Influences of Coherent Structures on Validity of the Constant Flux Layer Assumptions in the Unstable Atmospheric Surface Layer

在天气和气候模拟等许多应用中，假定包括热量、水蒸气、二氧化碳和其他温室气体在内的标量（即标量通量）的传输速率相对于大气表层（大气的最底层）的高度是恒定的。这种所谓的恒定通量层假设也广泛用于陆地表面通量测量，以量化进出陆地表面的标量通量。然而，报告中这一假设的失败意味着，在大气表层测量和模拟的标量通量不等于穿过地表-大气界面的通量，导致这些应用中测量和模拟的通量存在不确定性。尽管对大型湍流涡旋（即相干结构）对标量通量的作用研究较多，但对于大型湍流涡旋对大气表层通量随高度变化的影响却知之甚少。该项目的目的是研究哪些物理机制调节大涡流的属性，这些属性导致通量随高度的变化，从而导致在广泛的大气条件下恒定通量层假设的失败。利用现有设施，在通量预算方程的指导下，将在密西西比州里奇兰的罗斯巴内特水库的大水体上进行现场实验，通量塔配备了五级涡动相关系统和其他仪器。现场实验将提供一个独特的数据集，以最大限度地减少平流项的影响，并能够更精确地检查大型湍流漩涡的属性。结合几种分析方法，如快速傅立叶变换、小波变换和集合经验分解模式，将用于表征大型湍流涡流的属性及其在不稳定范围内随高度的变化。象限分析将用于量化大型湍流涡流扫掠和喷射产生的不对称通量贡献，从而能够分析调节通量随高度变化的潜在机制。此外，这种分析将允许研究导致不同标量的高度变化通量的不同行为的机制。野外实验将产生一个独特的数据集，用于研究微气象学、水文气象学、边界层湍流、湖泊蒸发、水-大气相互作用、内陆水域碳排放、淡水管理的水预算和水生生态系统等广泛的主题。参与的博士生将获得设计、准备和进行微气象实验的第一手经验，并学习涡旋相关技术、数据分析理论和工具以及许多其他研究技能。使用本项目收集的数据集的小型研究项目将被开发并纳入PI教授的一门本科课程和两门研究生课程的课程中，作为学生的学期项目。有关实地实验和研究成果的短片和报告将在科学会议和研讨会上以及PI的部门网站上传播给不同的受众。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

In many applications such as weather and climate modeling, it is assumed that the transfer rate for scalars (i.e., scalar fluxes), including heat, water vapor, CO2, and other greenhouse gases, is constant with respect to height in the atmospheric surface layer, the lowest layer of the atmosphere. This so-called constant flux layer assumption is also widely used in land-surface flux measurements to quantify scalar fluxes to and from land surfaces. However, the reported failure of this assumption implies that scalar fluxes measured and modeled in the atmospheric surface layer are not equivalent to the fluxes across the surface-atmosphere interface, leading to uncertainty in measured and modeled fluxes in these applications. Despite the abundant studies on the roles of large turbulent eddies (i.e., coherent structures) in contributing to scalar fluxes, it is not well understood as to how large turbulent eddies contribute to changes in fluxes with height in the atmospheric surface layer. The objective of this project is to study what physical mechanisms regulate the attributes of large eddies across height that lead to varying contributions to fluxes with height, contributing to the failure of the constant flux layer assumption across a wide range of atmospheric conditions. By leveraging the existing facilities and as guided by flux budget equations, a field experiment will be conducted over the large water body of Ross Barnett Reservoir in Ridgeland, Mississippi, with a flux tower equipped with five levels of eddy covariance systems and other instruments. The field experiment will provide a unique dataset that minimizes the influence of advective terms and enables more precise examination of the attributes of large turbulent eddies. A combined approach of several analysis methods, such as fast Fourier transform, wavelet transform, and ensemble empirical decomposition mode, will be used to characterize the attributes of large turbulent eddies and their variations with height across instability ranges. Quadrant analysis will be used to quantify asymmetric flux contributions from sweeps and ejections of large turbulent eddies, enabling an analysis of the underlying mechanisms that modulate changes in fluxes with height. Further, such analyses will allow the study of mechanisms that lead to different behaviors of height-varying fluxes for different scalars. The field experiment will generate a unique dataset valuable for studying a wide range of topics in micrometeorology, hydrometeorology, boundary-layer turbulence, lake evaporation, water-atmosphere interactions, carbon emissions from inland waters, water budget for freshwater management, and aquatic ecosystems. An involved Ph.D. student will gain first-hand experience in designing, preparing, and conducting micrometeorological experiments and learn eddy covariance techniques, data analysis theories and tools, and many other research skills. Mini-research projects using the datasets collected in this project will be developed and incorporated into the curriculum of one undergraduate course and two graduate courses being taught by the PI for students’ term projects. Short films and presentations about the field experiment and research findings will be disseminated in scientific conferences and seminars as well as the PI’s department websites for diverse audiences.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.