• DEBASHIS MITRA

      Articles written in Journal of Earth System Science

    • Trend analysis of atmospheric temperature, water vapour, ozone, methane and carbon-monoxide over few major cities of India using satellite data

      POOJA JINDAL PRADEEP KUMAR THAPLIYAL MUNN VINAYAK SHUKLA SOM KUMAR SHARMA DEBASHIS MITRA

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      In this study, decadal trend analysis of atmospheric temperature, water vapour, ozone, methane and carbon-monoxide has been presented over few major cities of India using Aqua-AIRS products from 2003 to 2012. The atmospheric column is studied in few atmospheric layers, viz., surface-850, 850–500, 500–100, 100–50 and 50–1 hPa for temperature, water vapour and ozone. However, $\rm{CH_{4}}$ and $\rm{CO}$ results are presented in total column amounts. Non-parametric Mann–Kendall test has been applied to investigate the trends of annual means of parameters and Sen’s slope estimate has been used to find the rate of the change, if there is a trend. The layer average temperature (LAT) has been found to be increasing in lower troposphere (surface-850 hPa) and decreasing in lower stratosphere (100–50 hPa). The warming trend over Chennai is found to be not limited in lower tropospheric region, but extended in 850–500 hPa layer also. However, LAT(850–500 hPa) has decreasing trend over Thiruvananthapuram. LAT in 500–100 hPa has significant decreasing trend only over Ahmedabad. The decreasing LAT trend in 100–50 hPa is quite prominent with significant decreasing trends over Mumbai, Ahmedabad, Kolkata and Hyderabad. The layer integrated water vapour (LIWV) is found be increasing mainly in surface-850 hPa and 850–500 hPa layers. The decreasing trend of LIWV has been observed only over Ahmedabad in 500–100 hPa layer. For total column water vapour, the trends are mostly increasing, however, it is statistically significant only over Hyderabad. The layerintegrated ozone has been found to be increasing in troposphere and decreasing in lower stratosphere. The increasing trend of ozone in troposphere is most prominent in lower-mid tropospheric region (850–500 hPa layer). No significant trend has been observed for total column ozone. Total column methane has shown significant increasing trend over all cities with very good significance level. However, for total column carbonmonoxide, the trends are decreasing and the decreasing trends are significant over Delhi and Mumbai.

    • Detection and tracking of tropical cyclone using NCEP-GFS model analysis and forecasts

      SANJEEV KUMAR SINGH CHARU SINGH DEBASHIS MITRA

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      The techniques based on wind pattern matching index (WPMI) and pressure centroid have been utilised to identify and track the tropical cyclone (TC) using National Centers for Environmental Prediction- Global Forecast System (NCEP-GFS) model analysis and forecasts. For the WPMI, a threshold value is used to detect the center of the cyclonic storm. The threshold value is determined by maximising the probability of detection (POD) and minimising the false alarm ratio (FAR). The POD and FAR are computed using 226 cases of cyclonic conditions that include 11 TCs and 300 cases of non-cyclonic conditions during the period 2015–2017. Low-level (at 850 hPa) wind fields and surface pressure from GFS analysis as well as forecasts have been used to detect the center location of analysed cyclones. The pressure centroid is estimated within a circumference that covers the TC inner core. The detected center position is used to generate 6-hourly track up to 120 h forecasts for 15 cyclones, which formed during the period 2018–2020 and subsequent to this track forecast, errors obtained from both the schemes are estimated with respect to the best track of cyclone provided by India Meteorological Department. The performance of both the schemes has been examined and based on the averaged error analysis of 15 cyclones, it is noted that the track forecast produced from the WPMI scheme was improved by 0.8–21.8% in 12–120 h forecasts with respect to the pressure centroid method. In addition, landfall error for all the cyclones has also been calculated and verified against observations. It was found that the WPMI scheme is able to predict the landfall position with least error.

    • Reflectance spectroscopic and geochemical characteristics of hydrocarbon microseepage-induced sediments from Assam–Arakan Fold Belt, India: Implications to hydrocarbon exploration

      SANTOSH GARAIN DEBASHIS MITRA PRANAB DAS

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      Characterizations of hydrocarbon microseepage-induced surface sediments are essential to utilize remote sensing and geochemistry as effective petroleum exploration tools. However, only few researchers have attempted spectroscopic and geochemical characterizations of microseepage-affected sediments and that too are focussed on arid or semi-arid climatic regions. This article aims to delineate the spectral, mineralogical and geochemical characteristics of the microseepage-affected sediments in Assam–Arakan Fold Belt (AAFB), NE India, dominated by intense precipitation. The analytical approach followed consists of diffuse reflectance spectroscopy, X-ray diffraction (XRD), X-ray Fluorescence (XRF), and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analyses. ReCectance spectroscopy reveals that microseepage-induced sediments are enriched in illitic clay and deficient in goethite than the unaffected regions. XRD studies support the presence of illitic clays in the sediments. Major element analyses indicate higher Al$_2$O$_3$ and K$_2$O and lower Na$_2$O and SiO$_2$ in the altered sediments. Trace element patterns point out enrichment of V, Cu, Zn, Ga, Zr, and Mo and depletion of Li, Cr, Co, Ni, Rb, Sr, Sc, and Y in the microseepage-affected sediments. Normalized rare earth element (REE) patterns are similar for both the altered and unaltered sediments, but the former are deficient in REE contents. Though based on limited sample analysis, the study indicates microseepage affected and unaffected sediments differ in spectral and geochemical characters. This may help in strengthening the microseepage model for hydrocarbons in high precipitation areas and thereby may help in reducing exploration risks in the basin.

      $\bf{Highlights}$

      $\bullet$ The study uses spectroscopy and geochemistry to characterize hydrocarbon microseepage-induced sediments.

      $\bullet$ Spectroscopic study indicates abundance of clay minerals and deficiency of ferric iron in microseepage-affected sediments.

      $\bullet$ Geochemical analysis shows minor compositional differences between microseepage-affected and unaffected sediments in terms of major and trace element distribution patterns.

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