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. 1 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. __________________________ Corresponding author address: Michael Umscheid, NWS, Weather Forecast Office, 104 Airport Rd, Dodge City, KS 67801. E-mail: [email protected] Figure 1: A large firewhirl over a burning field of wheat stubble; the view is looking southeast from approximately 800 m away. Photo by Michael UmscheidUmscheid, 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 2. Firewhirls and Nonmesocyclone Tornadoes Strong rotation can occur in the convective columns associated with fires. Countryman (1971) shows various configurations of fire, wind, and terrain that are favorable for what are popularly known as “firewhirls” and that these vortices occur across a range of fire sizes and terrain conditions. Firewhirls also have been observed in association with fires occurring in surface meteorological conditions ranging from quiescent (i.e., light winds, clear skies) (see e.g. Clark et al. 1999) to stormy (i.e., strong winds, thunderstorms). Typically, however, although modeling studies of fire growth and evolution include initial meteorological conditions featuring a range of wind speeds, other aspects of the initial meteorological state are relatively benign. One of the earliest documentations of firewhirls in the refereed literature appears in Hissong (1926). He describes large firewhirls that developed after lightning struck an oil storage facility near San Luis Obispo, CA. Several firewhirls were observed to develop on the margins of the fire, and significant damage occurred to surrounding structures, with the loss of two lives. Since severe thunderstorms were in the vicinity, and there is anecdotal evidence that tornadoes also occurred in other areas away from the fire, this instance can be thought to represent an extreme example of firewhirl formation in an environment already conducive for tornado formation. This instance also underscores that there may be meteorological ingredients associated with the subsynoptic or mesoscale environment that can augment whatever vertical vorticity generating processes are associated with fires themselves in the development of firewhirls. Recent analyses of video imagery during a crown fire (i.e., fire restricted to the tops of trees) (Clark et al., 1999) show derived vertical vorticity on the order of 4 to 10 s-1 along the fire front, values similar to those observed in weak tornadoes. Observations from the study by Clark et al. (1999) also indicate that vorticity associated with this fire was caused largely by tilting and stretching of horizontal vortices produced by the fire’s horizontal thermal gradients on the forward side of the fire line. Vortices occurred near the nose of the fire, with the main vortex tilting occurring at that location. Counter-rotating vortices were also observed on either side of the fire nose, caused by ingestion, convergence, and tilting on the flanks of the fire line. Visible rotation in each of these cases likely would be described as “firewhirls.” Models that simulate wildland fires often depict large firewhirls (Coen 2005) for a range of fire sizes and topographic configurations. The National Center for Atmospheric Research’s (NCAR) coupled atmosphere fire model has been used to simulate the large-scale interactions between fire and local winds. Clark et al. (2004) used the model to capture the spread of fires of different sizes and across a range of surface conditions. Of particular relevance to the present study was the modeled fire on flat terrain with a fire line 140 m long in a light surface wind environment (3 m s-1) (Clark et al. 2004). The fire size and surface conditions