Microscale Wind Extrema Effects on Wildfires

A sequence of recent California wildfires, driven by extreme Santa Ana and Diablo wind events, have overrun communities adjacent to or intermixed with wildland environments. Reports have linked the ignition of many of these events to electrical malfunctions associated with high winds. Driven by regional pressure gradients, winds are accelerated as they pass over or between topographic features. Recent work by Coen investigates the mechanisms generating microscale wind extrema in these events, in which peak wind speeds can exceed 30-40 m s-1 and which are frequently located at wildfire origins, and subsequent fire growth. Analysis of simulations using the CAWFE® coupled weather-wildland fire model showed that high speed surface winds reaching 30-40 m s-1 affected the fire's origin area and, generated by a shear instability caused by a velocity difference across the interface between two low-level stable layers, high speed wind bursts (Figure Xa) rapidly drove the fire downslope. Figure Xa shows the simulated fire extent and wind bursts late morning, as the western edge of the fire has entered the town of Paradise, CA. Figure Xb shows coincident Landsat Operational Land Imager (OLI) imagery.

(a) VAPOR visualization of a CAWFE® coupled weather-wildland fire model simulation of the November 8, 2018, Camp Fire in Paradise, CA (indicated in figure). The image shows the heat flux produced by the fire, colored according to the color scale at left (top).  The arrows indicate the simulation wind speed near the surface, colored by the wind speed magnitude (according to lower color bar at left). (b) A natural color image using visible and shortwave infrared bands to highlight the active fire using data acuired by the Operational Land Imager on Landsat 8 on Nov. 8 at 10:45 a.m.
Figure A: (a) VAPOR visualization of a CAWFE® coupled weather-wildland fire model simulation of the November 8, 2018, Camp Fire in Paradise, CA (indicated in figure). The image shows the heat flux produced by the fire, colored according to the color scale at left (top). The arrows indicate the simulation wind speed near the surface, colored by the wind speed magnitude (according to lower color bar at left). (b) A natural color image using visible and shortwave infrared bands to highlight the active fire using data acuired by the Operational Land Imager on Landsat 8 on Nov. 8 at 10:45 a.m.