Stable isotopic (δ~(18)O and δ~2H) and geospatial approach for evaluating extreme rainfall events

摘要

Natural disasters are more frequent in the tropical region in the recent past due to anthropogenic forcing and consequent climate change. In this context, understanding the genesis and pattern of extreme rainfall events are important to devise disaster management plans. Two natural disaster events, Cyclone Ockhi (05 December 2017) and a torrential rainfall event (944 mm in 24 h) occurred on 26 July 2005, that had caused large scale devastation, loss of lives and properties, in Mumbai, India were evaluated using stable isotopes (delta O-18 and delta H-2) of precipitation and geospatial data. Both the events were analysed and compared to understand their occurrences, moisture source regions and deduce the atmospheric factors responsible for their formation. Distinctly depleted stable isotopic content in rains were observed during the Cyclone Ockhi, while, during the extreme rainfall event, it was same as that of the normal monsoon rains, which is attributed to the dynamic nature of the moisture. The depleted isotopic signature of the cyclone may be because of the isotopic fractionation effect caused by the organized convective condition and vapour recycling along the trajectory. The genesis, structure and pattern of both the extreme events were evaluated using Sea Surface Temperature (SST), Outgoing Longwave Radiation (OLR) and Relative Humidity (RH) data. Wind pattern and moisture transport pathways were derived using HYSPLIT backward trajectory model. Prevalence of high SST over the southwest Bay of Bengal and adjoining areas of Sri Lanka as well as in the Arabian Sea was responsible for the genesis and intensification of the Cyclone Ockhi. The extremely low OLR value indicated intense and deep convection over the region during the cyclone. The large scale altitudinal variation in RH created unstable condition in the troposphere, consequently, strengthened the cyclone. The intensive shallow convection, continuous supply of moisture from the Arabian Sea and the orographic uplifting of clouds at the Western Ghats were responsible for the occurrence of torrential rainfall on 26 July 2005 in Mumbai. The wind pattern during the cyclone was distinct; it was circular and ascending upward, while, it was more or less horizontal and was descending downwards during the torrential rainfall event. The atmospheric processes responsible for the occurrence of two natural disaster rain events were also conceptualized. Although, the isotope data is limited, the study is first of its kind and it provides better insights into the genesis, intensification of the extreme rainfall events that are essential for forecasting, planning disaster mitigation measures and in warning strategies of this region.