Some Requirements for Simulating Wildland Fire Behavior Using Insight from Coupled Weather—Wildland Fire Models

Abstract

A newer generation of models that interactively couple the atmosphere with fire behavior have shown an increased potential to understand and predict complex, rapidly changing fire behavior. This is possible if they capture intricate, time-varying microscale airflows in mountainous terrain and fire-atmosphere feedbacks. However, this benefit is counterbalanced by additional limitations and requirements, many arising from the atmospheric model upon which they are built. The degree to which their potential is realized depends on how coupled models are built, configured, and applied. Because these are freely available to users with widely ranging backgrounds, I present some limitations and requirements that must be understood and addressed to achieve meaningful fire behavior simulation results. These include how numerical weather prediction models are formulated for specific scales, their solution methods and numerical approximations, optimal model configurations for common scenarios, and how these factors impact reproduction of fire events and phenomena. I discuss methods used to adjust inadequate outcomes and advise on critical interpretation of fire modeling results, such as where errors from model limitations may be misinterpreted as natural unpredictability. I discuss impacts on other weather model-based applications that affect understanding of fire behavior and effects.

Meteorological Applications Benefiting from an Improved Understanding of Atmospheric Exchange Processes over Mountains

“…On the low ends of those scales, outflow from a pyrocumulonimbus generated by a fire can, in turn, modify that same fire's immediate environment [56,57]. To be most effective, modeling systems for simulating wildfires and the smoke they generate (see Section 8) must account for these interactions [58][59][60]. How well such interactions are represented depends on our understanding of the exchange processes, the surface and boundary layers, and the free atmosphere in and around wildfires.…”

The Generation and Forecast of Extreme Winds during the Origin and Progression of the 2017 Tubbs Fire

“…Its solution methods and options were designed to allow it to perform well at fine scales (tens to hundreds of meters) in extremely complex terrain. Impacts of the design of CAWFE and other models on simulated airflow and fire behavior are discussed in [26].…”

Computational modeling of extreme wildland fire events: A synthesis of scientific understanding with applications to forecasting, land management, and firefighter safety

“…NWP models have been used for decades to simulate weather phenomena that are also linked to critical wildland fire behavioratmospheric convection, cloud production, outflows, gust fronts, and microbursts, mountain circulations, and downslope windstorms. Though NWP models may solve the same set of equations, different approximations, solution methods, and discretization can produce solutions with different properties, with impacts on simulations of fire behavior (discussed in [20]). For example, the WRF model produces an atmospheric energy spectrum that adheres closely to the expected natural spectra across synoptic and mesoscales [22], but components included to reduce numerical divergence severely dissipate fine-scale motions and smooth fine-scale gradients.…”

“…Studies have shown this has led to underestimates in peak wind extrema in wind-driven fire events [23], smoothed gradients across interfaces precluding WRF from producing gravity wave overturning (discussed in Coen 2018) -a primary mechanism for producing strong downslope winds driving fire events such as the 2012 High Park Fire [24] and shear instabilities [25], and smoothing gradients underlying simulation of the full development of fire phenomena such as fire whirls [20,26]. [41] and demonstrated over wildfires [42] though their broader use is still restricted.…”