Articles written in Journal of Earth System Science

    • Uncertainties in river discharge simulations of the upper Indus basin in the Western Himalayas


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      This study focuses on understanding and quantifying uncertainties in simulating river discharge in the Upper Indus Basin (UIB) of the Western Himalayas using a macro-scale semi-distributed hydrology model forced with multiple observed precipitation datasets and reanalysis products of near-surface wind-speed, maximum and minimum temperature during 2010–2012. We performed a suite of numerical simulations using a high-resolution setup of the variable infiltration capacity (VIC) hydrology model for the UIB. This model takes into account the balance of both water and surface energy budgets within each grid cell and incorporates sub-grid variability of topography to represent the effects of orographic precipitation and temperature lapse rate essential for hydrological modelling in the complex Himalayan terrain. While river discharges over non-mountainous basins are known to be generally sensitive to precipitation variations, it is noted that both precipitation and snowmelt processes critically influence seasonal river discharge in the UIB during the northern summer through surface temperature and wind-speed variations. Our study found that a marginal difference in temperature forcing can create large difference in snowmelt over UIB during summer season, which in turn increases the uncertainty in the summer monsoon river discharge. This analysis highlights the equally important need for the incorporation of realistic temperature data as that of precipitation product for the better simulation of land surface processes during various seasons, especially during summer, over snow covered UIB. Further analysis of daily simulations of the VIC model during 2010–2012 indicates that low and medium intensity river discharges tend to be associated with relatively lower spread among the ensemble members, as compared to the high intensity discharges which exhibit large ensemble spread. In particular, we noted a large increase in the spread of high flow simulations over the UIB during the flood episodes in the summer of 2010, arising from uncertainties in the precipitation forcing across multiple datasets. Our results emphasize the need for improved representation of precipitation and hydrological processes over the Himalayan region in weather and climate models for better management of water resources and flood forecasting in the UIB region under a changing climate.

    • On the dynamics of cyclogenesis, rapid intensification and recurvature of the very severe cyclonic storm, Ockhi


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      A very severe cyclonic storm (VSCS), ‘Ockhi’ started its journey from southwest Bay of Bengal (BoB) over Comorin area (7.5$^{\circ}$N–77.5$^{\circ}$E) as a low-pressure area on 28 November, 2017. Initially it moved north-westwards across Lakshadweep Islands without hitting Tamil Nadu or Kerala coast, then moved north-eastward over the Arabian Sea and dissipated upon hitting south Gujarat coast on 06 December, 2017. Rapid intensification (RI), clockwise recurvature and a longer track were the major features of the VSCS, Ockhi. In the present study, an attempt has been made to understand the mechanism of cyclogenesis, RI and recurvature of the Ockhi using satellite and reanalysis datasets. Initially, role of easterly waves (EW) and Madden Jullian Oscillations (MJO) on the cyclogenesis of tropical cyclone Ockhi is addressed. Our analysis suggests that the EW (MJO) played a seminal (insignificant) role in preconditioning the atmosphere for the cyclogenesis of the Ockhi. Our detailed analysis using various oceanic parameters indicate that, the passage of the cyclonic storm over the regions of high thermal energy, especially warmer ocean mean temperature (OMT) at 100 m depth, was instrumental in its rapid intensification. Further, we addressed the recurving feature of the VSCS Ockhi using steering flow analysis. It is found that strong north-eastward steering winds, embedded in subtropical westerlies with deep southward extent, favoured the recurving of the Ockhi towards north-eastward by suppressing the conventional westward (north-westward) track movement.


      $\bullet$Easterly wave activity facilitated the cyclogenesis.

      $\bullet$Upper ocean thermal energy contributed in rapid intensification.

      $\bullet$Trough in upper level westerlies facilitated in recurvature of the VSCS Ockhi.

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