Impact of forest gaps on wind turbulence and potential wildfire behavior at the rural-urban interface

Forest canopy can impact fire behavior through modulating both subcanopy atmospheric turbulence and wake turbulence at the forest edge. The 2020 Lake ÅŒhau fire happened at a rural-urban interface where the heavily fire-damaged village was surrounded by a heterogeneous forest canopy. Evidence shows that high wind speeds with strong gusts were present throughout the fire outbreak, indicating a strong wind-driven wildfire. Since the village was surrounded by complex terrain including a forest canopy, the forest canopy might play an important role in modifying the wind conditions within the village which could impact the fire spread behavior. This research uses Large Eddy Simulation (LES) to study wind gusts and other near-surface wind characteristics in the Lake ÅŒhau area under the weather conditions of the 2020 Lake ÅŒhau fire. The work especially focuses on how the forest canopy around the village could change the wind gust within and around the village area. To do so, a LiDAR field campaign was also undertaken to obtain ultra-high resolution (grid resolution < 10m) forest canopy and other geoinformation. Results also show that the village was impacted by two mesoscale wind systems - an offshore westerly downslope wind from west of the village and a northerly wind coming down over Lake ÅŒhau. The LES results were also used to further explore how simulated forest gap orientation might impact the downstream wind. Previous studies have shown that creating forest gaps or firebreaks can significantly change the wind characteristics within and at the edge of the forest. Three additional scenarios were studied by modifying the forest canopy around the village, including one without any forest canopy, one with horizontal (West-East orientation) forest gaps and one with vertical (North-South orientation) forest gaps. Total removal of the forest canopy increased the average wind gust in almost all areas within the village while the changes were spatially more heterogeneous in the horizontal and vertical gap scenarios. Differences of the spatial wind gust distribution might be related to how different forest gap alignments change the dominant inflow wind component. Next steps of this work will utilize passive tracers and temporal analysis to identify the processes driving these differences.