• S VISHNU

Articles written in Journal of Earth System Science

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

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.

• Petrographic and geochemical characteristics of the granitic basement rocks below Deccan Traps obtained from scientific drilling to 3014 m depth in the Koyna region, western India

The composition of the Precambrian granitic basement below a few hundred to a few thousand meters of the Deccan Traps in western and central India remain poorly characterized, essentially due to lack of samples for laboratory analyses. In the present work, petrographic and geochemical characteristics of the basement granitoids from the Koyna region of western India have been studied through coresamples and cuttings made available from the scientific drilling to the depth of 3014 m. The basement rock comprises dominantly of granite, granitic gneiss and tonalite. The major minerals are quartz, plagioclase and orthoclase, whereas the minor minerals include epidote, chlorite and hornblende. In the feldspar and quartz grains, signatures of brittle deformation, as well as shearing effects, are observed. Also, fractures and joints are noticed frequently throughout the basement rock with possibilities of more than one dominant stress direction during fracturing. Below 1100 m depth in the western and 1300 m depth in the eastern side of the Western Ghats escarpment, the basement cores are weakly foliated as well as sheared due to tectonic deformation. The bulk rock geochemistry of the basement granitoids shows range from tholeiitic to calc-alkaline series of magma types which mainly belong to adamellite–granodiorite–tonalite suites. The basement rocks are predominantly peraluminous in nature and have an overall predominance of sodium over potassium. Fractionated REE patterns with enriched light REE (LREE) and depleted heavy REE (HREE) along with a smallnegative Eu-anomaly are observed in the samples. Overall, the Precambrian basement rocks are lithologically heterogeneous and apparently genetically related by fractional crystallization and partially derived from magmas.

• # Journal of Earth System Science

Volume 132, 2023
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Posted on July 25, 2019