Turbulent Fluxes and their Relation to Surface Heterogeneity and Transient Mesoscale Motions

9310576马赫特这项工作的目的是分析几个新的和现有的数据集，以提高我们对边界层流动对非均匀表面的响应的理解，从多个、定义明确的不连续面到更复杂的表面非均质性。新的数据包括飞机、塔楼和有限的遥感观测，并允许详细检查以前没有考虑过的异质性类别。其中一个目标是研究对区域平均地表通量的主要影响。这些影响包括：1)散布的树木、林缘和其他障碍物引起的与压力阻力相关的动量通量的增强；2)地面条件变化引起的下游过渡区的非平衡效应；3)与空间变异性和通量与梯度之间的非线性关系有关的额外雷诺项；4)中尺度运动的直接输送。为了解决这一问题，将飞机在同一轨迹上重复运行所观测到的流动形式分解为湍流、瞬变中尺度运动和定常中尺度运动(主要与地表非均质性有关)。文中将详细讨论这两个中尺度分量对湍流通量的调制。从更实际的角度来看，上述对面平均通量的影响将隐含在有效粗糙长度和对应于面平均通量的交换系数的计算中。在这些计算之前，将对通量采样误差进行分析，其中将包括对估算通量误差的技术的一些研究。拟议的理论工作将仅限于根据现有的几个线性和非线性预测来解释非均匀地形上的中尺度环流。***

9310576 Mahrt The objective of this work is to analyze several new and existing data sets toward the goal of improving our understanding of the response of boundary-layer flow to heterogeneous surfaces ranging from multiple, well defined, discontinuities to more complex surface heterogeneity. The new data include aircraft, tower and limited remote sensing observations and allow examination of classes of heterogeneity in detail not previously considered. One of the goals is to examine the main influences on the area averaged surface fluxes. These influences include: 1) enhancement of the momentum flux associated with pressure drag induced by scatter trees, forest edges and other obstacles, 2) non-equilibrium effects in transition zones downstream from changes of surface conditions, 3) extra Reynolds terms associated with spatial variability and the nonlinear relationship between fluxes and gradients and 4) direct transport by mesoscale motions. To approach this problem, the flow observed by repeated aircraft runs over the same track is formally decomposed into turbulence, transient mesoscale motions and stationary mesoscale motions (primarily associated with surface heterogeneity). The modulation of the turbulence flux by the two mesoscale components will be considered in detail. From a more practical point of view, the above influences on the area averaged flux will be implicitly incorporated into calculation of effective roughness lengths and exchange coefficients corresponding to the area averaged fluxes. These calculations will be preceded by an analysis of flux sampling errors which will include some research on the techniques for estimating flux errors. Proposed theoretical work will be limited to interpretation of the mesoscale circulations over heterogeneous terrain in terms of several existing linear and nonlinear predictions. ***