastic half-space caused by uniform movement along a long vertical tensile fracture is well known. In this paper, we study the problem of static deformation of a homogeneous, isotropic, perfectly elastic half-space caused by a nonuniform movement along a long vertical tensile fracture of infinite length and finite depth. Four movement profiles are considered: linear, parabolic, elliptic and cubic. The deformation corresponding to the four non-uniform movement profiles is compared numerically with the deformation due to a uniform case, assuming the source potency to be the same. The equality in source potency is achieved in two ways: One, by varying the depth of fracture and keeping the surface discontinuity constant and the other way, by keeping the depth of fracture constant and varying the surface discontinuity. It is found that the effect of non-uniformity in movement in the near field is noteworthy. The far field is not affected significantly by the non-uniformity in movement. In the first case, horizontal displacement is significantly affected rather than vertical displacement. In the second case, non-uniformity in movement changes the magnitude of the displacement at the surface. Also, the displacements around a long vertical tensile fracture for different movement profiles are plotted in three dimensions.
pp 1065-1080 August 2013
In hard rock areas, conventional apparent resistivity measurement using Schlumberger resistivity sounding fails to detect thin conducting structures (2-D and 3-D fractures filled with groundwater and mineral aggregate) concealed at a large depth. In the present study, an attempt is made to way-out the detection problem of deep seated thin conducting layer. It is proposed to study the apparent conductance simultaneously with resistivity sounding to detect such conductive zones qualitatively. Apparent conductance is defined as the magnitude of current flowing in the subsurface for a unit applied voltage through current electrodes. Even though such measurement is of qualitative importance, it gives extremely valuable information for the presence of conductive zones at depth in challenging hard rock terrain. It has been observed that apparent conductance increases significantly when groundwater bearing fractures and conductive bodies are encountered in the subsurface. Field data from different locations are presented to demonstrate the efficacy of such measurement. The measurement assists to the conventional resistivity sounding for successful prediction of groundwater zones at large depth in different hard rock areas and is of enormous importance. The approach is also used for possible solution of suppression problem in the DC resistivity sounding when intermediate layer is not reflected in the resistivity sounding curve. Finally, the approach can be used together with resistivity sounding to solve many practical problems.
pp 1081-1089 August 2013
Sandıklı-Hüdai geothermal field is one of the geothermal systems in Afyon and environ, located approximately 40 km southwest of Afyon. The study area consists of volcanic, sedimentary and metamorphic rocks. Kestel greenschist formation of Paleozoic age forms the basement rock in the area while quartzite which is a member of the Kestel greenschist formation serves as the reservoir rock of Sandıklı-Hüdai geothermal system. Geothermal waters from the study area are classified as Na–SO4–HCO3 type waters. The waters plot along SO4–HCO3 end of the Cl–SO4–HCO3 triangle diagram suggesting same origin for the geothermal waters. 𝛿18O and $\delta D$ isotope ratios of the Sandıklı waters plot along the continental meteoric water line, indicating meteoric waters that were unaffected by evaporation. The tritium values imply that the fluids were deep circulating and recharged from older waters. Furthermore, oversaturation of the geothermal fluids with quartz confirms these findings showing long time residence of these groundwaters.
pp 1091-1105 August 2013
This study investigated the hydrogeochemical processes of groundwater in the Tasuj plain, Iran. The Tasuj plain is one of the 12 marginal plains around Urmia Lake which is currently under a critical ecological condition. In the last decades, the Tasuj plain aquifer suffered from severe groundwater level declination and caused degradation of groundwater quality. To better understand hydrogeochemical processes in the Tasuj plain, this study adopted graphical methods and multivariate statistical techniques to analyze groundwater samples. A total of 504 groundwater samples was obtained from 34 different locations (qanats, wells, and springs) over 12 years (1997–2009) and analyzed for 15 water quality parameters. From the results, the Piper diagram indicated four groundwater types and the Stiff diagram showed eight different sources of groundwater samples. The Durov diagram identified five major hydrogeochemical processes in the aquifer. However, hierarchical cluster analysis (HCA) identified five water types in the groundwater samples because HCA was able to analyze more chemical and physical data than graphical methods. The HCA result was checked by discriminant analysis and found consistency in all samples that were classified into correct groups. Using factor analysis, we identified three factors that accounted for 81.6% of the total variance of the dataset. Based on the high factor loadings of the variables, factors 1 and 2 reflected the natural hydrogeochemical processes and factor 3 explained the effect of agricultural fertilizers and human activities in the Tasuj plain. Dendrograms from 2000 to 2009 were studied to understand the temporal variation of groundwater quality. Comparing the distributions of groundwater types in 2000 and 2009, we found that the mixing zone was expanded. This may be due to artificial groundwater recharge in the recharge area and the effect of inverse ion exchange in the discharge area.
pp 1107-1124 August 2013
Desertification risk assessment is important in order to take proper measures for its prevention. Present research intends to identify the areas under risk of desertification along with their severity in terms of degradation in natural parameters. An integrated model with fuzzy membership analysis, fuzzy rulebased inference system and geospatial techniques was adopted, including five specific natural parameters namely slope, soil pH, soil depth, soil texture and NDVI. Individual parameters were classified according to their deviation from mean. Membership of each individual values to be in a certain class was derived using the normal probability density function of that class. Thus if a single class of a single parameter is with mean 𝜇 and standard deviation 𝜎, the values falling beyond 𝜇 + 2𝜎 and 𝜇 − 2𝜎 are not representing that class, but a transitional zone between two subsequent classes. These are the most important areas in terms of degradation, as they have the lowest probability to be in a certain class, hence highest probability to be extended or narrowed down in next or previous class respectively. Eventually, these are the values which can be easily altered, under extrogenic influences, hence are identified as risk areas. The overall desertification risk is derived by incorporating the different risk severity of each parameter using fuzzy rule-based interference system in GIS environment. Multicriteria based geo-statistics are applied to locate the areas under different severity of desertification risk. The study revealed that in Kota, various anthropogenic pressures are accelerating land deterioration, coupled with natural erosive forces. Four major sources of desertification in Kota are, namely Gully and Ravine erosion, inappropriate mining practices, growing urbanization and random deforestation.
pp 1125-1139 August 2013
River pollution has tremendously increased in the major cities of South Asia, where the rivers have become a repository for domestic, agricultural, municipal and industrial wastes. This study presents the evaluation of benthic macroinvertebrate assemblage as a means of assessing ecological status, determining different disturbance zones and identifying environmental variables and stressors that deteriorate the river ecosystem. In total, 20 sites in 36-km stretch of the main stem of the Bagmati River and 7 sites on its tributaries were selected for sampling in the post-monsoon and pre-monsoon seasons during the time period 2008–2010. The Ganga River System Biotic Score (GRSbios) index was applied to determine the ecological status. The ecological status of the different Bagmati River stretches ranged from reference, class 1 to class 5 (extremely polluted). We identified three types of disturbance zones along the river, ranging from minimally polluted to extremely polluted. A river corridor survey was conducted to identify any river stressing factors, revealing a sharp deterioration of the river from upstream to downstream with increasing concentrations of chloride and ortho-phosphate phosphorus. Effluents and Activities and Facilities were found to be the major stressing factors to the river ecosystem. The information gained should help water managers find the most time-efficient and cost-effective measures to address river degradation.
pp 1141-1152 August 2013
Present study aims at reconstructing the paleomonsoonal rainfall, paleovegetation and provenance change during the late Quaternary. Towards this, Bhognipur core, collected from the southern Ganga Plain, have been sampled for soil carbonate (SC) and soil. The 𝛿18O values of SC (𝛿18OSC) range from −7.6 to −4.9‰. The variations in 𝛿18OSC values suggest that during the late Quaternary, the monsoon intensified during MIS 3 and MIS 1 and the maximum lowering of rainfall intensity is observed during MIS 2. The 𝛿13C value of SC (𝛿13CSC), organic matter dispersed in the soil (𝛿13CSOM) and occluded in the carbonate nodules (𝛿13CNOM) ranges from −4.1 to +1.4‰, −25.6 to −16.3‰, and −27.7 to −25.0‰, respectively, implies mixed C3$–$C4 vegetation over the Ganga Plain. Variations in 𝛿13CSOM and 𝛿13CNOM values at same depth imply preservation problem of pristine organic matter signature. Therefore, it is important to assess the preservation of residual organic matter before using it for paleovegetational reconstruction. The monsoon-vegetation relationship indicates that relative abundances of C3$–$C4 vegetation were mainly driven by variations in monsoonal rainfall intensity. Using 87Sr/86Sr in SC, we show that the Himalayan river was supplying sediments in the southern part of the Ganga Plain during MIS 3.
pp 1153-1159 August 2013
Shell weights of planktonic foraminifera species Globigerinoides ruber in the size range of 300–355 μm were measured from sediment traps in the western and eastern Arabian Sea which represent upwelling and non-upwelling conditions respectively. In the Western Arabian Sea Trap (WAST), G. ruber flux ranged from 33.3 to 437.3#/m2/day and shell weights ranged from 6.7 to 14.2 𝜇g. Whereas, in the Eastern Arabian Sea Trap (EAST), flux ranged from 0.7 to 164.6#/m2/day and shell weights ranged from 10.4 to 14.8 𝜇g. Shell weights of G. ruber versus flux showed significant correlation at both the sites which reveals that shell calcification mainly depends on optimal growth conditions. Though the WAST and EAST location have distinct difference in pCO2 and sea surface temperature (SST), the shell weights of G. ruber are similar in these two regions which suggest that surface water pCO2 and SST do not show dominant influence on shell calcification on a seasonal timescale.
pp 1161-1171 August 2013
Five hundred and eighteen cosmic spherules were identified among the 672 spherules handpicked from deep sea sediments by using Scanning Electron Microscope-Energy Dispersive Spectrometry (SEM-EDS). One of the spherules is found to enclose a spherical chondrule-like object that can be distinguished from the rest of the spherule by its shape, texture and composition and whose petrographic features, size and chemical composition are similar to chondrules from a chondritic meteorite, probably of carbonaceous chondritic nature. The present finding suggests that a small fraction of the particulate extraterrestrial matter enters the earth as fragments of larger meteorites.