Characterising forest wind profiles for utilisation in fire spread models

Abstract

Fire behaviour is strongly affected by wind speed and direction. Wind affects fire spread rate bending the flames closer to unburnt fuel, increasing the rate of combustion and by blowing embers ahead of the main fire front lighting spotfires. Stronger winds blow flames forward, lowering the flame angle and subsequent distance between the flame and the fuel. In open areas there is typical a logarithmic vertical wind profile; winds become faster as height increases. However forest vegetation disrupts wind flow, and a logarithmic profile cannot be assumed. When predicting forest fire spread, wind reduction factors are typically used to reduce open area wind speeds to the more sheltered sub-canopy winds that directly affect fire spread. These methods typically assume a consistent vertical wind profile that does not change with wind speed. In this study we investigated these assumptions. We characterised the variation in wind speed at different heights in different vegetation types and at different open area wind speeds. To do this, we used cup anemometers to measure the horizontal wind speed at different heights in four differently structured forest areas. Our results show that the use on a single wind reduction factor is a gross over-simplification; horizontal wind speeds can vary by a factor of three depending of the height above ground. We have also found that even for a given height, the wind reduction factor can vary by a factor of two or more depending on the strength of the winds in the open. This is in addition to a factor of 10 or more variation in wind speed between different vegetation types. The variation in wind speed in different vegetation type, height above ground and wind speed in the open means that there is potentially a large margin of error in predicting fire behaviour using wind speeds measured or forecast for 10 m in the open for fires in forests or shrublands. Some strategies for improving estimates of wind speed for fire spread prediction are discussed.