FE~RU^RY 1992 PICTURE OF THE MONTH 371 PICTURE OF THE MONTHWhirlwind Formation at a Burning Oil Supertanker in the Gulf of Mexico VINCENT W. WOOD NOAA, Environmental Research Laboratories, National Severe Storms Laboratory, Norman, Oklahoma 5 August 1991 and 6 August 19911. Introduction Atmospheric vortices have been observed to occurin the vicinity of man-made and natural fires. Suchvortices are produced in forest fires (Graham 1955),refinery explosions (Hissong 1926), gas well fires( Desserts 1963 ), volcanic eruptions (Thorarinsson andVonnegut 1964), area conflagrations in cities (Ebert1963), bonfires (Glaser 1959; Minsinger 1980), andexperimental oil burners (Church et al. 1980). On Saturday, 9 June 1990, the 885-ft Norwegiansupertanker Mega Borg exploded in the Gulf of Mexicoat 0600 UTC. The explosion occurred while crude oilwas being transferred to a smaller tanker, and producedtwo fatalities. During the continuing crude oil spill fromthe Mega Borg, fire went out of control for several daysuntil firefighting ships finally put it out. While visiting my family in Houston, Texas, I sawin The Houston Chronicle a news photo (Fig. 1) ofwhirlwind formation from the supertanker fire. It isworth documenting here, because it is very rare thatburning oil spills cause whirlwinds over the ocean. Thepurpose of this note is to describe briefly the whirlwindphenomenon, and to compare this observation to similar ones made elsewhere.2. Visual observation Figure I vividly illustrates two simultaneous whirlwinds in the downstream plume. The location of theburning wreckage of the Mega Borg (far to the left andout of Fig. 1) was 28.33-N, 94.08-W, or about 54nautical miles southeast of Galveston, Texas. Thesewhirlwinds were very similar to those observed by Hissong (1926) at an oil tank fire, Thorarinsson and Vonnegut (1964) in the thermal plume from the Surtesyvolcano, and Church et al. (1980) in experimental oilburners. Corresponding author address: Vincent T. Wood, National SevereStorms Laboratory, 1313 Halley Circle, Norman, OK 73069.3. Physical mechanisms on the formations of atmo spheric vortices Church et al. (1980) offered a step-by-step explanation of vortex generation processes. They observedthe three following types of vortices generated by theexperimental oil burners: 1) large, counterrotatingvortices in the downstream plume, 2) intense smallscale vortices resembling very strong dust devils seenat the surface on the downwind side of the plume, and3) very large columnar vortices produced when thelower portion of the plume goes into rotation as awhole. Based on their observations and physical reasoning, Church et al. hypothesized that there were threemechanisms producing the concentration of vorticityfor these three types of vortices. These mechanisms aretilting and stretching of horizontal vorticity present inthe environmental wind field, generation of vorticitywithin the plume by the action of buoyancy and dragforces, and convergence of preexisting backgroundvorticity from the environment. According to Steve Campbell, the Houston Chroniclephotographer who took this news photo, vortices emanated periodically from burning oil slicks and waveredover the water for a few minutes before dissipating.The time interval between formation of successivevortices was approximately 20-30 s. The approximatedistance between successive vortices was about 50 ft. Mechanism I of Church et al. (1980) may be theone responsible for the formation of columnar vorticesin the downstream plume. In the presence of environmental wind shear (as revealed by the sloping plumein Fig. 1 ), the thermal plume probably acquired cyclonic and anticyclonic rotations on its right and leftsides from the tilting and stretching of initially horizontal vortex tubes near the ocean surface. The photographer was unable to determine the sense of rotationin either the plume or the columnar vortices in Fig. 1from his vantage point. However, it seems likely thatvortices on the right downwind side were cyclonic,while the vortices on the left downwind side were anticyclonic.372 MONTHLY WEATHER REVIEW VOLUM1E 120 FIG. 1. Two simultaneous vortices generated by the burning oil slicks near the wrecked Norwegian supertanker Mega Borg. This photowas taken between 1700 and 1800 UTC 12 June 1990, 3 days after the explosion. The U.S. Coast Guard and firefighting ships are in thebackground. (Photo by Steve Campbell, courtesy of The Houston Chronicle.) In Church et al.'s controlled oil-burning experiment,the nearby surface was complex and the ambient windfield probably contained significant vertical vorticity.In contrast, Fig. 1 shows that the wind field over thenearly smooth, uniform ocean surface was probablyvery uniform in the horizontal. In this sense, the MegaBorg fire was the "cleaner experiment." How smoke gets into the vortex cores is not obvious.Since there does not appear to be any smoke beingdrawn into the cores from a surface inflow layer (Fig.1 ), there are only two other alternatives, according toSnow ( 1991, personal communication): a) Smoke persists in the core as the vortex movesdownstream from the parent plume. This implies little,if any, vertical motion in the core. b) Smoke is being drawn down from aloft. This implies a two-celled vortex structure, with an inner coreof downdrafi. Two-celled structure has been observedin dust devils (Sinclair 1973) and laboratory vortexchambers (Church et al. 1979). Acknowledgments. The author would like to thankDrs. Bob Davies-Jones at NSSL and John Snow atPurdue University for reviewing and making beneficialcomments on this manuscript. He also acknowledgesthe support of Don Burgess and other personnel atNSSL, and of Michael Young at the National WeatherService Office at Galveston, Texas. Steve Campbell ofThe Houston Chronicle took and provided the photograph. REFERENCESChurch, C. R., J. T. Snow, G. L. Baker, and E. M. Agee, 1979: Characteristics of tornado-like vortices as a function of swirl ratio: A laboratory investigation. J. Atmos. Sci., 36, 1755-1776. , and J. Dessens, 1980: Intense atmospheric vonSces as sociated with a 1000 MW fire. Bull. Amer. Meteor, Soc., 61, 682-694.Dessens, J., 1963: Tourbillon de sable pr6s de l'incendie du puits de Gassi-Touil. J. Rech. ~ltrnos., 1, 209. Ebert, C. H. V., 1963: The meteorological factor in the Hamburg fire storm. Weatherwise, 16, 70-73.Glaser, A. H., 1959: Tornado studies, Meteor. Abstr., 10, No. 2, 303.Graham, H. E., 1955: Fire whirlwinds. Bull. Arner. Meteor. Soc., 36, 99-103.Hissong, J. E., 1926: Whirlwinds at oil tank fire, San Luis Ohispo, California. Mon. Wea. Rev., 54, 161-165.Minsinger, W. E., 1980: Whirlwinds at a bonfire. Weather,vise, 33, 92.Sinclair, P. C., 1973: The lower structure of dust devils. J. Atmos. Sci., 30, 1599-1619.Thorarinsson, S., and B. Vonnegut, 1964: Whirlwinds produced by the eruption of Surtsey volcano. Bull. Arner. Meteor. Soc., 45, 440-444.
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