An Improved Canopy Wind Model for Predicting Wind Adjustment Factors and Wildland Fire Behavior

The ability to rapidly estimate wind speed beneath a forest canopy or near the ground surface in any vegetation is critical to practical wildland fire behavior models. The common metric of this wind speed is the ``mid-flame" wind speed, U_MF. But the existing approach for estimating U_MF has some significant shortcomings. These include the assumptions that both the within canopy wind speed and canopy structure are uniform with depth throughout the canopy and that the canopy roughness length [z_0] and displacement height [d] are the same regardless of canopy structure and foliage density. The purpose of this study is to develop and assess a model of canopy wind and turbulence that eliminates these shortcomings and thereby provide a more physically realistic method for calculating U_MF. The present model can be used for canopies of arbitrary foliage distribution and leaf area and the single function that describes the within-canopy wind speed is shown to reproduce observed canopy wind speed profiles across a wide variety of canopies. An equally simple analytical expression for the within canopy turbulence (Reynolds stress) also provides a very good description of the observed vertical profiles of Reynolds stress. In turn, the Reynolds stress profile is used to calculate z_0 and d. Tests of operational performance are also discussed.