非均匀地表空气动力学有效粗糙度的统计特征

利用基于大气边界层Monin-Obukhov相似理论、通量守恒和质量守恒原理设计的非均匀地表空气动力学有效粗糙度计算方案, 研究了3种不同地表类型情况下有效粗糙度的统计特征, 并分析了大气层结稳定度和粗糙变率对有效粗糙度的影响. 结果表明, 有效粗糙度总是大于面积加权平均粗糙度, 大部分情况下有效拖曳系数比平均拖曳系数大10%以上; 有效粗糙度虽然和大气层结稳定度有关, 但对粗糙变率更敏感, 粗糙变率加倍将使有效粗糙度相对变化百分比达到加倍前的4倍, 有效拖曳系数相对变化百分比达到加倍前的3倍. 因此, 非均匀下垫面的数值模式中, 不能简单地采用面积加权平均粗糙度, 需要采用能表示下垫面非均匀性综合效应的有效粗糙度. 关键词： 非均匀性 有效粗糙度 粗糙变率 层结稳定度

Statistical features of aerodynamic effective roughness length over heterogeneous terrain

With a new scheme of effective roughness length for heterogeneous terrain, based on the atmospheric boundary layer Monin-Obukhov similarity theory as well as flux and mass conservation principles, the statistical features of effective roughness length and its sensitivity to atmospheric stratification stability and roughness step for three surface category case are investigated. The results show that the effective roughness length is greater than the area-weighted logarithmic average one and the effective drag coefficient is more than 10% greater than the average one in most cases. The effective roughness length is much more sensitive to the roughness step, though it is dependent on the atmospheric stratification stability, and the relative percentage of effective roughness length and the effective drag coefficient will be 4 times and 3 times, respectively, for the double roughness step case. Therefore, the area-weighted average roughness length should be replaced by the effective one when the surface heterogeneity is considered in numerical models, which can represent the integrated effect of heterogeneous terrain.