西南水汽通道上昆明站降水中的稳定同位素

STABLE ISOTOPES IN PRECIPITATION IN THE VAPOR TRANSPORT PATH IN KUNMING OF SOUTHWEST CHINA

位于西南水汽通道上的昆明站降水中的稳定同位素比率具有显著的季节变化.旱季(11～4月)降水中平均δ18O明显高于雨季(5～10月).显著的降水量效应说明昆明站降水的水汽主要来源于低纬度海洋.与全球大气水线相比,昆明站大气水线的斜率和常数项均较小.这与雨滴在未饱和大气中降落时重同位素的蒸发富集作用有关.统计分析显示,近地面温度露点差△Td与降水中稳定同位素比率存在显著的正相关关系.在旱季,受大陆性气团的影响,空气干燥,降水量小,大气中△Td大,因此蒸发强,重同位素的富集作用强,从而降水中稳定同位素比率高;在雨季,受来自海洋水汽的影响,空气湿润,降水量大,大气中△Td小,因此蒸发弱,重同位素的富集作用轻,从而降水中稳定同位素比率低.据此推测,降水量效应可能是不同水汽来源对降水中稳定同位素影响的产物.

The stable isotopic ratios in precipitation have notable seasonal variations in the vapor transport path in Kunming in Southwest China, with a higher mean δ^18O in precipitation in dry season （from November to April） than that in rain season （from May to October） . The vapor for precipitation in Kunming is originated primarily from low-latitude oceans. The slope and the constant item in the meteoric water line （MWL）, compared to the global MWL, are both smaller in Kunming, and are related to the evaporation enrichment of heavy isotopes as raindrops fall in unsaturated atmosphere. There is a notable positive correlation between the stable isotopic ratios in precipitation and the dew-point deficit in land △Td. In dry season with the dry air, light precipitation, great △Td in atmosphere, strong evaporation and thus strong enrichment of heavy isotopes, influenced by continental air-mass, the rainfall in Kunming is characterized with high stable isotopic ratio. In rain season with the moist air, heavy precipitation, small △Td in atmosphere, light evaporation and thus light enrichment, influenced by low-latitude marine vapor, the rainfall is characterized with low stable isotopic ratio. Therefore, it can be deduced that the effect is the outcome of the impact of different vapor origins on stable isotopes in precipitation.