The Nature of Fire Ash Particles: Microwave Material Properties, Dynamic Behavior, and Temperature Correlation

Abstract

This paper focuses on the investigation of a number of physical and electromagnetic properties of fire generated ash particles, with relation to radar observations of forest fire smoke columns. Emphasis is placed on understanding the physical properties of the ash, which have direct effects on their scattering ability. Coupled with the electromagnetic properties, these physical properties describe the scatter generated when a number of dispersed ash particles are volumetrically interacting with radar signals. Due to their planar geometry, a study of ash particles originating from the eucalyptus genus has been conducted. Particular focus is placed on this genus due to its high population and role in fueling large bushfires within the Australian continent. The fundamental scattering mechanisms required for describing the radar reflectivity in horizontal, vertical, and cross-polarization have been explored by breaking down and analyzing three distinct properties of an individual ash particle. These include its geometric, dynamic, and electromagnetic properties. Statistical distributions from all three areas have been included to aid in the development of modeling tools.

Wildfire and Weather Radar: A Review

“…As seen in equation , the radar scattering of any object will depend on its RCS ( η ). Baum et al () described three categories of factors that affect the RCS of pyrogenic scatterers: geometric properties including surface area, cross section, aspect ratio, and general shape (such as if the particle is plate‐like, needle‐like, and spheroidal‐like); dynamic properties including its orientation during ascent and descent of the particle, as is significant in snowflake RCS (Matrosov, ); and electromagnetic properties, which are a function of the material molecular composition and the degree to which it polarizable, as well as the mass, moisture content, and porosity of the biomass material. Notably, these properties can be differentiated by the temperature history, as well as any ongoing combustion (flaming or smoldering) or cooling of the particle.…”

“…Argued that the permittivity would require moisture contents in ash as high as 30%, which they state as unlikely based on the observed moisture absorption rate of eucalypt ash particles exposed to different temperatures (between 150 and 400°C, based on Ghorbani et al, 2012). Baum et al (2015) Ash of Messmate Stringybark (Eucalyptus obliqua).…”

Pyrocumulonimbus lightning and fire ignition on Black Saturday in southeast Australia

“…Reflectivity data are used to derive echo-top altitudes, based on a 5 dBZ threshold, with prior radar analyses of pyroCbs [Fromm et al, , 2012Rosenfeld et al, 2007] showing how echo top data are informative for examining both the internal structure and the convective injection height of pyroCb events. The radar echoes come not only from cloud hydrometeors but also from debris lofted by the fires [Jones, 1950;Lindley et al, 2011;Baum et al, 2015].…”

Of Fire and Smoke Plumes, Polarimetric Radar Characteristics

“…Radar observations of pyroconvection combined with lightning mapping array indicates that lightning occurred whenever the smoke plume grew to 10 km [9] of mean sea level. These scientists capitalized on the polarimetric data (CSU CHILL, Colorado State University and Chicago Illinois radar) to identify the smoke part of plumes by noting low reflectivities (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25), increased Z DR (1-5 dB), and small correlation coefficient between returns at orthogonal polarizations, ρ hv of about 0.6.…”

“…These characteristics are extremely hard to determine from in situ observations. Nevertheless, measurements in a laboratory setting are feasible and several studies document these [13][14][15]. Most consider the frequency band 8-12 GHz, but [15] considers 10 GHz and 38 GHz, whereas [16] covers the 8-12 GHz and 26.5 to 40 GHz range.…”