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Abstract: Before using a fluid dynamics physically based wildfire model to study wildfire, validation is necessary and model results need to be systematically and objectively analyzed and compared to real [...]

The purpose of this study is to identify what effects are responsible for the observed temperature field, in particular the lower-level cold core, in the trough region of a convectively active tropical easterly wave disturbance. GATE Phase III A/B- and B-scale data were used in the analysis, and the divergence equation, a first-order balance condition proposed by Cho and Jenkins applicable to slowly evolving or near-steady-state large-scale convectively active tropical circulation systems, and the standard nonlinear and linear balance equations were chosen as the framework in which to assimilate the observational data in order to understand the spatial anticorrelation between temperature changes and latent heating. It is shown that all three balance conditions reproduced the thermal structure of the easterly waves that passed over GATE during Phase III. Despite the differences in formulation and form, the simpler standard balance equations were as accurate as the first-order divergence equation in diagnosing the temperature field. The central result of the analysis is that the lower cold-core temperature anomaly observed in the near-trough region of easterly wave disturbances is not a direct consequence of the distribution of latent heat released by cumulus clouds, but reflects instead a balance of forces that dominate the momentum field and that the agreement between the observed and diagnosed temperature fields is dominated by the rotational component of the flow. This has possible implications for a first-order model of the interaction between cumulus-scale and large-scale equatorial wave motions, and for the distinction between developing and nondeveloping easterly waves in terms of the ambient vorticity field, which may aid future numerical investigations of tropical cyclogenesis.

The major source of uncertainty in wildfire behavior prediction is the transient behavior of wildfire due to changes in flow in the fire's environment. The changes in flow are dominated by two factors. The first is the interaction or 'coupling' between the fire and the fire-induced flow. The second is the interaction or 'coupling' between the fire and the ambient flow driven by turbulence due to wind gustiness and eddies in the atmospheric boundary layer (ABL). In the present study, coupled wildfire-atmosphere large-eddy simulations of grassland fires are used to examine the differences in the rate of spread and area burnt by grass fires in two types of ABL, a buoyancy-dominated ABL and a roll-dominated ABL. The simulations show how a buoyancy-dominated ABL affects fire spread, how a roll-dominated ABL affects fire spread, and how fire lines interact with these two different ABL flow types. The simulations also show how important are fire-atmosphere couplings or fire-induced circulations to fire line spread compared with the direct impact of the turbulence in the two different ABLs. The results have implications for operational wildfire behavior prediction. Ultimately, it will be important to use techniques that include an estimate of uncertainty in wildfire behavior forecasts. [ABSTRACT FROM AUTHOR]

Physics-based coupled fire-atmosphere models are based on approximations to the governing equations of fluid dynamics, combustion, and the thermal degradation of solid fuel. They require significantly more computational resources than the most commonly used fire spread models, which are semi-empirical or empirical. However, there are a number of fire behaviour problems, of increasing relevance, that are outside the scope of empirical and semi-empirical models. Examples are wildland-urban interface fires, assessing how well fuel treatments work to reduce the intensity of wildland fires, and investigating the mechanisms and conditions underlying blow-up fires and fire spread through heterogeneous fuels. These problems are not amenable to repeatable full-scale field studies. Suitably validated coupled atmosphere-fire models are one way to address these problems. This paper describes the development of a three-dimensional, fully transient, physics-based computer simulation approach for modelling fire spread through surface fuels. Grassland fires were simulated and compared to findings from Australian experiments. Predictions of the head fire spread rate for a range of ambient wind speeds and ignition line-fire lengths compared favourably to experiments. In addition, two specific experimental cases were simulated in order to evaluate how well the model predicts the development of the entire fire perimeter. [ABSTRACT FROM AUTHOR]

Focuses on the Haines Index, an operational fire?weather index introduced in 1988 and based on the observed stability and moisture content of the near-surface atmosphere, has been a useful indicator of the potential for high-risk fires in low wind conditions and flat terrain. How the Haines Index is of limited use, however, as a predictor of actual fire behavior; Ways to develop a fire?weather index to predict severe or erratic wildfire behavior, an understanding of how the ambient lower-level atmospheric stability and moisture affects the growth of a wildfire is needed.

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