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2786 MONTHLY REVIEW VOLUME 136

NOTES AND CORRESPONDENCE

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

HOWARD B. BLUESTEIN School of , 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 formed along a line of convective towers during the 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) from an area of 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 has appeared in Tornadoes are frequently associated with larger-scale the literature. One must identify days on which surface parent vortices, such as in , and boundaries marked by vertical 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 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 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 and funnel clouds are often ob-

DOI: 10.1175/2007MWR2357.1

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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 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 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 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

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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 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 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 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-

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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. (Data analysis courtesy of Plymouth State College Weather Center.)

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FIG. 4. Radar reflectivity (dBZ)at0.5° elevation angle, from the Weather Surveillance Radar-1988 Doppler (WSR-88D) at Dodge City at 2309 UTC 19 Aug 2006. Color scale is indicated at the bottom. Range markers are shown at 15-km intervals. The central azimuth line points to the north, toward the top of the image. tive cells extending from west of Garden City to near had very low convective available potential energy Hays (Fig. 4). (CAPE), ϳ100JkgϪ1 or less. The synoptic-scale flow The nearest proximity sounding was taken at 0000 aloft was generally from the west or southwest around UTC at Dodge City separated in time by an hour or so an whose center was located to the south and in space by several hundred kilometers. It is seen and east (Fig. 9). At 700 hPa there was a zone of con- from this sounding that the deep-layer vertical shear fluence along the Nebraska–Kansas border, but there was relatively weak, but there was a local maximum in was no temperature gradient, and so the quasigeo- shear (as evidenced by a shift in wind direction from strophic forcing aloft was nonexistent or very weak. At east-northeasterly wind to southwesterly) around 800 North Platte, there was weak, warm advection at low hPa (ϳ2 km MSL) associated with the frontal zone levels, as evidenced by the veering of the wind with aloft (Fig. 8a). [The frontal zone was located higher up, height below 750 hPa (Fig. 8b); at Dodge City, there at ϳ750 hPa (ϳ2.7 km MSL), to the north at North was only a shallow layer of veering wind with height Platte, Nebraska (Fig. 8b)]. The vertical shear near the near 800 hPa. ground was northeasterly, as the wind speed increased Ϫ1 from 5 to 12.5 m s in the lowest 500 m, consistent 3. Summary and discussion qualitatively (according to thermal wind consider- ations) with the observed surface temperature gradient. The funnel cloud formed in a manner consistent with Even accounting for a surface temperature of 23°C that of many other landspout funnels: It formed in a (see Fig. 2), the Dodge City and North Platte soundings line of relatively shallow convective towers in an envi-

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FIG. 5. Evolution of the mesoscale areas of precipitation in western and central Kansas, and in far northern and northwestern Oklahoma, as depicted by the WSR-88D radar at 0.5° elevation angle, at Dodge City at approxi- mately 30-min intervals: (a) 2131, (b) 2205, (c) 2231, (d) 2257, and (e) 2332 UTC 19 Aug 2006 and (f) 0001 UTC 20 Aug 2006.

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FIG. 6. Meteograms for (top) Hays and (bottom) Russell for 19–20 Aug 2006. Temperature and dewpoint (°F), visibility (mi), wind (one barb ϭ 10 kts, one-half barb ϭ 5 kts), cloud height (ft), and pressure (hPa; for Russell, Kansas only) as a function of time (UTC) are shown. The vertical scales for identical variables in the meteograms for both time periods depicted are not all the same. (Courtesy of Plymouth State College Weather Center.) ronment of relatively weak deep-layer shear. However, with strong low-level vertical shear (evidenced by the it did not form along a surface front, dryline, orographi- tilt of the funnel and the Dodge City sounding; the cally induced surface boundary, or outflow boundary. Dodge City sounding, however, was far from the funnel The cloud line, instead, formed on the warm side of a cloud and this evidence is therefore very tentative), zone of temperature gradient created by the juxtaposi- which may have been tilted by the updraft in one of the tion of the evaporatively cooled air in the vicinity of a convective towers. It is not known why such a process region of stratiform precipitation with the relatively may have occurred only in the cell east of Hays and not warmer air on the cold side (i.e., to the rear) of a sur- in any of the other cells in the curved band to the west face cold front. There was no evidence of a well-defined and southwest. Funnel clouds/tornadoes may have oc- wind shift (or or zone of cyclonic curred there also, but they were not reported. The one vertical vorticity) along this zone of temperature gradi- observed by the author was, to the best of his knowl- ent, as there often is along outflow boundaries. edge, not reported by anyone else and it occurred near Because many landspouts acquire their vorticity from a heavily traveled interstate highway. If the funnel the surface boundary along which they form, it is spec- cloud observed was the only one that occurred, then ulated that the source of the vorticity for the funnel either the relative isolation of its parent cell compared cloud may have been horizontal vorticity associated with that of the others to its west and southwest (Fig. 4)

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FIG.7.Geostationary Operational Environmental Satellite-12 visible satellite image at 2312 UTC 19 Aug 2006. (Courtesy of Cooperative Institute for Research in the /National Oceanic and Atmospheric Admin- istration.) The location of Garden City and Dodge City are labeled “GCK” and “DDC,” respectively. The curved band of convective cells seen in Fig. 3 corresponds to a curved cloud line (just to the southeast of the label). The approximate location of the funnel cloud is indicated. may have been important, or perhaps its proximity to tal precipitation and/or warm advection at low levels the MCS may have been important. It is also possible could produce rising motion along the warm side of the that there was a very narrow band of vorticity not re- “reverse” frontal zone. solved by the surface-observing network (and not asso- Some evidence is available to support these hypoth- ciated with an outflow boundary) and that stretching of eses in Fig. 3. Because the wind speed varied little over preexisting vorticity was responsible for the funnel much of the region centered over western Kansas (Fig. cloud. 2), but the wind direction did vary considerably on the The speculations presented here must be viewed with mesoscale and synoptic scale (there was pronounced caution, because of the lack of finescale data in the diffluence), the axis of dilatation of the wind field was vicinity of the funnel cloud. However, because the par- oriented approximately along the axis of diffluence ent cloud line and line of radar echoes were organized (Fig. 3b). The orientation of the surface potential tem- on the mesoscale, it is more likely that the data were perature gradient was directed from east to west, be- able to resolve the major features associated with the cause of the cooler air to the east and the higher eleva- convective line. tion to the west. Therefore, there was frontogenetical The tentative implications of this case study are that forcing that could have induced rising motion along the surface baroclinic zones to the rear of cold fronts, be- western edge (the warm side) of the potential tempera- cause of evaporative cooling of postfrontal precipita- ture gradient, in far western Kansas. However, this tion, may play a role in triggering convection on the mechanism neither can account for the sharpness of the “warm side” of the baroclinic zone, even though the line of convection that was forced, nor can it account warm side is well behind the much warmer air lying for the curvature of the convective cloud line (Figs. 4 ahead of the front and the mesoscale area of precipita- and 7). In addition, at the surface, there was some warm tion just to the rear of the front. It is suggested that advection (near and northeast of the “X” in Fig. 3a). frontogenetical forcing on the rear side of the postfron- However, even if the warm advection were dynamically

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FIG. 8. Soundings for Dodge City (DDC) and North Platte (LBF) at 0000 UTC 20 Aug 2006. (Courtesy of Plymouth State College Weather Center.)

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FIG. 9. Plotted regional maps for 500, 700, and 850 hPa, centered on Kansas, for 0000 UTC 20 Aug 2006. Temperature and dewpoint (°C), and 500-, 700-, and 850-hPa heights (m, with the leading “5,”“3,” and “1” omitted, respectively), are plotted.

significant, it is not possible to link synoptic-scale forc- Severe Local , Indianapolis, IN, Amer. Meteor. Soc., ing to the narrow, mesoscale convective band. Never- 267–270. theless, it is concluded that long lines of convection that Brady, R. H., and E. J. Szoke, 1989: A case study of nonmesocy- clone tornado development in northeast Colorado: Similari- form to the rear of evaporatively cooled air in MCSs ties to formation. Mon. Wea. Rev., 117, 843–856. may, under circumstances not completely understood, Davies-Jones, R. P., R. J. Trapp, and H. B. Bluestein, 2001: Tor- produce funnel clouds and even tornadoes. nadoes. Severe Convective Storms, Meteor. Mongr., No. 50, Amer. Meteor. Soc., 167–221. Acknowledgments. This work was supported by NSF Golden, J. H., 1971: Waterspouts and tornadoes over South Florida. Mon. Wea. Rev., 99, 146–154. Grants ATM-0241037 and ATM-0637148 to the Uni- ——, and H. B. Bluestein, 1994: The NOAA-National Geo- versity of Oklahoma. The author is grateful to the Me- graphic Society Waterspout Expedition (1993). Bull. Amer. soscale and Microscale Meteorology (MMM) Division Meteor. Soc., 75, 2281–2288. of the National Center for Atmospheric Research Lee, B. D., and R. B. Wilhelmson, 1997: The numerical simulation (NCAR) for hosting his summer 2007 visit. Jeff Snyder of nonsupercell . Part II: Evolution of a family of tornadoes along a weak outflow boundary. J. Atmos. Sci., assisted in making the WSR-88D radar data available 54, 2387–2415. for perusal using SOLO. Anonymous reviewers pro- Marquis, J. M., Y. P. Richardson, and J. M. Wurman, 2007: Kine- vided very helpful comments. matic observations of misocyclones along boundaries during IHOP. Mon. Wea. Rev., 135, 1749–1768. Szoke, E. J., and R. H. Brady, 1989: Forecasting implications of REFERENCES the 26 July 1985 northeastern Colorado tornadic thunder- case. Mon. Wea. Rev., 117, 1834–1860. Bluestein, H. B., 1985: The formation of a “landspout” in a “bro- Wakimoto, R. M., and J. W. Wilson, 1989: Non-supercell torna- ken-line” line in Oklahoma. Preprints, 14th Conf. on does. Mon. Wea. Rev., 117, 1113–1140.

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