下垫面蒸发和云中凝结分馏对降水稳定同位素影响的数值试验——时间变化的比较(以长沙降水同位素为例)

利用稳定同位素大气水平衡模式（iAWBM）的模拟数据，分析了在不同的下垫面蒸发和不同的凝结分馏条件下降水中δ¹⁸O的时间变化、降水量效应、负温度效应和大气水线。并通过与长沙站5年实测数据的比较以及模拟试验结果之间的相互比较，揭示下垫面蒸发水汽中稳定同位素的季节性变化和云中稳定同位素分馏对降水中稳定同位素变化的可能影响，增进对季风区水稳定同位素效应的理解和认识。iAWBM给出的4个模拟试验均很好地再现了监测站降水中δ¹⁸O的时间变化，模拟出季风区降水中稳定同位素在暖半年被贫化、在冷半年被富集的基本特点。与平衡分馏相比，动力分馏下降水中稳定同位素被贫化的程度加强、季节差和离散程度减小；由下垫面蒸发水汽中稳定同位素δ_e季节性变化所引起的降水中稳定同位素的变化在不同季节完全相反：在长沙，暖半年降水中δ¹⁸O更低，冷半年降水中δ¹⁸O更高，使得降水中稳定同位素季节差和离散程度增大。4个模拟试验均很好地再现了季风区的降水量效应和负温度效应。与平衡分馏相比，动力分馏下模拟的降水量效应和负温度效应的斜率相对较小；δ_e季节性变化导致模拟的降水量效应和负温度效应的斜率增大。利用iAWBM，模拟出季风区湿热气候条件下的MWL。动力分馏以及δ_e季节变化均使模拟得到的MWL的斜率和截距减小。

Numerical tests on the impacts of surface evaporation and condensation fractionation on stable isotopes in precipitation:a comparison of temporal variation (taking the isotopes in precipitation at Changsha as an example)

Using the simulation data of the isotope enabled atmospheric water balance model (iAWBM), the temporal variations of δ¹⁸O in precipitation, amount effect, negative temperature effect and meteoric water line (MWL) are analyzed in order to reveal the impact mechanism of the in-cloud isotopic fractionation and vapor isotopic composition from surface evaporation on simulated precipitation isotopic characteristics, improve the understanding of stable isotopic effects in monsoon areas, through the comparison between simulations and 5-year observations in Changsha and intercomparisons among the simulations. The results simulated by four iAWBM tests all reproduce the observed seasonal variation of δ¹⁸O in precipitation. The basic characteristics in monsoon areas that stable isotopes in precipitation are depleted during warm months and enriched during cold months. Compared with the simulations under equilibrium fractionation, the depleted degree of stable isotopes in precipitation is enhanced under kinetic fractionation, in company with a decrease of isotopic seasonality and inter-event variability. The variations of stable isotopes in precipitation, caused by the seasonal variation of stable isotopes in vapor evaporated from the surface (δ_e), are opposite between different seasons. In Changsha, a lower δ¹⁸O in precipitation during warm months and a higher δ¹⁸O in precipitation during cold months lead to the increase of isotopic seasonality in precipitation and inter-event variability. Four simulations all produce the amount effect and negative temperature effect commonly observed in monsoon areas. Under kinetic fractionation, the slopes of simulated amount effect and negative temperature effect decrease relative to under equilibrium fractionation; but increase under the δ_e seasonality. The MWLs for warm and humid climate in monsoon areas are well simulated too. The slopes and intercepts of the simulated MWLs decrease under kinetic fractionation, as well as under δ_e seasonality.