More Observations of Small Funnel Clouds and Other Tubular Clouds

3714 MONTHLY WEATHER REVIEW VOLUME 133

PICTURE OF THE MONTH

HOWARD B. BLUESTEIN School of Meteorology, University of Oklahoma, Norman, Oklahoma

(Manuscript received 14 March 2005, in final form 20 May 2005)

ABSTRACT

In this brief contribution, photographic documentation is provided of a variety of small, tubular-shaped clouds and of a small funnel cloud pendant from a convective cloud that appears to have been modified by flow over high-altitude mountains in northeast Colorado. These funnel clouds are contrasted with others that have been documented, including those pendant from high-based cumulus clouds in the plains of the United States. It is suggested that the mountain funnel cloud is unique in that flow over high terrain is probably responsible for its existence; other types of small funnel clouds are seen both over elevated, mountainous terrain and over flat terrain at lower elevations.

1. Introduction which the benign funnel clouds occur so that if they are observed, severe weather warnings are not issued. Fur- Bluestein (1994) and others (e.g., Doswell 1985, p. thermore, the small funnel clouds are of interest in their 107; McCaul and Blanchard 1990) have documented, in own right because their dynamics are not well under- the plains of the United States, small funnel clouds pen- stood, and they have not been discussed in the litera- dant from convective clouds whose updrafts appear to ture to the much greater extent that the dynamics of be rooted above the boundary layer. Above many of tornadoes have (e.g., Davies-Jones 1986). these funnel clouds, the base of the parent cloud is The purpose of this note is to provide documentation ragged, while the parent cloud itself has dissipating el- of other small funnel clouds or tubular-shaped cloud ements. Many low-precipitation (LP) supercells [see elements, including those occurring over mountainous Rasmussen and Straka (1998) for a discussion of the terrain. To this extent, this note extends the discussion differences among classic, LP, and high-precipitation by Bluestein (1994). It is suggested that the dynamics of (HP) supercells] produce these “high based” funnel some of the vortices associated with these funnel clouds clouds, especially when they dissipate (e.g., Doswell might be a result of the interaction of flow over orog- 1985, p. 104; Bluestein and Parks 1983). Other tubular- raphy with local, buoyant convection. Photographic shaped clouds have been seen underneath cumulonim- documentation is provided in section 2. A summary and bus clouds (E. W. McCaul Jr. 2005, personal commu- discussion are found in section 3. nication) and in the clear-air boundary layer near or within a field of cumulus humilis clouds. Since these funnel clouds appear to be benign, they 2. Photographic documentation of funnel clouds are in sharp contrast with the condensation-funnel clouds that are associated with the intense columnar a. High-based funnel clouds pendant from ragged vortices of many tornadoes and waterspouts. It is there- bases of cumuliform clouds fore important to understand the circumstances under An example is shown of a common type of high- based funnel cloud, pendant from the bottom of a dissipating convective tower having a ragged base (Fig. 1). Corresponding author address: Dr. Howard B. Bluestein, School of Meteorology, University of Oklahoma, 100 E. Boyd, This ragged-based tower is in contrast to the flat bases Rm. 1310, Norman, OK 73019. of active convective towers nearby (to the right). A E-mail: [email protected] supercell was forming well to the east of this high-based

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FIG. 1. Funnel cloud pendant from a high-based cumulus congestus cloud, viewed to the northwest, in Briggsdale, CO, on 24 Aug 2002 at 1615 mountain daylight time (MDT). (Copyright H. Bluestein.)

FIG. 2. Two upside-down u-shaped tubular cloud structures in funnel cloud and was not associated with it. The parent the clear-air boundary layer, alongside cumulus humilis clouds, cloud formed over the plains of northeast Colorado south of Leadville, CO, on 26 Sep 1986. (Copyright H. Bluestein.) near a southward propagating surface boundary. Al- though the funnel cloud was rotating rapidly, no debris cloud was evident at the ground. The funnel cloud c. The “right-side-up u-shaped tubular cloud” lasted for several minutes. This type of funnel cloud is A variation on the upside-down u-shaped tube is the similar to those discussed by Doswell (1985), Bluestein right-side-up u-shaped tubular cloud seen only on rare (1994), McCaul and Blanchard (1990), and others, es- occasions (Ludlum 1991, plate 207), pendant from a pecially in recent years posted on personal Web sites. high cloud base in a cumulonimbus cloud (Fig. 5). Many storm chasers have called referred to this cloud struc- b. Upside-down “u”-shaped tubes or partial loops ture as a “bowtie funnel.” E. W. McCaul Jr. (2005, amongst cumulus clouds personal communication) refers to this rare cloud struc- Other tubular-shaped cloud elements have been ture as a “devil’s trapeze.” noted in the clear-air boundary layer during the day- time (Figs. 2–4). They appear to be upside-down u- d. Tubular cloud elements pendant from or near shaped (or partially looped) cloud elements that persist cumulus clouds over mountainous terrain for several minutes in the vicinity of cumulus humilis Other tubular-shaped clouds have been visible from clouds. Some storm chasers have called the tubes/loops the summits of mountains. Some were pendant from “horseshoe vortices.” None of the funnel-like clouds, convective clouds having ragged bases (Fig. 6) near the however, was attached to cumulus clouds at all or ap- convergence zone separating winds having an easterly peared to evolve from cumulus clouds. component east of the Continental Divide and winds

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FIG. 3. Upside-down u-shaped tubular cloud structure, as in Fig. 2, in Norman, OK, on 27 Mar 1987 at 1630 central standard time (CST); viewed to the northwest. (Copyright H. Bluestein.) having a westerly component west of the Continental part of the diurnal mountain–valley wind, while the Divide. These funnel clouds are probably similar to westerly winds were associated with the ambient flow those seen underneath dissipating, high-based cumuli- above mountaintop level. In Fig. 7 a horizontally ori- form clouds over the plains (Fig. 1). Inferences about ented funnel cloud that lasted for several minutes is the winds were made based on visual observations of seen. It seems to be similar to that shown in Figs. 6 cloud motion. It is thought that the easterly winds were and 1.

FIG. 4. As in Fig. 3, but in Boulder, CO, on 9 Feb 2003 at 1235 mountain standard time (MST). (Copyright H. Bluestein.)

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Other tubular-shaped clouds (e.g., Fig. 8) were visible as cloud fragments separated completely from their parent cloud, which had earlier evaporated completely. These funnel clouds also persisted for several minutes or longer.

e. Funnel cloud pendant from a cumulus fractus cloud over mountainous terrain An example of what might be yet another variety of funnel cloud is shown in Fig. 9. The parent cloud of this funnel cloud was a ragged, broken, roiling convective cloud mass composed of many elements, which formed over high mountain peaks in Colorado (Fig. 9a). The cloud elements were rapidly rotating and thereby easily caught my attention. The parent cloud then developed vertically oriented tendril-like features underneath it (Fig. 9b); the cloud looked like a jellyfish. Soon one of the tendrils developed into a laminar funnel cloud al- most overhead. The laminar funnel cloud, which was connected to its parent cloud, became turbulent looking above (Fig. 9c). Such a structure is reminiscent of the transition to vortex breakdown sometimes seen in tornadoes near the ground (Pauley and Snow 1988). The funnel cloud persisted for several minutes, but did not produce any visible surface debris cloud. After the FIG. 5. U-shaped, tubular cloud element underneath the down- shear side of a supercell in eastern Oklahoma, on 26 May 1997, at funnel cloud had dissipated, an upside-down u-shaped 1854 central daylight time (CDT); viewed to the northwest from fragmented cloud mass remained (Fig. 9d) as the cloud south of Okemah. (Copyright H. Bluestein.) drifted to the northeast.

FIG. 6. Funnel cloud pendant from a small cumulus cloud near Long Lake, in northeastern Colorado, on 26 Jul 2003 at 1231 MDT, looking to the northeast. (Copyright H. Bluestein.)

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FIG. 8. Vertically oriented funnel cloud segment suspended in FIG. 7. Horizontal funnel cloud (arrow) underneath the base of clear air (top, center), viewed to the south from the summit of cumulus congestus clouds near the Continental Divide, viewed to James Peak, in central Colorado, on 30 Aug 2002, around 1300 the south-southwest from Niwot Ridge, near Ward, CO, on 8 Jul MDT. (Copyright H. Bluestein.) 2002 around 1200–1300 MDT. (Copyright H. Bluestein.)

3. Summary and discussion that the vorticity associated with them probably origi- nates in boundary layer shear (horizontal vorticity) that A number of tubular-shaped cloud elements or small is tilted by the internal eddies associated with clear-air funnel clouds have been photographically documented convection in the manner described by Prandtl (Scorer both over the plains and over mountainous terrain. The 1972). It is also possible that tilted Kelvin–Helmholtz feature common to all the hitherto noted funnel clouds rolls may be associated with these loop clouds. is their proximity to small cumuliform clouds having In the case of the funnel clouds pendant from ragged ragged bases. Some were connected to cumulus clouds, bases, the source of vorticity is simply not known while others were detached from them. None was as- (Bluestein 1994). That the funnel clouds sometimes sociated with deep, convective storms. It is likely then persist longer than their parent clouds is very enigmat- that turbulent, shallow convection played a role in their ic, though the stabilizing role of rotation may be im- development. portant. However, since they may appear both verti- In the case of the detached cloud loops appearing cally and horizontally oriented, it is suggested that tilt- near a field of cumulus humilis clouds, it is speculated ing of ambient vorticity associated with environmental

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FIG. 9. The development of a funnel cloud over Long’s Peak and Mt. Meeker, in northeastern Colorado, on 30 Sep 2002, around 1200–1300 MDT, as viewed to the west from the summit of Twin Sisters. (a) A roiling, cumulus fractus; (b) the cumulus fractus assumes an upside-down u shape with tendrils; (c) a funnel cloud appears in the southernmost vertical column, making up the u-shaped cloud; (d) after the funnel cloud had dissipated, an upside-down u-shaped cloud mass is left behind. (Copyright H. Bluestein.)

Unauthenticated | Downloaded 09/30/21 02:25 PM UTC 3720 MONTHLY WEATHER REVIEW VOLUME 133 vertical shear or vorticity produced baroclinically at (NCAR) and two anonymous reviewers for helpful cloud edges may be important. comments on the manuscript. The author also thanks In the case of the cumulus fractus cloud over the Mark Laufersweiler (OU) for his computer assistance. mountains discussed last, it is speculated that a hori- Most of the images depicted in this contribution were zontal roll associated with an orographic wave aligned taken with a Nikon FM2 camera and Kodachrome 25 along the mountain range was distorted into a u shape and Fuji Velvia film; a few were taken with a Fuji A303 by the convective eddy associated with the cumulus digital camera. Color slides were digitized with a Nikon fractus cloud as a whole. It is thought that horizontal, Super Coolscan 9000 scanner. orographically induced rolls are quite common over REFERENCES Front Range of the Rocky Mountains (Bedard 1978) when there is a component of wind normal to it. One Bedard, A. J., Jr., 1978: Infrasound originating near mountainous regions in Colorado. J. Appl. Meteor., 17, 1014–1022. side of the vortex tube may have been stretched suffi- Bluestein, H. B., 1994: High-based funnel clouds in the southern ciently as a result of a local, convective updraft, to plains. Mon. Wea. Rev., 122, 2631–2638. lower the pressure enough to produce a smooth con- ——, and C. R. Parks, 1983: A synoptic and photographic clima- densation funnel. Shapiro and Markowski (1999) have tology of low-precipitation severe thunderstorms in the demonstrated how such a funnel cloud might build southern plains. Mon. Wea. Rev., 111, 2034–2046. Davies-Jones, R. P., 1986: Tornado dynamics. Thunderstorm Mor- downward from a parent vortex aloft. phology and Dynamics, E. Kessler, Ed., University of Okla- The small-scale, short-lived, and relatively rare fun- homa Press, 197–236. nel clouds discussed in this paper present a particularly Doswell, C. A., III, 1985: The operational meteorology of convec- difficult challenge for direct observations and/or re- tive weather. Vol. II, NOAA Tech. Memo. ERL ESG-15, 240 pp. mote sensing. It is suggested that they might best be Ludlum, D. M., 1991: Audubon Society Field Guide to North studied using moist, large-eddy simulation (LES) American Weather. Knopf, 656 pp. models. McCaul, E. W., Jr., and D. O. Blanchard, 1990: A low- precipitation cumulonimbus along the dryline in Colorado. Acknowledgments. This work was supported by NSF Mon. Wea. Rev., 118, 2768–2773. Pauley, R. L., and J. T. Snow, 1988: On the kinematics and dy- Grants ATM-9912097 and -0241037. Some of this work namics of the 18 July 1986 Minneapolis tornado. Mon. Wea. was done while the author was on sabbatical leave at Rev., 116, 2731–2736. the National Center for Atmospheric Research Rasmussen, E. N., and J. M. Straka, 1998: Variations in supercell (NCAR) in the Mesoscale and Microscale Meteorology morphology. Part I: Observations of the role of upper-level Division. NCAR is supported by the National Science storm-relative flow. Mon. Wea. Rev., 126, 2406–2421. Scorer, R. S., 1972: Clouds of the World: A Complete Colour En- Foundation. The author is indebted to Peggy LeMone cyclopedia. David & Charles, 176 pp. (NCAR) and Prof. Steven Thorpe (United Kingdom) Shapiro, A., and P. Markowski, 1999: Dynamics of elevated vor- for stimulating discussions and to Greg Thomson tices. J. Atmos. Sci., 56, 1101–1122.

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