The use of spectrum width radar data for bushfire model verification

Abstract

In wildland fires, the turbulent motions of air in the convective plume can significantly affect fire spread through processes such as spotting, local wind field modifications and the initiation of pyrocumulus and pyrocumulonimbus (i.e., cumuliform cloud types associated with fire activity). Radar has been used in a variety of studies to gauge the escalation of such processes. However, these studies have relied heavily on the radar moment of reflectivity despite a poor understanding of the nature of the particles producing the back-scatter. In the last decade, many models have emerged capable of simulation of fire-atmosphere interactions. Validation of these models requires observations of the turbulent motions in plumes, but such observations from very large fires, including those that develop pyrocumulonimbus, are very rare. Doppler velocity can be used to estimate radial windspeeds, but specialised manual interpretation is required to characterise the coherent vortices, which offers little utility to modellers. Reflectivity and Doppler velocity therefore are limited in the modelling of to bushfire. The use of spectrum width, defined as the standard deviation of the Doppler velocity, has received no attention in the literature to date in relation to its potential to quantifiably validate modelling of fire-atmosphere interactions. This variable has long been known to act as a proxy for the sub-grid turbulence in radar sampling volumes. The Bushfire Convective Plume Experiment (BCPE) involved a 2.5 year field campaign to capture observations of pyroconvection primarily with a mobile X-band Doppler radar, to collect a range of different types of observed fields including spectrum width. This field campaign involved collaborating with agencies responding to wildfires through the bushfire season, as well as attending prescribed burning for bushfire hazard reduction. The mobile radar observations were supported by other field observations, including weather balloons to observe vertical wind and stability structure of the atmosphere, as well as rapiddeployable Automatic Weather Stations (AWS), time-lapse cameras and fire severity reconstructions. One particular finding from the BCPE relevant to the modelling community is associated with the Doppler velocity and spectrum width results. In effect, it was found that spatial approximations of the turbulence distributions within bushfire plumes can be produced, along with resolving the structures of large vortices within the plume through the two variables. Here we present a summary of some previous radar-based studies of plume dynamics, as well as observations from one of the BCPE fire cases, showing the spectrum width and Doppler analyses of the Mt Bolton bushfire (in Victoria, southeast Australia). The Mt Bolton fire event was captured in high resolution using a portable X-band radar in the course of the BCPE field campaign. The resolution allowed for the calculation of Plume Relative Winds (or PRW). These were calculated by subtracting an estimate of the advection in the wind field from the downwind Doppler returns. The initial results of PRW and spectrum width illustrate a potential new avenue for model validation. The proposed validation includes tying high resolution turbulence parameters and structures in the radar data in bushfire simulators, Large Eddy Simulation and in fire-atmosphere coupled models.