基于Stewart模型改进方案的新疆降水同位素的云下蒸发效应比较

A comparison of sub-cloud secondary evaporation effect of precipitation isotope in Xinjiang based on the Stewart model improvement scheme

干旱半干旱区降水稀少,蒸发能力大,雨滴在云下易发生蒸发现象并改变其同位素比率,明确雨滴降落过程中稳定氢氧同位素的变化在同位素水文学研究中具有重要意义。Stewart模型常被用于评估雨滴中同位素比率的变化,云下雨滴降落过程中的气温、相对湿度等是模型输入的关键参数。目前广泛采用的大气均质假设往往与自然条件有差异,这种均质假设对模拟结果产生的影响有待明确。基于2016年7月至2017年6月新疆66个地面气象站逐小时观测资料与8个探空站的高空定时观测资料,通过三种Stewart模型算法方案(方案①以地面气象资料代替大气气象参数,方案②假设地面气象资料与预测的云底气象参数均匀变化,以其平均值代入模型,方案③根据高空探测资料,分层计算大气参数)的对比,深入研究新疆降水同位素的云下二次蒸发效应。结果表明:(1)三种方案计算的新疆各分区云下二次蒸发存在明显的季节差异,蒸发剩余比(f)均呈现出秋冬较大、春夏较小的趋势,各分区的ΔδD、Δδ18O和Δd均为春夏较大、秋冬较小。(2)从空间来看,Δd均值在准噶尔盆地西部和塔里木盆地东侧北缘较小。对于蒸发剩余比(f)均值、ΔδD均值、Δδ18O均值和Δd均值来说,三种方案的差异性主要体现在塔里木盆地。(3)蒸发剩余比(f)与d-excess的变化量存在相关性,整体来看,三种方案的斜率均表现为塔里木盆地>阿尔泰山地>准噶尔盆地>天山山地。无论是新疆各分区(除天山山地方案②)还是新疆全境,三种方案的f与Δd的线性关系均>1.0‰/%,最为干旱的塔里木盆地方案③的斜率高达1.496‰/%,这可能与新疆地处干旱半干旱气候区有关。毋庸置疑方案③是后续云下二次蒸发效应研究的方向。

The sub-cloud secondary evaporation is easy to occur and change its isotope ratio in the arid and semi-arid areas with low precipitation and high evaporation.In the isotope hydrology study it is necessary to have a better understanding of variability in stable hydrogen and oxygen isotopes of raindrops from cloud base.The Stewart model is often used for evaluation the change in isotope ratios in raindrops.Temperature and relative humidity of raindrops falling under the cloud are the key input parameters of the model.Now,the widely used atmospheric mean hypothesis is different from natural conditions,the impact of which on the simulation results remains to be determined.Based on the hourly meteorological observations at surface as well as the daily radiosonde observation in Xinjiang from July 2016 to June 2017,the effect of sub-cloud secondary evaporation in Xinjiang was analyzed using the Stewart model improvement schemes(i.e.,schemes①,②and③).It is found that:(1)There are obvious seasonal variation of sub-cloud evaporation found by the three schemes,the trend of f is larger in autumn and winter and smaller in spring and summer,andΔδD,Δδ18O andΔd is smaller in autumn and winter and larger in spring and summer.(2)Spatially,Δd is less in the western region of Junggar Basin and northern edge of Turpan-Hami Basin.In terms of f,ΔδD,Δδ18O andΔd,the differences among the three schemes are obvious in the Tarim Basin.(3)There is correlation between f andΔd.On the whole,the slopes of the three Stewart model improvement schemes are highest in Tarim Basin,following by the Altai region and the Junggar Basin,and then the Tianshan Mountains.It can be found that the linear relationship between f andΔd of the three Stewart model improvement schemes is larger than 1.0‰/%in Xinjiang,except the second scheme in the Tianshan Mountains,the slope of the third scheme in Tarim Basin reaches up to 1.496‰/%,which may be due to Xinjiang located in the arid and semi-arid climate zone.There is no doubt that schemes③is the research direction of sub-cloud secondary evaporation in the future.