Differences in Stable Isotopes in Precipitation between Day and Night: A Case Study of Changsha

The relative abundance of stable isotopes in precipitation plays a vital role in the water cycle analyses and diagnosing atmospheric circulation patterns. The analysis of stable isotopes in precipitation in a short time scale is beneficial in the study of controlling factors of stable isotopes in precipitation with increasing time resolution, in the acquisition of detailed information of stable isotope fractionation during phase transition, and in understanding the fractionation mechanism. As such, the characteristics of diurnal and night precipitation were analyzed and compared under different seasons (warm half-year and cold half-year), different precipitation types (convective precipitation and large-scale precipitation), and different precipitation intensities based on the observation data on stable isotopic composition (2H and 18O) in precipitation within Changsha. We aimed to determine the differences of stable isotopes in precipitation between day and night and their influencing factors or enhance the understanding of the regularity of stable isotopes in precipitation within the monsoon region. Results showed that δ18O in precipitation was more enriched during the night than in the day during the warm half-year; the opposite occurred during the cold half-year and negatively correlated with precipitation intensity. Whether in the warm or cold half-year, the d-excess in precipitation is more positive in the night than in the daytime because of the low temperature and high relative humidity at night, as well as the weak evaporation and enrichment of heavy isotopes, while the LMWL slope was higher during the day than during the night. Under convective precipitation, water vapor with relatively rich isotopes in the lower layer is lifted in strong updraughts, and compared to the convective precipitation, δ18O in precipitation is positive. Furthermore, the weak vertical motion of the air in the stratiform clouds make the water vapor with relatively depleted isotopes converge in the upper layer, and the δ18O in the precipitation is more negative; the LMWL slope of the former is lower than that of the latter. The slope and intercept of LMWL increased with precipitation intensity, while the stable isotopes in precipitation decreased with the increase in precipitation intensity. When the precipitation intensity was greater than or equal to 0.1 mm/12 h, the LMWL slope was higher during the day, and the intercept was higher during warm half-year night and cold half-year day. When the precipitation intensity was greater than or equal to 5.0 mm/12 h, the slope and intercept of the LMWL were higher at warm half-year days and cold half-year nights