Validation of Coupled Atmosphere-Fire Behavior Models

Recent advances in numerical modeling and computer power have made it feasible to simulate the dynamical interaction and feedback between the heat and turbulence induced by wildfires and the local atmospheric wind and temperature fields. At Los Alamos National Laboratory, the authors have developed a modeling system that includes this interaction by coupling a high resolution atmospheric dynamics model, HIGRAD, with a fire behavior model, BEHAVE, to predict the spread of wildfires. The HIGRAD/BEHAVE model is run at very high resolution to properly resolve the fire/atmosphere interaction. At present, these coupled wildfire model simulations are computationally intensive. The additional complexity of these models require sophisticated methods for assuring their reliability in real world applications. With this in mind, a substantial part of the research effort is directed at model validation. Several instrumented prescribed fires have been conducted with multi-agency support and participation from chaparral, marsh, and scrub environments in coastal areas of Florida and inland California. In this paper, the authors first describe the data required to initialize the components of the wildfire modeling system. Then they present results from one of the Florida fires, and discuss a strategy for further testing and improvement of coupled weather/wildfire models

Coupled Weather and Wildfire Behavior Modeling at Los Alamos: An Overview

Over the past two years, researchers at Los Alamos National Laboratory (LANL) have been engaged in coupled weather/wildfire modeling as part of a broader initiative to predict the unfolding of crisis events. Wildfire prediction was chosen for the following reasons: (1) few physics-based wild-fire prediction models presently exist; (2) LANL has expertise in the fields required to develop such a capability; and (3) the development of this predictive capability would be enhanced by LANL`s strength in high performance computing. Wildfire behavior models have historically been used to predict fire spread and heat release for a prescribed set of fuel, slope, and wind conditions (Andrews 1986). In the vicinity of a fire, however, atmospheric conditions are constantly changing due to non-local weather influences and the intense heat of the fire itself. This non- linear process underscores the need for physics-based models that treat the atmosphere-fire feedback. Actual wildfire prediction with full-physics models is both time-critical and computationally demanding, since it must include regional- to local-scale weather forecasting together with the capability to accurately simulate both intense gradients across a fireline, and atmosphere/fire/fuel interactions. Los Alamos has recently (January 1997) acquired a number of SGI/Cray Origin 2000 machines, each presently having 32 to 64 processors. These high performance computing systems are part of the Department of Energy`s Accelerated Strategic Computing Initiative (ASCI). While offering impressive performance now, upgrades to the system promise to deliver over 1 Teraflop (10(12) floating point operations per second) at peak performance before the turn of the century

Regional climatology sensitivity studies

Recent interest in understanding climate and climate change at regional-scales has led to the application of mesoscale models for regional climatology studies. These models can provide an understanding of climate processes in a physically consistent way at much higher resolution than presently offered by GCMs. In recent work, we have tried to address questions in the RAMS mesoscale model to establish confidence in our modeling procedure. A more rigorous comparison of our modeling results with various data sets is reported in Roads et al. (1992). In the present paper, we use two simple numerical experiments to examine the impact of grid configuration on the predicted precipitation field from the RAMS model. We intend to demonstrate that the choice of the lateral boundaries and grid configurations can significantly impact the predicted fields of interest

Modeling Wind Fields and Fire Propagation Following Bark Beetle Outbreaks in Spatially-Heterogeneous Pinyon-Juniper Woodland Fuel Complexes

We used a physics-based model, HIGRAD/FIRETEC, to explore changes in within-stand wind behav- ior and fire propagation associated with three time periods in pinyon-juniper woodlands following a drought-induced bark beetle outbreak and subsequent tree mortality. Pinyon-juniper woodland fuel complexes are highly heterogeneous. Trees often are clumped, with sparse patches of herbaceous veg- etation scattered between clumps. Extensive stands of dead pinyon trees intermixed with live junipers raised concerns about increased fire hazard, especially immediately after the trees died and dead needles remained in the trees, and later when the needles had dropped to the ground. Studying fire behavior in such conditions requires accounting for the impacts of the evolving heterogeneous nature of the wood- lands and its influence on winds that drive fires. For this reason we used a coupled atmosphere/fire model, HIGRAD/FIRETEC, to examine the evolving stand structure effects on wind penetration through the stand and subsequent fire propagation in these highly heterogeneous woodlands. Specifically, we studied how these interactions changed in woodlands without tree mortality, in the first year when dried needles clung to the dead trees, and when the needles dropped to the ground under two ambient wind speeds. Our simulations suggest that low wind speeds of 2.5 m/s at 7.5-m height were not sufficient to carry the fire through the discontinuous woodland stands without mortality, but 4.5 m/s winds at 7.5-m height were sufficient to carry the fire. Fire propagation speed increased two-fold at these low wind speeds when dead needles were on the trees compared to live woodlands. When dead needles fell to the ground, fine fuel loadings were increased and ambient wind penetration was increased enough to sustain burn- ing even at low wind speeds. At the higher ambient wind speeds, fire propagation in woodlands with dead needles on the trees also increased by a factor of ∼2 over propagation in live woodlands. These simulations indicate that sparse fuels in these heterogeneous woodlands can be overcome in three ways: by decreasing fuel moisture content of the needles with the death of the trees, by moving canopy dead needles to the ground and thus allowing greater wind penetration and turbulent flow into the woodland canopy, and increasing above-canopy wind speeds. 2012 Elsevier B.V. All Rights Reserve

Modelling fire behaviour in heterogeneous fuels resulting from bark beetle outbreaks

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Fires Following Bark Beetles: Factors Controlling Severity and Disturbance Interactions in Ponderosa Pine

Previous studies have suggested that bark beetles and fires can be interacting disturbances, whereby bark beetle–caused tree mortality can alter the risk and severity of subsequent wildland fires. However, there remains considerable uncertainty around the type and magnitude of the interaction between fires following bark beetle attacks, especially in drier forest types such as those dominated by ponderosa pine (Pinus ponderosa Lawson & C. Lawson). We used a full factorial design across a range of factors thought to control bark beetle−fire interactions, including the temporal phase of the outbreak, level of mortality, and wind speed. We used a three-dimensional physics-based model, HIGRAD/FIRETEC, to simulate fire behavior in fuel beds representative of 60 field plots across five national forests in northern Arizona, USA. The plots were dominated by ponderosa pine, and encompassed a gradient of bark beetle–caused mortality due to a mixture of both Ips and Dendroctonus species. Non-host species included two sprouting species, Gambel oak (Quercus gambelii Nutt.) and alligator juniper (Juniperus deppeana Steud.), as well as other junipers and pinyon pine (Pinus edulis Engelm.). The simulations explicitly accounted for the modifications of fuel mass and moisture distribution caused by bark beetle–caused mortality. We first analyzed the influence of the outbreak phase, level of mortality, and wind speed on the severity of a subsequent fire, expressed as a function of live and dead canopy fuel consumption. We then computed a metric based on canopy fuel loss to characterize whether bark beetles and fire are linked disturbances and, if they are, if the linkage is antagonistic (net bark beetle and fire severity being less than if the two disturbances occurred independently) or synergistic (greater combined effects than independent disturbances). Both the severity of a subsequent fire and whether bark beetles and fire are linked disturbances depended on the outbreak phase of the bark beetle mortality and attack severity, as well as the fire weather (here, wind). Greater fire severity and synergistic interactions were generally associated with the “red phase” (when dead needles remain on trees). In contrast, during the “gray phase” (when dead needles had fallen to the ground), fire severity was either similar to, or less than, green-phase fires and interactions were generally antagonistic, but included both synergistic and neutral interactions. The simulations also revealed that the magnitude of the linkage between these two disturbances was smaller for fires occurring during high wind conditions, especially in the red phase. This complexity might be a reason for the contrasted or controversial perception of bark beetle−fire interactions reported in the literature, since both fire severity and the type and magnitude of the linkage can vary strongly among studies. These results suggest that, for fires burning in the gray phase following moderate levels of mortality, bark beetle–caused mortality may buffer rather than exacerbate fire severity. However, for fires burning under high wind speeds, regardless of the outbreak phase or level of mortality, the near complete loss of canopy fuels may push this ecosystem into an alternative state dominated by sprouting species