Pyrocumulonimbus Firepower Threshold: Assessing the Atmospheric Potential for pyroCb
Pyrocumulonimbus Firepower Threshold: Assessing the Atmospheric Potential for pyroCb
复制标题
火积雨云火力阈值：评估大气中发生pyroCb 的潜力
作者：
K. Tory;J. Kepert
火积雨云（pyroCb）云很难预测，可以产生极端和意想不到的野火行为，这对消防人员来说是非常危险的。许多预报员修改了传统的雷暴诊断方法，通过添加温度（Δθ）和湿度增量（Δq）来表示接近预期羽流凝结水平的烟羽热力学，从而预测pyroCb潜力。然而，估算这些Δθ和Δq增量是一个高度主观的过程，需要对可能影响未来火灾大小和强度的所有因素有专业知识。在本文中，不是试图预测特定火灾的这些Δθ和Δq增量，而是考虑给定大气环境下产生pyroCb所需的最小火力。这个概念，被称为pyroCb火力阈值（PFT），只需要大气信息，消除了对火力贡献的主观估计的需要。本文提出了一种计算PFT的简单方法，该方法仅结合了基本的羽流上升物理特性，从而得到了一个解析解，为了解羽流行为和pyroCb的形成提供了重要的见解。在热力学图上诊断出深层潮湿对流所需的最小增量Δθ和Δq，加上最小云底高度（zfc）。Briggs的羽流上升方程用于将Δθ， zfc和平均水平风速U转换为PFT的测量值：进入羽流底部的最小热通量。这个PFT正比于U Δθ和zfc的平方的乘积。讨论了布里格斯方程提供的羽流行为见解，并给出了一些PFT示例。
Pyrocumulonimbus (pyroCb) clouds are difficult to predict and can produce extreme and unexpected wildfire behavior that can be very hazardous to fire crews. Many forecasters modify conventional thunderstorm diagnostics to predict pyroCb potential, by adding temperature (Δθ) and moisture increments (Δq) to represent smoke plume thermodynamics near the expected plume condensation level. However, estimating these Δθ and Δq increments is a highly subjective process that requires expert knowledge of all factors that might influence future fire size and intensity. In this paper, instead of trying to anticipate these Δθ and Δq increments for a particular fire, the minimum firepower required to generate pyroCb for a given atmospheric environment is considered. This concept, termed the pyroCb firepower threshold (PFT) requires only atmospheric information, removing the need for subjective estimates of the fire contribution. A simple approach to calculating PFT is presented that incorporates only basic plume-rise physics, yielding an analytic solution that offers important insight into plume behavior and pyroCb formation. Minimum increments of Δθ and Δq required for deep, moist convection, plus a minimum cloud-base height (zfc), are diagnosed on a thermodynamic diagram. Briggs’s plume rise equations are used to convert Δθ, zfc, and a mean horizontal wind speed U to a measure of the PFT: the minimum heat flux entering the base of the plume. This PFT is proportional to the product of U, Δθ, and the square of zfc. Plume behavior insights provided by the Briggs’s equations are discussed, and a selection of PFT examples presented.