Turbulent Flows and Scalar Transport in the Forest-Atmosphere Interface over a Complex Terrain

复杂地形对森林冠层以上涡旋相关方差测量提出了重要的问题。改进复杂地形上的涡动相关方差测量，需要更好地了解复杂地形如何影响森林冠层上方和内部湍流的时空变异性。这将导致大气与植被之间动量、热量和尺度交换测量的改进，从而可以在不同的时空尺度上对复杂地形的地表能量平衡、水循环和碳收支进行可靠的解释/评估。目前，对于复杂地形、流入的动力和热力学条件以及植物冠层如何同时作用调节湍流结构，从而调节动量、热量、水蒸气和二氧化碳的输送，还没有明确的认识。在这项研究中，将通过对欧洲EGER实验（山区交换过程）与大涡模拟（LES）相结合的测量数据进行分析，研究复杂地形上森林冠层-大气界面的湍流。2011年6月和7月，华盛顿州立大学（WSU）的研究小组与其他六个国际小组一起参加了在德国巴伐利亚州东北部Weidenbrunnen Waldstein （DE-Bay）的FLUXNET站点进行的EGER实验。每个小组都贡献了不同的仪器和研究活动，以最大限度地绘制现场的三维湍流结构。除了数据分析，多层冠层模块将被纳入天气研究与预报模型(WRF) -LES (WRF-LES)，以探索平均/湍流的空间变化和时间演变，并量化不同机制对动量/热量/H2O/CO2转移的相对贡献。总的来说，本研究将研究：1)与理想山丘上观察到的结构相比，“真实”地形诱导的压力扰动和冠层的相互作用如何改变湍流结构，包括相干结构和高阶湍流统计，如速度方差、湍流应力/通量、压力方差和冠层以上和冠层内TKE的产生/损失？2)与以往在中性大气条件下的结果相比，不同的大气稳定性条件如何改变冠层内外平均/湍流主要特征的时空变化？湍流的主要特征包括剪切层、拐点、TKE、二阶统计量、u型和w型的偏度和峰度、尾迹区域和尾迹深度（仅背风侧）以及再循环（仅背风侧）。3)在地形、冠层和稳定性的共同作用下，平均/湍流如何导致CO2通量、水平和垂直平流、通量散度以及CO2源和汇的时空变化？这些CO2通量时空变化的动力机制是什么？对CO2通量塔测量的影响是什么？学术价值：总的来说，这项研究将提高对山区林冠和大气之间的平均和湍流流动以及动量、热量和标量（如二氧化碳）交换的理解。应用包括：1)复杂地形下的碳循环模拟；2)复杂地形下的风能预测；3)复杂环境下的污染物扩散。更广泛的影响：这项工作的结果将提高我们在复杂地形中量化碳、水和能量流动的整体能力，从而提高我们对全球碳科学重要组成部分的理解。这些结果将有助于碳循环科学和FLUXNET社区在复杂地形上限制碳预算和高档二氧化碳通量，从塔到景观尺度甚至到区域尺度。基于多层冠层模块的WRF-LES模型系统将有助于研究复杂地形上的冠层流和PBL流；在确定风力涡轮机的潜在地点方面的风能应用；森林管理，以确定在大风条件下树木受损的高风险地点；森林火灾行为在火灾传播量化中的应用；这项研究将直接有助于WSU博士研究生的教育研究培训。研究结果将通过FLUXNET社区、会议和研讨会传播给更广泛的受众，并将用于与华盛顿州立大学本科和研究生课程相关的课程和讲习班，以及以大气化学、空气质量和气候变化为重点的夏季REU项目。

Complex terrain poses significant problems to eddy covariance measurements above forest canopies. Improving eddy covariance measurements over complex terrain requires a better understanding of how complex terrain influences spatial and temporal variability in turbulent flows above and within forest canopies. This will lead to improvements in measurements of the exchange of momentum, heat, and scalars between the atmosphere and vegetation, so that reliable interpretations/assessments of the surface energy balance, water cycle, and carbon budget over complex terrain can be made over various temporal and spatial scales. Currently, there is no clear understanding of how the simultaneous action of complex terrain, dynamic and thermodynamic conditions of inflows, and plant canopies modulate turbulence structures and thus transport of momentum, heat, water vapor, and carbon dioxide. In this research, turbulence in the forest canopy-atmosphere interface over a complex terrain will be examined by conducting analyses of the data measured in a European EGER experiment (ExchanGE processes in mountainous Regions) integrated with large-eddy simulations (LES). The Washington State University (WSU) research team along with six other international groups participated in the EGER experiment that was conducted in June and July of 2011 at the FLUXNET site in Weidenbrunnen Waldstein (DE-Bay), located in North-Eastern Bavaria, Germany. Each group contributed different instruments and research activities to map, to the fullest extent possible, three-dimensional turbulence structures at the site. In addition to data analysis, a multi-layer canopy module will be incorporated into the Weather Research and Forecasting Model (WRF) - LES (WRF-LES) to explore spatial variations and temporal evolutions of mean/turbulent flows and quantify relative contributions of different mechanisms to momentum/heat/H2O/CO2 transfer. Collectively, this research will examine:1) How does the interaction of 'real' topography-induced pressure perturbations and canopy alter turbulence structures, including coherent structures and high-order turbulent statistics such as velocity variances, turbulent stresses/fluxes, pressure variance, and production/loss of TKE above and within the canopy, as compared with the structures observed over idealized hills?2) How do different atmospheric stability conditions alter spatial and temporal variations in the main features of mean/turbulent flows within and above the canopy, as compared with previous results under neutral atmospheric conditions? The main features of turbulent flows include shear layer, inflexion point, TKE, second-order statistics, skewness and kurtosis of u and w profiles, wake region and wake depth (lee side only), and recirculation (lee side only).3) How do mean/turbulent flows as a result of the simultaneous actions of topography, canopy, and stability, lead to spatial and temporal variations in CO2 fluxes, horizontal and vertical advections, flux divergence, and CO2 sources and sinks? What are the dynamic mechanisms for these spatial and temporal variations in CO2 fluxes and the implications for tower measurements of CO2 fluxes?Intellectual Merit: Overall, the study will provide an improved understanding of mean and turbulent flows and exchange of momentum, heat, and scalars (e.g., CO2) between the canopy and the atmosphere over mountainous regions. Applications include: 1) simulation of carbon cycling in complex terrain, 2) wind energy predictions in complex terrain, and 3) pollutant dispersion in complex environments.Broader Impacts: Results from this work will improve our overall ability to quantify carbon, water, and energy flows in complex terrain and thus improve our understanding of important components of global carbon science. The results will be beneficial to carbon cycle science and the FLUXNET community in helping constrain the carbon budget and upscale CO2 fluxes from tower to landscape scale and even to regional scale over complex terrain. The updated WRF-LES modeling system with a multi-layer canopy module will be beneficial to a variety of research communities in studying canopy flows and PBL flows over complex terrain; wind energy applications in terms of identifying potential locations for wind turbines; forest management in identifying locations of high risks of tree damage in windy conditions; and forest fire behaviors in quantifying fire propagation; The research will contribute directly to the educational research training of Ph.D. students at WSU. The results will be disseminated to a broader audience through the FLUXNET community, conferences, and seminars, and will be used in courses and workshops related to WSU's undergraduate and graduate curriculum as well as the summer REU program which is focused on atmospheric chemistry, air quality, and climate change.