Measuring and Modeling Interactions of the Turbulent Atmospheric Boundary Layer with Multiscale Ground Topology

大气中的气流和输送过程受到地形的强烈影响，地形会产生力并导致复杂的气流扭曲。在较小尺度上，大气表层也受到植被冠层的实质性影响。以前的大部分工作都集中在以单一长度尺度为特征的山丘和植被地形的影响上，例如，特定大小的单个山丘，或由植物组成的冠层，通常使用规定的叶面积密度分布进行建模。然而，众所周知，像山脉和树冠这样的流动障碍物具有广泛的长度尺度。然而，如何参数化这类多尺度目标对低层大气动力学的影响尚不清楚。本研究通过一个综合的实验室实验和计算程序来解决这个问题，重点是分形形状的大气边界层流动。分形在一定的尺度范围内，为山脉和植被几何形状固有的多尺度特征提供了方便的理想化。实验包括在“光学折射率匹配”设施中进行流动结构和阻力的实验室模型研究，在该设施中，可以在整个复杂区域内进行无障碍、详细的流动和力测量。多平面粒子图像测速测量将提供应力和速度梯度张量的所有分量。实验工作的一个关键动机是需要验证和支持进一步发展和改进一种新的预测工具，重归一化数值模拟（RNS）。该技术利用在计算网格上明确解析的大尺度上确定的阻力系数，从未解析的地面拓扑特征中对力进行建模，然后进行动态重新缩放。RNS将用于模拟分形树木和山脉之间的水流，并将预测的力和水流特征与测量结果进行比较。有了详细的流量数据，差异的原因将被识别并用于改进。该项目的主题是研究如何完善陆地-大气相互作用子网格参数化的科学基础，对大气和气候科学的基础设施具有广泛的影响。在大气流动现象的背景下，使用新颖的光学指数匹配方法进行测量，提供了在细节水平上的量子步骤的可能性，从而可以绘制和理解流动。此外，在多尺度地面拓扑上，开发适当验证的分形边界流动RNS可能会对湍流边界层以外的区域产生更广泛的影响。分形已被用作许多学科的描述工具，例如生物学（分支血液网络，肺结构，珊瑚），天体物理学（宇宙的大规模结构，行星际磁场的间歇性）和其他地球科学方面（分形海岸线，云）。RNS将分形的几何思想扩展到流体动力学。这项工作的教育影响将集中在研究生教育/培训上，强调物理实验和模拟之间的相互作用。作为教育推广工作的一部分，与巴尔的摩市学校系统的互动将继续并加强。具体来说，来自巴尔的摩理工学院的高中生将参与我们实验室为期一年的研究经验，作为他们必修的研究实习的一部分。参与分形边界上大气流动的研究将有助于激励有才华的高中生考虑未来在这一领域的职业。

Flow and transport processes in the atmosphere are strongly influenced by orography that generates forces and causes complex flow distortions. At smaller scales, the atmospheric surface layer is also affected substantially by vegetation canopies. Most previous work has focused on effects of hills and vegetated terrain characterized by a single length scale, e.g. a single hill of a particular size, or canopies consisting of plants, often modeled using a prescribed leaf-area density distribution. It is well known, however, that flow obstructions such as mountain ranges and canopies are characterized by a wide range of length scales. Yet, it is not known how to parameterize the effects of such multi-scale objects on the lower atmospheric dynamics. This research addresses this issue with an integrated laboratory experimental and computational program, focusing on atmospheric boundary layer flow over fractal shapes. Fractals provide convenient idealizations of the inherently multi-scale character of mountain range and vegetation geometries, within certain ranges of scales. The experiments consist of laboratory model studies of flow structure and drag forces in an "optically index-matched" facility, where unobstructed, detailed flow and force measurements can be performed within the entire complex domain. Multi-plane particle image velocimetry measurements will provide all components of the stress and velocity gradient tensors. A key motivation for the experimental work is the need to validate and support further development and improvements of a new prediction tool, Renormalized Numerical Simulation (RNS). This technique models forces from unresolved features of the ground topology using drag coefficients determined from interrogation of the large scales that are explicitly resolved on the computational mesh, followed by dynamic rescaling. The RNS will be used to model the flow across fractal trees and mountain ranges, and compare the predicted forces and flow features with the measurements. With the detailed flow data available, causes for discrepancies will be identified and used for improvements.Research on improving scientific foundations of sub-grid parameterizations of land-atmosphere interactions, the subject of the project, has a broad impact on the infrastructure of atmospheric and climate sciences. Measurements using novel, optically index-matched, methods in the context of atmospheric flow phenomena provide the possibility of a quantum step in the level of detail with which flows can be mapped and understood. Moreover, the development of properly validated RNS applied to flow with fractal boundaries may yield broader impact in areas other than turbulent boundary layers over multi-scale ground topology. Fractals have been used as a descriptive tool in many disciplines, such as biology (branching blood network, pulmonary structures, corals), astrophysics (large-scale structure of the universe, intermittency of interplanetary magnetic fields), and other geosciences aspects (fractal coastlines, clouds). RNS extends the geometric idea of fractals to fluid dynamics. Educational impact of the work will focus on graduate education/training that stresses the interplay between physical experimentation and simulation. As part of the educational outreach effort, interactions with the Baltimore City School system will continue and strengthen. Specifically, senior high-school students from the Baltimore Polytechnic Institute will be involved in yearlong research experiences in our laboratory, as part of their required Research Practicum. Involvement in research on atmospheric flows over fractal boundaries will help motivate talented senior high-school students to consider future careers in this field.