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    Author(s): W. J. MassmanJ. M. ForthoferM. A. Finney
    Date: 2017
    Source: Canadian Journal of Forest Research. 47: 594-603.
    Publication Series: Scientific Journal (JRNL)
    Station: Rocky Mountain Research Station
    PDF: View PDF  (1.0 MB)

    Description

    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, UMF. However, the existing approach for estimating UMF has some significant shortcomings. These include the assumptions that both the within-canopy wind speed and the canopy structure are uniform with depth (z) throughout the canopy and that the canopy roughness length (z0) 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 Reynolds stress that eliminates these shortcomings and thereby provide a more physically realistic method for calculating UMF. The present model can be used for canopies of arbitrary plant surface 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 Reynolds stress, u2(z), also provides a reasonable description of the observed vertical profiles of Reynolds stress. In turn, u2(z) is used to calculate z0 and d. Tests of operational performance are also discussed.

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    Citation

    Massman, W. J.; Forthofer, J. M.; Finney, M. A. 2017. An improved canopy wind model for predicting wind adjustment factors and wildland fire behavior. Canadian Journal of Forest Research. 47: 594-603.

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    Keywords

    fire spread modeling, canopy foliage distribution, Rothermel model

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