青藏高原多年冻土区土壤冻融过程对地表能量通量的影响研究

Impacts of Freeze/Thaw Processes on Land Surface Energy Fluxes in the Permafrost Region of Qinghai-Xizang Plateau

利用国家重大科学研究计划项目"青藏高原沙漠化对全球变化的响应"北麓河站2014-2015年陆面过程观测资料,根据5 cm土壤日最高和最低温度将冻土分为融化过程、完全融化、冻结过程和完全冻结四个阶段,分析了地表感热通量Hs、潜热通量LE、地表土壤热通量G_0和波文比在不同冻融阶段的季节和日变化特征,并探讨了土壤冻融过程对地表能量及其分配的影响。结果表明,波文比和G_0的季节变化受土壤冻融阶段转变的影响显著,其中土壤完全融化使波文比减小,G_0变为正值;土壤冻结使波文比增大,G_0变为负值。冻结过程对Hs和LE变化趋势的影响不明显,但是使波文比显著增大;融化过程使Hs停止增长并出现减小趋势,使LE增大,从而使波文比显著减小。Hs的日变化在不同冻融阶段差异较小。LE的日变化主要与浅层土壤含水量的大小和日变化有关,其中完全融化和完全冻结阶段土壤含水量的日变化较小,土壤含水量越大,LE越大;在融化过程和冻结过程阶段,土壤含水量的日变化较大,且与R_(net)的日变化相反,限制了LE的增长。在冻结过程阶段,受冻融过程的影响,G_0的日变化小于其他阶段。

Permafrost is ubiquitous on the Qinghai-Xizang Plateau.Soil freezing/thaw processes change the surface condition,which significantly influences the material and energy exchange between land and atmosphere.Observational data of land surface processes from the National Basic Research Program of China ＇ The response of desertification over Tibet Plateau to global changes＇ at Beiluhe during 2014-2015 were used in this paper.The permafrost is divided into four stages based on the daily maximum and minimum soil temperature at 5 cm depth,i.e.the thawing stage,the completely thawed stage,the freezing stage and the completely frozen stage.Characteristics of the seasonal and diurnal variations of surface sensible heat flux（Hs）,latent heat flux（LE）,ground soil heat flux（G_0） and Bowen ratio at different freeze/thaw stages are analyzed and the impacts of freeze/thaw processes on land surface energy fluxes and energy partitioning are investigated.The results show that：the transition of soil freeze/thaw stages has remarkable effect on the seasonal variations of Bowen ratio and G_0.Bowen ratio decreases and G_0 changes from positive to negative at the beginning of the completely thawed stage and conversely when the soil starts to freeze.The impact of freezing process on the trend of Hs and LE is not obvious but Bowen ratio increases significantly.Thawing process leads to the reduction in Hs and increase in LE,which makes Bowen ratio decrease obviously.The diurnal variation of Hs is similar during different stages.The diurnal variation of LE is related to the near surface soil water content and its diurnal variation.Due to weak diurnal variation,the higher soil water content leads to larger LE during completely frozen and thawed stages.LE is constrained during the freezing and thawing stage by the combined effects of the large diurnal variation of soil water content and the out-of-phase changes of soil water content and R_（nct）.Influenced by freeze/thaw processes,the diurnal variation of G_0 is weak during the freezing stage.