Modeling interactions between fire and atmosphere in discrete element fuel beds

Abstract

In this text we describe an initial attempt to incorporate discrete porous element fuel beds into the coupled atmosphere–wildfire behavior model HIGRAD/FIRETEC. First we develop conceptual models for use in translating measured tree data (in this case a ponderosa pine forest) into discrete fuel elements. Then data collected at experimental sites near Flagstaff, Arizona are used to create a discontinuous canopy fuel representation in HIGRAD/FIRETEC. Four simulations are presented with different canopy and understory configurations as described in the text. The results are discussed in terms of the same two discrete locations within the canopy for each simulation. The canopy structure had significant effects on the balance between radiative and convective heating in driving the fire and indeed sometimes determined whether a specific tree burned or not. In our simulations the ground fuel density was the determining factor in the overall spread rate of the fire, even when the overstory was involved in the fire. This behavior is well known in the fire meteorology community. In the future, simulations of this type could help land managers to better understand the role of canopy and understory structure in determining fire behavior, and thus help them decide between the different thinning and fuel treatment strategies available to them.

Evaluating Crown Fire Rate of Spread Predictions from Physics-Based Models

“…Further analysis of the points that fell outside the predictive bounds was limited due to a lack of information regarding many of the independent variables known to influence crown fire behavior. However, four of the six FIRETEC points that showed an over-prediction bias were from a study conducted by Linn et al [22]. In this study, the authors represented the initial atmospheric boundary conditions as a constant no shear wind profile.…”

“…For example, often an investigator will provide a single number when describing the wind speed near the fire or the fuels complex, despite both wind speed magnitude and canopy fuel loading having a strong dependence on the height above ground and significant horizontal variability. Recent numerical simulations have shown that rate of spread predictions from detailed physics-based models are sensitive to small variations in both the spatial pattern of the fuels complex [22,23] and assumptions regarding the atmosphere boundary layer [46]. Clearly, further assessment of detailed physics-based models would benefit from additional data regarding spatial and temporal variability of key fuel and environmental characteristics (i.e.…”

“…The emitted energy from each hot cell is deposited according to the distribution. For more details on the description of FIRETEC, the reader is referred to [1,15,21,22,33,34].…”

“…Of the four approaches discussed by Morvan [19], we consider two here: FIRETEC [15] and WFDS [2,20]. These two models were chosen in part because they can simulate fire behavior in three-dimensional domains large enough to contain a crown fire (i.e., a volume 100 s of meters on a side) and in part because they have been utilized to predict crown fire behavior in Fire Technology 2015 a number of published studies [21][22][23][24][25][26]. The two other detailed physics-based approaches considered by Morvan [19] have only been applied, to our knowledge, to fires in smaller domains or are limited to two dimensions.…”

Assessment of crown fire initiation and spread models in Mediterranean conifer forests by using data from field and laboratory experiments

“…Although current physical models (Linn et al, 2005;Mell et al, 2009) represent substantial advances in crown fire modelling, from an operational point of view, two main empirical approaches in predicting crown fire spread continue being the base of the commonly used crown fire prediction systems. The first approach is that used in the models proposed by Rothermel (1991) and Finney (1998), based on the relationship between the predicted rate of fire spread at the surface and the fire rate of spread observed in crown fires in the Rocky Mountains (North America).…”