• Volume 118, Issue 3

      June 2009,   pages  181-272

    • Equatorial modes observed in atmospheric variables

      L Kiranmayi G S Bhat

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      Wavenumber –frequency spectral analysis of different atmospheric variables has been carried out using 25 years of data.The area considered is the tropical belt 25°S $–$25°N. A combined FFT- wavelet analysis method has been used for this purpose.Variables considered are outgoing long- wave radiation (OLR),850 hPa divergence,zonal and meridional winds at 850,500 and 200 hPa levels,sea level pressure and 850 hPa geopotential height.It is shown that the spectra of different variables have some common properties,but each variable also has few features different from the rest. While Kelvin mode is prominent in OLR and zonal winds,it is not clearly observed in pressure and geopotential height fields;the latter two have a dominant wavenumber zero mode not seen in other variables except in meridional wind at 200 hPa and 850 hPa divergences.Different dominant modes in the tropics show significant variations on sub-seasonal time scales.

    • A diagnosis of rainfall over South America during 1997/98 El Niño and 1998/99 La Niña events: Comparison between TRMM PR and GPCP rainfall estimates

      Sergio H Franchito V Brahmananda Rao Ana C Vasques Clovis M E Santo Jorge C Conforte

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      A comparison between TRMM PR rainfall estimates and rain gauge data from ANEEL and combined gauge/satellite data from GPCP over South America (SA)is made.In general,the annual and seasonal regional characteristics of rainfall over SA are qualitatively well reproduced by TRMM PR and GPCP.It is found that over most of SA GPCP exceeds TRMM PR rainfall.The largest positive differences between GPCP and TRMM PR data occur in the north SA,northwestern and central Amazonia.However,there are regions where GPCP rainfall is lower than TRMM PR,particularly in the Pacific coastal regions and in southern Brazil.We suggest that the cause for the positive differences GPCP minus TRMM PR rainfall are related to the fact that satellite observations based on infrared radiation and outgoing longwave radiance sensors overestimate convective rainfall in GPCP and the cause for the negative differences are due to the random errors in TRMM PR.Rainfall differences in the latter phases of the 1997/98 El Ni ˜no and 1998/99 La Ni ˜na are analyzed.The results showed that the rainfall anomalies are generally higher in GPCP than in TRMM PR,however,as in the mean annual case,there are regions where the rainfall in GPCP is lower than in TRMM PR.The higher positive (negative)differences between the rainfall anomalies in GPCP and TRMM PR,which occur in the central Amazonia (southern Brazil),are reduced (increased) in the El Ni ˜no event.This is due to the fact that during the El Ni ˜no episode the rainfall decreases in the central Amazonia and increases in the southern Brazil.Consequently,the overestimation of the convective rainfall by GPCP is reduced and the overestimation of the rainfall by TRMM PR is increased in these two regions,respectively.

    • Quasi-periodic VLF emissions observed during daytime at a low latitude Indian ground station Jammu

      K K Singh J Singh R P Patel A K Singh R P Singh Rejesh Singh P A Ganai

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      This paper reports quasi-periodic pulsing hiss emissions recorded during daytime in the frequency range of 50 Hz –15 kHz at low latitude station Jammu (geomag.lat.=22° 26′N; 𝐿 =1.17). It is noted that pulsing VLF emissions are a rare phenomena at low latitudes.The various spectrograms of pulsing VLF hiss emissions presented in this paper clearly show band limited spectrums regularly pulsing with almost equal period of the order of few seconds in the frequency range of ∼3-8 kHz. Generation and propagation mechanism of these emissions are briefly discussed.

    • Appraisal of groundwater resources in an island condition

      N C Mondal V S Singh D V Sarwade M V Nandakumar

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      A group of 36 coral islands is being scattered in the Arabian Sea of the western coast of India. On such islands,groundwater is the only source of fresh water for the islanders.The demand for groundwater is increasing every year due to growing population and urbanization.On the other side the peculiar hydrologic,geologic and geomorphic features restrict the availability of groundwater. Thus a proper understanding of the groundwater condition is important in order to meet this increasing demand and also to formulate future development and management strategies.Detailed hydrogeological,geophysical and hydrochemical studies had been carried out to identify potential fresh groundwater resources and quantify vulnerable parts of Andrott Island,Union Territory of Lakshadweep.Systematic collection and analysis of hydrological,geophysical and hydrochemical data gives an early signal of deterioration in groundwater quality in the peripheral parts of eastern and western coasts of this island and it suggests immediate measures for arresting the deterioration in groundwater quality as well as augmentation for restoration of aquifer in some parts of the island.

    • Aquifer characteristics and its modeling around an industrial complex, Tuticorin, Tamil Nadu, India: A case study

      N C Mondal V S Singh R Rangarajan

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      Anthropogenic pollution of shallow groundwater resources due to industrial activities is becoming a cause of concern in the east coastal belt of the state of Tamil Nadu,India.Integrated hydrogeological,geophysical and tracer studies were carried out in the coastal region encompassing an industrial complex.The objective has been to gain knowledge of aquifer characteristics,ascertaining groundwater movement and its flow direction,which would in turn reveal the possibility of contamination of groundwater regime and its better management.The results of multi-parameters and model study indicate that the velocity of groundwater flow ranges from 0.013 m/d to 0.22 m/d in and around the industrial complex in upstream western part of the catchment and 0.026 m/d to 0.054 m/d in the downstream eastern part,near the coast.These parameters are vital for the development of groundwater management scheme.

    • Tectonic controls on the geomorphic evolution of alluvial fans in the Piedmont Zone of Ganga Plain, Uttarakhand, India

      Pradeep K Goswami Charu C Pant Shefali Pandey

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      The Piedmont Zone is the least studied part of the Ganga Plain.The northern limit of the Piedmont Zone is defined by the Himalayan Frontal Thrust (HFT)along which the Himalaya is being thrust over the alluvium of the Ganga Plain.Interpretation of satellite imagery,Digital Terrain Models (DTMs)and field data has helped in the identification and mapping of various morpho-tectonic features in the densely forested and cultivated Piedmont Zone in the Kumaun region of the Uttarakhand state of India.The Piedmont Zone has formed as a result of coalescing alluvial fans,alluvial aprons and talus deposits.The fans have differential morphologies and aggradation processes within a common climatic zone and similar litho-tectonic setting of the catchment area. Morphotectonic analysis reveals that the fan morphologies and aggradation processes in the area are mainly controlled by the ongoing tectonic activities.Such activities along the HFT and transverse faults have controlled the accommodation space by causing differential subsidence of the basin,and aggradation processes by causing channel migration,channel incision and shifting of depocentres.The active tectonic movements have further modified the landscape of the area in the form of tilted alluvial fan,gravel ridges,terraces and uplifted gravels.

    • Tectonomagnetic study in the seismoactive area of Narmada–Son lineament, central India: Preliminary results on repeat field observations

      S Y Waghmare S D Pimprikar P B Gawali L Carlo A G Patil

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      Repeated measurements of the total geomagnetic field on the five profiles have revealed a picture of stress-induced tectonomagnetic effect in the form of secular variation of the total geomagnetic field in the tectonically and seismically active area of Jabalpur and adjoining areas of the Narmada-Son lineament (NSL),central India.For this experiment,a reference base station was established within the study area at Jabalpur.Using proton precession magnetometers with a sensitivity of 0.1 nT, simultaneous measurements of total geomagnetic field were made annually at the base and all field stations.Five cycles of repeated observations have been performed between 2003 and 2007.For data analysis,a difference method has been applied and the residuals have been calculated as secular variations of the total geomagnetic field with values ranging from ± 0.1nT to about > ± 14.6$nT/yr$ over the different stations.The anomalies in secular variation of the total geomagnetic field may be related to anomalous accumulation of tectonic stresses and tensions on the deep fault zones and crustal blocks due to recent geodynamic processes and active geological inhomogeneities in the NSL.

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