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  • Wiley  (51)
  • Oxford University Press  (21)
  • 2015-2019  (70)
  • 2000-2004  (2)
  • 11
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    Trends in wind-wave climate over the head Bay of Bengal region (2016)
    Wiley
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    Publication Date: 2016-01-22
    Description: ABSTRACT The average population density is significantly higher along the nearshore coastal zones compared to inland areas. Therefore, proper assessment and evaluation of the impact of climate change on coastal belts has immense societal benefits. The head Bay region located in the north Indian Ocean is a thickly populated area and highly vulnerable to threats from sea-level rise and extreme weather events. In this study, the wave climatology for the north Indian Ocean specifically covering the head Bay region was examined, utilizing the past 21 years of satellite altimeter data for the period ranging from 1992 to 2012. In addition, the study also examines the significant wave heights obtained from the WAVEWATCH-III (WW3) model to substantiate and evaluate the findings of the observed variability from altimeter records. The study used daily altimeter data from eight satellite missions to understand the annual and seasonal variability in wind speed and significant wave heights for the head Bay region. The annual distribution of these parameters follow the climatology, whereas the percentage variability in both wind speed and significant wave height show a clear contrasting trend exhibiting a zonal dipole. The study establishes the fact that the responsible mechanism for this contrasting trend is the variations in mean sea level pressure over the head Bay region. In addition, a comprehensive analysis using empirical orthogonal function (EOF) shows that the second mode that represents the inter-seasonal variations substantiates the trend observed in the percentage distribution of these parameters. Interestingly, the study clearly signifies that trends in both wind speed and significant wave height was lower for the western side, unlike that noticed over the eastern portions in the head Bay of Bengal region.
    Print ISSN: 0899-8418
    Electronic ISSN: 1097-0088
    Topics: Geosciences , Physics
    Published by Wiley
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  • 12
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    Coincident airglow, VHF radar and ionosonde observations of electrified medium‐scale traveling ionospheric disturbances in the equatorial latitudes (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract The occurrence of late night spread F in the low and equatorial latitude regions during solar minimum is not properly understood. Radar observations have revealed occurrence of postmidnight echoes at low and equatorial latitudes during solar minimum periods. This work discusses the coordinated airglow, VHF radar and ionosonde observations of nighttime electrified medium scale traveling ionospheric disturbances (EMSTIDs) made from the low and equatorial latitude Indian region. Two nights of observations are discussed in detail in which EMSTIDs are seen to coexist with spread F in ionograms and VHF backscatter. These observations reveal that the EMSTIDs propagate to latitudes very close to dip equator at times and they can be responsible for a reasonable portion of the postmidnight radar echoes and spread F events observed at low and equatorial latitudes during low solar activity period.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 13
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    Dynamic probabilistic hazard mapping in the Long Valley Volcanic Region CA: integrating vent opening maps and statistical surrogates of physical models of pyroclastic density currents (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Ideally, probabilistic hazard assessments combine available knowledge about physical mechanisms of the hazard, data on past hazards, and any precursor information. Systematically assessing the probability of rare, yet catastrophic hazards adds a layer of difficulty due to limited observation data. Via computer models, one can exercise potentially dangerous scenarios that may not have happened in the past, but are probabilistically consistent with the aleatoric nature of previous volcanic behavior in the record. Traditional Monte Carlo based methods to calculate such hazard probabilities suffer from two issues: they are computationally expensive, and they are static. In light of new information newly available data, signs of unrest, new probabilistic analysis describing uncertainty about scenarios the Monte Carlo calculation would need to be redone under the same computational constraints. Here we present an alternative approach utilizing statistical emulators that provides an efficient way to overcome the computational bottleneck of typical Monte Carlo approaches. Moreover, this approach is independent of an aleatoric scenario model, yet can be applied rapidly to any scenario model making it dynamic. We present and apply this emulator‐based approach to create multiple probabilistic hazard maps for inundation of pyroclastic density currents in the Long Valley Volcanic Region. Further we illustrate how this approach enables an exploration of the impact of epistemic uncertainties on these probabilistic hazard forecasts. Particularly, we focus on the uncertainty of vent opening models and how that uncertainty both aleatoric and epistemic impacts the resulting probabilistic hazard maps of pyroclastic density current inundation.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 14
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    Evaluating Simulations of Interhemispheric Transport: Interhemispheric Exchange Time Versus SF6 Age (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Two recent studies using sulfur hexafluoride (SF6) observations to evaluate interhemispheric transport in two different ensembles of atmospheric chemistry models reached different conclusions on model performance. We show here that the different conclusions are due to the use of different metrics and not differences in the performance of the models. For both model ensembles, the multimodel mean interhemispheric exchange time τex agrees well with observations, but in nearly all models the SF6 age in the southern hemisphere is older than observed. This occurs because transport from the northern extratropics into the tropics is too slow in most models, and the SF6 age is more sensitive to this bias than τex. Thus, simulating τex correctly does not necessarily mean that transport from northern midlatitudes into the southern hemisphere is correct. It also suggests that more attention needs to be paid to evaluating transport from northern midlatitudes into the tropics.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 15
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    Vertical Coupling From the Lower Atmosphere to the Ionosphere: Observations Inferred From Indian MST Radar, GPS Radiosonde, Ionosonde, Magnetometer, OLR (NOAA), and SABER/TIMED Instrument Over Gadanki (2019)
    Wiley
    Details
    Publication Date: 2019
    Description: Abstract Simultaneous wind observations from Mesosphere Stratosphere Troposphere (MST) radar collectively with Global Positioning System (GPS) radiosonde over Gadanki, covering altitude range of 3.6–20 km (January–December 2009; 365 days), divulge the propagation of lower atmospheric waves up to the ionosphere. It is combined with temperature data (20–110 km) observed from Sounding of the Atmosphere using the Broadband Emission Radiometry (SABER) instrument onboard Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite, ionosonde observations, and daily Outgoing Longwave Radiation (OLR) data acquired from the National Ocean and Atmospheric Administration (NOAA; centered around Gadanki [13.5°N, 79.2°E]) for the same time period. Long‐period oscillations with periodicities of ~64, ~32, and ~21 days are witnessed along with the well‐known oscillations of ~16, ~6.4, and ~5.3 days. Most of the long‐period oscillations are dominantly perceived during the summer months (April–June 2009), which can even exist up to September. These long‐period oscillations are found to propagate from lower tropospheric heights up to ionospheric heights with large vertical wavelengths (~300–400 km, in some cases) near to transition zones of atmospheric layers (e.g., tropopause, stratopause, and mesopause). Signatures of vertical coupling of atmosphere through large vertical wavelengths (indicating possible intraseasonal connections) is clearly observed in the equatorial electrojet current and the peak plasma frequencies of ionospheric layers (E and F regions). Noticeable reduction of the wave oscillations in spatial scale with upsurge in spatial damping is evidently visible, in the tropical stratosphere and mesosphere, which can be attributed to stratospheric ozone.
    Print ISSN: 2169-9380
    Electronic ISSN: 2169-9402
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 16
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    On the seasonal variations of reflectivity and turbulence characteristics of low latitude mesospheric echoes over Gadanki (2016)
    Wiley
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    Publication Date: 2016-05-24
    Description: Gadanki radar observations of the low latitude mesospheric echoes studied earlier have shown that while both occurrence rate and signal-to-noise ratio (SNR) of the mesospheric echoes peak in the equinoxes turbulent kinetic energy (TKE) dissipation rate and eddy diffusivity, estimated using spectral width of these echoes, peak in the summer. This seasonal difference is apparently inconsistent with the understanding that the mesospheric echoes are generated by turbulence. In this paper, we analyze Gadanki radar observations of mesospheric echoes made during 2011 and 2012 and study seasonal variations in reflectivity and TKE dissipation rate in an attempt to address the aforementioned puzzle. We show that both reflectivity and TKE dissipation rate in the mesosphere show semiannual variations peaking in the equinoxes, which are vastly different from those reported earlier. We also show that seasonal variations in reflectivity and TKE dissipation rate have a close correspondence with gravity wave activity. These results are found to be consistent with the gravity wave breaking hypothesis generating turbulence and radar echoes in the low latitude mesosphere.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 17
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    Dual Band Resonance in Tetragonal BaTiO3/NBR Composites for Microwave Absorption Applications (2016)
    Wiley
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    Publication Date: 2016-05-24
    Description: Tetragonal BaTiO 3 bulk samples are prepared using the solid-state route in conjunction with intermediate high-temperature annealing steps. The (002) and (200) X-ray diffraction peaks near 2Ɵ~45° and 310, 520, and 720 cm −1 characteristic vibrational modes in Raman spectroscopic measurements confirm the tetragonal crystallographic structure of BaTIO 3 bulk samples. The 1100°C annealed BaTiO 3 sample showed optimal tetragonality ~1.016 and the same is used for BaTiO 3 –acrylonitrile butadiene rubber (NBR) composites at different BaTiO 3 loading fractions in parts per hundred (PHR). These BaTiO 3 /NBR composite systems exhibit dual band microwave resonance, widening the operating window for microwave absorption applications. Eighty PHR BaTiO 3 /NBR composite exhibits microwave reflection losses (RL) at 9.5 and 16.5 GHz with ~−9 and ~−18 dB reflection losses, respectively. The onset of dual band is attributed to the ferroelectric-induced dipolar relaxation at 9.5 GHz and its second-order resonance at 16.5 GHz in such composite systems.
    Print ISSN: 0002-7820
    Electronic ISSN: 1551-2916
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 18
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    Highly localized unique electrodynamics and plasma irregularities linked with the 17 March 2015 severe magnetic storm observed using multi-technique common-volume observations from Gadanki, India (2016)
    Wiley
    Details
    Publication Date: 2016-11-06
    Description: In this paper we study equatorial electrodynamics and plasma irregularities linked with the 17 March 2015 severe magnetic storm in the Indian sector using common volume observations made by the Gadanki Ionospheric Radar Interferometer, airglow imager, digisonde and GPS receiver established at Gadanki (13.5 o N, 79.2 o E). Observations show that with the initiation of the storm at ~6 UT on 17 March, which happened to be midday in the Indian sector, the low latitude ionosphere responded in tune with the storm induced electric field and by the sunset time the base of the F layer ascended to an altitude of 470 km with a peak upward velocity of 50 m s -1 eventually manifesting equatorial plasma bubble and irregularities causing strong GPS scintillation. The most important finding found in this study is the confinement of plasma bubble and irregularities in a narrow longitude zone of 69 o -98 o E. Results also show reversal of zonal drift of the irregularities from ~120 m s -1 eastward drift to ~120 m s -1 westward drift in a time span of ~30 min. Both observations are shown to be linked with very special electrodynamical conditions induced by the magnetic storm related electric field in the dusk sector. Intriguing details of the longitudinally localized electrodynamics and plasma irregularities are discussed in terms of prompt penetration and disturbed dynamo electric field effects.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 19
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    Unexpected characteristics of the 150 km echoes observed over Gadanki and their implications (2016)
    Wiley
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    Publication Date: 2016-10-27
    Description: Recent discovery of two distinct types of 150 km echoes, namely type-A and type-B, and subsequent progress in the large-scale kinetic simulation of photoelectron induced plasma waves have begun a new era in resolving the five decades long 150 km echoing riddle. In this paper, we present hitherto unrevealed three important and unexpected findings on the two distinct types of 150 km echoes based on Gadanki radar observations. Our observations show unexpected predominance of type-A echoes, strong seasonal dependence of both type-A and type-B echoes, and a surprising connection of the type-B echoes to the unusually deep solar minimum of 2008-2009. We discuss how these results provide important new clues in tethering the competing processes involved in the daytime 150 km echoes and have significance in the recently proposed photoelectron-induced plasma fluctuations as a potential mechanism for the 150 km echoes.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 20
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    Characterization of the gluconate utilization system of Vibrio cholerae with special reference to virulence modulation (2016)
    Oxford University Press
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    Publication Date: 2016-11-04
    Description: Orthologs search identified that the Vibrio cholerae gluconate (Gnt) utilization system minimally consisted of the Entner–Doudoroff (ED) pathway ( edd and eda ) and three other genes, namely gntU , gntK and gntR . This system appeared unique by genomic organization of component genes into two operons transcribed in opposite directions. In silico analysis indicated GntU as an inner-membrane protein functioning for transport and GntK as a kinase with cytosolic localization that generates Gnt6P, which is then metabolized through the ED pathway. Enzyme 6-phosphogluconate dehydratase encoded by edd converts Gnt6P to 2-keto-3-deoxy-6-phosphogluconate (KDPG), which is metabolized by the action of KDPG-aldolase encoded by eda . Transcriptional upregulation of the Gnt utilization genes in the gntR mutant matched well to a predicted repressor role of GntR. GntR displayed DNA binding to a region in the promoters of two bi-directionally transcribed operons. Growth defect of mutants in Gnt-supplemented media confirmed obligate involvement of these genes in Gnt utilization and such defect was restored upon complementation. Defective Gnt utilization resulted in attenuation of colonization potential and reduction of cholera toxin secretion in V. cholerae . The ED pathway mutants showed the highest level of virulence attenuation. Overall, this study established a minimal requirement of the V. cholerae Gnt utilization system, which played a critical role in pathogenesis.
    Print ISSN: 0928-8244
    Topics: Biology , Medicine
    Published by Oxford University Press
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