中国季风区降水构成对降水中稳定同位素的影响:以长沙为例

The influence of the composition of precipitation on the stable isotopes in precipitation in the monsoon region of China:a case study of Changsha

降水构成是指对流或平流降水量在总降水量中所占比例,不同的降水构成对降水稳定同位素的丰度会产生重要的影响。基于长沙地区实测的日降水稳定同位素数据和GNIP月降水同位素数据,对不同时间尺度、不同季节时段以及不同降水强度下降水构成对降水中稳定同位素影响的变化特征进行分析和比较,旨在揭示不同的降水构成对降水中稳定同位素影响的差异,深化对季风区降水形成过程中稳定同位素变化规律的认识。结果表明:云中对流强度的差异造成了不同季节对流降水量占总降水量的比例(convective precipitation fraction,CPF)与总降水量之间相关关系的差异。在日和月时间尺度下,在暖半年,CPF随总降水量的增大而减小,而在冷半年,CPF随总降水量的增大而增大。无论是在暖半年还是在冷半年,日时间尺度下长沙站降水中的δ18O随CPF增大不断偏正。在月时间尺度下,包括长沙站在内的中国季风区大部,降水中δ18O与CPF均呈正相关。由于不同季节对流活动的强度不同会造成降水构成的不同,也使得相对于暖半年,冷半年时段降水构成对降水中稳定同位素的影响要强于降水量效应的影响,因此在冷半年长沙地区δ18O—CPF线性回归方程的斜率随降水强度的增大而增大,而在暖半年,δ18O—CPF斜率随降水强度的增大而减小。

Background,aim,and scope As the most typical monsoon region in the world,the study area,monsoon region in China,is the main part of the East Asia monsoon region,and the variation characteristics of precipitation isotopes are directly influenced by monsoon circulation.Changsha belongs to the typical sub-tropical monsoon climate.The process of precipitation formation is complex and diverse under different weather or different seasons,therefore,Changsha is an important place to study the characteristics and influence mechanism of stable isotopes of precipitation in monsoon region.Precipitation composition refers to the fraction of convective or advection precipitation in the total precipitation,and different precipitation composition exerts large impact on the stable isotopes composition of precipitation.Our aims were to reveal the differences of the influence of different precipitation compositions on the precipitation stable isotopes,and to deepen the understanding of the variation regulation of stable isotopes in the process of precipitation formation in the monsoon region.Materials and methods From January 1,2014,to December 31,2019,the precipitation was collected at the meteorological station of Hunan Normal University at the foot of Yuelu Mountain in Changsha.All collected water samples were tested for stable isotope ratios of oxygen and hydrogen using a Isotopic Water Analyzer(DLT-IWA-35EP,USA)at the Meteorological and Climatic Laboratory of Hunan Normal University.Based on the observed daily stable precipitation isotopes data in Changsha and monthly GNIP precipitation isotopes data,by using the method of mathematical statistics,this paper analyzed and compared the variation characteristics of the influence of different timescales,different season periods and different precipitation intensities on the stable precipitation isotopes.Results The results showed that,on the daily and monthly timescales,in the warm half-year,CPF decreases with the increase of total precipitation,while in the cold half-year,CPF increases with the increase of total precipitation.Whether in the warm or cold half-year,δ18O in the precipitation at Changsha station constantly increased with the increase of CPF on the daily timescale.The slope ofδ18O—CPF linear regression equation increased with the increase of precipitation intensities in cold half-year at Changsha,and decreased with the increase of precipitation intensities in warm half-year.Discussion Changsha is located in the East Asian monsoon region.Due to the seasonal movement of planetary pressure belt and wind belt,the intensity of convective activity varies significantly in different seasons.The difference of convective intensity in cloud resulted in the difference of the correlation between convective precipitation fraction(CPF)and the total precipitation in different seasons.In the warm half-year,CPF decreases with the increase of total precipitation,while in the cold half-year,CPF increases with the increase of total precipitation.The formation processes of convective precipitation and advection precipitation are different,and the sources of water vapor and the condensation temperature during the formation of precipitation are also different,so the stable isotope abundances in precipitation are significantly different.Usually,stable isotopes in precipitation are relatively positive when CPF is large,and relatively negative when advection precipitation fraction(APF)is large.Because the intensity of convective activity in different seasons will result in different precipitation composition,the influence of precipitation composition on stable isotopes in precipitation was stronger than that of amount effect in cold half-year compared with warm half year.Conclusions Regardless of the seasonal periods and time scales,the precipitation composition in the whole monsoon region of China including Changsha has a significant impact on the stable isotopes in precipitation,and theδ18O in precipitation increases with the increase of CPF.Recommendations and perspective The results can provide a theoretical basis for the reconstruction of paleoclimate,strengthen the understanding of the importance of precipitation composition on the influence of stable isotope composition in precipitation,and further the understanding of the influence mechanism of precipitation formation process in monsoon region on stable isotope composition in precipitation.