青藏公路沿线多年冻土区活动层起始冻融时间的时空变化特征和影响因素

多年冻土区活动层冻融格局对气候系统、能量平衡、水文过程和生态系统有重要的影响，地表冻融时间是反映冻融格局时空变化的重要指标。为了探明多年冻土区活动层起始冻融时间的影响因素和机制，通过对青藏公路沿线8个典型活动层观测场地表起始融化时间（OOT）和起始冻结时间（OOF）进行研究，分析了不同观测场起始冻融时间的时空差异及其影响因素。结果表明：（1）青藏高原多年冻土区活动层起始融化主要发生在4月中下旬，起始冻结主要发生在10月中下旬。OOT的年际变化幅度远大于OOF，每年起始冻结的发生较起始融化更为准时。（2）起始融化发生时的气温普遍比起始冻结发生时高1~4℃。气温对OOT的影响要比对OOF大，其中OOT的变化主要与春季气温有关，冬季气温对其影响不大。（3）植被和土壤水分对OOT和OOF有重要调节作用，土壤含水率越高，植被状况越好，起始融化和冻结的发生时间往往越迟。（4）在起始融化和冻结阶段，厚度较大和持续时间较长的积雪对地温变化有明显的抑制作用，对OOT和OOF有延迟作用。

Spatial and temporal variation characteristics on the onset dates of freezing and thawing of active layer and its influence factors in permafrost regions along the Qinghai-Tibet Highway

The situation of freezing and thawing within the active layer has a vital influence on the climate system, energy balance, hydrological processes and ecological system, and the onset date of freezing and thawing is an important index to describe the spatial and temporal variations of freezing and thawing condition. In order to find out the influence factors and process on onset of freezing and thawing within the active layer, eight typical observation sites in permafrost regions along the Qinghai-Tibet Highway were selected, and their onset date of freezing (OOF) and the onset date of thawing (OOT) at each site are analyzed. The analyses showed that:(1) Within the active layer in permafrost on the Tibetan Plateau, the initial thawing occurs mainly in the middle to late April, and the initial freezing occurs mainly in the middle to late October; the annual variation of OOT is greater than that of OOF, and the freezing of the active layer occurs more regular than thawing. (2) The air temperature when the initial thawing of the active layer occurs is generally 1-4℃ higher than that when the initial freezing occurs; the effect of air temperature at OOT is greater than that at OOF; the variation of OOT is mainly related to the mean air temperature in spring, and barely influenced by the mean air temperature in winter. (3) Vegetation and soil moisture play an important role to OOT and OOF; high soil moisture, as well as nice vegetation, generally delays the onset of freezing and thawing. (4) When the freezing or thawing is about to start, snow cover with great depth or rather long duration obviously inhibits the rising of ground temperature and then delays the OOT and OOF, however, this is an occasional event.