Fire Whirl Formation 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 the wind reversal occurred after the fire whirl formed as shown by images from time-lapse photography and video. 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 seen and was approximately 300 to 400 m AGL in height. It is hypothesized that the vortex formed due to vertical directional shear that formed aloft since the observed break-in of the sea breeze front occurred later at the surface. This period was most likely a transitional period between the two wind regimes. This paper will discuss in more detail the wind and turbulence structure observed during the fire whirl formation.