Daisuke Seto

Fire Whirl Evolution Observed during a Valley Wind-Sea Breeze Reversal

This observational study documented the atmospheric environment of a prescribed fire conducted in a narrow valley when a small fire whirl developed during a mesoscale wind reversal. Based on analysis of in situ meteorological measurements, it is hypothesized that the fire whirl formed due to the presence of strong vertical wind shear caused by the interaction of a sea breeze front with a weaker up-valley wind. Vorticity generated by the interaction of the wind shear and the fire front was estimated to be ~0.2 s−1. Peak turbulence kinetic energy was caused by the wind shear rather than the buoyancy generated by the fire front. It was also found that the convective Froude number itself may not be sufficient for fire whirl prediction since it is less relevant to the near-surface boundary-layer turbulence generated by environmental wind shear. Observations from this case study indicate that even low-intensity prescribed fires can result in the formation of fire whirls due to mesoscale changes in the ambient atmospheric environment.

Fire weather conditions and fire–atmosphere interactions observed during low-intensity prescribed fires – RxCADRE 2012

The role of fire-atmosphere coupling on fire behaviour is not well established, and to date few field observations have been made to investigate the interactions between fire spread and fire-induced winds. Therefore, comprehensive field observations are needed to better understand micrometeorological aspects of fire spread. To address this need, meteorological observations were made during the Prescribed Fire Combustion and Atmospheric Dynamics Research Experiment (RxCADRE) field campaign using a suite of meteorological instrumentation to measure both the ambient fire weather conditions and the fire-atmosphere interactions associated with the fires and plumes. Fire-atmosphere interactions are defined as the interactions between presently burning fuels and the atmosphere, in addition to interactions between fuels that will eventually burn in a given fire and the atmosphere (Potter 2012).