Polarimetric Observations of Prescribed Bushfires in South Australia using an X-Band Phased Array Radar

Wildfires in Australia, commonly bushfires, are a natural part of the country's ecosystem and help to clean out and recycle the local flora. Although necessary, these bushfires can pose a significant danger to nearby people and property. To prevent the onset of uncontrolled bushfires near population centers, local authorities regularly conduct controlled burns each year. During April and May of 2013, an X-Band Phased Array Radar was deployed to South Australia where it made polarimetric observations of eight prescribed bushfires conducted within and around the Adelaide Hills region. Utilizing the “phase-tilt” architecture, the Phase-Tilt Weather Radar is an X-band (9.41GHz) phased array radar that electronically scans in the azimuthal plane and mechanically tilts in elevation. At each prescribed bushfire, the radar nominally performed fixed-elevation azimuth scans, as well as intermittent RHI (Range-Height Indicator) and volume scans, during the entire evolution of the burn. Narrow bandwidth (50m range resolution) and high bandwidth (7m range resolution) waveforms were alternately transmitted throughout each burn. The prescribed burns observed were smaller in scale than uncontrolled bushfires, on average covering less than 50 hectares, but they still exhibited similar phenomena and smoke dynamics. The use of high resolution waveforms enabled the analysis of smoke plume dynamics for these small-sized burns. Time-synchronized photography was also collected to document visual observations throughout. This talk presents measurements and polarimetric products observed from the prescribed bushfires. Copolar correlation coefficient within the plume is observed to be somewhat low, consistent with lofted debris particles. Regions of higher correlation coefficient nearest the active combustion suggest the presence of either re-condensed water droplets or possibly refractive index turbulence scattering. Doppler velocity measurements and time-synchronized photography show the rotation of the smoke plume, and the measured radar moments indicate patterns which distinguish localized areas above fire sources from particle drift amidst ground-level winds. Rapid azimuth scanning indicates the velocity of ambient ground winds, confirmed against local in situ measurements near the burn sites. These measurements provide a real-time evolution of the smoke plume dynamics at a spatial and temporal resolution that has never been observed with an X-band radar.