新疆夏季云下二次蒸发对雨滴稳定同位素影响的定量研究

Quantitative study of sub-cloud secondary evaporation effect on stable isotopes in raindrops during summer in Xinjiang

在干旱区,雨滴在云下降落易受到二次蒸发的影响,明确降水从云层底部降落到地面过程中稳定氢氧同位素的变化在同位素水文学研究中很有必要。基于新疆地面气象站逐小时的观测资料,采用改进后的Stewart模型,研究了新疆雨滴云下蒸发剩余比(从云下到近地面雨滴蒸发后剩余体积占原体积的百分率f)、雨滴中δD变化量(ΔδD)和过量氘变化量(Δd)的时空特征,并分析了Δd与气象要素的关系。结果表明:(1)新疆降水中Δd和蒸发剩余比存在明显的空间差异。(2)当在气温低、相对湿度大、降水强度大、雨滴直径大的情况下,蒸发剩余比较大,Δd接近于0,蒸发剩余比与Δd间的线性关系明显,斜率较低。不同气象条件下,蒸发剩余比与Δd的关系并不固定,利用这一线性关系反推蒸发剩余比应慎重。(3)敏感性分析可以得出,如果各气象站的气温升高2℃,Δd降幅为0.26%～3.10%;如果相对湿度升高5%,Δd升幅为1.23%～8.34%;如果雨强增大10%,Δd升幅为0.06%～0.89%;如果雨滴直径增加0.2 mm,Δd升幅为0.98%～8.16%,但雨滴直径增加量大于1.2 mm时,Δd变化量趋于稳定。

In the arid area, the sub-cloud secondary evaporation usually occurs to the raindrops. It is important to understand the variation of the stable hydrogen and oxygen isotopes of the raindrops when they travel from cloud-base to the ground in the isotope hydrological studies. Based on the hourly meteorological observations from the meteorological stations in Xinjiang, China, the temporal and spatial characteristics of the sub-cloud evaporation residue ratio （which is the ratio of the residual volume of the raindrops after the evaporation as they travel from cloud-base to the near-surface to the original volume, f）, the δ D variation （△δD） of the raindrops and the deuterium excess change （△d）were analyzed by using the modified Stewart model. The relationship between Ad and meteorological parameters as well as the sensitivity analysis of Ad were also analyzed. The results showed as follows： （1）The values of Ad and fpresent spatial dependency in Xinjiang. （2） Under the conditions of low temperature, high relative humidity, high rainfall intensity and large raindrop diameter, fis usually large and Ad is close to 0. The linear relationship between land Ad is evidenced and the slope is low. However the regression coefficients are not always stable under different meteorological conditions, thus the inversion computation of the parameter f based on this linear relationship should be treated with caution. （3） The sensitivity analysis indicates that, if the temperature was increased by 2 ℃, Ad was decreased by 0.26‰- 3.10‰ ; If the relative humidity was increased by 5%, Ad was increased by 1.23‰-8.34‰; If precipitation intensity was increased by 10%, Ad was increased by 0.06‰-0.89‰; If the raindrop diameter was increased by 0.2 mm, Ad was increased by 0.98‰- 8.16‰. However, if the raindrop diameter was larger than 1.2 mm, Ad tended to be stable.