Factors influencing the development of violent pyroconvection. Part II: fire geometry and intensity.

Fire spread associated with violent pyrogenic convection is highly unpredictable and difficult to suppress. Wildfire-driven convection may generate cumulonimbus (storm) clouds, also known as pyrocumulonimbus (pyroCb). Research into such phenomena has tended to treat the fire on the surface and convection in the atmosphere above as separate processes. We used a numerical model to examine the effect of fire geometry on the height of a pyroconvective plume, using idealised model runs in a neutral atmosphere. The role of geometry was investigated because large areal fires have been associated with the development of pyroCb. Complementary results (detailed in Part I) are extended by considering the effect that fire shape can have on plume height by comparing circular, square, and rectangular fires of varying length and width, representing the difference between firelines and areal fires. Results reveal that the perimeter/area ratio influenced the amount of entrainment that the plume experiences and therefore the height to which the plume rises before it loses buoyancy. These results will aid in the prediction of blow-up fires (whereby a fire exhibits a rapid increase in rate of spread or rate of spread) and may therefore be useful in determining where fire agencies deploy their limited resources.

Fig. 12. Cross-sections of circular and 4:1, and 64:1 rectangular fires at 70 min of simulation. The long axis of each of the rectangular fires is oriented perpendicular to the wind. Top row shows no wind, middle row shows 5 ms^-1 and bottom row shows 10 ms^-1. Each cross-section is through the centre in the x-direction. The thick solid black contour is the 0.01 g kg^-1 mixing ratio contour, indicating the cloud outline. The thin black contours are vertical velocity in ms^-1 with positive values shown by a solid line and negative by dashed. Contour spacing is 5 ms^-1 and the zero contour has been removed. Red shading is temperature perturbation in K. The dotted line indicates the tropopause.