• ARINDAM CHAKRABORTY

      Articles written in Journal of Earth System Science

    • Orographic control of the Bay of Bengal cold pool rainfall

      P V Arushi Arindam Chakraborty Ravi S Nanjundiah

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      In boreal summer (June–September), most of the Indian land and its surroundings experience rainrates exceeding 6 mm day−1 with considerable spatial variability. Over southern Bay of Bengal (BoB) along the east coast of the Indian peninsula (henceforth referred to as the Bay of Bengal cold pool or BoB-CP), the rain intensity is significantly lower (<2 mm day−1) than its surroundings. This low rainfall occurs despite the fact that the sea surface temperature in this region is well above the threshold for convection and the mean vorticity of the boundary layer is cyclonic with a magnitude comparable to that over the central Indian monsoon trough where the rainrate is about 10 mm day−1. It is also noteworthy that the seasonal cycle of convection over the BoB-CP shows a primary peak in November and a secondary peak in May. This is in contrast to the peak in June–July over most of the oceanic locations surrounding the BoB-CP. In this study, we investigate the role of the Western Ghat (WG) mountains in an Atmospheric General Circulation Model (AGCM) to understand this paradox. Decade-long simulations of the AGCM were carried out with varying (from 0 to 2 times the present) heights of the WG. We find that the lee waves generated by the strong westerlies in the lower troposphere in the presence of the WG mountains cause descent over the BoB-CP. Thus, an increase in the height of the WG strengthens the lee waves and reduces rainfall over the BoB-CP. More interestingly in the absence of WG mountains, the BoB-CP shows a rainfall maxima in the boreal summer similar to that over its surrounding oceans. The WG also impacts the climate over the middle and high latitude regions by modifying the upper tropospheric circulation. The results of this study underline the importance of narrow mountains like the WG in the tropics in determining the global climate and possibly calls for a better representation of such mountains in climate models.

    • Evaluation of ESACCI satellite soil moisture product using in-situ CTCZ observations over India

      SHUBHI AGRAWAL ARINDAM CHAKRABORTY

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      Recently available European Space Agency Climate Change Initiative (ESACCI) soil moisture dataset, derived by merging soil moisture values calculated using measurements from satellite-based active and passive sensors, is validated over the Indian region using in-situ observations from 117 Continental Tropical Convergence Zone (CTCZ) Programme stations spread across India. The dataset is compared for the monsoon season (June–September: JJAS) of two years – 2011–2012, over six regional domains which differ in soil characteristics and mean soil moisture values, thus taking the spatial heterogeneity into account. Evaluation shows that the mean JJAS ESACCI volumetric soil moisture is $25.5% (\rm{m^{3} m^{-3}})$, with an intra-seasonal standard deviation of 6%. The root mean squared difference (RMSD) between ESACCI soil moisture product and CTCZ observations is 10% over the Indian region. Over smaller homogeneous regions, the RMSD values between the two products are smaller than 5%, except over southern India and north-east India. Overall, the ESACCI soil moisture dataset is in good agreement with the CTCZ in-situ soil moisture observations, and has relatively higher accuracy over the plains of northern and central India, as compared to other regions. However, the ESACCI soil moisture dataset shows higher intra-seasonal variability at shorter time-scale of 2–4 days, as compared to the CTCZ observations, possibly due to the difference in the soil sampling depths between the two datasets.

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