Fire-Modified Winds during the Lahaina Fire

The Hawaiian island of Maui suffered its worst wildfire in modern history as flames burned through the historically significant town of Lahaina on 8-9 August 2023. The fire initially progressed east-to-west through the town, then rapidly spread north-south along the coastline, burning the majority of structures in town. Previous Weather Research and Forecasting (WRF) and uncoupled Streamlined Wildland-Urban Interface Fire Tracing (SWUIFT) simulations revealed a downslope windstorm causing the initial east-to-west spread with the subsequent inland migration of the hydraulic jump, where the flow detaches from the surface, causing both meridional and eastward movement of the fire. These simulations, however, do not account for the potential for fire heat fluxes to modify the wind flow and thus aspects of the fire behavior.

To fill this gap, this study examines fire-modified winds in the presence of large fire heat fluxes from the combustion of buildings and vegetation. This is accomplished using a coupled fire-atmosphere model (WRF-Fire) run in large-eddy simulation mode (100 m horizontal grid cell spacing). Fire-generated winds are quantified using differences between two simulation groups: (1) a fireless control run, and (2) a “forced fire” run with prescribed fire spread (i.e., time of ignition) and modified fuel loads. The time-of-ignition of model grid points is based on previously simulated fire spread from SWUIFT. This step bypasses uncertainties in the fire spread and allows us to isolate the atmospheric response to the fire. Since existing fuel categories in WRF-Fire consider urban structures as unburnable, we create a new housing/structure fuel category to account for structures being the dominant fuel type during the fire. These urban fuels have very high spatially averaged fuel loads (~20-40 kg m^-2) and thus are capable of producing large heat fluxes that perturb the atmospheric flow.

By contrasting the control and forced-fire simulations, our results help isolate the role that urban burning may have played in the meteorology contributing to one of Hawaii’s most devastating tragedies. These results also inform our understanding of other WUI fires where houses/structures become a dominant fuel type, such as California’s Camp fire and Colorado’s Marshall Fire.