Browsing by Author "S.K. Dash"

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A new phase transition in supercooled TBDA probed by low-temperature Raman study
(John Wiley and Sons Ltd, 1999) S.K. Dash; Ranjan K. Singh; B.P. Asthana; P.R. Alapati; A.L. Verma
Raman spectra of terephthalidenebis-p-n-decylaniline (TBDA) were recorded in the regions 925-1000 and 1120-1220 cm-1 from room temperature down to 20 K during both cooling and heating cycles. The subtle changes in the spectral features of the bands at ca 975 and ca 1195 cm-1 at 47 K were attributed to a hitherto unreported stable-metastable phase transition in TBDA. The dynamics of the new phase transition are explained in terms of splitting of the non-planar mode at ca 975 cm-1 due to strong steric hindrances caused by increased intermolecular interactions due to close packing in the low-temperature phase. Copyright © 1999 John Wiley & Sons, Ltd.
Projected Change and Variability Assessment of Indian Summer Monsoon Precipitation in South Asia CORDEX Domain Under High-Emission Pathway
(Birkhauser, 2020) P.K. Rai; G.P. Singh; S.K. Dash
The regional climate model version 4 (RegCM4) is analyzed in this study to assess Indian summer monsoon precipitation (ISMP) over six homogeneous precipitation regions and various meteorological subdivisions of India embedded therein during a reference period (1976–2005) and mid- (2031–2060) and far-future (2070–2099) periods under the RCP8.5 scenario over the South Asia CORDEX domain. A Coupled Model Intercomparison Project (CIMIP5) global model GFDL-ESM2M provides initial and boundary conditions to the RegCM4 under the high-emission scenario RCP8.5. RegCM4 precipitation fields are validated against observed India Meteorological Department (IMD) and Asian Precipitation-Highly Resolved Observational Data Integration Toward Evaluation (APHRODITE) precipitation datasets, while wind and specific humidity fields obtained from RegCM4 are validated against National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis and the parent GFDL model integrated fields. Model comparisons indicate that RegCM4 captures the regional characteristics of ISMP satisfactorily in terms of biases, trends, interannual variability, and circulation patterns. RegCM4 precipitation fields show high correlations of 0.9 and 0.8 with those of IMD and APHRODITE, respectively, and RegCM shows better skill in comparison with GFDL over about 68% of meteorological subdivisions. RegCM4 projects increases in precipitation by about 15.3% (28.4%), 5.1% (16.2%), and 5.4% (18.4%) respectively over the Northwest (NW), Northeast (NE) and Hilly Regions (HR) in the mid-(far-)future but decreases in precipitation over the Peninsular (PE) region by about −1.5% (−15.7%) during the same period. A similar precipitation change pattern is also found when analyzing the probability distribution functions (PDFs) over the same regions. The precipitation intensity (95th percentile) shows increases above 40% over numerous subdivisions of the West Central (WC), NW, and HR regions. The present analysis also reveals significant increases of more than 50% in mean precipitation over several meteorological subdivisions. Analysis of the circulation fields depicts a northward shift of the high-precipitation belt while high-pressure systems dominate the peninsular region when approaching the central India global warming scenario. It is interesting to note that the extreme precipitation index Rx5day exactly follows the pattern of projected increase in mean precipitation. In addition, it is noted that the projected variability and change in the mean precipitation are less frequent than for RX5day, while a consistently stronger spread in variability is projected in the mid- to far-future under the warming scenario. © 2019, Springer Nature Switzerland AG.
Projected changes in extreme precipitation events over various subdivisions of India using RegCM4
(Springer, 2020) P.K. Rai; G.P. Singh; S.K. Dash
Present study attempts to project extreme precipitation indices over 34 different meteorological subdivisions and six homogeneous regions such as Northwest, Central Northeast, Northeast, West Central, Peninsular India and Hilly Region during summer monsoon season in the twenty-first century. For this purpose, the Regional Climate Model version4 (RegCM4) had been run at 50 km horizontal resolution forced with the global model GFDL-ESM2 M, during reference period 1976–2005 for the model validation, and the mid- (2031–2060) and far-future (2070–2099) for projections under RCP8.5 scenario over the South Asia CORDEX domain. In this paper, model simulated precipitation has been validated against IMD, APHRODITE and NCEP/NCAR data sets. The results indicate that RegCM4 captures the important features of seasonal precipitation and various extreme indices over the study area. The RegCM4 has projected an increase in the mean seasonal precipitation by 0.56 mm/day whereas in case of GFDL model the rate is 0.39 mm/day during the far-future relative to the reference period. The heavy precipitation indices are projected to increase more frequently (0.264/decade) than the mean precipitation rate (0.01/decade) over India. The correlations between the extreme precipitation indices and the seasonal mean precipitation are found to be strong. In addition, the consecutive dry days are projected to occur more frequently (3–5 days) over West Central (Telangana, Vidarbha and Marathwada) and west Rajasthan while consecutive wet days are projected to decrease over larger parts of India during far-future. Similarly, 1 day maximum precipitation and the simple daily intensity index are projected to increase consistently from mid- to far- futures over some sub-divisions of West coast, Hilly and Northeast regions. From a spatial probability perspective, model projection indicates more frequent severe drought and flood conditions over India. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
Statistical Method of Forecasting of Seasonal Precipitation over the Northwest Himalayas: North Atlantic Oscillation as Precursor
(Birkhauser, 2020) Usha Devi; M.S. Shekhar; G.P. Singh; S.K. Dash
Dynamical and Statistical models are operationally used by Snow and Avalanche Study Establishment (SASE) for winter precipitation forecasting over the Northwest Himalayas (NWH). In this paper, a statistical regression model developed for seasonal (December–April) precipitation forecast over Northwest Himalaya is discussed. After carrying out the analysis of various atmospheric parameters that affect the winter precipitation over the NWH two parameters are selected such as North Atlantic Oscillation (NAO) and Outgoing Long wave Radiation (OLR) over specific areas of North Atlantic Ocean for the development of statistical regression model. A set of 27 years (1990–1991 to 2016–2017) of observed precipitation data and parameters (NAO and OLR) are utilized. Out of 27 years of data, first 20 years (1990–1991 to 2009–2010) are used for the development of regression model and remaining 7 years (2010–2011 to 2016–2017) are used for the validation purpose. Precipitation over NWH mainly associated with Western Disturbances (WDs) and the results of the present study reveal that NAO during SON has negative relationship with WDs and also with the winter precipitation over same region. Quantitative validation of the multiple regression model, result shows good Skill Score and RMSE-observations standard deviation ratio (RSR) which is 0.79 and 0.45 respectively and BIAS − 0.92. © 2020, Springer Nature Switzerland AG.
Study of a high-temperature Raman-sensitive mode of TBDA at the solid-smectic G transition
(John Wiley and Sons Ltd, 2000) Ranjan K. Singh; S.C. Singh; S.K. Dash; B.P. Asthana; A.L. Verma
Raman spectra of terephthal-bis-p-n-decylaniline (TBDA) were recorded in the wavenumber region 900-1050 cm-1 at room temperature and higher temperatures up to 473 K. The Raman-sensitive mode at ca 975 cm-1 splits into two bands, the new band appearing at ca 968 cm-1. The variation of the deconvoluted linewidths of these two bands with temperature exhibits clearly a phase transition at 346 K. The dynamics of the phase transition are discussed in terms of large-amplitude vibration of the alkyl tail causing a strain on the core consisting of three staggered benzene rings and hence reduces the inter-planar spacing. The study reveals that the selection of a suitable Raman mode to characterize and study a phase transition is crucial. Copyright (C) 2000 John Wiley and Sons, Ltd.
Variability of Diurnal Temperature Range During Winter Over Western Himalaya: Range- and Altitude-Wise Study
(Birkhauser Verlag AG, 2018) M.S. Shekhar; Usha Devi; S.K. Dash; G.P. Singh; Amreek Singh
The current trends in diurnal temperature range, maximum temperature, minimum temperature, mean temperature, and sun shine hours over different ranges and altitudes of Western Himalaya during winter have been studied. Analysis of 25 years of data shows an increasing trend in diurnal temperature range over all the ranges and altitudes of Western Himalaya during winter, thereby confirming regional warming of the region due to present climate change and global warming. Statistical studies show significant increasing trend in maximum temperature over all the ranges and altitudes of Western Himalaya. Minimum temperature shows significant decreasing trend over Pir Panjal and Shamshawari range and significant increasing trend over higher altitude of Western Himalaya. Similarly, sunshine hours show significant decreasing trend over Karakoram range. There exists strong positive correlation between diurnal temperature range and maximum temperature for all the ranges and altitudes of Western Himalaya. Strong negative correlation exists between diurnal temperature range and minimum temperature over Shamshawari and Great Himalaya range and lower altitude of Western Himalaya. Sunshine hours show strong positive correlation with diurnal temperature range over Pir Panjal and Great Himalaya range and lower and higher altitudes. © 2018, Springer International Publishing AG, part of Springer Nature.