The dynamics of pyro-tornadogenesis using a coupled fire-atmosphere model

The Australian wildfires during 2003 that reached the outer suburbs of Canberra produced a series of pyro-cumulonimbus cells and the first recorded Australian pyro-tornado during the afternoon of 18 January. Here the dynamics of these pyro-tornadoes are investigated using the model CM1. Sensible and latent fluxes are specified at the lower boundary of the model through a parameterized representation of a wildfire based on the McArthur formula and coupled to the cloud model CM1. The coupled model is initialized using the sounding taken from Wagga Wagga, located 163 km west of Canberra along with an initial fire profile. The fire-spread model then receives the required atmospheric data and updates the burning area. The burning area data is then sent to CM1 as updated sensible and latent heat fluxes where the atmospheric conditions respond to this new source of surface heat and moisture, resulting in a fully coupled fire-atmosphere system. The resulting pyro-cumulonimbus and structure of the vertical vortices are consistent with previous literature. Spatial resolution dependencies are tested, and it is shown that the fundamental structure of the generated vortex is consistent for both: uniform grid spacing of 100 m, and non-uniform grid spacing with horizontal grid spacing of 25 m in the region of the parameterized fire. Passive tracer parcels are seeded into the domain such that they enter the vortex as it reaches tornadic strength. A vorticity budget is calculated along the forward trajectories of these parcels and results show that the dominant physical process in the formation of intense vertical vorticity is the vertical tilting and stretching of buoyancy-produced horizontal vorticity near the surface. The sensitivity of vortex generation to the availability of moisture is also tested, by removing the latent heat release of the fire.