Identifying Plume Mode via WSR-88D Observations of Wildland Fire Convective Plumes and Proposed Tactical Decision Support Applications

To date, the use of Doppler radar (WSR-88D) in wildland fire operations has been limited, with tactical applications focused on analyzing ambient atmospheric features. This paper presents geographically diverse analysis of radar-observed wildland fire convective plumes to determine indicators of plume mode for tactical decision support. Through the visualization of buoyancy via thermal bubbles and vertical plumes, plume mode is revealed via WSR-88D interrogation of three Southern Great Plains grass/shrub fires and two timber fires in Texas and California. Analogous to thunderstorm convective modes, past research has identified two distinct plume modes of wildland fire: multicell and intense convective plume. Multicell plume mode is characterized by a series of shallow discrete cells that move away from the fire’s main buoyancy source, with successive cells rising, expanding, and replacing cells from the updraft source. This process, known as the thermal bubble concept, occurs most notably in strong vertical wind profile environments with a strong advection component. These cells or thermal bubbles are observed via WSR-88D data for three Southern Great Plains cases. Intense convective plumes are observed to be vertical with the low-level reflectivity maximum and maximum echo top juxtaposed and occurrence is confined to weak wind environments; these plume structures are identified in the two timber fire cases. An important WSR-88D signature, the back-sheared convective plume (hereafter BSCP), is identified in terms of transverse vortices and vortex rings, which may imply enhanced combustion rates due to increased turbulent mixing. Determination of plume convective mode via radar offers meteorologists the ability to detect changes in plume mode and to provide important tactical decision support information about fire behavior.

Mobile X-Pol Radar: A New Tool for Investigating Pyroconvection and Associated Wildfire Meteorology

The process of pyroconvection occurs when fire-released heat, moisture, and/or aerosols induce or augment convection in the atmosphere. Prediction of pyroconvection presents a set of complex problems for meteorologists and wildfire managers. In particular, the turbulent characteristics of a pyroconvective plume exert bidirectional feedback on fire behavior, often with resulting severe impacts on life and property. Here, we present the motivation, field strategy, and initial results from the Bushfire Convective Plume Experiment, which through the use of mobile radar aims to quantify the kinematics of pyroconvection and its role in fire behavior. The case studies presented include world-first observations from two wildfires and one prescribed burn using the University of Queensland’s portable, dual-polarized X-band Doppler radar (UQ-XPOL). The initial analyses of reflectivity, Doppler winds, polarimetric variables, and spectrum width data provide insights into these relatively unexplored datasets within the context of pyroconvection. Weather radar data are supported by mesonet observations, time-lapse photography, airborne multispectral imaging, and spot-fire mapping. The ability to combine ground-validated fire intensity and progression at an hourly scale with quantitative data documenting the evolution of the convective plume kinematics at the scale of hundreds of meters represents a new capability for advancing our understanding of wildfires. The results demonstrate the suitability of portable, dual-polarized X-band Doppler radar to investigate pyroconvection and associated plume dynamics.

The Mean and Turbulent Properties of a Wildfire Convective Plume

The time-mean and time-varying smoke and velocity structure of a wildfire convective plume is examined using a high-resolution scanning Doppler lidar. The mean plume is shown to exhibit the archetypal form of a bent-over plume in a crosswind, matching the well-established Briggs plume-rise equation. The plume cross section is approximately Gaussian and the plume radius increases linearly with height, consistent with plume-rise theory. The Briggs plume-rise equation is subsequently inverted to estimate the mean fire-generated sensible heat flux, which is found to be 87 kW m−2. The mean radial velocity structure of the plume indicates flow convergence into the plume base and regions of both convective overshoot and sinking flow in the upper plume. The updraft speed in the lower plume is estimated to be 13.5 m s−1 by tracking the leading edge of a convective element ascending through the plume. The lidar data also reveal aspects of entrainment processes during the plume rise. For example, the covariation of the radial velocity and smoke perturbations are shown to dilute the smoke concentration with height.