基于氢氧稳定同位素的黄土高原云下二次蒸发效应

Effect of Below-cloud Secondary Evaporation in Precipitations over the Loess Plateau Based on the Stable Isotopes of Hydrogen and Oxygen

根据GNIP提供的1985年1月至2004年12月黄土高原8个站点的409个降水同位素数据及同期的气象资料，结合HYSPLIT4．9模型，分析了该区δ18O的和d的时空分布特征，计算了二次蒸发的比率，并对不同气象因子影响二次蒸发的程度作了探讨，得到以下结论：①冬季风和夏季风期间，在黄土高原内由南向北δ18O的呈上升趋势，d呈下降趋势；自东向西δ18O仅在夏季风期间有上升趋势，而在冬季风期间呈现下降趋势，d总呈现下降的趋势，δ18O和d的变化幅度可以指示季风的路径．②该区全年存在二次蒸发效应，夏季风期间最为明显，蒸发比率介于1．51％～5．88％之间，平均值为3．87％．冬季风期间蒸发比率总体较低，介于1．06％～5．46％之间，平均值也降为3．03％．黄土高原边缘站点蒸发比率受季风变化影响较大，而中部站点受季风变化影响较小．③温度对二次蒸发的影响较大，降水量和水汽压其次，相对湿度较小．此外，风速和海拔对二次蒸发的影响较弱．

Based on stable isotopes in 409 precipitation samples provided by GNIP and meteorological records at the eight stations in Loess Plateau from January 1985 to December 2004, as well as the trajectory model of HYSPLIT 4.9, the spatial and temporal variations of d-excess and δ18O were analyzed. The spatial distribution of secondary evaporation rate and the impact of meteorological factors on below-cloud secondary evaporation were also discussed. The result showed that： ① During summer and winter monsoon periods, δ18O showed an uptrend variation and d-excess showed a downtrend variation from south to north in Loess Plateau. From east to west, δ18O showed an uptrend variation only in summer monsoon period and a downtrend variation in winter monsoon period. The value of d-excess also showed a downtrend variation. Amplitude of variation δ18O and d-excess could indicate the routes of monsoon. ② Secondary evaporation existed on an annual basis, and it was relatively significant during the summer monsoon period, with ranges from 1.51% to 5.88% and an average rate of 3.87%. While winter monsoon became lower, the rates ranged from 1.06% to 5.46% , and the average rate dropped to 3.03%. Monsoon had larger influence on secondary evaporation in margin area of the plateau, while the influence on the central stations was little. ③ Temperature had the highest contribution to secondary evaporation, followed by precipitation amount and water vapor pressure, and relative humidity had a small contribution. Moreover, the influence of wind speed and altitude on secondary evaporation was weak.