| 摘要: |
| 为探究寒潮过程中大气水汽同位素的响应特征及其动力机制，基于2021年11月4—9日兰州市一次典型寒潮事件在2 m高度处采集的144组高时间分辨率水汽同位素观测数据，结合气象资料与再分析数据，探究寒潮期间δ18O、δ2H、δ17O、d-excess和17O-excess的演变规律及其与天气系统的内在联系。结果表明：（1）寒潮入侵导致水汽同位素急剧贫化：δ18O降低15.33‰（从-20.25‰降至-35.58‰），δ2H降低116.54‰（从-139.21‰降至-255.75‰），δ17O降低8.19‰（从-10.58‰降至-18.77‰）；相反，17O-excess升高200.79 per meg（从101.75 per meg升至302.54 per meg），而d-excess的昼夜变化规律被打破。同位素演变呈现明显的3阶段模式，准确记录了水汽来源由局地暖湿气团→远源干冷气团→混合气团的转换过程。（2）寒潮期间局地大气水汽线为δ2H=(7.09±0.17)δ18O+(2.67±4.46)（R2=0.96），其斜率和截距均高于季风期，反映水汽经历更强的非平衡分馏过程以及西伯利亚-蒙古高原源区的低温低湿条件。（3）δ18O与气温和相对湿度的相关性在寒潮不同阶段表现不一致，整体上受大尺度天气系统动力过程主导，局地气象因子的控制有限。17O-excess与相对湿度的相关性由阶段1的R2=0.25降至阶段2的R2=0.15，成功分离源区信号与局地干扰，验证了其作为水汽源区相对湿度示踪指标的有效性。（4）在动力机制方面，西风槽与低涡系统通过平流输送与动力抬升，直接输入贫化水汽同位素并增强云内分馏，同时高低空急流耦合驱动垂直混合，促使高空贫化水汽下传，抑制局地蒸发，凸显源区信号。研究证实水汽同位素是揭示寒潮事件中水汽来源、输送路径和相变过程的有效示踪剂，为理解极端冷事件相关的水文循环过程提供了新的同位素视角。 |
| 关键词: 寒潮 水汽同位素 大气环流系统 水汽来源 |
| DOI：10.7515/JEE2025051 |
| CSTR：32259.14.JEE2025051 |
| 分类号: |
| 基金项目:国家自然科学基金联合基金项目（U23A2013） |
| 英文基金项目: |
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| Water vapor isotope characteristics during cold wave events and their implications for the dynamic mechanism of weather systems |
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ZHANG Rongrong，LIU Jingfeng
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College of Geography and Environmental Science, Northwest Normal University, Lanzhou 730070 , China
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| Abstract: |
| Background, aim, and scope Cold waves are extreme winter events from southward polar air movement, yet their moisture dynamics remain unclear. Water vapor isotopes trace evaporation, fractionation, and mixing, helping separate remote advection from local effects. This study examines multi-stage isotopic evolution during a cold wave in Lanzhou, evaluates local versus large-scale processes, and identifies key mechanisms controlling isotopic composition, offering an isotopic perspective on moisture cycling in extreme cold events. Materials and methods This study focuses on a typical cold wave event in Lanzhou from November 4 to 9, 2021. A total of 144 water vapor isotope samples (δ2H, δ18O, and δ17O) were measured at 2 m height using a Picarro L2140-i ultra high precision water isotope analyzer, with data at this height selected to represent near surface conditions and avoid surface interference. Meteorological data were obtained from the automatic weather station at Northwest Normal University and the NCEP FNL reanalysis dataset. Isotopic parameters (d-excess and 17O-excess) were calculated to investigate water vapor sources and transport processes. Results The cold wave event induced marked isotopic depletion in ambient water vapor. During the cold wave period, δ18O decreased by 15.33‰ (from -20.25‰ to -35.58‰), δ2H decreased by 116.54‰ (from -139.21‰ to -255.75‰), and δ17O decreased by 8.19‰ (from -10.58‰ to -18.77‰). In contrast, δ17O-excess increased by 200.79 per meg (from 101.75 per meg to 302.54 per meg), while the normal diurnal oscillation of d-excess was suppressed. The isotopic time series exhibited three distinct stages. The local meteoric water vapor line during the cold wave was fitted as δ2H = (7.09 ± 0.17)δ18O + (2.67 ± 4.46) (R2 = 0.96), with slope and intercept higher than those of the monsoon-period water vapor line. Regression analysis showed that for δ18O versus temperature and relative humidity, the correlation coefficients varied across stages, with R2 values of 0.03 and 0.05 for the entire event (p ＞ 0.05). The R2 between δ17O-excess and relative humidity decreased from 0.25 in Stage 1 to 0.15 in Stage 2. At the synoptic scale, the 500 hPa geopotential height field revealed a transition from a “two-ridge-one-trough” pattern to a southward-cutting trough, accompanied by enhanced vertical motion (up to -0.5 Pa/s at mid-troposphere) and low-level cyclonic vorticity (up to 14×10-5 s-1), with divergence showing low-level convergence and upper-level divergence during Stage 2. Discussion The three-stage isotopic evolution closely mirrored the sequential replacement of local moist air by advected cold-dry air and subsequent mixing, indicating that synoptic-scale circulation shifts were the primary control on water vapor isotopic composition during this cold wave. The elevated slope and intercept of the cold-wave water vapor line, relative to the monsoon-period line, suggest more intense kinetic fractionation during vapor transport. This is consistent with the cold and arid conditions over the Siberian-Mongolian Plateau source region, where low relative humidity enhances diffusion-related fractionation, as reflected by the concurrent rise in 17O-excess. The weak and statistically insignificant correlation between δ18O and local meteorological factors (temperature and humidity) throughout the event implies that local surface processes played a minor role compared with large-scale advective forcing. In contrast, the stage-dependent decline in the 17O-excess-humidity correlation from Stage 1 to Stage 2 demonstrates that 17O-excess effectively decouples the source-region humidity signal from local environmental interference, confirming its utility as a source-region humidity tracer. Dynamically, the southward-propagating westerly trough and associated low-vortex system contributed to isotopic depletion through three intertwined mechanisms: (1) long-range advection of isotopically depleted dry air from high latitudes, which directly lowered ambient δ18O; (2) enhanced Rayleigh distillation within the cloud layer, driven by vigorous uplift that preferentially removed heavy isotopes; (3) vertical mixing between the boundary layer and free troposphere, facilitated by the coupling of upper-level and low-level jets, which transported more depleted vapor downward while suppressing local evaporative input. These processes collectively amplified the source-region isotopic signal at the surface. These findings are broadly consistent with previous isotope-based studies of extratropical cyclones and cold surges, but they further demonstrate that the combination of δ18O, d-excess, and 17O-excess can resolve moisture-source transitions at sub-daily timescales, offering a more detailed picture of the hydrological response to baroclinic disturbances. Conclusions This study demonstrates that high-resolution water vapor isotope observations can effectively resolve the moisture-source evolution and dynamic fractionation processes embedded in cold-wave weather systems. The integrated use of δ18O, d-excess, and 17O-excess provides a robust diagnostic framework for tracing the hydrological fingerprints of synoptic-scale disturbances, with implications for improving the understanding and forecasting of extreme cold events. Recommendations and perspectives Future research should integrate isotope-enabled numerical models to quantify the contributions of different weather systems to isotopic variations. Expanding the spatial and temporal coverage of isotope observations could further enhance the understanding of cold wave dynamics. |
| Key words: cold wave water vapor isotopes atmospheric circulation system moisture source |
