The evaluation of flux aggregation methods using aircraft measurements in the surface layer
The evaluation of flux aggregation methods using aircraft measurements in the surface layer
复制标题
使用飞机在表层测量的通量聚合方法的评估
DOI：
10.1016/s0168-1923(99)00093-3
复制
发表时间：
1999
影响因子：
6.2
通讯作者：
A. Jochum
中科院分区：
文献类型：
--
作者：
M. Frech;A. Jochum
使用整体空气动力学关系来预测非均匀地形表面动量、热量或水分的湍流通量可能导致交换系数依赖于平均尺度，即依赖于模型网格尺寸。本文通过比较四种不同通量聚合方法的结果和飞机在大气近地层的数据，研究了在森林为主的异质北方景观中动量和热量交换系数的尺度依赖性。我们使用了NOPEX（北方半球陆面气候过程试验）期间进行的11次飞行中的41次飞机飞行（每次飞行长度为60公里）。我们发现动量和标量转移的不同行为。虽然动量通量是由任何单一网格框（或飞机跑道段）中最粗糙的元素（在我们的情况下是森林）决定的，但感热通量是由占主导地位的土地覆盖类型决定的。动量传递的有效交换系数似乎与平均尺度无关。这意味着有效粗糙度长度可以用于动量通量参数化目的，并且可以使用标准相似理论。动量传递的有效粗糙长度为1.5 m。与此相反，显热交换的有效交换系数则依赖于平均尺度。因此，亚网格尺度的陆面和上覆边界层的不均匀性必须明确解决，以正确的网格平均地表热通量预测。敏感性分析和比较的数据采取了一个温暖和干燥的陆地表面表明，这种规模的依赖似乎仅限于地面温度接近的温度覆盖的空气表面。当存在子网格水面时，情况尤其如此。
The use of the bulk aerodynamic relationship to predict the turbulent flux of momentum, heat or moisture at the surface over heterogeneous terrain may lead to a dependence of the exchange coefficient on averaging scale, i.e. on model grid size. This study investigates the scale dependence of exchange coefficients for momentum and heat transfer in a forest dominated heterogeneous boreal landscape by comparing the results of four different flux aggregation methods with aircraft data in the atmospheric surface layer. We have used a total of 41 aircraft runs (with a length of ≈60km each) from 11 flights performed during NOPEX (a NOrthern hemisphere land-surface climate Processes Experiment). We find a different behavior for momentum and scalar transfer. While momentum flux is governed by the roughest elements (in our case forest) in any single grid box (or aircraft run segment), the sensible heat flux is determined by the dominant land cover type. The effective exchange coefficients for momentum transfer appear to be independent of the averaging scale. This implies that an effective roughness length can be used for momentum flux parameterization purposes and standard similarity theory can be used. The effective roughness length for momentum transfer is on the order of 1.5m. In contrast, the effective exchange coefficient for sensible heat tranfer does depend on averaging scale. Therefore, the subgrid scale heterogeneity of the land-surface and the overlying boundary layer must be resolved explicitly in order to predict the correct grid averaged surface heat flux. Sensitivity analysis and comparison with data taken over a warmer and drier land-surface show, that this scale dependence seems to be confined to land surfaces with a surface temperature close to the temperature of the overlying air. This is particularly the case when sub-grid water surfaces are present.