中纬度地区混合云中稳定同位素分馏的数学模拟——以乌鲁木齐降水为例

SIMULATIONS OF STABLE ISOTOPIC FRACTIONATION IN MIXED CLOUD IN MIDDLE LATITUDES——A CASE OF THE PRECIPITATION AT URUMQI

文中介绍的数学模型考虑了混合云中液、固相共存时以及冰面过饱和环境下稳定同位素的动力分馏效应。利用该数学模型,模拟不同冷却条件下稳定同位素的温度效应。在相同的湿度条件下,湿绝热冷却过程中δ18O随温度的变化率小于等压冷却过程。冰面过饱和比Si的增大意味着动力分馏效应的增大。与平衡态过程相比,它的作用使得稳定同位素的综合分馏系数减小,从而使得降水中δ18O随温度的变化趋缓。模拟结果显示,湿绝热冷却过程中大气水线(MWL:δD=bδ18O+d)的梯度项b和常数项d均大于等压冷却过程。不同的冰面过饱和比对大气水线的影响是不同的。与冰面过饱和比相比,b和d的大小对云中含水量的变化敏感性较低。模拟结果还显示,乌鲁木齐的降水不是来自海洋水汽的初始凝结。在经历了长途输送之后,乌鲁木齐降水中的稳定同位素成分在很大程度上被衰减。模拟的稳定同位素比率/温度、δD/δ18O曲线分别与乌鲁木齐实测的稳定同位素比率/温度回归线以及大气水线有非常好的一致性。

The introduced mathematical model takes into account the action of the kinetic fractionation effect in super saturation environment at ice surface as liquid and solid phases coexist in mixed cloud. The kinetic fractionation effect is controlled by the super saturation ratio Si at ice surface in the mixed cloud. The greater the Si, the stronger the kinetic fractionation effect. The kinetic fractionation action makes the synthetic fractionation factor decreased and the change of the δ18 O with temperature flatted, compared to that at equilibrium state. The δO values in the wet adiabatic cooling process are greater than in the isobaric cooling process under the same temper- ature, showing that the fractionation rate of the latter is faster than that of the former. The change rate of the δ18O against temperature in the wet adiabatic cooling process is smaller than in the isobaric cooling process under same humidity. Compared to the δ18O in the liquid phase in the equilibrium process ( Si=1. 0), the δ18O in the solid phase is of a smaller change rate for greater Si and a greater one for smaller S;. With increasing S;, the kinetic fractionation effect is strengthened, and perhaps αe, which means that the δ18 O in the solid phase will maintain unchanged or increase with decreasing temperature. The liquid water contents in the cloud are of heavy δ18 O values because the isotopic memories from the previous condensation process are preserved in the liquid water. Consequently, the curves of the δ18O against temperature have low slopes, compared to ones in which the liquid - water contents in the cloud are not considered. The more the liquid - water contents in the cloud, the lower the slope. The contribution of the liquid - water contents in the cloud is only to change the magnitude of stable isotopic ratio in water, but not the variation trend with temperature. The different super saturation ratios have the different impacts on MWL. The slope parameter b and the intercept d of the meteoric water line (MWL), δD= bδ18O+ d, in the wet adiabatic cooling are greater than those in the isobaric cooling. The global MWL lies between two MWLs simulated under the wet adiabatic and the isobaric cooling processes, respectively. Compared to the super saturation ratio at ice surface, the b and d have the low sensitivity to the liquid- water contents in the cloud. Using the kinetic fractionation model, the variation of stable isotopes in precipitation at Urumqi is simulated. The simulated stable isotopic ratio/temperature and the δD/δ18O curves have very good consistency with actual regressions and MWL at Urumqi, respectively. The difference between them is related to evaporation and isotopic exchange of raindrops with surrounding atmosphere during falling. The precipitation at Urumqi is not the outcome of the initial condensation of o-cean vapor. The stable isotopic compositions in precipitation are greatly depleted after vapor undergoes long distance transportation.