基于ERA5-LAND的中国东北地区近地表土壤冻融状态时空变化特征

土壤冻融交替是陆地表层极其重要的物理过程,土壤冻融状态的频繁变化对地气能量交换、地表径流、植被生长、生态系统及土壤碳氮循环等均具有重要的影响。本文基于1981—2019年ERA5-LAND逐小时土壤温度数据,借助GIS空间分析功能,利用Python编程处理分析了中国东北地区近地表土壤冻融状态的时空变化特征。结果表明：从不同冻融状态起始日期的空间分布来看,近地表不同阶段的起始日期主要受纬度和地形的影响,具有明显的纬度地带性和垂直地带性。春季冻融过渡期和完全融化期的起始日期由东南向西北均呈逐渐推迟趋势,而秋季冻融过渡期与完全冻结期起始日期则由东南向西北随纬度升高越来越早。就不同冻融状态发生天数的空间分布而言,研究区南部春季冻融过渡期发生天数多于北部,西部多于东部,年均发生天数均在30 d以内;秋季发生冻融的天数空间差异不大,研究区一半以上的地区年均发生天数在10 d以内。完全融化期发生天数最多,从东南向西北呈逐渐减少趋势,年均发生天数主要介于150~240 d之间;完全冻结期发生天数则由南向北日益增多,其空间分布表现为一向南开口的簸箕形,各地年均发生天数集中于90~180 d之间。从时间变化趋势来看,近年来春季冻融过渡期起始日期以提前趋势为主,而秋季冻融过渡期起始日期总体表现为延后,致使完全融化期发生天数以增加趋势为主,年均变化速度高达0.2 d/a;大兴安岭以西、呼伦贝尔高原以北地区及辽河平原春季冻融过渡期发生天数呈减少趋势,其他地区为增加趋势;大兴安岭以西地区、呼伦贝尔高原以北地区完全融化期起始日期明显提前;松嫩平原和长白山区秋季冻融过渡期起始日期推迟显著,发生天数的变化趋势呈北增南减的空间分异特征;不同地区完全冻结期起始日期的变化趋势差异显著,中部广大的平原区呈不显著的推迟趋势,而大、小兴安岭、长白山、辽东半岛和辽西丘陵则提前进入完全冻结状态;研究区完全冻结期发生天数呈减少趋势,研究区中部的季节冻土区完全冻结期明显变短,年均减少速度大于0.2 d/a。

Spatiotemporal variations of soil freeze-thaw state in Northeast China based on the ERA5-LAND dataset

The soil freeze-thaw cycle plays an important role in land surface processes. Repeated freeze-thaw cycles can have profound effects on land-atmosphere energy exchange, surface runoff, plant growth, ecosystems and soil carbon & nitrogen cycles. Using spatial analysis functions of geographical information system and python programming language, this paper analyzed the spatial distributions and temporal variations of soil freeze-thaw state in Northeast China based on the ERA5-LAND hourly soil temperature dataset for the period 1981-2019. The results suggest that the start dates of the four soil freeze-thaw periods for the near-surface layer are mainly determined by latitude and topography. The start dates of freeze-thaw transition period in spring (SFTTP) and complete thawing period (CTP) show a southeast-northwest gradient with later starts in the northwest part, while the start dates of freeze-thaw transition period in autumn (AFTTP) and complete freezing period (CFP) exhibit a latitudinal pattern with earlier starts in the north. For most parts of the study area, the average annual number of days for SFTTP is less than 30, with higher values in the south and west compared to the north and east. The number of days for AFTTP, however, is below 10 per year for most parts of the region, with just a slight difference in the study area. The CTP is the longest compared to the other three periods, varying from 150 days in the northwest to 240 days in the southeast. The CFP, which comes next, ranges from 90 to 180 days per year, presenting a dustpan-shaped spatial pattern with higher values in the north and lower values in the south. Trend analysis shows that with the advance of start date for SFTTP and the delay of start date for AFTTP, the number of days for CTP has increased with a rate of 0.2 d/a. The number of days for SFTTP in the Liaohe Plain, the western part of the Da Hinggan Mountains and the northern part of Hulun Buir Plateau shows a decreasing trend, while in other regions an increasing trend is observed. In the western part of the Da Hinggan Mountains and the northern part of the Hulun Buir Plateau, the CTP starts earlier. The start date of AFTTP is significantly delayed in the Songnen Plain and Changbai Mountains, and the trend for the number of days varies substantially with an increase in the north and a decrease in the south. The start date for CFP occurs later in the vast area of the Northeast China Plain and occurs earlier in the Da Hinggan Mountains, Xiao Hinggan Mountains, Changbai Mountains, Eastern Liaoning Peninsula and Western Liaoning Hills. The number of days for CFP shows a declining trend throughout the study area, especially in the seasonally frozen area located in the central part with an annual decreasing rate of more than 0.2 d/a.