Meteorological analysis on extremely depleted 18O rainfall events during the summer in Adelaide, Australia

Abstract

The transport of atmospheric water vapor plays a crucial role in the production of precipitation and the variation of precipitation isotopic composition (δ18Op). This study investigates three precipitation events with extremely depleted precipitation isotopes in the summer rainfall of the Adelaide, Australia. Using fundamental water vapor diagnostic and moisture calculation methods, this research analyzes the impact of rainout levels 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 generating extremely depleted δ18Op at hourly time scale, and to understand the influence of water vapor transport on δ18Op. The results show the diversity and complexity of δ18Op variation in summer precipitation events in Adelaide. The rainout caused by local and upstream large precipitation may be the main reason for the steep drop to an extremely low value of δ18Op. The phenomenon of sub-cloud 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 δ18Op values. The oxygen stable isotopic composition of water vapor (δ18Oa) would usually become higher, when the air mass mixes with new moisture with relatively high δ18Oa suppressing the influence of the previous rainout. The evapotranspiration(ET) from the underlying surface along water vapor transport pathways modulates the isotopic composition of atmospheric water vapor . When the δ18O in ET exceeds that in precipitation, δ18Oa gradually becomes enriched.