• P SIRISHA

      Articles written in Journal of Earth System Science

    • Evaluation of the impact of high-resolution winds on the coastal waves

      P SIRISHA P G REMYA ANURADHA MODI RABI RANJAN TRIPATHY T M BALAKRISHNAN NAIR B VENKATESWARA RAO

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      This study discusses the impact of high-resolution winds on the coastal waves and analyses the effectiveness of the high-resolution winds in recreating the fine-scale features along the coastal regions during the pre-monsoon season (March–May). The influence of the diurnal variation of winds on waves is studied for the Tamil Nadu coastal region using wind fields from weather research and forecast (WRF) (3 km) and European Centre for Medium-Range Weather Forecasts (ECMWF) (27.5 km). The improvement in the coastal forecast is then quantified with wave rider buoy observations. The high-resolution wind fields simulated fine-scale features like land–sea breeze events and showed good agreement with observation results. The error in the wave height and period is reduced by 8% and 46%, respectively, with the use of high-resolution forcing winds WRF over ECMWF, although the overestimation of wave energy on high frequencies due to overestimated WRF winds remains as a challenge in forecasting. The analysis also shows the importance of accurate wave forecast during a short-duration sudden wind ($\sim$12 m/s) occurrence in southern Tamil Nadu near Rameswaram during the pre-monsoon period. Low pressure forms over Tamil Nadu due to the land surface heating, resulting in a sudden increase of winds. High winds and steep waves which cause damage to the property of the coastal community near Rameswaram also were well simulated in the high-resolution forecast system with WRF winds.

    • Ocean state forecasting during VSCS Ockhi and a note on what we learned from its characteristics: A forecasting perspective

      R HARIKUMAR P SIRISHA ANURADHA MODI M S GIRISHKUMAR S VISHNU K SRINIVAS RAKHI KUMARI G YATIN P DINESH KUMAR T M BALAKRISHNAN NAIR M MOHAPATRA

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      Tropical Cyclone Ockhi was an intense cyclone, with a peculiar and long track, in the Arabian Sea in 2017. It caused severe damage to coastal infrastructure and death of 282 people. Indian National Centre for Ocean Information Services (INCOIS) issued the Joint INCOIS-IMD (India Meteorological Department) bulletins on the Ocean State Forecasts (OSF) and alerts/warnings during Ockhi. Validation of the OSF from INCOIS using buoys reveals that the forecasts were in good agreement with the observations [average correlation 0.9, RMSE ${\le}$0.8 m (for larger waves), and scatter index ${\le}$25%]. Climatological analysis of Genesis Potential Index (GPI) suggests that the southeast Arabian Sea, where the TC-Ockhi was intensified, had all the favourable conditions for intensification during November/December. Moreover, it was found that four days before the genesis of Ockhi, the environmental vorticity and relative humidity were more favourable for the cyclogenesis compared to vertical wind shear and potential intensity. The intensification rate was rapid as experienced by earlier cyclones in this region. Also, the cyclone track closely matched the background tropospheric winds. The present study suggests that the forecasters should look into the background dynamic and thermodynamic conditions extensively in addition to multi-model guidance to better predict the genesis, intensity and track of the cyclones.

      $\bf{Higlights}$

      $\bullet$ In the Arabian Sea, during the TC-Ockhi, the forecasts of wave parameters from the model forced with bias-corrected ECMWF winds resulted in very good agreement with observations.

      $\bullet$ Climatologically, TC-Ockhi region has large potential for the genesis and intensification of TC due to an enhanced low-level cyclonic vorticity and the reduction in vertical wind shear.

      $\bullet$ During the TC-Ockhi period, low-level vorticity and mid-tropospheric relative humidity were the dominant contributing factors, which lead to an enhanced GPI in the Arabian Sea.

      $\bullet$ TC-Ockhi also had rapid intensification in a similar fashion the earlier cyclones in this region behaved.

      $\bullet$ There is no abnormality also in the TC-Ockhi track, as the TC-Ockhi track matches well with the background tropospheric flow.

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