Christopher R. Church and intense Atmospheric vortices John T. Snow Department of Geosciences, Purdue University Associated with a 1000 MW Fire W. Lafayette, Ind. 47907 and Jean Dessens Centre de Recherches, Atmospheriques Henri Dessens, University de Clermont, Campistrous—CIDEX B47, 65300 Lannemezan, France Abstract The special case of the fire whirl, with its central core filled with burning material, is at once impressive Observations of vortices of various types produced in a large thermal plume are described. The apparatus used to generate and dangerous, and is particularly intriguing. the plume is the M£teotron, an array of 105 fuel oil burners The dynamic structure of these and other atmo- with a total heat output of approximately 1000 MW. Three spheric vortices (tornadoes, waterspouts, and dust types of vortices have been observed: 1) large counter- devils) is not well known, primarily due to the lack rotating rolls in the downstream plume, 2) intense small-scale vortices resembling very strong dust devils seen at the sur- of direct measurements within the actual flows. Such face on the downwind side of the plume, and 3) very large measurements have been precluded by several ob- columnar vortices produced when the lower portion of the stacles. These include the design of instrumentation plume goes into rotation as a whole. Three mechanisms to survive in the adverse environment of the vortex leading to the concentration of vorticity necessary to produce core, the logistical problem of following and inter- these vortex types are discussed. These include tilting and stretching of horizontal vorticity present in the environ- cepting a moving natural vortex, and considerations mental wind field, generation of vorticity within the plume of personnel safety. Here we report on an opportunity by the action of buoyancy and drag forces, and convergence for generating and making direct measurements of of preexisting background vorticity from the environment. large man-made atmospheric vortices and discuss the It is concluded, based on these observations and physical considerations, that the generation of vortices of moderate results of a recent preliminary effort to observe and intensity is to be expected in large plumes, be their source document the vortices that formed in the plume from a forest fire or an industrial operation. a large controlled fire. It is the intent of this and future work to rectify, in part, the current lack of direct observations of intense vortical flows in a natural 1. Introduction environment. It is well established that intense atmospheric vortices occur in the vicinity of large man-made and natural 2. Experimental program fires. The literature documents the production of such vortices in forest fires (Graham, 1955), refinery ex- The work reported herein was carried out at the plosions (Hissong, 1926), gas well fires (Dessens, 1963), Centre de Recherches Atmospheriques Henri Dessens, volcanic eruptions (Thorarinsson and Vonnegut, 1964), located on the plateau of Lannemezan (^600 m MSL), area conflagrations in cities (Ebert, 1963), and at- some 30 km north of the central Pyrenees. The current tendant to a number of industrial operations involving series of experiments was begun in 1978 as a joint the large-scale release of heated gases into the atmo- research venture between the Observatoire du Puy de sphere. Such vortices vary greatly in intensity, ranging Dome and Electricite de France (EdF), the French from tornado-like flows strong enough to throw tree national electrical power company. (EdF has an trunks around and destroy small homes to much more interest in assessing the environmental impact of the modest whirls that resemble very small dust devils. release of waste dry heat in amounts of the order of 1000 MW into the atmosphere.) A number of re- 0003-0007/80/070682-13$07.25 searchers from private industry and neighboring French © 1980 American Meteorological Society universities participated in the program. The present 682 Vol. 61, No. 7, July 1980 Unauthenticated | Downloaded 09/23/21 06:12 PM UTC Bulletin American Meteorological Society 683 project is the successor of two previous programs: aging 5-10 m in length and 1 m in diameter. Fuel is 1961-64, dealing with the creation of cumulus clouds consumed at a rate of 1 m3 h-1 per burner, with each (Dessens and Dessens, 1964) and the formation of burner having a calorific power output of some 10 MW. tornado-like vortices (Dessens, 1962); and 1971-73, Thus a combined output of some 1000 MW for the dealing with a quantitative study of an artificial con- array as a whole is achieved. An experimental period vective plume initiated from the ground (Benech, at full pressure normally lasts from 20 to 30 min. 1976). In the current program, experimental burns Under background conditions of even very moderate were carried out in June, July (15 burns), and No- conditional instability, the large thermal plume gen- vember (15 burns) 1978, and in May and June (15 erated by the Meteotron triggers the production of burns) 1979. The experiments were performed under moderate size cumulus clouds. These can occasionally a wide variety of background meteorological condi- grow to the cumulus congestus stage. As a consequence, tions. Two of us (Church and Snow) participated in much of the supporting instrumentation is focused on the 1979 phase for the specific purpose of investigating the downstream, nearly horizontal portion of the plume. the large vortices that have been observed in the plume. Due to the interests of the Observatoire, a more ex- The array of burners used for producing the buoyant tensive set of instrumentation has recently been in- plume is called the Meteotron. Since the debut of the stalled in the immediate vicinity of the plume base initial design by H. Dessens in 1961 (see Dessens, in order to measure the flow (convergence and mean 1962), the Meteotron has undergone several changes vorticity) in the immediate vicinity of the fire. How- in burner design, number of burners used, and con- ever, the main scientific thrust (and hence the bulk figuration of burner deployment. As shown in Fig. 1, of the instrumentation) continues to focus on the far the array currently consists of 105 fuel oil burners downstream portion. deployed in a three-armed spiral pattern within a Instrumentation for observing the base of the plume 140 m X 140 m square. A burner of the type used in during the 1979 summer program consisted of 42 in- the present series of experiments is shown in Fig. 2. strument assemblages suspended over the burner array Three diesel-powered pumps supply fuel oil to the by means of an elaborate cable assembly, supplemented burners at pressures up to 5900 kPa. Each burner is by 16 similar ground-based assemblages (see Fig. 3). equipped with a set of spark gap electrodes and Each assemblage contained a three-component ane- a step-up transformer so as to be ignited electrically mometer and a temperature sensor. The primary by striking an arc next to the fuel injection nozzle. purpose for which this extensive array was installed At full pressure, each burner produces a flame aver- was the collection of baseline wind and temperature FIG. 1. The M£t£otron, looking to the northeast. For scale, the pylon in the far corner is approximately 280 m away and 60 m tall. Unauthenticated | Downloaded 09/23/21 06:12 PM UTC 684 Vol. 61, No. 7, July 1980 data concerning the source region of the plume. Thus details, such as occur with the development of small the assemblages were positioned just outside the region vortices, could not be resolved in the data field. wherein the details of the individual burners were Other supporting experimental equipment in 1979 important. The 16 ground-based packages were each consisted of time-lapse camera systems, 8.6 mm and mounted on a 4 m fold-down tower, and spaced four 10 cm radar, and Doppler sodar. A B23 research to a side along the flanks of the fire area to obtain aircraft from the University of Washington, equipped a picture of the strength of the low-level convergence with an array of environmental monitoring and particle into the base of the plume. The suspended assemblages sampling equipment, provided data on the micro- were arranged in two levels, 21 to a level, at 30 and physical aspects of the downwind portion of the plume 60 m AGL. Horizontal grid spacing was 40 m. These and cloud. Frequent rawinsonde observations were served to define the main features of the vertical taken from a site 4 km to the southwest of the burner motion and temperature field in the lowest portion array. of the plume. Additional instrumentation was mounted In order to investigate more fully the small-scale on each of the four 60 m tall towers supporting the details of the low-level plume structure and to observe cable assembly. All data from this array were digitized, the vortices reported therein, several hand-held still multiplexed, and recorded on magnetic tape. However, and movie cameras were used and the observers the spacing of the instruments is such that small-scale (Church and Snow) worked in close to the base of FIG. 2. A close-up view of one of the 105 burners in the array. The diameter of the cylin- drical drum is 90 cm. Also visible is the fuel injection nozzle and the elements of the arc ignition system. The burners were spaced 4 m apart. Unauthenticated | Downloaded 09/23/21 06:12 PM UTC Bulletin American Meteorological Society 685 FIG. 3. A schematic (not to scale) of the anemometer and temperature sensor array used to investigate the base of the plume.