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

复杂的地形给森林冠层上方的涡动协方差测量带来了很大的问题。改进复杂地形上的涡旋协方差测量需要更好地了解复杂地形如何影响森林树冠上方和内部湍流的空间和时间变异性。这将改进对大气和植被之间的动量、热量和标量交换的测量，以便能够在不同的时间和空间尺度上对复杂地形上的地表能量平衡、水循环和碳收支进行可靠的解释/评估。目前，对于复杂的地形、流入的动力和热力学条件以及植物树冠如何同时作用于湍流结构，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(山区的交换过程)与大涡模拟(LES)相结合的测量数据的分析，来检验复杂地形上森林树冠-大气界面的湍流。华盛顿州立大学(WSU)的研究团队与其他六个国际组织一起参加了2011年6月和7月在德国巴伐利亚州东北部魏登布朗宁·瓦尔德斯坦(DE-Bay)的FLUXNET网站进行的Eger实验。每个小组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图。除了数据分析外，天气研究和预报模式(WRF-LES)还将加入一个多层冠层模块，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传输的相对贡献。总而言之，这项研究将检验：1)与在理想化的丘陵上观察到的结构相比，“真实”地形引起的压力扰动和冠层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数据，如速度变化、湍流应力/通量、压力变化和TKE的产生/损失？2)与以前在中性大气条件下的结果相比，不同的大气稳定条件如何改变冠层内和冠层以上平均/湍流流动主要特征的空间和时间变化？湍流的主要特征包括切变层、拐点、TKE、二阶统计量、u和w剖面的偏度和峰度、尾迹区域和尾迹深度(仅限于背风面)和再环流(仅限于背风面)。3)由于地形、冠层和稳定度的共同作用，平均/湍流流动如何导致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？二氧化碳通量的这些时空变化的动力学机制是什么，以及对二氧化碳通量的塔式测量的影响？智力价值：总的来说，这项研究将提供对山区冠层和大气之间的平均和湍流流动以及动量、热量和标量(例如，二氧化碳)交换的更好的理解。应用包括：1)复杂地形中碳循环的模拟，2)复杂地形中的风能预测，3)复杂环境中的污染物扩散。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的重要组成部分的理解。这一结果将有利于碳循环科学和FLUXNET社区帮助限制碳收支和高端二氧化碳通量，从塔楼到景观尺度，甚至复杂地形上的区域尺度。更新的WRF-LES模拟系统具有多层树冠模块，将有助于不同的研究团体研究复杂地形上的树冠流动和PBL流动；风能在识别风力涡轮机潜在位置方面的应用；森林管理在识别多风条件下树木破坏的高风险位置；以及森林火灾行为在量化火灾传播方面的作用；该研究将直接有助于西斯加州大学博士生的教育研究培训。研究结果将通过FLUXNET社区、会议和研讨会向更广泛的受众传播，并将用于与西澳州立大学本科生和研究生课程以及重点关注大气化学、空气质量和气候变化的夏季REU项目相关的课程和研讨会。

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.