An hourly-scale assessment of sub-cloud evaporation effect on precipitation isotopes in a rainshadow oasis of northwest China

In arid northwest China where the precipitation intensity is relatively low, the stable hydrogen and oxygen isotopes (62H and 618O) in precipitation are usually impacted by the sub-cloud evaporation. To understand the sub-cloud evaporation effect under an arid climate, we used the hourly meteorological data at 14 stations in the Kaxgar-Yarkant River Oasis, a rainshadow oasis of northwest China, and estimated the monthly and hourly isotopic change in falling drops from the cloud base to the ground. The results showed that the hourly meteorological observations are an effective way to assess the spatiotemporal pattern of sub-cloud evaporation effect. Across the 14 stations, the annual mean changes in deuterium excess below cloud base ranged from -19.1%o to -6.8%o. The impacts of sub-cloud evaporation during spring and autumn are larger than those during winter and weaker than those during summer. The exponential regression (R2 = 0.96), instead of linear regression (R2 = 0.85), is better at describing the relationship between the raindrop remaining fraction and the isotopic changes from the cloud base to the ground. The sensitivity analysis of isotopic changes to different relative humidity scenarios shows that the drying scenario may lead to slightly larger sensitivity than the wetting scenario. The impact of sub-cloud evaporation on stable isotope compositions in precipitation may be underestimated when low precipitation events (especially less than 1 mm/h) are not sampled.

Automated summary

In arid northwest China, sub-cloud evaporation affects the stable isotope compositions in precipitation. Analysis of meteorological station data in the Kaxgar-Yarkant River Oasis revealed that sub-cloud evaporation has greater impact in spring and autumn, less impact in winter, and stronger impact in summer. Exponential regression was found to be more accurate in describing the relationship between raindrop remaining fraction and isotopic changes. Sampling of low precipitation events is crucial for accurately assessing the impact of sub-cloud evaporation on isotopic compositions.