青藏高原降雨增加和气温升高对多年冻土水热动态贡献研究

Contributions of increased rainfall and rising air temperature on hydrothermal dynamics in the permafrost of the Qinghai-Xizang Plateau

在人类活动和全球气候变化驱动下,青藏高原气候整体呈现暖湿化变化趋势,由此引发的多年冻土活动层水热变化对寒区生态环境和寒区工程稳定性产生显著影响。目前,温度升高对多年冻土的影响机制较为明确,但降雨增加、降雨增加与气温升高共同作用下的多年冻土水热响应过程和机制尚不明确。在考虑雨水感热作用的地表能水平衡-冻土水热耦合模型的基础上,对比研究气温升高、降雨增加单一作用及其共同作用对活动层水热影响机制。结果表明:相比气温升高和降雨增加单一作用,暖湿化复合作用导致地表净辐射通量和蒸发潜热通量增长显著,地表感热降低更加明显,雨水感热影响较小,地表土壤热通量呈增加趋势;暖湿化复合作用下温度梯度液态水通量增长显著,基质势梯度液态水通量在浅层增幅也大于单独升温作用,但小于单一降雨增加作用,暖湿化导致暖季土壤含水率增幅小于单独降雨作用;暖湿化作用下活动层热传导通量在冷季增加显著且增幅小于单独升温作用,而液态水对流传热在暖季增加明显且增幅小于单独湿化作用;降雨增加促使土体暖季降温显著,暖湿化与单一气温升高均导致土体在冷季升温效果高于暖季;气温升高1.0℃引起多年冻土上限下移10cm,降雨增加100 mm促使上限抬升8 cm,暖湿化共同作用导致冻土上限下移6 cm;在暖湿化作用下,降雨增加对冻土降温作用较小,气温升高对冻土的升温效应仍占据主导。

Driven by human activities and global climate change,the climate on the Qinghai-Xizang Plateau is experiencing a warming and humidifying trend.It significantly impacts the thermal-moisture dynamics in the active layer of the permafrost,which in turn affects the ecological environment of cold regions and the stability of cold region engineering.While the effect of air temperature on permafrost thaw has been well quantified,the processes and mechanisms behind the thermal-moisture response of the permafrost under the combined influence of increased rainfall and rising air temperature remain contentious and largely unknown.A coupled model was applied to quantify the impacts of increased rainfall,rising air temperature,and their compound effects on the thermal-moisture dynamics in the active layer,considering the sensible heat of rainwater in the ground surface energy balance and water balance process.The results indicate that the compound effect of warming and humidifying resulted in a significant increase in surface net radiation and evaporation latent heat,a more significant decrease in surface sensible heat,and a smaller impact of rainfall sensible heat,leading to an increase in surface soil heat flux.The compound effect of warming and humidifying leads to a significant increase in the liquid water flux with temperature gradient.The increase in liquid water flux due to the temperature gradient is larger than that of warming alone but smaller than the effect of humidifying alone.Warming and humidifying result in a smaller increase in soil moisture content during the warm season compared to rainfall increases alone.The thermal conductivity heat flux in the active layer increases significantly during the cold season but less than the effect of warming alone.The convective heat flux of liquid water flux increases noticeably during the warm season but less than the effect of rainfall increases alone.Increased rainfall significantly cools the soil during the warm season,while both warming and humidifying lead to a more pronounced warming effect on the soil during the cold season than during the warm season.An increase in the average annual temperature by 1.0℃leads to a downward shift of the permafrost table by 10 cm,while an increase in rainfall by 100 mm causes an upward shift of the permafrost table by 8 cm.The combined effect of warming and humidifying results in a downward shift of the permafrost table by 6 cm.Under the influence of climate warming and humidifying,the cooling effect of increased rainfall on permafrost is relatively small,and the warming effect of increased temperature still dominates.