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The Annual Variation and Its Influencing Mechanism of Surface Roughness Length of Yuzhong in Semi-arid Areas
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摘要: 利用2006年6月—2010年12月兰州大学半干旱气候与环境观测站 (SACOL) 观测资料，分析了黄土高原自然植被下垫面榆中地表粗糙度时间变化特征，考虑到地形、植被物理特征以及降水和热力条件的影响，分析了东南风向和西北风向粗糙度年变化规律及其影响机理，并分别给出这两个风向归一化粗糙度与时间的拟合关系式。研究发现:对于非均一下垫面，由于地形起伏和下垫面植被差别造成的不同风向粗糙度差异显著。选取东南风向和西北风向，这两个风向的地表粗糙度无论是量级还是年变化特征都有很大差别，且由于地形和植被的差别，东南风向粗糙度年变化趋势与稳定度年变化趋势一致，粗糙度与稳定度存在一定相关关系，而西北风向粗糙度年变化趋势与降水量年变化趋势一致，粗糙度与降水量相关性较好。Abstract:
Based on data observed at the Semi-arid Climate and Environment Observatory of Lanzhou University (SACOL) from June 2006 to December 2010, temporal characteristics of aerodynamic roughness length over the natural vegetation surface of Yuzhong are analyzed. Annual change characteristics of roughness length and influencing mechanisms in the southeast and northwest are analyzed, taking the impact of terrain, vegetation, precipitation and thermal conditions into account, and the fitting relationships between normalized roughness and time are given. It shows that for heterogeneous underlying surface, the difference of aerodynamic roughness length in different wind directions caused by undulating terrain and vegetation difference is very significant. According to the prevailing wind direction, two wind directions which are southeast and northwest are selected. Both magnitude and changing trends of roughness length of two selected wind directions are remarkable different. The averaged roughness length in southeast is 0.015 m, whose magnitude is equal to the roughness length over sparsely vegetated underlying surface like deteriorated grassland, while the averaged roughness length in northwest is 0.123 m, whose magnitude is equal to the roughness length over farmland underlying surface. To eliminate effects of the inter-annual variation of roughness length, the normalized roughness length is injected into the discussion. The time-distributing characteristics in two wind directions vary considerably, which can be considered showing opposite trends. The annual changing trend of roughness length in southeast decreases first and then increases, while it increases first and then decreases in northwest. And due to differences in terrain and vegetation, influencing mechanisms of the time variation of roughness length in the two wind directions are different. The annual variation trend of normalized roughness is consistent with the annual variation of atmospheric stability and the roughness length has a certain relationship with atmospheric stability in southeast due to the stunted sparse vegetation. While the annual variation trend of roughness length is consistent with the annual variation of precipitation and the roughness length has a good relationship with precipitation in northwest due to the impact of vegetation, and the vegetation is mainly effected by the precipitation. The time parametric relationship between normalized roughness and time in two directions can be described by a set of sinusoidal functions, and the related coefficient can reach 0.49 and 0.82, respectively.
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图 1 SACOL站观测环境 (a) 及地形高度 (b)
Fig. 1 Observation environment (a) and topography (b) of SACOL Site
图 2 不同风向区间粗糙度2006年6月—2010年12月月平均值 (a) 及年变化 (b)
Fig. 2 Monthly mean change (a) and monolithic annual variation (b) of aerodynamic roughness length in different wind direction intervals from Jun 2006 to Dec 2010
图 3 不同风向区间平均粗糙度 (单位：m)
Fig. 3 The averaged aerodynamic roughness length in different wind direction intervals (unit:m)
图 4 生长季和非生长季粗糙度随风向区间的变化 (单位：m)
Fig. 4 Variation of aerodynamic roughness length along with wind direction intervals in growing season and non-growing season (unit:m)
图 6 归一化粗糙度年变化及其与时间拟合曲线 (a) 东南风向，(b) 西北风向
Fig. 6 Annual variation of normalized aerodynamic roughness length and fits for relation between roughness length and time in two wind directions of southeast (a) and northwest (b)
图 7 稳定度 (a) 与降水量 (b) 年变化
Fig. 7 Annual variations of stability (a) and precipitation (b)
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