Diurnal variation of stable isotopes in rainfall observed at Bengkulu for the YMC-Sumatra 2017

Abstract

Abstract Diurnal variability of stable isotopes in precipitation and its controlling factors are poorly understood, especially in the tropics. This study investigated the diurnal variability of stable isotopes in rainfall observed at Bengkulu in southern Sumatra, Indonesia. Rainfall samples were collected at 6-h intervals from 1 December 2017 to 15 January 2018 for the Year of the Maritime Continent (YMC-Sumatra 2017). The oxygen and hydrogen isotopic compositions δ18O and δH) of the rainwater varied from –15.1%o to –1.8%o and from –108.9%o to –1.3%o, respectively, and the d-excess varied from 1.9%o to 17.1%o. The observations at Bengkulu revealed that diurnal variation in rainfall amount was not significant for the entire period; however, the δ18O was lighter in both the early morning and the afternoon and heavier before noon and overnight. The δ18O became gradually depleted in the periods of continuous rain that occurred during 1–4 and 21–24 December. The amount of morning rain was much greater than evening rain in these periods, whereas the amount of evening rain was greater than morning rain in other periods. Based on satellite-derived CMORPH data, it is assumed that morning rain and evening rain corresponded to weak rainfall over surrounding sea region and strong rainfall over inland regions of Sumatra, respectively. The differences in moisture source and ratio of stratiform/convective rain rate between morning and evening might affect the isotopic composition of rainfall in the tropics.