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Using Gridded Winds (BFB, STFB)
Gridded winds are spatial wind field grids that are useful for examining and calculating fire spread in complex terrain where winds are influenced by topography. Without gridded winds, each cell on the landscape will have a single windspeed and direction. When using gridded winds, outputs on ridgetops will be stronger than winds in the valley bottom and wind direction vectors will 'bend' around the lee side of terrain features. Proper use of gridded winds in an analysis can produce more realistic fire behavior modeling results.
Gridded winds are an optimal input in (BFB) and Short-Term Fire Behavior (STFB) models and are always run in the Automated versions of these models. Gridded winds are currently not available in Near-Term Fire Behavior (NTFB) but are planned as a future enhancement.
Only two user-defined inputs are required to generate gridded winds: wind speed and wind direction. These inputs are automatically populated (depending on historic or forecast data), but users can override these values if needed. When selecting a wind speed input for gridded winds it is important to note that this value will represent an average across the entire landscape. After running models with gridded winds, users should query the gridded wind outputs to evaluate whether the 'landscape average' wind speed input resulted in reasonable gridded wind outputs at various point locations.The gridded wind model automatically applies a dominant vegetation to the landscape (grass, brush or timber) to account for some level of vegetation drag on the wind field. The model assumes that the selected vegetation type covers the entire landscape. An island of timber in a landscape that is predominantly grass will be treated as grass and the localized effects of the stand of trees will not be represented in the gridded wind outputs.
Gridded winds are generated after an analysis is launched using the WindNinja model. The dominant vegetation type is chosen automatically from the fuels layer and the model utilizes the user-defined wind speed and direction inputs to adjust wind vectors according to the terrain. Wind vectors are created at the (user defined) resolution of the landscape in the model outputs as a color-coded grid with legends that describe the range of modeled wind speeds and directions.
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Last updated on 1/28/2019 4:14:25 PM.
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