Dynamics of dry convection above wildland fires: sensitivity to fireline details

Firelines are complex phenomena with a broad range of scales of cross-line dimension, undulations, and along-line variation in heating rates. While some earlier studies have examined parcel processes in two-dimensional simulations, the complexity of firelines in nature motivates a study in which the impact of three-dimensional fireline details on parcel processes is examined systematically. This numerical modeling study aims to understand how fundamental processes identified in 2D simulations operate in 3D. The first step is to perform simulations in a 3D model, with no fireline undulations or inhomogeneity. In general, convective modes simulated in the 2D model are reproduced in the 3D model. In one particular case with strong vertical wind shear, new convection develops separate from the main line of convection as a result of local changes to parcel speed and heating. However, in general the processes in the 2D and 3D simulations are identical.

The second step is to examine 3D experiments wherein fireline shape and along-line inhomogeneity are varied. It is found that for a fireline of uniform intensity consisting of a series of undulations, or fingers, the shape of the fireline leads to stronger parcel heating between fingers. In a separate simulation with a more realistic fire structure wherein surface heat fluxes are strongest within the fingers, parcel heating and convection are weaker than for a straight, uniformly heated fireline.

This work represents an intermediate stage between earlier 2D experiments designed to examine fundamental processes, and more complex simulations using 3D coupled atmosphere-fire models. The goal is to better understand how convection develops near wildland fires and, ultimately, better anticipate convective evolution and any subsequent changes in fire behavior.