Role of Horizontal Eddy Diffusivity within the Canopy on Fire Spread

Yana Bebieva,Nicholas S Skowronski,Bryan Quaife,Warren E Heilman,Julia Oliveto,Kevin Speer

doi:10.3390/atmos11060672

Abstract

Wind profile observations are used to estimate turbulent mixing in the atmospheric boundary layer from 1 m up to 300 m height in two locations of pine forests characteristic of the southeast US region, and to 30 m height at one location in the northeast. Basic turbulence characteristics of the boundary layers above and within the canopy were measured near prescribed fires for time periods spanning the burns. Together with theoretical models for the mean horizontal velocity and empirical relations between mean flow and variance, we derive the lateral diffusivity using Taylor’s frozen turbulence hypothesis in the thin surface-fuel layer. This parameter is used in a simple 1D model to predict the spread of surface fires in different wind conditions. Initial assessments of sensitivity of the fire spread rates to the lateral diffusivity are made. The lateral diffusivity with and without fire-induced wind is estimated and associated fire spread rates are explored. Our results support the conceptual framework that eddy dynamics in the fuel layer is set by larger eddies developed in the canopy layer aloft. The presence of fire modifies the wind, hence spread rate, depending on the fire intensity.

Highlights

Numerous factors affect the spread of fire in a wildland environment, beginning with the vegetation or fuel present, and the underlying surface topography
The purpose of this paper is to examine the role of lateral mixing within the canopy and fuel layer in fire spread
Strong downdrafts behind the fire line revealed by both numerical simulation Sun et al [27] and direct measurement Clements et al [2] are thought to play a key role in fire spread and fire-atmosphere interactions

Summary

Introduction

Numerous factors affect the spread of fire in a wildland environment, beginning with the vegetation or fuel present, and the underlying surface topography. Fluctuations at many time scales are typically present, with a dominant eddy length and time scale increasing with height away from a smooth wall. Such eddies are responsible for much of the turbulent flux in the boundary layer. Over rough forest canopies of height h, a background mean flow U produces a time scale h/U that describes eddy mixing in neutral stability [1]. The presence of fire within the canopy generates its own circulation which interacts with the background flow in which it is embedded, enhancing turbulence levels [2] and modifying the mean flow structure near the boundary. Horizontal Diffusivity L Tign Tmax Finit b u BG u0

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