• Volume 119, Issue 5

      October 2010,   pages  561-751

    • Spatial and temporal variations in the occurrences of wet periods over major river basins in India

      N R Deshpande N Singh

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      This study highlights the hydro-climatic features of the five wet periods contributing in different percentages to the annual rainfall total over major river basins in India.Spatial and temporal variations in the parameters such as starting date,duration and rainfall intensity of these wet periods throughout India have been discussed using daily gridded rainfall data for the period 1951 –2007.An attempt is also made here,to assess the impact of global SSTs on the start and duration of the wet periods in Indian river basins.

      It is observed that,for almost all river basins in India,the 10%wet period occurs in the months of July/August with an average duration of 1 –3 days and rainfall intensity varying from 44 to 89 mm/day.The duration of the wet period contributing 90%to the annual rainfall varies from 112 days in the central parts of India to 186 days in the northern parts of the country.Signi ficant increase in the rainfall intensity has been observed in the case of some river basins of central India. The late start of 75%wet period along the West Coast and in peninsular river basins has been observed with increase in Nino 3.4 SSTs (MAM),while increase in the duration of the 75%wet period over the Krishna basin is associated with increase in Nino 3.4 SSTs (concurrent JJAS).

    • Diurnal variation of precipitation over the Carolina Sandhills region

      A Wootten S Raman A Sims

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      The Carolina Sandhills are known to have an area of maximum precipitation on its western boundary during the summer mainly due to differences in soil types.Statistical analysis was performed on summer precipitation data from automated weather stations in the Carolinas,along the Sandhills for the years 2001 to 2006.Statistically significant difference was observed between the day and night precipitation amounts.A case study also revealed the diurnal pattern of convective precipitation.

      North American Mesoscale (NAM)model forecasts for the summers of 2004 to 2006 were evaluated using observations.The model underpredicted precipitation significantly during nights. A numerical simulation using Weather Research and Forecast (WRF)model was performed for August 9 –11,2001 and the forecasts were compared with observed precipitation data.The model precipitation forecasts were better for daytime as compared to the night.This feature is attributed to model physics not capturing cloud –radiation interaction processes dominant during nights. Although this study is for a specific region in the US,results are applicable for other regions for similar conditions.

    • Use of objective analysis to estimate winter temperature and precipitation at different stations over western Himalaya

      Jagdish Chandra Joshi Ashwagosha Ganju

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      Temperature and fresh snow are essential inputs in an avalanche forecasting model.Without these parameters,prediction of avalanche occurrence for a region would be very difficult.In the complex terrain of Himalaya,nonavailability of snow and meteorological data of the remote locations during snow storms in the winter is a common occurrence.In view of this persistent problem present study estimates maximum temperature,minimum temperature,ambient temperature and precipitation intensity on different regions of Indian western Himalaya by using similar parameters of the neighbouring regions.The location at which parameters are required and its neighbouring locations should all fall in the same snow climatic zone.Initial step to estimate the parameters at a location,is to shift the parameters of neighbouring regions at a reference height corresponding to the altitude of the location at which parameters are to be estimated.The parameters at this reference height are then spatially interpolated by using Barnes objective analysis.The parameters estimated on different locations are compared with the observed one and the Root Mean Square Errors (RMSE)of the observed and estimated values of the parameters are discussed for the winters of 2007 –2008.

    • Thunderstorms over a tropical Indian station, Minicoy: Role of vertical wind shear

      H S Chaudhari G K Sawaisarje M R Ranalkar P N Sen

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      In this study,an attempt has been made to bring out the observational aspects of vertical wind shear in thunderstorms over Minicoy.Case studies of thunderstorm events have been examined to find out the effect of vertical wind shear and instability on strength and longevity of thunderstorms.Role of vertical wind shear in thunderstorms and its mechanism has been explored in this study.Results reveal that for prolonged thunderstorms high and low instability along with moderate to high vertical wind shear (moderate:0.003S−1 ≤ vertical wind shear ≤ 0.005S−1 and high: < 0.005S−1) play a significant role in longevity and strength of thunderstorms.The mechanism of vertical wind shear in thunderstorms was investigated in a few cases of thunderstorm events where the duration of thunderstorm was covered by the radiosonde/rawin ascent observation taken at Minicoy. Empirical model has been developed to classify thunderstorm type and to determine the strength and longevity of thunderstorms.Model validation has been carried out for selected cases.Model could classify thunderstorm type for most of the cases of thunderstorm events over island and coastal stations.

    • Anomalous electric field changes and high flash rate beneath a thunderstorm in northeast India

      S D Pawar P Murugavel V Gopalakrishnan

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      In spite of many experimental and theoretical studies the relationships between storm dynamics, severe weather,and lightning activity have been least understood.Measurements of electric field made under a severe thunderstorm at a northeastern Indian station,Guwahati,India are reported. Lightning flash rate increases drastically to about 84 flashes per minute (fpm)during the active stage which lasted for about 7 minutes,from about 15 flashes per minute during the initial phase of thunderstorm.Sudden increase in lightning flash rate (‘lightning jump ’)of about 65 fpm/min is also observed in the beginning of the active stage.The dissipating stage is marked by slow and steady decrease in lightning frequency.Despite very high flash rate during the active stage, no severe weather conditions are observed at the ground.It is proposed that the short duration of the active stage might be the reason for the non-observance of severe weather conditions at the ground.Analysis of Skew-t graph at Guwahati suggests that vertical distribution of Convective Available Potential Energy (CAPE)also may play some role in non-occurrence of severe weather at ground in spite of large lightning flash rate and lightning jump observed in this thunderstorm.Further,all electric field changes after a lightning discharge indicates the presence of strong Lower Positive Charge Centers (LPCC)in the active and dissipation stages. This suggests that LPCC plays an important role in initiation of lightning discharges in these stages.

    • Numerical study for production of space charge within the stratiform cloud

      A K Srivastava S N Tripathi

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      One dimensional numerical model has been developed to predict the production of space charge and variations in other electrical parameters within the low level stratiform type of cloud having very weak vertical motion.Non-linear coupled differential equations which govern ion concentrations,charged and neutral droplet concentrations and electric field were used.Symmetry has been observed in all the electrical parameters within the cloud.The magnitude of average positive ion concentrations was observed to be high as compared to the negative ion concentrations,which is due to low scavenging rate of positive ions than the negative ions,highly attributed to their mobilities.The rate of scavenging of ions affects the concentration of charged droplets,which eventually influence the electric field and thus the space charge density within the cloud.Maximum electric field (𝐸_max) was observed at middle of the cloud whereas minimum was observed at both the edges of the cloud.Minimum electric field (𝐸_min) was found to be equal and constant (∼27Vm−1)for any drop concentration.Net positive and negative space charges were observed at the top and bottom of the cloud,respectively.The simulated results show some discrepancies to the natural condition, which are due to simulations made under some basic assumptions and limitations and that will be incorporated in the future studies for natural cloud condition.

    • Variations in long term wind speed during different decades in Arabian Sea and Bay of Bengal

      V Sanil Kumar C Sajiv Philip

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      A study has been carried out by comparing the extreme wind speeds estimated based on NCEP/NCAR reanalysis data for 100 years return period using Fischer Tippet-1 (commonly known as Gumbel)and Weibull distributions for three locations (off Goa,Visakhapatnam and Machilipatnam)in the north Indian Ocean.The wind dataset for Goa is compared with that from ERA-40 data.For higher wind speeds ($12-20 m s^{−1}$ ),NCEP wind speed has higher percentage of occurrence than that of ERA-40.Analysis has shown slight upward trend in the annual maximum wind for location off Machilipatnam with an increase of $1.2cms^{−1}$ per year and a decreasing trend of $−1.3cms^{−1}$ per year in the case of Goa.The Weibull distribution with shape parameter 2 fits the annual maximum wind data better than FT-1 distribution.

    • An experimental investigation to characterise soil macroporosity under different land use and land covers of northeast India

      Sangeeta Shougrakpam Rupak Sarkar Subashisa Dutta

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      Saturated macropore flow is the dominant hydrological process in tropical and subtropical hilly watersheds of northeast India.The process of infiltration into saturated macroporous soils is primarily controlled by size,network,density,connectivity,saturation of surrounding soil matrix, and depthwise distribution of macropores.To understand the effects of local land use,land cover and management practices on soil macroporosity,colour dye infiltration experiments were conducted with ten soil columns (25 × 25 × 50 cm)collected from different watersheds of the region under similar soil and agro-climatic zones.The sampling sites included two undisturbed forested hillslopes,two conventionally cultivated paddy fields,two forest lands abandoned after Jhum cultivation,and two paddy fields,one pineapple plot and one banana plot presently under active cultivation stage of the Jhum cycle.Digital image analyses of the obtained dye patterns showed that the infiltration patterns differed significantly for different sites with varying land use,land cover,and cultivation practices.Undisturbed forest soils showed high degree of soil macroporosity throughout the soil profile,paddy fields revealed sealing of macropores at the topsoil due to hard pan formation,and Jhum cultivated plots showed disconnected subsoil macropores.The important parameters related to soil macropores such as maximum and average size of macropores,number of active macropores,and depthwise distribution of macropores were estimated to characterise the soil macroporosity for the sites.These experimentally derived quantitative data of soil macro- porosity can have wide range of applications in the region such as water quality monitoring and groundwater pollution assessment due to preferential leaching of solutes and pesticides,study of soil structural properties and infiltration behaviour of soils,investigation of flash floods in rivers, and hydrological modelling of the watersheds.

    • Evaluation of phase chemistry and petrochemical aspects of Samchampi–Samteran differentiated alkaline complex of Mikir Hills, northeastern India

      Abhishek Saha Sohini Ganguly Jyotisankar Ray Nilanjan Chaterjee

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      The Samchampi –Samteran alkaline complex occurs as a plug-like pluton within the Precambrian granite gneisses of Mikir Hills,Assam,northeastern India and it is genetically related to Sylhet Traps.The intrusive complex is marked by dominant development of syenite within which ijolite – melteigite suite of rocks is emplaced with an arcuate outcrop pattern.Inliers of alkali pyroxenite and alkali gabbro occur within this ijolite –melteigite suite of rocks.The pluton is also traversed by younger intrusives of nepheline syenite and carbonatite.Development of sporadic,lumpy magnetite ore bodies is also recorded within the pluton.Petrographic details of the constituent lithomembers of the pluton have been presented following standard nomenclatorial rules.Overall pyroxene compositions range from diopside to aegirine augite while alkali feldspars are typically orthoclase and plagioclase in syenite corresponds to oligoclase species.Phase chemistry of nepheline is suggestive of Na-rich alkaline character of the complex.Biotite compositions are typically restricted to a uniform compositional range and they belong to ‘biotite ’field in the relevant classification scheme.Garnets (developed in syenite and melteigite)typically tend to be Ti-rich andradite,which on a closer scan can be further designated as melanites.Opaque minerals mostly correspond to magnetite.Use of Lindsley ’s pyroxene thermometric method suggests an equilibration temperature from ∼450°$–$600°C for melteigite/alkali gabbro and ∼400° C for syenite.Critical assessment of other thermometric methods reveals a temperature of equilibration of ∼700°$–$1350°C for ijolite –melteigite suite of rocks in contrast to a relatively lower equilibration temperature of ∼600° C for syenite. Geobarometric data based on pyroxene chemistry yield an equilibration pressure of 5.32 –7.72 kb for ijolite,melteigite,alkali pyroxenite,alkali gabbro and nepheline syenite.The dominant syenite member of the intrusive plug records a much higher (∼11 kb)equilibration pressure indicating a deeper level of intrusion.Major oxide variations of constituent lithomembers with respect to differentiation index (D.I.)corroborate a normal magmatic differentiation.A prominent role of liquid immiscibility is envisaged from field geological,petrographic and petrochemical evidences. Tectonic discrimination diagrams involving clinopyroxene chemistry strongly suggest within plate alkaline affinity for the parental magma which is in conformity with the regional plume tectonics.

    • Palynodating of subsurface sediments, Raniganj Coalfield, Damodar Basin, West Bengal

      Srikanta Murthy B Chakraborti M D Roy

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      The Gondwana sediments comprising fine-grained shales,carbonaceous shales,sandstones and the coal horizon in borecore RT-4 (approximately 547.00 m thick)from Tamra block,Raniganj Coal field,Damodar Basin,are analyzed palynologically.Based on the distribution pattern of marker palynotaxa,two assemblage zones are identi fied.In the Barren Measures Formation,dominance of enveloping monosaccate (Densipollenites) along with striate bisaccate (Striatopodocarpites,Fauni- pollenites) pollen taxa,and the FAD ’s of Kamthisaccites and Arcuatipollenites observed at 30.75, have equated this strata (30.75 –227.80 m thick)with the Raniganj Formation of Late Permian in age.Downwards in the Barakar Formation,between 423.80 –577.70 m depths,an abundance of non-striate (Scheuringipollenites )and striate(Faunipollenites and Striatopodocarpites )bisaccate pollen taxa is observed,that dates late Early Permian in age.

      Fair occurrences of hyaline,distorted and blackish-brown plant matter is observed within 231.00 –408.40 m depths.Present study infers the existence of the Raniganj Formation in the lithologically delimited Barren Measures Formation in the study area,and the underlying unproductive strata (approx.177.40 m)might represent the part of the Barren Measures Formation.

    • Chemical composition and palaeobotanical origin of Miocene resins from Kerala–Konkan Coast, western India

      Suryendu Dutta Monalisa Mallick Runcie Paul Mathews Ulrich Mann Paul F Greenwood Rakesh Saxena

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      The terpenoid composition of resins from the Miocene lignite horizons from the Kerala –Konkan Coast,western India was analyzed by Curie-point pyrolysis –gas chromatography –mass spectrometry (Cupy –GC –MS).The major pyrolysates were cadalene-based bicyclic sesquiterpenoids including some C30-C31 bicadinenes and bicadinanes typical of dammar resin from angiosperm plants of Dipterocarpaceae family.These plants are typically supported by tropical climates which the western Indian region was known to have experienced during early Tertiary period.The present study suggests that these palaeoclimatic conditions persisted until at least the Miocene epoch.These sesquiterpenoids which are commonly detected in many SE Asian crude oils may be utilised as useful biomarkers for petroleum exploration in the western Indian region.

    • Assessment of flexural analysis applied to the Sumatra–Java subduction zone

      R T Ratheesh Kumar Tanmay K Maji Rajesh R Nair

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      Indian Ocean subduction zone is one of the most active plate margins of the globe as evident from its vast record of great magnitude earthquake and tsunami events.We use Bouguer admittance (Morlet isostatic response function)in Sumatra –Java subduction zones comprising both the subduction and over-riding plates to determine the lithospheric mechanical strength variations. We determine effective elastic thickness $(T_e)$ for five oceanic windows (size 990 × 990 km2) by analyzing the admittance using Bouguer gravity and bathymetry data. The results show bimodal $T_e$ values > 20 km for Sumatra and 20 –40 km for Java.The lower bimodal values obtained for Sumatra appears to correlate well with the zones of historical seismicity.This is in sharp contrast with Java subduction zone,which shows higher $T_e$ values (20 –40 km)and apparently associated with low magnitude earthquakes.We suggest a strong and wide interseismic coupling for Sumatra between the subducting and over-riding plates,and deeper mantle contributing to low strength,shallow focus –high magnitude seismicity and vice versa for Java,leading to their seismogenic zonation.

    • Estimates of source parameters of 𝑀 4.9 Kharsali earthquake using waveform modelling

      Ajay Paul Naresh Kumar

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      This paper presents the computation of time series of the 22 July 2007 𝑀 4.9 Kharsali earthquake. It occurred close to the Main Central Thrust (MCT)where seismic gap exists.The main shock and 17 aftershocks were located by closely spaced eleven seismograph stations in a network that involved VSAT based real-time seismic monitoring.The largest aftershock of 𝑀 3.5 and other aftershocks occurred within a small volume of 4 × 4 km horizontal extent and between depths of 10 and 14 km. The values of seismic moment $(M_o)$ determined using P-wave spectra and Brune’s model based on $f^2$ spectral shape ranges from $10^{18}$ to $10^{23}$ dyne-cm.The initial aftershocks occurred at greater depth compared to the later aftershocks.The time series of ground motion have been computed for recording sites using geometric ray theory and Green ’s function approach.The method for computing time series consists in integrating the far-field contributions of Green ’s function for a number of distributed point source.The generated waveforms have been compared with the observed ones.It has been inferred that the Kharsali earthquake occurred due to a northerly dipping low angle thrust fault at a depth of 14 km taking strike N279°E, dip 14° and rake 117°. There are two regions on the fault surface which have larger slip amplitudes (asperities)and the rupture which has been considered as circular in nature initiated from the asperity at a greater depth shifting gradually upwards.The two asperities cover only 10%of the total area of the causative fault plane.However,detailed seismic imaging of these two asperities can be corroborated with structural heterogeneities associated with causative fault to understand how seismogenesis is influenced by strong or weak structural barriers in the region.

    • Soil-gas helium and surface-waves detection of fault zones in granitic bedrock

      G K Reddy T Seshunarayana Rajeev Menon P Senthil Kumar

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      Fracture and fault networks are conduits that facilitate groundwater movement in hard-rock terrains.Soil-gas helium emanometry has been utilized in Wailapally watershed,near Hyderabad in southern India,for the detection of fracture and fault zones in a granite basement terrain having a thin regolith.Based on satellite imagery and geologic mapping,three sites were selected for detailed investigation.High spatial resolution soil-gas samples were collected at every one meter at a depth of <1.5m along 100 m long profiles (3 in number).In addition,deep shear-wave images were also obtained using the multichannel analysis of surface waves.The study clearly indicates several soil-gas helium anomalies (above 200 ppb)along the pro files,where the shear-wave velocity images also show many near-surface vertical low velocity zones.We thus interpret that the soil-gas helium anomalous zones and the vertical low-velocity zones are probable traces of fault/fracture zones that could be efficient natural recharge zones and potential groundwater conduits.The result obtained from this study demonstrates the efficacy of an integrated approach of soil-gas helium and the seismic methods for mapping groundwater resource zones in granite/gneiss provinces.

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