A Local-Scale Modelling System to Simulate Smoke Dispersion

Currently there is a growing awareness about the effects that smoke from wildland fires can have in the environment. This concern is also associated to prescribed fires, namely in Australia and North America where this fire management technique is frequently used. Smoke emitted can considerably decrease local air quality and affect human health. Although the long term health effects from occupational smoke exposure remain unknown, the evidence to date suggests that brief, intense smoke exposures can easily exceed short-term exposure limits in peak exposure situations such as direct attack and holding fire lines downwind of an active wildfire or prescribed burn (Sandberg et al., 2002).

The main purpose of this paper is to present a fire behaviour system, developed to estimate fire progression, smoke dispersion and visibility impairment, in a local scale, and to evaluate its performance comparing results with measurements from experimental field fires.

The system is an improvement of two numerical tools already available, DISPERFIRE (Miranda et al., 1994) and FireStation (Lopes et al, 2002), which were integrated together. Figure 1 presents a simple scheme of this integrated system.

Figure 1 ¨Scheme of the developed fire behaviour system.

FireStation is a software system aimed at the simulation of fire spread over complex topography. It implements a model for fire rate of spread, which takes as input local terrain slope, parameters describing fuel properties as well as the wind speed and direction. The fire shape is described with recourse to an ellipse type model. Wind field is simulated using the diagnostic model NUATMOS (Ross et al., 1988).

DISPERFIRE is a real time system developed to simulate the dispersion in the atmosphere of the pollutants emitted during a forest fire. It is also based in the diagnostic wind model NUATMOS, and in a lagrangean dispersion model. Wind and dispersion models were adapted to work together and to simulate specific conditions of forest fire behaviour. Also, a model for the estimation of visibility impairment, based on the relationship between the air pollutants concentration and visibility, was included in DISPERFIRE.

DISPERFIRE and FireStation were combined aiming to obtain a more complete and powerful system of models. The whole system was developed under a graphical interface, previously developed for FireStation, allowing a friendly use and providing easily readable output to facilitate its application under operational conditions.

The system was applied to an experimental field fire and main results were compared with experimental values. The burning experiments performed since 1998 at Central Portugal, Gestosa, aim to collect a large range of different but complementary experimental data, which can be used to support the development of new concepts and models and to validate existing methods or models in various fields of fire management (Viegas et al., 2002), constituting a particularly important opportunity to measure and analyse air pollutants concentrations during experimental field fires. Experiments undertaken in 2004 are the basis for the system evaluation. Two mobile laboratories were parked near the experimental burning plots (15 plots with 0.1 ha each) measuring in continuum concentrations of particles with an aerodynamic diameter smaller than 2.5 µm (PM2.5), and smaller than 10 µm (PM10), nitrogen dioxide and monoxide (NO2 and NO), and carbon monoxide (CO).

Air quality and visibility impairment, during the GESTOSA 2004 experimental fires were simulated with the new integrated tool. The fire spread, emission and dispersion of pollutants, and their effect on visibility were calculated. Comparison between estimated and measured values indicates a good performance of the modelling system. Despite the small size of these experimental burns, the resulting air quality and the visibility impairment reached levels of some concern, mainly when evaluating the safety conditions of firemen's work.

References:

Lopes, A.M.G., Cruz, M.G. & Viegas, D.X. (2002) - FireStation ¨C an integrated software system for the numerical simulation of fire spread on complex topography. Environmental Modelling & Software. Vol 17. pp 269-285.

Miranda, A.I., Borrego, C. and Viegas, D. - Forest fire effects on the air quality. In International Air Pollution Conference, II, Barcelona, Spain, 27 ¨C 29 September 1994 - Air Pollution II, Computer Simulation. Computational Mechanics Publications, Southampton: J.M. Baldasano, C.A. Brebbia, H. Power and P. Zannetti, 1994, Vol. 1, p. 191-199. Ross, D.; Smith, I.; Manins, P. & Fox, D. (1988) - Diagnostic Wind Field Modelling for Complex Terrain: Model Development and Testing. Journal of Applied Meteorology, Vol. 27, 785-796.

Sandberg, D.; Ottmar, R.; Peterson, J. & Core, J. (2002) - Wildland fire on ecosystems: effects of fire on air. Gen. Tech. Rep. RMRS-GTR-42-vol. 5. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 79 p.

Viegas DX, Cruz MG, Ribeiro LM, Silva AJ, Ollero A, Arrue B, Dios R, G¨®mez-Rodr¨ªguez F, Merino L, Miranda AI, Santos P (2002) Gestosa fire spread experiments. In 'Proceedings of IV International Conference on Forest Fire Research', Luso-Coimbra, Portugal, 18-23 November 2002. Forest Fire Research & Wildland Fire Safety. (Ed. DX Viegas), Millpress, Rotterdam, Netherlands. pp. 121