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Photographs and Analysis of an Unusually Large and Long-Lived Firewhirl

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Photographs and Analysis of an Unusually Large and Long-Lived Firewhirl Umscheid, M.E., J. P.Monteverdi, and J.M. Davies, 2006: Photographs and Analysis of an Unusually Large and Long-lived Firewhirl. Electronic J. Severe Storms Meteor. Photographs and Analysis of an Unusually Large and Long-Lived Firewhirl MICHAEL E. UMSCHEID NOAA/NWS, Weather Forecast Office, Dodge City, KS JOHN P. MONTEVERDI Department of Geosciences, San Francisco State University, San Francisco, CA JONATHAN M. DAVIES Private Meteorologist, Wichita, KS (Submitted February 6, 2006) ABSTRACT On 30 June 2005, a large and long-lived firewhirl was observed and photographed over a field being burned to remove wheat stubble in central Kansas. With a well-defined boundary focusing vertical vorticity in the immediate vicinity, the meteorological setting appeared to have at least some similarity to those associated with many nonmesocyclone tornadoes. This paper photographically documents the firewhirl and its evolution. In addition, an examination of the synoptic and local meteorological environment suggests that a pre-existing frontal boundary contributed to the occurrence and longevity of the firewhirl in this interesting and unusual case. Although they are clearly different phenomena, firewhirls and nonmesocyclone tornadoes appear to share some similarities in formation mechanisms that are illustrated by this case. –––––––––––––––––––––––– 1. Introduction During the late afternoon of 30 June 2005, a long-lived firewhirl (see Fig. 1) occurred over a wheat stubble field prescribed burn in central Kansas. The fire front was approximately 300 m wide at the time of the firewhirl development. The firewhirl, which towered approximately 200 m and lasted around 20 minutes , occurred in the vicinity of a slow moving cold front that we hypothesize played an important role in the evolution, longevity, and strength of this vortex. The unusual duration and size of this fire- spawned vortex, combined with the presence of the frontal boundary, suggest nonmesocyclone tornado processes supplemented local fire vortex generation to support this unique and unusual event. The purpose of this paper is to present photographic documentation of the evolution of the firewhirl along with supporting radar, satellite, and surface observation analyses. __________________________ Figure 1: A large firewhirl over a burning field Corresponding author address: of wheat stubble; the view is looking southeast Michael Umscheid, NWS, Weather Forecast from approximately 800 m away. Photo by Office, 104 Airport Rd, Dodge City, KS 67801. Michael Umscheid E-mail: [email protected] 1 Umscheid, M.E., J. P.Monteverdi, and J.M. Davies, 2006: Photographs and Analysis of an Unusually Large and Long-lived Firewhirl. Electronic J. Severe Storms Meteor. 2. Firewhirls and Nonmesocyclone Tornadoes the main vortex tilting occurring at that location. Counter-rotating vortices were also observed on Strong rotation can occur in the convective either side of the fire nose, caused by ingestion, columns associated with fires. Countryman convergence, and tilting on the flanks of the fire (1971) shows various configurations of fire, line. Visible rotation in each of these cases likely wind, and terrain that are favorable for what are would be described as “firewhirls.” mike umscheid ! 5/3/06 2:31 PM popularly known as “firewhirls” and that these Comment: Changed to present tense vortices occur across a range of fire sizes and Models that simulate wildland fires often depict terrain conditions. large firewhirls (Coen 2005) for a range of fire sizes and topographic configurations. The Firewhirls also have been observed in National Center for Atmospheric Research’s association with fires occurring in surface (NCAR) coupled atmosphere fire model has meteorological conditions ranging from been used to simulate the large-scale interactions quiescent (i.e., light winds, clear skies) (see e.g. between fire and local winds. Clark et al. (2004) Clark et al. 1999) to stormy (i.e., strong winds, used the model to capture the spread of fires of thunderstorms). Typically, however, although different sizes and across a range of surface modeling studies of fire growth and evolution conditions. include initial meteorological conditions featuring a range of wind speeds, other aspects of Of particular relevance to the present study was the initial meteorological state are relatively the modeled fire on flat terrain with a fire line benign. 140 m long in a light surface wind environment (3 m s-1) (Clark et al. 2004). The fire size and One of the earliest documentations of firewhirls surface conditions are similar to those in the case in the refereed literature appears in Hissong documented here. Results show that the (1926). He describes large firewhirls that convective circulation rooted at the head of the mike umscheid ! 5/3/06 2:35 PM developed after lightning struck an oil storage fire advected low level air upwind to the back of Comment: Present tense facility near San Luis Obispo, CA. Several the fire. Non-linear processes due to terrain or John Monteverdi ! 5/4/06 3:54 PM firewhirls were observed to develop on the fuel inhomogeneities induced perturbations on Comment: Minor word change. margins of the fire, and significant damage the edges of the fire line, affecting the spread mike umscheid ! 5/4/06 2:37 PM occurred to surrounding structures, with the loss rate into unburned areas downwind. Clark et al. Comment: John, could you re-word this sentence of two lives. Since severe thunderstorms were in (2004) hypothesize that these perturbations per Banacos minor pt.#5? the vicinity, and there is anecdotal evidence that augmented horizontal vorticity generation along mike umscheid ! 5/3/06 2:37 PM tornadoes also occurred in other areas away from the fire line through tilting and stretching into the Comment: Present tense the fire, this instance can be thought to represent fire updraft, producing firewhirls. an extreme example of firewhirl formation in an environment already conducive for tornado It is also well known that tornadoes can occur formation. This instance also underscores that along convergent wind shift boundaries with there may be meteorological ingredients newly developing, multicell storms (e.g., associated with the subsynoptic or mesoscale Burgess and Donaldson 1979). On radar, environment that can augment whatever vertical signatures associated with these tornadoes are vorticity generating processes are associated with smaller and typically shallower than those fires themselves in the development of associated with supercell storms and their firewhirls. associated mesocyclones (e.g., Burgess et al. 1993), and are called misocyclones (Fujita Recent analyses of video imagery during a crown 1981). In the 1980s, the term “landspout” fire (i.e., fire restricted to the tops of trees) (Bluestein 1985) came into use for tornadoes (Clark et al., 1999) show derived vertical from these circulations because of their vorticity on the order of 4 to 10 s-1 along the fire similarity to waterspouts. Additional mike umscheid ! 5/3/06 2:33 PM front, values similar to those observed in weak observations and research suggest that these Comment: Present tense tornadoes. Observations from the study by Clark “nonmesocyclone tornadoes” (Brady and Szoke mike umscheid ! 5/3/06 2:39 PM et al. (1999) also indicate that vorticity 1989), also called “nonsupercell tornadoes” Comment: Present tense associated with this fire was caused largely by (Wakimoto and Wilson 1989), develop from pre- mike umscheid ! 5/3/06 2:33 PM tilting and stretching of horizontal vortices existing low-level vertical vorticity circulations Comment: Present tense produced by the fire’s horizontal thermal along sharp wind shift boundaries that are gradients on the forward side of the fire line. stretched by expanding updrafts above the Vortices occurred near the nose of the fire, with circulations. Modeling experiments by Lee and 2 Umscheid, M.E., J. P.Monteverdi, and J.M. Davies, 2006: Photographs and Analysis of an Unusually Large and Long-lived Firewhirl. Electronic J. Severe Storms Meteor. Wilhelmson (1997, 2000) confirm that this low moved south through the central plains nonmesocyclone tornadoes occur when during the morning. mike umscheid ! 5/3/06 2:39 PM horizontal shearing perturbations in vertical Comment: Present tense vortex sheets along boundaries are stretched by updrafts. Wilczak et al. (1992) also found that tilting of local horizontal vorticity near these boundaries could contribute to nonmesocyclone tornadoes. Donaldson and Burgess (1982) noted that boundaries associated with nonmesocyclone tornadoes are often detectable as clear air echoes (lines) on radar. Recent studies, such as Pietrycha and Manross (2003) and Caruso and Davies (2005), show the importance of boundary detection and evolution using radar reflectivity mike umscheid ! 5/3/06 2:42 PM and velocity products in nonmesocyclone Comment: Present tense tornado settings. Brady and Szoke (1989) and Davies (2006) also link nonmesocyclone tornadoes to steep low-level lapse rates (near the mike umscheid ! 5/3/06 2:42 PM dry-adiabatic rate) that could generate rapidly Comment: Present tense rising low-level parcels and stretching beneath updrafts, similar to boundary-layer thermodynamic profiles associated with dust devils. Figure 2: Composite map of synoptic scale meteorological conditions valid 1200 UTC on 30 There are some evident similarities between fire June 2005. Isotachs at 250mb are dotted blue and vortices and nonmesocyclone tornadoes. As in the jet core is indicated
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