• A K MITRA

      Articles written in Journal of Earth System Science

    • Assessment of NEMO simulated surface current with HF radar along Andhra Pradesh coast

      IMRANALI M MOMIN A K MITRA BHATLA R

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      Simulating upper layer of Bay of Bengal through three-dimensional ocean circulation models is a challenging task. In this study, the surface current from the Nucleus European Modelling of Ocean (NEMO) based global ocean assimilation system is assessed against the high frequency (HF) radar data along the Andhra Pradesh coast on a daily scale during southwest monsoon 2016. The temporal variation of NEMO simulated surface current with HF radar data shows that the NEMO model captures the zonal current better than the meridional current. Both NEMO and HF radar show that the mean surface current average over latitude (15.8$^{\circ}$–16.3$^{\circ}$N) is westward for zonal surface current and southward for meridional current with maximum at 40–60 km away from the coast. Further, the monthly mean HF radar derived surface current indicates the strong south-westward flow of surface current dominated during July 2016 with speed more than 50 cm/s which is also well simulated by NEMO analysis. It also captures the cold core eddy during 15–25 July 2016 with very small north-eastward shift with respect to HF radar. The scatter plot of collocated surface zonal and meridional current average over the box (81.5–82.5$^{\circ}$E; 15.5–16.5$^{\circ}$N) clearly shows that NEMO analysis has the correlation of more than 0.5 for both zonal and meridional current.

      $\bf{Highlights}$

      $\bullet$ The simulation of upper layer of Bay of Bengal (BoB) through three dimensional ocean circulation models is a challenging task. In this study, the surface current from the high resolution NEMO based global ocean assimilation system is compared against the observed High Frequency (HF) radar data along the Andhra Pradesh Coast during the southwest monsoon 2016.

      $\bullet$ NEMO analysis captures the mean and variability of surface current very well with HF radar. However, it underestimates the mean surface current which may be due to coarser model resolution and complex non-linear processes in the coastal region.

      $\bullet$ The strong cold core eddy during 15–25 July 2016 is observed along the coast which is well simulated in NEMO model with small north-eastward shift with respect to HF radar.

      $\bullet$ The scatter plot of collocated surface current from NEMO analysis and HF radar data average over the Andhra Pradesh Coast (APCO; 81.5–82.5$^{\circ}$E; 15.5–16.5$^{\circ}$N) clearly shows that NEMO analysis has the correlation of more than 0.5 for surface current.

    • Impact of locally modified cloud microphysics over Tibetan plateau on the Indian summer monsoon

      A JAYAKUMAR SAJI MOHANDAS JOHN P GEORGE A K MITRA E N RAJAGOPAL

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      Tibetan Plateau (TP), a high elevation region in the Asian subcontinent, play an influential role in the Indian summer monsoon. In this numerical model study, sensitivity to the local changes in the microphysics over Tibet on the model forecast of circulation and precipitation over Indian monsoon regions is assessed. The local modification of the cloud microphysical parameters, riming, over TP is attempted. The simulation experiments have been carried out for different synoptic situations during the summer monsoon season. The riming gave differing responses in the two synoptic cases with the ice to rain conversion displaying a uniform distribution throughout the atmospheric column for the active monsoon case, whereas it is restricted up to an altitude of 8000 m in pre-monsoon case. The experiment over TP gives a 1.97% increase (0.54% reduction) in the all India rainfall for the pre-monsoon (active monsoon) case, which are mainly driven by the changes in the monsoon core zone. The maximum impact is found in Western Ghats rainfall with a 3.74% reduction (10.49% increases) for the pre-monsoon (active monsoon) case. Modulations in Tropical Easterly Jet and surface circulations in the experiments have substantial effect over the head Bay and the Western Ghats.

      $\bf{Highlights}$

      $\bullet$ Tibet Plateau (TP) played a significant role in the circulation and precipitation over Indian monsoon region. In this study, the impact of locally modified microphysics parameters (riming) over TP is estimated through NCMRWF Unified Model sensitivity experiments.

      $\bullet$ Precipitation distribution in both pre-monsoon and active monsoon synoptic situation shows a different response with the riming modification.

      $\bullet$ The experiment gives a 1.97% increase (0.54% reduction) in the all India rainfall for the pre-monsoon (active monsoon) case, with maximum impact in Western Ghat region.

      $\bullet$ Current modelling approach may be useful for the different cloud observation projects.

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