青藏高原中部典型下垫面活动层水热动态及其热扩散率研究

多年冻土活动层， 尤其是浅层土壤的水热传输机制， 以及冻融过程的时空异质性是研究地-气间能水交换的关键。利用位于青藏高原中部的唐古拉和通天河两个活动层观测场2013年的土壤温度和水分数据， 比较了不同下垫面浅层土壤日冻融循环过程的差异， 以及不同冻融阶段的地温日变化及热扩散率特征。结果表明： 根据一日之内地温的正负波动， 浅层土壤的冻融过程可以划分为解冻期、 完全融化期、 始冻期和完全冻结期四个时期， 其中解冻期和始冻期统称为日冻融循环发生期。解冻期的持续天数和深度明显高于始冻期， 高寒草原的日冻融循环天数和发生深度明显高于高寒草甸。浅层土壤（0 ~ 20 cm）日地温变化普遍呈现明显的正弦波动趋势， 且不同冻融阶段的振幅差异较大， 由于相变的缘故， 解冻期的日地温变化振幅最小。高寒草甸的日地温振幅显著低于高寒草原， 说明日地温动态与土壤质地和土壤水分密切相关， 植被作为热绝缘层， 减弱了地温对气温波动的响应。地表下5 ~ 10 cm的热扩散率显著大于10 ~ 20 cm深度， 且5 - 10月融化季的热扩散率显著大于冻结季。热传导对流方程可以描述多年冻土区典型下垫面在季节冻融循环周期内不同月份的水分迁移方向。

Hydro-thermal dynamic and soil thermal diffusivity characteristics of typical active layer on the central Tibetan Plateau

The spatiotemporal heterogeneity of freezing and thawing processes within the active layer， especially on the ground surface， is the key to study the energy and water exchange between permafrost and atmosphere. Therefore， this study used the daily data of ground temperature and moisture from the Tanggula and Tongtianhe active layer observation sites in 2013 to compare variation of the daily freezing-thawing cycles， as well as the soil thermal diffusivity characteristics. The results showed that the seasonal freezing-thawing processes could be divided into four stages： unstable thawing stage， stable thawing stage， unstable freezing stage and stable freezing stage. The duration of unstable thawing stage was significantly longer than that of unstable freezing stage. The duration and depth of daily freezing-thawing cycles in alpine steppe were significantly bigger than those in alpine meadow. Most of the daily ground temperature dynamics showed obvious sinusoidal fluctuations， and the amplitudes of all freezing-thawing stages were quite different. The amplitude of the daily ground temperature changes in the unstable thawing stage was the smallest. Moreover， the amplitude of daily ground temperature in alpine meadow was significantly lower than that in alpine steppe， which indicated that vegetation had a significant impact on daily ground temperature dynamics. As a thermal insulation layer， vegetation cover can weaken the influence of atmosphere temperature fluctuation on ground temperature. The thermal diffusivity within the 5 - 10 cm soil layer was significantly higher than that within the 10 - 20 cm soil layer， and the thermal diffusivity in the warm season from May to October was significantly higher than that in the freezing season. The conduction-convection algorithm can reflect the moisture transfer trend of alpine grassland in permafrost regions.