Scale Effects and Heterogeneity in Land-atmosphere Interactions: Large Eddy Simulation Studies, Parameterizations and Field Validations

1中国科学院环境与应用流体力学研究中心；约翰霍普金斯大学机械工程系，巴尔的摩MD 21218； 3洛桑联邦理工学院，瑞士；约翰霍普金斯大学地理与环境工程系，巴尔的摩MD 21218。摘要：知识价值：在水文学中，获取区域尺度蒸发的主要实用方法仍然是基于经典的大气边界层相似理论（ABL）。该理论假设了一个均匀的陆地表面，但由于ABL中的湍流，在非均匀的自然陆地表面上的流动也经常被发现是可以接受的，它有效地混合了景观中的各种来源和不均匀性。因此，了解和预测陆地异质性和大气分层如何影响与这一混合过程相关的长度尺度是至关重要的。具体来说，混合层的高度和表面温度和湿度通量的有效“平均”粗糙度长度是建立简化模型所必需的。在本研究中，将对ABL湍流流动和输运进行大涡模拟（Large Eddy Simulations， LES），以识别不同分层条件下湍流混合的混合特性和空间非均质粗糙度特性，包括分形分布。数值模拟将采用新开发和测试的拉格朗日尺度相关动态模型，该模型已被证明特别适合于捕获复杂环境中未解决的小尺度湍流物理，其中湍流偏离了各向同性，惯性范围行为的经典假设。动态过程消除了指定可调模型系数的需要。将实现考虑标量传输的动态模型的扩展。通过一系列参数化的高分辨率模拟（参数化LES），其中表面粗糙度、几何排列、表面温度和/或热通量是系统变化的，这些特征驱动了非均质地形上的陆地-大气交换动力学，并建立了相关的长度尺度。这一信息将用于获取区域尺度通量的新策略，如实际地形上的热通量和蒸发。LES和参数化的现场验证将使用计划中的瑞士马赛克实验期间收集的数据进行。更广泛的影响：用于模拟天气和地球气候以及预测未来几十年温度变化的大尺度（区域或全球尺度）地球系统模拟，严重依赖于参数化来表示大气边界层和陆地表面。具有精确测试的子网格模型的LES使我们能够对新的，仔细和单独测试的大型模型组件进行评估和开发。提高大规模模型的单个组成部分的可信度是决策机构更认真地对待预测的必要步骤，最终也会得到更广泛的公众的重视。拟议的研究活动的更广泛的影响也通过我们独特的研究生培训计划实现。学生们在各个院系的课程中接受严格的科学训练，有可能通过CEAFM的双学位课程获得机械工程硕士学位和地理与环境工程系的博士学位。并且，通过地理与环境工程课程，他们将在社会与环境之间的界面方面拥有更广泛的知识。这个项目的独特之处在于它关注人类行为与自然之间的相互作用，以及它的结构，它将政治和人类方面与环境科学相结合。霍普金斯大学的项目因其通过坚实的科学影响公共政策的成功而获得了相当大的认可。

1Center for Environmental and Applied Fluid Mechanics (CEAFM), 2Dept. of Mechanical Engineering, The Johns Hopkins University, Baltimore MD 21218, 3Ecole Polytechnique Federale Lausanne, Switzerland, and Dept. of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore MD 21218.Summary:Intellectual merit: In hydrology, the main practical approach to obtain regional scale evaporation continues to be based on classical similarity theory of the atmospheric boundary layer (ABL). The theory assumes a uniform land surface yet has also often been found acceptable in flows over heterogeneous natural land surfaces due to the turbulent flow in the ABL, which efficiently blendsthe various sources and inhomogeneities across the landscape. It is thus essential to understand and predict how land heterogeneity and atmospheric stratification affect the relevant length-scales associated with this blending process. Specifically, the height of blending layers, and effective 'average') roughness lengths for surface fluxes of temperature and humidity are of needed to formulate simplified models. In the proposed research, Large Eddy Simulations (LES) of turbulentflow and transport in the ABL will be conducted to identify the blending properties of turbulent mixing under various conditions of stratification and spatially heterogeneous roughness properties, including fractal distributions. Numerical simulations will employ the newly developed and testedLagrangian scale-dependent dynamic model, which has been shown to be particularly well suited to capture unresolved small-scale turbulence physics in complex environments in which turbulencedeviates from the classical assumptions of isotropic, inertial-range behavior. The dynamic procedure eliminates the need to specify tunable model coefficients. Extensions of the dynamic model to account for scalar transport will be implemented. Through a parametric series of high-resolution simulations (parametric LES) where surface roughness, geometric arrangements, and surface temperature and/or heat flux are varied systematically, those features which drive the dynamics ofland-atmosphere exchange over heterogeneous terrain will be quantified, and their relevant lengthscales established. This information will be used in new strategies for obtaining regional scale fluxes such as heat flux and evaporation over realistic terrain. Field validations of the LES and parameterizations will be conducted using data collected during the planned Swiss MosaicExperiment. Broader impacts: Large-scale (regional or global-scale) earth-system simulations, used to simulate weather and the Earth's climate and project temperature changes in the coming decades, rely heavilyon parameterizations to represent the atmospheric boundary layer and the land surface. LES with accurately tested subgrid models allows us to undertake assessments and developments of new,carefully and individually tested, components of the larger scale models. Increasing the trustworthiness of individual components of large-scale models is a necessary step for predictions to be taken more seriously by policy-making bodies, and ultimately also by the broader public. Broader impact of the proposed research activity is also achieved through our unique graduate studenttrainingprogram. The students are trained rigorously in science through course-work in various departments, possibly leading to a M.S. degree in Mechanical Engineering and a PhD in theDepartment of Geography and Environmental Engineering through the CEAFM's Dual DegreeProgram. And, through the program in Geography and Environmental Engineering they will have abroader knowledge at the interface between society and environment. This program is distinctive inthe scope of its focus on the interaction between human behavior and nature over time and in itsstructure, which combines political and human aspects with the science of the environment. TheHopkins program has garnered considerable recognition for its success in influencing public policythrough solid science.