Meteorological Analysis on Extremely Depleted¹⁸ O Rainfall Events during the Summer in Adelaide, Australia

The transport of atmospheric water vapor plays a crucial role in the production of precipitation and the variation of precipitation isotopic composition (d¹⁸O_p). This study investigates three precipitation events with extremely depleted heavy isotopes in the summer of Adelaide, Australia. Using fundamental water vapor diagnosis and moisture calculation method, this research analyzes the impact of rainout degrees along moisture transport paths, atmospheric circulation patterns, water vapor sources, and moisture transport on the extreme depletion of precipitation isotopes in the study area. The purpose of this study is to reveal the direct cause of extremely depleted d¹⁸O_p at the hourly time scale. The results show the diversity and complexity of d¹⁸O_p variation in summer precipitation events in Adelaide. The rainout caused by local and upstream large precipitation may be the main reason for extremely low values of d¹⁸O_p. The phenomenon of subcloud secondary evaporation, which is driven by the interaction between relatively low humidity and high temperature at near-surface levels, plays a pivotal role in the entire precipitation process. This mechanism is particularly pronounced during the onset or cessation of precipitation events, thereby resulting in the observed enrichment of d¹⁸O_p values. The oxygen stable isotopic composition of water vapor (d¹⁸O_a) would usually become higher, when the air mass mixes with new moisture with relatively high d¹⁸O_a, suppressing the influence of the previous rainout.