National Center for Atmospheric Research, P.O. Box 3000, Boulder, Colorado, 80307, USA.
California Institute of Technology, Jet Propulsion Laboratory, 4800 Oak Grove Drive, MS 233-300, Pasadena, California, 91109, USA.
Ecol Appl. 2018 Sep;28(6):1565-1580. doi: 10.1002/eap.1752. Epub 2018 Jun 29.
Hypotheses that megafires, very large, high-impact fires, are caused by either climate effects such as drought or fuel accumulation due to fire exclusion with accompanying changes to forest structure have long been alleged and guided policy, but their physical basis remains untested. Here, unique airborne observations and microscale simulations using a coupled weather-wildland-fire-behavior model allowed a recent megafire, the King Fire, to be deconstructed and the relative impacts of forest structure, fuel load, weather, and drought on fire size, behavior, and duration to be separated. Simulations reproduced observed details including the arrival at an inclined canyon, a 25-km run, and later slower growth and features. Analysis revealed that fire-induced winds that equaled or exceeded ambient winds and fine-scale airflow undetected by surface weather networks were primarily responsible for the fire's rapid growth and size. Sensitivity tests varied fuel moisture and amount across wide ranges and showed that both drought and fuel accumulation effects were secondary, limited to sloped terrain where they compounded each other, and, in this case, unable to significantly impact the final extent. Compared to standard data, fuel models derived solely from remote sensing of vegetation type and forest structure improved simulated fire progression, notably in disturbed areas, and the distribution of burn severity. These results point to self-reinforcing internal dynamics rather than external forces as a means of generating this and possibly other outlier fire events. Hence, extreme fires need not arise from extreme fire environment conditions. Kinematic models used in operations do not capture fire-induced winds and dynamic feedbacks so can underestimate megafire events. The outcomes provided a nuanced view of weather, forest structure, fuel accumulation, and drought impacts on landscape-scale fire behavior-roles that can be misconstrued using correlational analyses between area burned and macroscale climate data or other exogenous factors. A practical outcome is that fuel treatments should be focused on sloped terrain, where factors multiply, for highest impact.
假设大型火灾（megafires），即非常大、影响严重的火灾，是由气候影响（如干旱）或由于火灾排除导致的燃料积累引起的，同时伴随着森林结构的变化，这种说法由来已久，并指导着相关政策，但它们的物理基础仍未得到检验。在这里，利用独特的空中观测和使用耦合天气-野火-行为模型的微观模拟，对最近发生的一次大型火灾——金火（King Fire）进行了分解，并分别研究了森林结构、燃料负荷、天气和干旱对火灾规模、行为和持续时间的相对影响。模拟再现了观测到的细节，包括到达倾斜峡谷、25 公里的延伸以及后来较慢的增长和特征。分析表明，与环境风相等或超过环境风的火灾诱发风以及表面天气网络无法检测到的细尺度气流是火灾快速增长和扩大的主要原因。敏感性测试在广泛的范围内改变了燃料湿度和数量，表明干旱和燃料积累的影响都是次要的，仅限于有坡度的地形，在这些地形中，它们相互叠加，而且在这种情况下，无法显著影响最终的火灾蔓延范围。与标准数据相比，仅从植被类型和森林结构的遥感中得出的燃料模型提高了模拟火灾的进展，特别是在受干扰的地区，以及燃烧严重程度的分布。这些结果表明，自我强化的内部动态而不是外部力量是产生这种和其他可能的异常火灾事件的一种方式。因此，极端火灾不一定是由极端火灾环境条件引起的。在操作中使用的运动学模型无法捕获火灾诱发的风和动态反馈，因此可能低估了大型火灾事件。这些结果提供了一个细致入微的视角，展示了天气、森林结构、燃料积累和干旱对景观尺度火灾行为的影响——这些作用可能会因燃烧面积与宏观气候数据或其他外部因素之间的相关分析而被误解。一个实际的结果是，燃料处理应该集中在有坡度的地形上，因为这些地形上的因素会相互叠加，产生最大的影响。
