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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. 

Abstract Observations reveal extreme long‐range fire spotting occurred during California's Dixie Fire. Specifically, we describe the occurrence of remarkable 9, 12, and 16 km spotting events on 16 August 2021. Radar data reveal these spot fires are linked to bent‐over but deep convective plumes with plume tops reaching 10–12 km MSL. These plumes have characteristic lofting regions in the fire‐generated updrafts and pyrometeor fall out locations in the downwind subsiding portion of the plume. Infrared data indicate spot fires occur along the plume's central axis. The cross winds impacting the plume rise and pyrometeor transport were ∼15 m s⁻¹, and the inferred transit time firebrands causing the longest‐range spot fire is ∼18 min. We also provide photographic evidence for large, partially burned pyrometeors at a range of ∼20 km from the fire and link these data to Ka‐band radar observations showing pyrometeor pulses and fall out over the observing site. The results of the study suggest that operational and research radars may be able to isolate periods conducive to long range spotting in near real‐time. 

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. 

Recent research suggests fire-generated vortices are always present during wildfires. 

Abstract Fire-generated tornadic vortices (FGTVs) linked to deep pyroconvection, including pyrocumulonimbi (pyroCbs), are a potentially deadly, yet poorly understood, wildfire hazard. In this study we use radar and satellite observations to examine three FGTV cases during high-impact wildfires during the 2020 fire season in California. We establish that these FGTVs each exhibit tornado-strength anticyclonic rotation, with rotational velocity as strong as 30 m s −1 (60 kt), vortex depths of up to 4.9 km AGL, and pyroCb plume tops as high as 16 km MSL. These data suggest similarities to EF2+ strength tornadoes. Volumetric renderings of vortex and plume morphology reveal two types of vortices: embedded vortices anchored to the fire and residing within high-reflectivity convective columns and shedding vortices that detach from the fire and move downstream. Time-averaged radar data further show that each case exhibits fire-generated mesoscale flow perturbations characterized by flow splitting around the fire’s updraft and pronounced flow reversal in the updraft’s lee. All the FGTVs occur during deep pyroconvection, including pyroCb, suggesting an important role of both fire and cloud processes. The commonalities in plume and vortex morphology provide the basis for a conceptual model describing when, where, and why these FGTVs form.