• IMRANALI M MOMIN

      Articles written in Journal of Earth System Science

    • Arctic summer sea-ice seasonal simulation with a coupled model: Evaluation of mean features and biases

      Saheed P P Ashis K Mitra Imranali M Momin Rajagopal E N Helene T Hewitt Ann B Keen Sean F Milton

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      Current state of the art weather/climate models are representation of the fully coupled aspects of the components of the earth system. Sea-ice is one of the most important components of these models. Simulation of sea-ice in these models is a challenging problem. In this study, evaluation of the hind-cast data of 14 boreal summer seasons with global coupled model HadGEM3 in its seasonal set-up has been performed over the Arctic region from 9th May start dates. Along with the biases of the sea-ice variables, related atmosphere and oceanic variables have also been examined. The model evaluation is focused on seasonal mean of sea-ice concentration, sea-ice thickness, ocean surface current, SST, ice-drift velocity and sea-ice extent. To diagnose the sea-ice biases, atmospheric variables like, 10 m wind, 2 m air temperature, sea-level pressure and ocean sub-surface temperatures were also examined. The sea-ice variables were compared with GIOMAS dataset. The atmospheric and the oceanic variables were compared with the ERA Interim and the ECMWF Ocean re-analysis (ORAP5) datasets, respectively. The model could simulate the sea-ice concentration and thickness patterns reasonably well in the Arctic Circle. However, both sea-ice concentration and thickness in the model are underestimated compared to observations. A positive (warm) bias is seen both in 2 m air temperature and SST, which are consistent with the negative sea-ice bias. Biases in ocean current and related ice drift are not related to biases in the atmospheric winds. The magnitude of the oceanic subsurface warm biases is seen to be gradually decreasing with depth, but consistent with sea-ice biases. These analyses indicate a possibility of deeper warm subsurface water in the western Arctic Ocean sector (Pacific and Atlantic exchanges) affecting the negative biases in the sea-ice at the surface. The model is able to simulate reasonably well the summer sea-ice melting process and its inter-annual variability, and has useable skill for application purpose.

    • 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.

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