Effects of Topography on Turbulent Fluxes in Stable Boundary Layers Using a New Generation Large-Eddy Simulation

在大气模拟模式(例如，数值天气预报和气候模式)中预测区域尺度的动量和热量通量会受到相当大的误差，这与我们考虑次网格尺度地形和大气稳定性的综合影响的能力有限有关。特别是，由于地形引起的形状阻力和重力波对湍流通量的增强，通常使用的基于相似的参数化方法不能很好地捕捉到，这种方法严格地只适用于均匀平坦表面上的边界层流动。为了产生物理上逼真的结果，这些模型需要稳定性修正，这是“模型性能的启发”，几乎没有科学基础。在这个项目中，将通过风洞实验、现场测量和大涡模拟(LES)的新组合来获得关于自然地形和稳定层结大气边界层湍流之间复杂相互作用的物理见解。首先，将创建一个框架来评估新一代大涡模拟在复杂地形上的性能(包括在无需调谐的动态亚网格尺度模型方面的最新进展)。新一代大涡模拟系统将被用来模拟不同地形上的稳定边界层，以获得研究陆地-大气相互作用的物理机制所需的高分辨率三维瞬变信息。多尺度小波分析将被用来量化地形、大气稳定度和湍流通量之间相互作用的主要尺度。这将旨在对相似性理论进行基于物理的修正，其中包括与地形和热分层相关的时间和空间尺度。这些结果有可能极大地改善区域尺度通量的参数化，从而提高天气和气候模式的准确性。这项研究的学术价值在于它使用了风洞实验、现场测量、最先进的大涡模拟技术和统计工具的独特和全面的组合，这些都是更好地了解地形对稳定边界层中湍流通量的影响所需的。这项研究有可能为我们对陆地-大气相互作用的理解和参数化能力提供新的启示。更广泛的影响包括大幅改进大气边界层通量的参数化，从而提高大气模拟模型的精度。将开发一种新的基于网络的教育工具，以利用大涡模拟场的可视化/动画的教育潜力。新工具将用于首席调查员教授的本科生和研究生课程以及针对高中生的外联活动。此外，两名研究生将作为研究助理接受指导，还将参与教育工具的开发和实施。

Prediction of regional-scale fluxes of momentum and heat in atmospheric simulation models (e.g., numerical weather prediction and climate models) is subject to substantial errors associated with our limited ability to account for the combined effects of subgrid-scale topography and atmospheric stability. In particular, enhancement of turbulent fluxes due to topographically-induced form drag and gravity waves cannot be properly captured by commonly-used similarity-based parameterizations, which are strictly only valid for boundary layer flow over homogeneous flat surfaces. In order to produce physically realistic results these models require stability corrections, which are "inspired in model performance" and have little or no scientific foundation. In this project, a novel combination of wind tunnel experiments, field measurements and large-eddy simulations (LES) will be carried out to gain physical insight concerning the complex interactions between natural topography and stably stratified atmospheric boundary layer turbulence. First, a framework will be created to evaluate the performance of the new generation LES (including recent advances in tuning-free dynamic subgrid-scale models) over complex terrain. The new-generation LES will then be used to simulate stable boundary layers over different topographies to obtain high-resolution 3-D transient information needed to study the physics governing land-atmosphere interaction. Multiscale wavelet analysis will be used to quantify the dominant scales of interaction between topography, atmospheric stability and turbulent fluxes. This will be aimed at developing physically-based modifications to similarity theory that include relevant temporal and spatial scales associated with both topography and thermal stratification. These results have the potential to substantially improve the parameterization of regional-scale fluxes and, therefore, the accuracy of weather and climate models. The intellectual merit of the research lies in its use of a unique and comprehensive combination of wind tunnel experiments, field measurements, state-of-the-art large-eddy simulation techniques, and statistical tools needed to better understand the effects of topography on turbulent fluxes in stable boundary layers. The research has the potential to shed new light on our understanding of, and ability to parameterize, land-atmosphere interactions.The broader impacts include substantial improvement in the parameterization of atmospheric boundary layer fluxes and, as a result, improved accuracy of atmospheric simulation models. A new web-based educational tool will be developed to take advantage of the educational potential of visualizations/animations of large-eddy simulation fields. The new tool will be used in undergraduate and graduate courses taught by the principal investigator as well as outreach activities for high school students. Furthermore, two graduate students will be mentored as research assistants and will also participate in the development and implementation of the educational tool.