Fire-Generated Tornadic Vortices

Abstract

Fire-generated tornadic vortices (FGTVs) linked to pyrocumulonimbi (pyroCb) are a potentially deadly, yet poorly understood and seldom observed 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, USA. We establish that these FGTVs each exhibit tornado-strength anticyclonic rotation, with rotational velocity as strong as 30 m s-1 (60 kts), vortex depths of up to 5 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 meso-scale 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.

Satellite Evidence for Glyoxal Depletion in Elevated Fire Plumes

“…As mentioned above, R gf is computed while taking into account the different effective light paths in the respective spectral ranges of HCHO and CHOCHO retrievals, thereby eliminating radiative transfer effects. A similar CHOCHO depletion has been observed for other elevated fire plumes in Australia (e.g., on 30–31 December 2019 and 5 March 2019) and in California during intense fire episodes with reported PyroCb formation (Lareau et al., 2022). This is illustrated in Figure S2 in Supporting Information for the Creek fire of 5 September 2020.…”

Tracking Wildfires With Weather Radars

“…For example, in the present study, the radar data provide critical details of fire progression during the Camp Fire's run into Paradise that were otherwise unavailable due course satellite data (GOES) and weather conditions that precluded aircraft operations. An additional strength of the radar observations is that they contain more information than we have highlighted in this study, including information about plume height, plume geometry, plume microphysics, pyroCb initiation, and fire induced winds (e.g., Lareau et al., 2018, 2022). As such, a more comprehensive use of radar observations for fire situational awareness is warranted as is further investigation of how to best extract information about fires from these data.…”

“…However, during this time, the fire transitions from a narrow parabolic nose to a broader, blunter head fire, and then to a period where the progression along the flanks of the head fire is faster than the progression in the center of the head fire, causing a "folding over" of the fire shape (i.e., an inverted parabola). The physical processes driving this transition are linked to a region of strong flow reversal (i.e., a fire-generated wind opposing the mean wind), strong wind shear, and vortex generation as described in Lareau et al (2022). The pyrometeor laden vortex is particularly apparent as the localized radar reflectivity maxima and fire points at 0125 UTC (Figure 14e) near −121.2/39.65.…”

“…The physical processes driving this transition are linked to a region of strong flow reversal (i.e., a fire‐generated wind opposing the mean wind), strong wind shear, and vortex generation as described in Lareau et al. (2022). The pyrometeor laden vortex is particularly apparent as the localized radar reflectivity maxima and fire points at 0125 UTC (Figure 14e) near −121.2/39.65.…”

“…In addition, both fire's produced deep, more‐upright, pyrocumulus‐topped convective columns (see Figure 13 in Peterson et al., 2017). In the case of the King Fire, fire‐induced winds are also thought to have substantially contributed to the fire's rate and direction of spread (Coen et al., 2018; Lareau et al., 2022). These fires are also at a much larger range from the radar site (∼110 km vs. 30–45 km), yielding a lower beam‐to‐beam azimuthal resolution (Table 1), and experience spread in directions both along and across scans.…”