National Centre for Earth Science Studies

The extreme rainfall events observed at a High- Altitude Cloud Physics Observatory (HACPO) site in Rajamallay, Munnar (10° 9’ 19.94” N, 77° 1’ 6.65” E, 1820 m above MSL) over the southern Western Ghats (India) during the floods in 2018 and 2019 monsoon periods (16th August 2018 and on 8th August 2019) have been investigated. The drop size distribution (DSD) spectra during the 2018 event are characterized by a large number of small to medium-sized drops resulting in maximum reflectivity of 48 dBZ with mass-weighted mean diameter (Dm) value of 1.2 mm. At the same time, the 2019 event is characterized by larger drops and resulted in high reflectivity of 53 dBZ and Dm value shifted to 1.4 mm. The consistent increment of Dm with variation in number concentration of drops during the intensive rain hours on 8th August 2019 shows a mixed- phase microphysical process that can invigorate the production of convective rainfall from deep cloud bands (217 K of cloud top temperature) with enhanced rain water content (22 gm-3 ). The parameters of scaled raindrop size distributions corresponding to higher rain rates (>50mmhr-1 ) suggest that the microphysical process that control the variations in DSD is strongly number controlled during these extreme rainfall events. The DSDs are evolved from a consistent, widespread rainfall supported by anomalous moisture advection from the Arabian Sea in 2018 monsoon period. The moisture convergence occurred on the elevated terrains leads to an intense spell of rainfall in two consecutive hours satisfies the occurrence of mini-cloud burst events on 8th August 2019 causing flash flood in the region.

Bibliographic Info: Sumesh, R. K., Resmi, E. A., Unnikrishnan, C. K., Dharmadas Jash, Padmalal, D. (2022). The extreme precipitation events of August 2018 and 2019 over southern Western Ghats, India A microphysical analysis using in-situ measurements. Atmospheric Research, Vol. 277, Art. 106322. https://doi.org/10.1016/j.atmosres.2022.106322.

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Assessment of monsoon moisture sources and their variability by secondary environmental controls was carried out in South India using stable isotopic composition of oxygen and hydrogen in rainwater. The rainwater isotopic signature in southwest India reflected dual monsoon sources with moisture from Arabian Sea and equatorial Indian Ocean during summer monsoon season, and a few depressions and cyclones generated over the Bay of Bengal formed the winter monsoon sources. The predominant summer monsoon rainwater along the eastern Arabian Sea coast reflected rainfall from first condensate while the marine moisture signatures were altered by the evapotranspiration process in inland regions indicating continental moisture recycling effect. The calculated high deuterium excess values (d-excess = 10-18‰) for observed rainwater also suggested higher influence of local precipitation effect in humid tropical river basins of the Western Ghats which usually exhibited annual average relative humidity ~ 85 % and temperature ~ 25°C. The time and space variability of regional moisture circulation in controlling atmospheric water balance was deduced in this study. The findings of this study revealed microclimate manifestation in the Western Ghats region similar to other cloud forest ecosystems like in South America (for e.g., Amazon basin), Hawaii, Eastern and Central Africa, Indonesia, etc.

Bibliographic Info: Tripti Muguli., Lambs, L., Gurumurthy, G. P., Moussa, I., Balakrishna, K (2022). Isotopic fingerprinting of dual monsoon moisture sources, evapotranspiration process and microclimate manifestation over the tropical rainforest region, western part of the Western Ghats, India. Journal of Hydrology, 612, 128239. https://doi.org/10.1016/j.jhydrol.2022.128239.

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In this study, the characteristic features and possible precursors for wet and dry spells of rainfall over southern tip of India are investigated. We also explore the variability in monsoon low-level jet (LLJ) in relation to wet and dry spells over a coastal station Thiruvananthapuram (8.48°N, 76.95°E) in southwest India using in-situ observations and other ancillary datasets. The results show that a wet spell spanning 3-4 days contributes about 30% of seasonal rainfall.Wet spells are characterized by westerly wind anomaly in the southern tip of India and easterly wind anomaly in northern India, leading to anomalous cyclonic vorticity over the Indian subcontinent. The opposite happens during dry spells. These characteristics are prominent from two days prior to the initiation of the spells, suggesting they may be used as precursors for forecasting wet and dry spells over Thiruvananthapuram. Analysis of low- to mid-tropospheric (2 and 4 km) humidity reveals significant moistening (drying) during wet (dry) spells. Yet, both wet and dry spells experience humid (>80%) boundary layer. The differences in mid-level humidity and thermodynamical structures between wet and dry spells seem to contribute to distinct rainfall characteristics over the southern tip of India. These results indicate that the use of in-situ observations along with large-scale reanalysis datasets may provide valuable information on the precursors for wet and dry spells over the southern tip of India, which can help both in regional- and city-level planning and management of water resources.Composite anomaly of zonal winds (ms-1) overlaid with vector at 850 hPa on d-2 day in (a) wet spells and (b) dry spells. Anomalies were calculated after subtracting the climatological daily mean for 1981-2020. The southerly winds from the Arabian Sea and westerlies in the Bay of Bengal strengthened at 5°N latitude (marked with purple curved arrow) on d-2 day at 850 hPa level that acts as the primary precursor for the wet spells. The easterly wind anomalies developed over the equatorial Indian Ocean located to the south and southwest of the southern tip (marked with red curved arrow), act as a precursor for the onset of dry spells. The corresponding, temporal evolution (±8 days) of rainfall anomalies (mm) averaged over the southern region (70°-85°E, 5°S-15°N) in wet and dry spells are represented as bar graph.

Bibliographic Info: Resmi, E. A., Preethi, B., Ajayamohan, R. S., Ray, P., Unnikrishnan, C. K., Nita, S., Sumesh, R. K., & Dharmadas, J. (2023). Analysis of localized features during wet and dry rainfall episodes over southern tip of India. International Journal of Climatology, 1–20. https://doi.org/10.1002/joc.8267.

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The Ediacaran-Cambrian transition period was one of the most significant intervals in the Earth’s history. Widespread changes in the oceanic and atmospheric conditions characterized this interval, which likely triggered the radiation of metazoans. Much of our knowledge of these changes have come from geochemical studies of marine carbonates deposited during this time. In search of such information and to establish the true geographical extent of these events, we carried out a detailed field and geochemical investigation in the Bilara Group of the Marwar Supergroup, the youngest Proterozoic marine carbonate succession of peninsular India. The variations in the 87Sr/86Sr observed in the limestone formation of the Bilara Group (0.70810 to 0.70916) suggest an early Cambrian (520–539 Ma) depositional age for the Group. We identify two δ13C negative excursions of magnitudes ~6‰ and ~7‰, respectively, in the middle and lower parts of the Bilara Group. Since these δ13C anomalies are observed through-out the Marwar Basin that was connected to global oceans, we believe that they represent temporal changes in the chemistry of seawater. Using the principles of δ13C stratigraphy and the available geochronological information, we correlate the middle Bilara anomaly to the Shiyantou carbon isotope excursion (SHICE). Although, poor age constraints hinder a definitive correlation of the older Bilara anomaly, we speculate that it is the basal Cambrian carbon isotope excursion (BACE). These results place the Ediacaran-Cambrian boundary closer to the basal part of the Bilara Group in the Marwar Supergroup.

Bibliographic Info: Bivin G. George, Sanjeev Kumar, Jyotiranjan S. Ray [2021]. C-Sr isotope stratigraphy of carbonate formations of the late Neoproterozoic - Cambrian Marwar supergroup, western India. Precambrian Research, Vol. 364, Art. 106378. https://doi.org/10.1016/j.precamres.2021.106378

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The Madurai block which forms the central crustal block in the Precambrian Southern Granulite Terrane, south India preserves crustal evolution history with record of multiple magmatic and metamorphic episodes ranging from early Archaean to late Neoproterozoic. Documenting the precise chronology and tectonothermal evolution of these events are of paramount importance to correlate with other crustal blocks in Southern Granulite Terrane as well as to link with other continental fragments of East Gondwana. The present study focuses on hitherto unreported location within a key section of Madurai block along the transcrustal Suruli shear zone known as the Kambam ultrahigh-temperature belt. This belt is noticeable for the proliferous occurrence of HT-UHT metapelites with an overall clockwise P-T trajectory. Samples of garnet-spinel granulite, cordierite-spinel granulite and associated granite gneiss were subjected to detailed petrographic, thermobarometric and geochronologic analysis. Combined petrography, mineral reaction, geothermobarometry and pseudosection modelling of the granulite samples reveal HT-UHT metamorphic conditions with clockwise P-T path demonstrating significant decompression and cooling. LA-ICPMS U-Pb dating of zircon, monazite and rutile phases within these samples constrain the timing of metamorphism as well as cooling. U-Pb zircon ages of ~2500 Ma from the granulites points to new evidences for Paleoproterozoic high-grade metamorphism in the area where monazite U-Pb ages at ~593 and ~557 Ma throw light into the imprints on ~36 Ma prolonged Neoproterozoic thermal event attributed to Pan African collisional orogeny. LA-MC-ICPMS Hf isotopic record from zircon cores point to the role of significant juvenile crustal melting/anatexis during Paleoproterozoic. In addition, several cryptic metamorphic pulses are also identified evincing the polydeformed evolutionary history of these rocks. The ca. ~432 Ma cooling age record from rutile estimates an average cooling rate of ~3.0 °C/Ma over a time period of ~125 Ma, classifying Madurai block as a classic slow cooled granulite terrane with long lived UHT orogenic history. The results also bring to the fore the importance of Kambam ultrahigh-temperature belt and Suruli shear zone which can be projected as a major terrane boundary within the Southern Granulite Terrane.

Bibliographic Info: Amal Dev, J., Tomson, J. K., Nilanjana Sorcar, Nandakumar, V. [2021]. Combined U-Pb/Hf isotopic studies and phase equilibrium modelling of HT-UHT metapelites from Kambam ultrahigh-temperature belt, South India: Constraints on tectonothermal history of the terrane. Lithos, Vol. 406-407, Art. 106531. https://doi.org/10.1016/j.lithos.2021.106531

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