【摘要】 提出了湿度效应的概念,即降水中稳定同位素比率与大气的温度露点差ΔTd存在显著的正相关关系.对两个气候特征完全不同的取样站乌鲁木齐和昆明降水中δ18O与温度露点差之间的关系进行了分析,尽管两站的δ18O与ΔTd的季节变化存在差异,但它们的湿度效应是显著的.利用稳定同位素动力分馏模型并根据500hPa月平均温度的季节分布对昆明站云中凝结物中δ18O进行了模拟,模拟的月平均δ18O与月平均温度的变化具有非常好的一致性,说明昆明站云中凝结物中的氧稳定同位素具有温度效应.这个结果与地面降水中氧稳定同位素的降水量效应截然不同.昆明站降水中δ18O一定程度上指示大气的干湿状况,同时也间接地指示降水量的多寡或季风的强弱.湿度效应的存在,影响降落雨滴中稳定同位素蒸发富集的强度以及雨滴与大气之间稳定同位素物质迁移的方向.它不仅改变降水中稳定同位素比率的大小,也改变其季节分布的特点.更多还原

【Abstract】 The humidity effect, namely the markedly positive correlation between stable isotopic ratio in precipitation and dew-point deficit in atmosphere %ΔT%-d, is put forward firstly and the relationship between {δ~{18}O} in precipitation and %ΔT%-d is analyzed in this paper for the Urumqi and Kunming Stations, which climatic characteristics are completely different. Although the seasonal variations in {δ~{18}O} and %ΔT%-d have difference for the two stations, their humidity effect is notable. In Kunming Station, the correlation coefficient and its confidence level of the humidity effect are all higher than those of amount effect, showing a marked influence of the psychrometric conditions on stable isotopes in precipitation. Using kinetic model for stable isotopic fractionation and according to seasonal distribution of mean monthly temperature on the 500 hPa at Kunming Station, the variation of {δ~{18}O} in condensates in cloud at Kunming Station is simulated. A very good agreement between the simulated monthly {δ~{18}O} and the mean monthly temperature on the 500 hPa is obtained, showing that the oxygen stable isotope in condensates in cloud has temperature effect. Such a result is very different from amount effect of the oxygen stable isotope in precipitation in surface. Based on the simulation for seasonal variation of {δ~{18}O} in falling raindrops, it can be found that, in the dry season from November to April, the increase, with falling distance, of {δ~{18}O} in falling drops is remarkable corresponding to great %ΔT%-d, showing a strong evaporation enrichment function for stable isotopes in raindrop as falling; however, in the wet season from May to October, {δ~{18}O} value in falling raindrops displays an inapparent increase corresponding to small %ΔT%-d, except for that in May. By comparing the simulated mean monthly {δ~{18}O} in surface with the measured monthly {δ~{18}O} in precipitation, it can be found that both monthly {δ~{18}O} series have very good agreement. Averagely, {δ~{18}O} values are relatively lower because of high moist air, heavy rainfall, small %ΔT%-d and weak evaporation enrichment function of stable isotopes in falling raindrops, under the influence of vapor from oceans; but relatively higher because of dry air, light rainfall, great %ΔT%-d and strong evaporation enrichment function in falling raindrops, under the control of continental air mass. Therefore, {δ~{18}O} in precipitation at Kunming Station can indicate the psychrometric situation in atmosphere to a certain degree, and thus do the intensity of precipitation and strength of monsoon indirectly. The humidity effect changes not only the degree of stable isotopic ratios in precipitation but also its seasonal distribution due to shifting the strength of evaporation enrichment of stable isotopes in falling raindrops and the direction of net mass transfer of stable isotopes between atmosphere and raindrops. Consequently, it is inferred that humidity effect is probably one of the foremost causations of amount effect.更多还原