• A P DIMRI

      Articles written in Journal of Earth System Science

    • Energetics of Indian winter monsoon

      P Kumar A P Dimri

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      The Indian subcontinent is characterized by complex topography and heterogeneous land use-land cover. The Himalayas and the Tibetan Plateau are spread across the northern part of the continent. Due to its highly variable topography, understanding of the prevailing synoptic weather systems is complex over the region. The present study analyzes the energetics of Indian winter monsoon (IWM) over the Indian subcontinent using outputs of mesoscale model (MM5) forced with National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), US, initial and boundary conditions. MM5 modeling framework, designed to simulate or predict mesoscale atmospheric circulations, is having a limited-area, non-hydrostatic and terrain following 12 sigma levels. The IWMenergetics is studied using MM5 model outputs. Prior to this model’s validity and deviation from the corresponding observations (NCEP/NCAR) is assessed. The model’s overestimation/underestimation of wind, temperature and specific humidity at upper troposphere proves that the model has difficulty in picking up corresponding fields at all the model grid points because of terrain complexity over the Himalayas and Tibetan Plateau. Hence, the model fields deviate from the corresponding observations. However, model results match well with the winter global energy budget calculated using reanalysis dataset by Peixoto and Oort (1992). It suggests MM5 model’s fitness in simulating large scale synopticweather systems. And, thus the model outputs are used for calculation of energetics associated with IWM. It is observed that beyond 15◦N lower as well as upper level convergence of diabatic heating, which represents continental cooling and sinking of heat from atmosphere to land mass (i.e., surface is cooler than surrounding atmosphere) dominates. The diabatic heating divergence (cooling of continents)is found over ocean/sea and whole of the China region, Tibetan and central Himalayas (because of excess condensation than evaporation). The adiabatic generation of kinetic energy depends on the cross isobaric flow (north to south in winter, i.e., the present study shows strong circulation during IWM). It is foundthat wind divergence of model concludes lower level convergence over study region (i.e., strong winter circulation in the model fields).

    • Rainfall over the Himalayan foot-hill region: Present and future

      ARKADEB BANERJEE A P DIMRI KIREET KUMAR

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      Uttarakhand, one of the Himalayan foot-hill states of India, covers an area of $51,125 \rm{km}^{2}$. This region is enriched with bio-diversity and is one of the highly potential regions in the Central Himalayas for agroclimate, hydro power generation, food-processing, tourism, etc. Present study investigates the spatio-temporal rainfall distribution over the state during Indian summer monsoon period. Observational and modelled (under different Representative Concentration Pathways (RCPs) at radiative forcing 2.6, 4.5 and $8.5 \rm{W/m}^{2}$) rainfall distribution is studied to assess the present and future trends. Study uses standard observational rainfall estimates from APHRODITE, Tropical Rainfall Measuring Mission (TRMM 3B42) and India Meteorological Department (IMD) gridded rainfall datasets and inter-compare these products in order to Bnd out orographic responses during the monsoon months and elevation dependent mean rainfall pattern changes. It is found that rainfall pattern breaks near 3100 m elevation. Comparative analysis reflects that with respect to IMD, TRMM 3B42 rainfall underestimates more than 3 mm/day rainfall whereas, APHRODITE overestimates rainfall below 4.5 mm/day. Future trends in modelled monsoon rainfall are examined and mixed results are found and discussed with possible explanation.

    • A new Western Disturbance Index for the Indian winter monsoon

      T M MIDHUNA P KUMAR A P DIMRI

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      The Himalayas are storehouse of freshwater, which is of utmost importance for agriculture and power generation for billions of people in India. Winter (December, January and February: DJF) precipitation associated with Western Disturbances (WDs) influences Himalayan climate, glaciers, snow-water storage, etc. One-third of annual precipitation over northern Indian region is received during winter. Winter WDs are synoptic-scale systems embedded the subtropical westerly jet (SWJ). Their orographic interaction with the Himalayas intensifies precipitation over Pakistan and northern India. Precipitation due to WDs and associated dynamics are termed as Indian winter monsoon (IWM). The present study focuses on the WDs climatology using National Center for Environmental Prediction/National Center for Atmospheric Research, US (NCEP/NCAR) reanalysis data. The period of study spans over 29 years (1986–2016) during which $\sim500$ WDs were observed as per India Meteorological Department (IMD) daily weather report. Precipitation, vertical distribution of wind and geopotential height during the passage of these WDs are analyzed. Importantly, a new index, Western Disturbance Index (WDI), for measuring strength of IWM is proposed by using difference of geopotential height at 200 and 850 hPa levels. The index is able to capture changes in 500 hPa wind, air temperature and mean sea level pressure during the passage of WDs.

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