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Abstract Scanning Ka-band Doppler radar observations reveal the development and intensification of a counter-rotating vortex pair (CVP) embedded in an advancing fire front during California’s Dixie Fire in August 2021. The observations show that an initially isolated plume associated with a new spot fire develops flow splitting and a fire-generated inflow wind on the plume’s lee side. This inflow retards the fire progression and enhances the lateral wind shear along the plume flanks. The lateral shear evolves into quasi-symmetric cyclonic and anticyclonic vortices with winds > 40 m s⁻¹. This CVP spreads perpendicular to the wind direction, yielding a “y-shaped” fire perimeter, with fire intensity and direction of spread strongly linked to the vortices. Detailed snapshots of the vortices reveal associated radar “hook echoes” and orbiting subvortices of tornado-like intensity. Some vortices remain attached to the fire, while others shed downstream. Additional lidar observations show the structure and development of the fire’s inflow. We discuss the observed vortex evolution in the context of existing conceptualizations for CVPs in wildland fire, including their preferential occurrence on lee slopes and their role in generating lateral fire spread. 

BackgroundThere is an ongoing need for improved understanding of wildfire plume dynamics. AimsTo improve process-level understanding of wildfire plume dynamics including strong (>10 m s−1) fire-generated winds and pyrocumulus (pyroCu) development. MethodsKa-band Doppler radar and two Doppler lidars were used to quantify plume dynamics during a high-intensity prescribed fire and airborne laser scanning (ALS) to quantify the fuel consumption. Key resultsWe document the development of a strongly rotating (>10 m s−1) pyroCu-topped plume reaching 10 km. Plume rotation develops during merging of discrete plume elements and is characterised by inflow and rotational winds an order of magnitude stronger than the ambient flow. Deep pyroCu is initiated after a sequence of plume-deepening events that push the plume top above its condensation level. The pyroCu exhibits a strong central updraft (~35 m s−1) flanked by mechanically and evaporative forced downdrafts. The downdrafts do not reach the surface and have no impact on fire behaviour. ALS data show plume development is linked to large fuel consumption (~20 kg m−2). ConclusionsInteractions between discrete plume elements contributed to plume rotation and large fuel consumption led to strong updrafts triggering deep pyroCu. ImplicationsThese results identify conditions conducive to strong plume rotation and deep pyroCu initiation.