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A Funnel Cloud in a Convective Cloud Line to the Rear of a Surface Cold Front

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A Funnel Cloud in a Convective Cloud Line to the Rear of a Surface Cold Front 2786 MONTHLY WEATHER REVIEW VOLUME 136 NOTES AND CORRESPONDENCE A Funnel Cloud in a Convective Cloud Line to the Rear of a Surface Cold Front HOWARD B. BLUESTEIN School of Meteorology, University of Oklahoma, Norman, Oklahoma (Manuscript received 24 August 2007, in final form 28 November 2008) ABSTRACT This brief case study describes the unusually benign environment in which a funnel cloud formed along a line of convective towers during the summer in Kansas. The parent cloud line was solitary and very narrow, yet organized on the mesoscale. The cloud line appeared to be best correlated with a zone of horizontal temperature gradient to the northwest of cool (evaporatively produced) outflow from an area of precipitation located just to the rear of a cold front. Implications for forecasting such an event are noted. 1. Introduction common. To the best of the author’s knowledge, no comprehensive landspout climatology has appeared in Tornadoes are frequently associated with larger-scale the literature. One must identify days on which surface parent vortices, such as mesocyclones in supercells, and boundaries marked by vertical vorticity are likely loca- ordinary cells that are often in lines (Davies-Jones et al. tions for the initiation of nonsupercell convection, and 2001). Those that form in ordinary cells frequently de- then anticipate that landspouts may occur. rive their vorticity from preexisting vorticity in the On 19 August 2006, the author, while en route from boundary layer (Wakimoto and Wilson 1989; Lee and Boulder, Colorado, to Norman, Oklahoma, serendipi- Wilhelmson 1997), begin near the ground first and tously observed a relatively long-lived landspout funnel build upward, and look visually like many Florida wa- cloud pendant from a line of cumulus congestus in west- terspouts (Bluestein 1985; Brady and Szoke 1989); for ern Kansas, 13 km east of Hays, Kansas. Because the the latter reason, they are sometimes called “land- landspout formed under conditions that are not gener- spouts.” These landspouts are often observed prior to ally associated with landspouts, it is useful to document the onset of precipitation at the ground, when their the conditions under which it formed so that in the parent clouds are still growing overhead. The source of future such rare events might become better under- boundary layer vorticity in landspouts has been identi- stood and more successfully forecast. fied as that associated with orographic surface features, In section 2 a brief case study is presented using pho- such as the Denver, Colorado, convergence–vorticity tographic documentation and operational surface, up- zone (Szoke and Brady 1989), and that which formed per-air, and radar data, and satellite imagery. In the along surface fronts and, possibly, along other bound- concluding section 3, the results are summarized and aries, such as the dryline (e.g., Marquis et al. 2007) and hypotheses are offered for why the funnel cloud and its the sea-breeze front (Golden 1971). parent cloud line formed. Our ability to forecast landspouts is very difficult be- cause they are rare, even though the orographic bound- 2. Case study aries and fronts along which they form are very Before the funnel cloud was encountered, the author noted a line of cumulus congestus oriented approxi- mately in an east-northeast to west-southwest direction, Corresponding author address: Dr. Howard B. Bluestein, School of Meteorology, University of Oklahoma, 120 David L. ahead to the east. As the cloud line was approached, Boren Blvd., Suite 5900, Norman, OK 73072. the visual similarities to the cumulus congestus along E-mail: [email protected] which waterspouts and funnel clouds are often ob- DOI: 10.1175/2007MWR2357.1 © 2008 American Meteorological Society Unauthenticated | Downloaded 10/07/21 10:23 AM UTC MWR2357 JULY 2008 NOTES AND CORRESPONDENCE 2787 FIG. 1. A funnel cloud, ϳ13 km east of Hays northwest of Victoria, KS, on Interstate-70, at approximately 1805–1820 central daylight time (CDT; 2305–2320 UTC) 19 Aug 2006. The photographs were taken approximately every minute or two. The view is to the north. (Photographs from H. Bluestein.) served in south Florida and the Florida Keys (Golden the north/northeast). Alternatively, convective outflow 1971; Golden and Bluestein 1994) was noticed. Water- from the southwest could have also been responsible spouts and funnel clouds in south Florida and the for the observed tilt. Toward the end of its life, before Florida Keys frequently occur under synoptically qui- it dissipated, the funnel cloud bulged outward to the escent conditions in an environment of relatively weak east-northeast, and then assumed a highly tilted ap- deep-layer vertical shear. pearance. Such behavior is similar to that observed in The funnel cloud was pendant about halfway to the waterspouts and supercell tornadoes; the outward ground and leaned to the west-southwest with height, bulge and dissipation are probably indicative of outflow approximately in the direction of the surface wind from the parent cloud after the onset of precipitation, (Figs. 1 and 2). Owing to the observed vertical tilt, it is which was subsequently observed. No debris cloud was likely that the east-northeasterly surface winds in- visible at the ground, either because the circulation was creased in speed with height, which is indicative of a too weak at the surface to lift ground material or be- northeasterly thermal wind and a temperature gradient cause there was a dearth of lightweight ground mate- directed toward the northwest (Fig. 3a; the temperature rial, such as dust (or water), or because a fence and gradient at the surface was weak and directed toward terrain precluded a view of the ground (in which case Unauthenticated | Downloaded 10/07/21 10:23 AM UTC 2788 MONTHLY WEATHER REVIEW VOLUME 136 FIG. 2. Plotted surface data for 2300 UTC 19 Aug 2006 in Kansas and its surrounding area. Temperature and dewpoint (°C); sea level pressure (ϫ10 hPa, less the beginning “10.”); Hays, Garden City, Dodge City, Russell, and Great Bend, KS, are at the observation sites labeled “HYS,”“GCK,”“DDC,”“RSL,” and “GBD,” respectively. The dashed line represents zone of temperature difference of 4°C across adjacent to the observing sites. The funnel cloud was located ϳ13 km east of “HYS.” (Courtesy of Plymouth State College Weather Center.) the funnel cloud could have been a tornado had there edge of the curved line of convective cells progressed to actually been a debris cloud, the view of which was the west, while the northeastern edge of the line was blocked). nearly stationary. The individual cells, however, moved The most unusual aspect of the funnel cloud was that toward the southwest, along with the low-level winds its parent cloud was not situated along a well-defined (Fig. 2). No evidence was found from the time series of surface boundary. A surface cold front was located far observations at Hays to the west, or Russell, Kansas, to to the south in Oklahoma (not shown in Fig. 2, which the east, of the passage of any surface boundary such as depicts conditions just to the north of the front), to an outflow boundary or a front (i.e., no sudden wind north of which there was a mesoscale convective system shift, temperature drop, or pressure rise; Fig. 6). At (MCS; Fig. 4). The funnel cloud formed well to the Hays, there was some precipitation noted at 0000 UTC northwest of a stratiform area of precipitation (Fig. 4) as convective cells passed by, but the temperature did in a relatively cool surface environment (Figs. 2 and 3) not fall until over an hour later, when the wind speed and also just to the northeast of small-scale, parallel became very light (2.5 m sϪ1 or less); had there been an bands of precipitation, that might have been triggered outflow boundary passage, the wind speed would have by gravity waves over the cold pool of the MCS. The increased and the wind direction would have been from parent convective cell appeared to be near the north- the east or southeast, rather than north or northeast, as eastern-most extension of a curved band of convective was observed. cells, most of which had cores of ϳ30–35 dBZ that However, the parent cloud (Fig. 7) was located on extended well to the west of the MCS, but curved to a the warm side of the zone of the northwestward- location relatively near to the back side of the MCS directed temperature gradient associated with the where the funnel cloud was observed. The curved band boundary between evaporatively cooled air in the MCS of cells was most pronounced near the time the funnel and the warmer, ambient air behind the cold front cloud was observed (Fig. 5), having formed between (northwest of Garden City, Kansas; Figs. 2, 3a): There approximately 2130 and 2230 UTC. The cells along the wasa4°C difference in temperature between Garden band became more widely scattered after 2330 UTC. City and Dodge City, Kansas, and between Hays/RSL The tops of the radar echoes associated with these cells and Great Bend, Kansas. The parent cloud line (Fig. 7) were no higher than ϳ4 km (not shown). The western was collocated with the line of relatively weak convec- Unauthenticated | Downloaded 10/07/21 10:23 AM UTC JULY 2008 NOTES AND CORRESPONDENCE 2789 FIG. 3. Regional depiction of surface streamlines at 2300 UTC 19 Aug 2006, centered on Kansas; the approximate location of the funnel cloud is labeled with an “X.” (a) Surface isotherms (°F) and (b) surface potential temperature isotherms (K) are shown. The solid line segment in (b) denotes approximate orientation of the axis of dilatation of the surface wind field based only on the diffluence of the wind field.
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