Modeling wind adjustment factor and midflame wind speed for Rothermel's surface fire spread model

Abstract

Rothermel's surface fire spread model was developed to use a value for the wind speed that affects surface fire, called midflame wind speed. Models have been developed to adjust 20-ft wind speed to midflame wind speed for sheltered and unsheltered surface fuel. In this report, Wind Adjustment Factor (WAF) model equations are given, and the BehavePlus fire modeling system is used to demonstrate WAF calculation and effect on modeled fire behavior. There are differences in implementation of the same basic wind adjustment models in various fire behavior applications, including the Fireline Handbook and FARSITE. Differences are due to assumptions such as tree shape and rules for transition from sheltered to unsheltered conditions. Specifics are given for differences among WAF tables and calculation applications. This technical documentation is useful to analysts, system developers, fire weather meteorologists, and those who are interested in model background and foundation.

Fire Behavior Simulation from Global Fuel and Climatic Information

“…To do that, a wind adjustment factor (WAF) was assigned, which is as a function of the existing vegetation (considering if the surface fuels were sheltered or unsheltered from the wind), the surface fuel depth and the wind profile. As the wind information had a coarse resolution (0.5 • ) and at that scale it is not possible to take into consideration a particular vegetation profile, an intermediate value between sheltered and unsheltered fuels was used, corresponding to 0.4 for 20-ft winds (see Table 5 of Andrews [55]). This value was converted to 10-m winds using a conversion factor of 1.15 [56], to obtain a final WAF of 0.348.…”

“…Wind is also influenced by terrain, fuel depth and overstory vegetation sheltering, decreasing midflame wind speed up to 60% [55]. All these parameters could not be considered at the resolution of the climate information (0.5 • ) or even the fuelbed map (approx.…”

Current status and future needs of the BehavePlus Fire Modeling System

“…The approach will address inconsistencies in model implementation in existing systems (e.g. Scott and Reinhardt 2001;Andrews 2012) and will produce model code blocks that can be used by developers of collaborative systems such as BlueSky, which utilises a modular approach.…”

“…Flame dimensions are not needed to determine the wind speed for calculating spread rate. In application, hand-held measurements of wind at 'eye-level' are often used for midflame wind (Andrews 2012). Effective midflame wind speed is the combined effect of wind and slope according to the Rothermel model equations (Albini 1976a).…”

A Study of Wind Speed Probability Models: A Case of Sir Seretse Khama International Airport Monthly Maximums

“…Andrews [2] states that wind is among the most important influencing factors of wild land fire as fire behaviour is strongly affected by wind speed and direction. Wind speed is random; hence empirical statistical methods are useful in estimating it.…”