CAREER: CONIFER: Role of Canopy Turbulence in Wildland Fire Behavior

随着气候和土地利用模式的变化，野火的破坏性越来越大。少数非常大的火灾已经变成了极高强度的地狱，造成了大部分的损失。然而，对控制野火行为的物理过程的理解仍然有限。尽管在了解火灾如何在草原上蔓延方面取得了一些研究进展，但森林环境中火灾行为的几个方面仍然没有得到很好的理解。在某些情况下，发生在森林地面的地表火灾可以垂直上升并破坏森林冠层。此外，如果条件有利，火灾可以在树冠上横向蔓延，成为强度更高的树冠火灾。因此，了解野火的物理和动态气象学对于制定诸如规定燃烧等缓解战略和诸如森林间伐等有效的燃料管理做法至关重要和必要。因此，为了评估这种管理工作的有效性，了解森林冠层环境中的野火行为至关重要。该项目将描述野火事件期间森林冠层湍流气流的机制，这将导致(i)火灾行为建模的进步，（ii）开发燃料处理和规定火灾的决策支持工具，以及（iii）改进野火对空气质量影响的模型。此外，该项目还将培养一批初入职场的科学家和学生，让他们掌握野火科学的基础知识。在植被环境中，火灾发生在森林冠层以下，这意味着冠层诱导的湍流涡与火灾相互作用，而火-气相互作用被冠层湍流的基本性质所调节。森林树冠充当了上方空气动量的延伸汇，从而形成了长度和速度范围。此外，在冠层-大气界面处存在剪切引起的不稳定性，导致间歇性相干结构穿透冠层下空域，有助于冠层-大气相互作用。迄今为止，植被冠层湍流的物理学本身是一个活跃的研究领域。标准的大气边界层公式不足以解释冠层与大气之间的质量、动量和能量交换。由于火灾产生了额外的剪切和浮力引起的运动，这种情况变得更加复杂。利用实验室和实地实验数据集，本项目将探讨以下问题：(i)林冠施加的相干结构如何影响调节火灾蔓延的对流和辐射机制？（ii）林冠湍流输运和湍流扩散的独特性质如何影响野火期间湍流能量的再分配？（iii）冠层湍流的间歇性如何影响与火势蔓延相关的爆破运动？这个项目将改变目前以燃料为中心的观点，在这种观点中，植被似乎仅仅是被火焰消耗的燃料，而在一个新的范式中，树冠本身通过跨多个尺度的相互作用来主动调节风和火。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。

Wildfires have become increasingly more destructive with changing patterns of climate and land-use change. A small number of very large fires have become extremely high-intensity infernos and have been causing the majority of the damage. However, the understanding of the physical processes governing wildland fire behavior has remained limited. Although some research progress has been made in understanding how fires spread on grasslands, several aspects of fire behavior in forested environments are still not well understood. Under certain circumstances, surface fires occurring on a forest floor can ascend vertically and damage the forest canopy. Moreover, if the conditions are favorable, the fire can spread horizontally tree-to-tree across the canopy, becoming a much higher intensity crown fire. Understanding the physical and dynamic meteorology of wildland fires is consequently of utmost importance and necessary to develop mitigation strategies such as prescribed burns and effective fuel management practices such as forest thinning. To assess the effectiveness of such management efforts, it is, therefore, crucial to understand wildland fire behavior inside the forest canopy environment. This project will characterize the mechanisms of turbulent wind flow in the forest canopy during wildland fire events, which will result in (i) advances in fire behavior modeling, (ii) developing decision support tools for fuel treatments and prescribed fires, and (iii) improved models of air quality impacts from wildland fires. Moreover, the project will train a cohort of early-career scientists and students in the fundamentals of wildfire science.In a vegetated environment, the fire occurs below the forest canopy, which means that canopy-induced turbulent eddies interact with the fire and the fire-atmosphere interaction is modulated by the fundamental nature of canopy turbulence. The forest canopy acts as an extended sink of momentum for the air above, thereby imposing a range of length and velocity scales. Moreover, the presence of shear-induced instabilities at the canopy-atmosphere interface results in intermittent coherent structures that penetrate the sub-canopy airspace and aid in canopy-atmosphere interaction. The physics of turbulent flow in vegetated canopies itself is an active area of research to date. Standard atmospheric boundary layer formulations are inadequate in explaining canopy-atmosphere exchange of mass, momentum, and energy. This is rendered further complicated by the presence of the fire, which generates additional shear and buoyancy-induced motions. Using lab and field experimental datasets, this project will explore the following questions: (i) how do the coherent structures imposed by the canopy influence the convective and radiative regimes that regulate fire spread? (ii) how do the unique nature of turbulent transport and turbulent diffusivity in the canopy impact the redistribution of turbulent energy during wildfires? and (iii) how does the intermittent nature of canopy turbulence impact bursting motions associated with fire spread? This project will alter the current fuel-centric perspective, where the vegetation appears as mere fuel to be consumed by the flames, to a new paradigm, where the canopy itself actively modulates the wind and the fire through interactions spanning multiple scales.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.