Fire behavior observed during a valley wind reversal

In situ measurements were made using a micrometeorological tower during a prescribed fire in California. Just before the fire front reached the tower, a 180 degree shift in wind direction occurred as the up-valley wind was overcome by a mesoscale circulation related to the sea breeze. High-frequency wind velocity data indicate that during a rapid wind reversal that occurred at the surface, a fire whirl formed within the fire front. The fire whirl was approximately 5 m in height within the fire front, but soon the vortex entrained ash as it moved behind the fire front and into the burnt area. At this point the full length of the vortex could be observed and was approximately 300 to 400 m AGL in height. Vertical directional shear is responsible for the development of the fire whirl to occur at first aloft, since the observed break-in of the sea breeze front occurred later at the surface. The wind reversal and turbulence structure in relation to fire swirl formation will be presented during this presentation.