陇中黄土高原自然植被下垫面陆面过程及其参数对降水波动的气候响应

作为气候敏感区和生态脆弱区,黄土高原地区的陆面物理过程受气候波动影响十分明显.目前,虽然对黄土高原特定气候条件下的陆面物理过程有了一定认识,但对其随气候波动的动态变化规律认识却非常有限.本文利用黄土高原陆面过程观测试验在陇中黄土高原榆中SACOL站5年多的观测资料,系统分析了该地区自然植被下垫面陆面水分收支和能量平衡及地表反照率、土壤热传导率和粗糙度长度等陆面过程参数对气候波动的响应规律,发现陇中黄土高原自然植被下垫面的陆面过程特征及其参数对降水的气候波动十分敏感,不仅对降水量波动响应显著,而且与降水性质也密切相关.土壤湿度并不完全随年总降水量波动,而是随有效降水量增加而增大；陆面水分收支和土壤水分收支也随有效降水量波动变化明显；近地层垂直感热平流通量随年总降水量增加而减弱,地表能量不平衡差额却随年总降水量增加而增大.夏半年平均反照率随有效降水量增加而减小,但全年平均地表反照率却受冬季积雪时数影响十分明显,而且随积雪时数增加而增大；土壤热传导率和粗糙度长度均随年有效降水量增加而增加.并且,降水量越小,陆面过程参数对降水波动的响应就越敏感.不过,陆面过程参数基本在荒漠气候平均值和农田气候平均值之间波动,并没有突破气候约束,能够用拟合关系来表示它们对降水因子波动的响应规律,这种动态参数化关系可用来改善以往数值模拟或微气象分析时由于使用非动态陆面过程参数所带来的误差.

Responses of the land-surface process and its parameters over the natural vegetation underlying surface of the middle of Gansu in loess plateau to precipitation fluctuation

The Loess Plateau is well known as a specific region sensitive to global climate change, and thus its land-surface process is significantly influenced by climatic fluctuation. Up to now, the land-surface physical process over the Loess Plateau has been basically understood under a specific climatic condition, but the dynamic variation regularity of land-surface process in different climatic states is still lacking in its knowledge. Utilizing the continuous five-year data collected at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) supported by the program 'the Loess Plateau Land-surface Process Experiment (LOPEX)', in this paper, we systematically analyze the regularity of responses of factors including land-surface water and energy budget as well as surface albedo and roughness to climatic fluctuation over the natural vegetation surface of Loess Plateau. The results show that the land-surface process and its relevant parameters are very sensitive to the fluctuations of annual precipitation. Besides, they respond markedly to annual rainfall amount and are also closely related to the nature of rainfall. Soil moisture fluctuales not fully with total amount of rainfall but rises with the increase of effective precipitation with which land-surface water and energy budget also vary. In addition, the vertical sensible heat advection goes up with rainfall increasing, while the trend of surface energy imbalance is opposite. The averaged albedo in summer half-year comes down with the increase of the ratio of effective precipitation to the climatic average, but yearly averaged albedo is evidently affected by the snow-accumulating winter time and rises with the increase of snow-accumulating hours. Soil thermal conductivity and surface roughness both increase with annual effective precipitation increasing to the climatic average, but they are more sensitive to the precipitation fluctuation under a low precipitation condition than under the normal precipitation condition. However, land-surface process parameters over the natural vegetable surface of Loess Plateau basically fluctuate between those of deserts and farmland, and have not yet broken the climatic restrictions. Fitting relationship is used to quantitatively reflect the regularity of response of the parameters to rainfall fluctuations so as to evidently eliminate possible bias brought to numerical models and microclimatic analysis by the previous fixed and un-dynamic changeable land-surface parameters.