Coupled fire-atmosphere models intended to be used for landscape-scale fires (domains of hundreds of meters to 10s of kilometers), typically divide the simulation domain up into a mesh of grid cells and these grid cells typically range in size from 1-30 meters on a side. As the processes governing combustion of an individual fuel element occur on considerably smaller scales, the gridded model requires a means of describing these smaller scale, or subgrid, processes. Probability distributions are one method of describing the impact of such processes and is the approach used in the HIGRAD/FIRETEC model.

For the solid phase of combustion, the mass loss rate in the model is dominated by turbulent mixing and fraction of a cell combusting as determined by a probability density function based on a grid cell’s temperature. We employ both visible and infrared video analysis to estimate these quantities. For the turbulent mixing, optical flow techniques are used in conjunction with particle image velocimetry on the visible videos to estimate a centimeter scale flow field in the vicinity of the fire which can be used to estimate the turbulence. Mass loss rate is estimated from infrared remote sensing techniques developed first in the laboratory and later used with satellites to estimate fire radiative power and fuel consumption. Results from a set of simple idealized fuel beds are shown along with preliminary results from natural fuel beds.