长沙降水中稳定同位素的昼夜差别

基于长沙地区降水稳定同位素的实测数据，对不同季节（暖半年、冷半年）、不同降水类型（对流降水、平流降水）、不同降水强度下昼、夜降水中稳定同位素的变化特征进行分析和比较，旨在揭示昼夜降水中稳定同位素的差异及其影响因素，深化对季风区降水稳定同位素变化规律的认识。结果表明：降水中δ¹⁸O在暖半年时段夜间比白天偏正，而在冷半年时段白天比夜间偏正，且均与降水量呈反比；无论在暖半年还是冷半年，相较于白天，夜间温度低相对湿度大，降落雨滴中重同位素蒸发富集作用较弱，从而降水中过量氘d在夜间偏正；由于下垫面水汽再循环对大气水线（LMWL）的影响大于雨滴云下二次蒸发的影响，LMWL的斜率在白天较夜间偏正。在对流降水主导条件下，强烈的辐合气流携带低层具有相对富集同位素的水汽上升，形成降水中的δ¹⁸O明显偏正；相较于对流降水主导，平流云中微弱的空气垂直运动使得高层具有相对贫化同位素的水汽辐合，形成降水中的δ¹⁸O较为偏负；LMWL的斜率则在平流降水主导时更大。随降水强度的增加，降水中稳定同位素逐渐偏负，LMWL斜率和截距均明显增大；在降水强度≥0.1 mm/12 h时，LMWL的斜率在白天更大，截距在暖半年晚上和冷半年白天更大；降水强度≥5.0 mm/12 h时，LMWL的斜率和截距在暖半年白天和冷半年晚上更大。

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.