Coupled atmosphere-fire model dynamics of a fireline crossing a hill

A three-dimensional, nonhydrostatic atmospheric model has been coupled with forest fire model to create NCAR's wildfire simulation model to represent the complex interactions between the fire and local winds. These experiments apply an improved version of the fire tracer model that produces a better, smoother representation of the fireline. The idealized experiments reported here examine a fireline propagating up the relatively sharp slope of a small Gaussian hill (height 300 m). To imitate the surface flows often found in complex terrain, the ambient flow resembled a katabatic flow or gust front. Without a fire, this resulted in strong winds in the hill's lee due to reflective effects from an overlaying critical layer. When a fire was lit upwind of the hill, the fire first moved up the slope and then deviated sharply along the ridge line instead of continuing to propagate down the other side. A large vertically-oriented vortex forms in the lee of the ridge. A sequence of more vertically-oriented vortices formed at the upwind end of the fireline and propagated downstream. Since the flow was symmetric, these results suggest an instability amplified an asymmetric disturbance. Video animations of this simulation will be shown at the conference highlighting some of the interesting features of the fire and flow.