Mary Ann Jenkins

A physics-based approach to modelling grassland fires

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.

Modeling Kepler Transit Light Curves as False Positives: Rejection of Blend Scenarios for Kepler-9, and Validation of Kepler-9 d, A Super-earth-size Planet in a Multiple System

Light curves from the Kepler Mission contain valuable information on the nature of the phenomena producing the transit-like signals. To assist in exploring the possibility that they are due to an astrophysical false positive, we describe a procedure (BLENDER) to model the photometry in terms of a “blend” rather than a planet orbiting a star. A blend may consist of a background or foreground eclipsing binary (or star–planet pair) whose eclipses are attenuated by the light of the candidate and possibly other stars within the photometric aperture. We apply BLENDER to the case of Kepler-9 (KIC 3323887), a target harboring two previously confirmed Saturn-size planets (Kepler-9 b and Kepler-9 c) showing transit timing variations, and an additional shallower signal with a 1.59 day period suggesting the presence of a super-Earth-size planet. Using BLENDER together with constraints from other follow-up observations we are able to rule out all blends for the two deeper signals and provide independent validation of their planetary nature. For the shallower signal, we rule out a large fraction of the false positives that might mimic the transits. The false alarm rate for remaining blends depends in part (and inversely) on the unknown frequency of small-size planets. Based on several realistic estimates of this frequency, we conclude with very high confidence that this small signal is due to a super-Earth-size planet (Kepler-9 d) in a multiple system, rather than a false positive. The radius is determined to be 1.64 +0.19 −0.14 R⊕, and current spectroscopic observations are as yet insufficient to establish its mass.

A Coupled AtmosphereFire Model: Convective Feedback on Fire-Line Dynamics

Abstract The object of this paper is to describe and demonstrate the necessity and utility of a coupled atmosphere-fire model: a three-dimensional, time-dependent wildfire simulation model, based on the primitive equations of motion and thermodynamics, that can represent the finescale dynamics of convective processes and capture ambient meteorological conditions. In constructing this coupled model, model resolution for both the atmosphere and the fuel was found to be important in avoiding solutions that are physically unrealistic, and this aspect is discussed. The anelastic approximation is made in the equations of motion, and whether this dynamical framework is appropriate in its usual form for simulating wildfire behavior is also considered. Two simple experiments-the first two in a series of numerical simulations using the coupled atmosphere- fire model-are presented here, showing the effect of wind speed on fire-line evolution in idealized and controlled conditions. The first experiment considers a 42...

The importance of fire–atmosphere coupling and boundary-layer turbulence to wildfire spread

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.

Coupling Atmospheric and Fire Models

Publisher Summary This chapter attempts to describe a fairly recent and major advance in the modeling of wildfires: the coupling of a cloud-resolving numerical prediction model with a simple fire-spread and wildfire behavior model, so that the atmosphere-fire is treated as a single, dynamical system. With this modeling approach, it is possible to simulate the small-scale atmosphere–fire interactions and feedbacks that are important to wildfire behavior, especially severe wildfire behavior, and the possible impacts of evolving, larger scale atmospheric forcing on the fire and vice versa. The interactions of forest fires and air-flow are highly nonlinear. The heat and moisture supplied through the burning of ground and canopy fuel during a forest fire create extreme levels of buoyancy forcing. The horizontal gradients of buoyancy produce vortices, or fire whirls, of tornado strength, which in turn affect the nature of the fire-spread through advection of hot gases and burning material. Fire vortices also enhance mixing of air with the flame which leads to higher flame temperatures, increased combustion efficiency, and greater intensity. Winds at the fire scale can be either strongly modified or even solely produced by the fire, depending on the level of atmosphere-fire coupling. This coupling or feedback occurs over spatial scales from tens of meters at the flame front to kilometers on the scale of the total burn area.

Real time simulation of 2007 Santa Ana fires

Abstract In this study we test the feasibility of using a coupled atmosphere–fire model for real time simulations of massive fires. A physics-based coupled atmosphere–fire model is used to resolve the large-scale and local weather as well as the atmosphere–fire interactions, while combustion is represented simply using an existing operational surface fire behavior model. This model combination strikes a balance between fidelity and speed of execution. The feasibility of this approach is examined based on an analysis of a numerical simulation of two very large Santa Ana fires using WRF–Sfire, a coupled atmosphere–fire model developed by the Open Wild Fire Modeling Community (OpenWFM.org). The study demonstrates that a wind and fire spread forecast of reasonable accuracy was obtained at an execution speed that would have made real-time wildfire forecasting of this event possible.

An examination of the sensitivity of numerically simulated wildfires to low-level atmospheric stability and moisture, and the consequences for the Haines Index

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. The Haines Index is of limited use, however, as a predictor of actual fire behavior. 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. This study is a first step in this process. This study investigates, through four comparative numerical simulations with a coupled wildfire-atmosphere model, the sensitivity of wildland fires to atmospheric stability and moisture, and in the process explores the correspondence between atmospheric stability and moisture, wildfire behavior, and the Haines Index. In the first three fire simulations, the model atmosphere was initially set to identical moisture but different instability conditions that correspond to Haines Indexes for low, moderate, and high potential for severe fire development. In the fourth fire simulation, the initial atmospheric and moisture conditions were for a high-risk fire Haines Index rating, but different from the initial conditions of dryness and stability of the previous experiments. The study indicates that high-risk fire development is sensitive to near-surface atmospheric stability and moisture, and that there is a range of atmospheric stability and moisture conditions that is important to the development of severe or erratic fire behavior, and that this range is within the atmospheric stability and moisture conditions represented by a Haines Index for high potential for severe fire. The analyses also suggest that there is a substantial latitude of fire behavior for fires rated as this Index, indicating that this Index should be further divided, or refined.

Numerical Simulations of Grassland Fire Behavior from the LANL-FIRETEC and NIST-WFDS Models

Grassland fires on level terrain offer a good basic scenario for test wildland fire behavior models, due to the simplicity and homogeneity of the fuels and terrain. Two physics based models, FIRETEC and WFDS, are briefly described, applied fire spread in grassland fuel, followed by a discussion of the results. It is important to note that both models have undergone appreciable development since the writing of this conference paper in 2005.

Investigating the Haines Index using parcel model theory

By using parcel model theory to construct a two-dimensional parameter space formed by low-level atmospheric stability and moisture, a simple framework on which to examine certain fire parcel properties associated with vertical column development is established. This framework is used to investigate if the Haines Index has some skill at predicting wildfire severity, where wildfire severity is assumed to be directly connected to vertical column development and the result of significant fire parcel ascent. By modeling the ascent of a moist, entraining fire parcel in four different background states—a 3 km deep boundary layer, a 2 km deep boundary layer, a 3 km deep boundary layer topped by an inversion layer, and a 2 km deep boundary layer topped by an inversion layer—the study shows that parcel properties that describe ascent and vertical column development are most significant when the boundary layer temperature lapse rate is near adiabatic and lower-level atmospheric humidity is relatively high. A shallower instead of a deeper boundary layer lowers parcel ascent, and an upper-level inversion lowers parcel ascent even further. This study shows that entraining fire parcel properties and magnitudes associated with significant ascent do not necessarily correspond to a Haines Index for a potential for high fire severity. The results suggest that the Haines Index may need to be refined or reformed depending on the stability and humidity in the boundary layer and vertical structure of the atmosphere. This study is a start to understanding the influence of the background state on fire parcel convection and an attempt to explain how the Haines Index works from an elementary but physical point of view.

Toward an integrated system for fire, smoke and air quality simulations

In this study, WRF-Sfire is coupled with WRF-Chem to construct WRFSC, an integrated forecast system for wildfire behaviour and smoke prediction. WRF-Sfire directly predicts wildfire spread, plume and plume-top heights, providing comprehensive meteorology and fire emissions to chemical transport model WRF-Chem, eliminating the need for an external plume-rise model. Evaluation of WRFSC was based on comparisons between available observations of fire perimeter and fire intensity, smoke spread, PM2.5 (particulate matter less than 2.5 μm in diameter), NO and ozone concentrations, and plume-top heights with the results of two WRFSC simulations, a 48-h simulation of the 2007 Witch–Guejito Santa Ana fires and a 96-h WRF-Sfire simulation with passive tracers of the 2012 Barker Canyon fire. The study found overall good agreement between forecast and observed local- and long-range fire spread and smoke transport for the Witch–Guejito fire. However, ozone, PM2.5 and NO concentrations were generally underestimated and peaks mistimed in the simulations. This study found overall good agreement between simulated and observed plume-top heights, with slight underestimation by the simulations. Two promising results were the agreement between plume-top heights for the Barker Canyon fire and faster than real-time execution, making WRFSC a possible operational tool.