NOTES AND CORRESPONDENCE. Central Illinois Cold Air Funnel Outbreak

Transcription

1 2815 NOTES AND CORRESPONDENCE Central Illinois Cold Air Funnel Outbreak ROBERT M. RAUBER Department of Atmospheric Sciences, University of Illinois at Urbana Champaign, Urbana, Illinois ROBERT W. SCOTT Illinois State Water Survey, Champaign, Illinois 23 March 2001 and 22 May 2001 ABSTRACT Numerous cold air funnels developed in close proximity over central Illinois on 23 May 1988 as the core of an upper-level cutoff low pressure center passed over the region. Five separate funnels were observed by one of the authors from a single location over a period of 33 min. Images of these funnels captured from video are presented, one showing two funnels extending toward one another from the same cloud and two others illustrating stages in the life cycle of one of the funnels. Surface and upper air analyses, radar data, and two soundings taken at the time of the outbreak are presented to illustrate the environment in which these funnels formed. The family of cold air funnels occurred along a weak stationary front. The fact that so many funnels occurred along the same line is suggestive of a vortex sheet breaking down due to the development of horizontal shearing instability. If this is the case, then the cold air funnels observed on 23 May are similar dynamically to nonsupercell tornado families that develop along cold fronts and outflows. 1. Introduction Cooley and Soderberg (1973) introduced the term cold air funnel into the meteorological lexicon to describe funnel clouds pendant from the base of convective clouds that form in the vicinity of cold pools of air aloft. Cooley (1978) noted that cold air funnels typically form in the rear quadrant of well-developed cold-core low pressure centers. The closed cold-core lows often extended well above the 500-mb level in cases included in their studies. Thunderstorms and showers were present in the vicinity of cold air funnels, although, according to Cooley, the thunderstorms seldom exceeded moderate intensity. Cooley observed that cold air funnels occur in spring through fall, primarily during the midday hours. These funnels last only a few minutes and rarely were observed to touch down. Short discussions of cold air funnels based on Cooley s work have since appeared in Davies-Jones (1986), Doswell and Burgess (1993), and Branick (1993), and a brief summary of forecasting issues related to cold air funnels appeared in O Hara et al. (1999). Trapp and Davies- Corresponding author address: Robert M. Rauber, Department of Atmospheric Sciences, University of Illinois at Urbana Champaign, 105 S. Gregory Ave., Urbana, IL r-rauber@uiuc.edu Jones (1997) also briefly mention cold air funnels as a possible example of a vortex formed through a dynamic pipe effect (Leslie 1971) in which a midlevel vortex in cyclostrophic balance descends as a result of an upwarddirected rotationally induced vertical pressure gradient force. To the authors knowledge, no further documentation of these vortices has appeared in the scientific or technical literature. On 23 May 1988, an outbreak of cold air funnels occurred over central Illinois. The National Weather Service Office in Springfield, Illinois (now at Lincoln, IL) identified the funnels as cold air funnels, and local television broadcasts and newspapers used the term cold air funnels to describe the funnels occurring over the area. The number of funnels that occurred is uncertain; however, one of the authors (RWS) observed five separate funnels over a period of 33 min from a single location 6 km north of Champaign, Illinois, and public reports of additional funnels were received over a fivecounty area. Four of the funnels observed by the author were videotaped. Figure 1, a panorama extracted from the video, shows two funnels emerging from a flat cloud base and extending toward one another. Different stages during the life cycle of the leftmost funnel, which developed at 0017 UTC and dissipated 10 min later, are illustrated in Figs. 2 and 3. This funnel is similar in 2001 American Meteorological Society

2 2816 MONTHLY WEATHER REVIEW VOLUME 129 FIG. 1. Two cold air funnels located approximately 6 km north of Champaign, IL. The image, captured from video, was taken at 0019 UTC on 24 May 1988 (local time: May). Note that there is an approximate 2 wide sector missing along the center line (cf. ground structures in Fig. 3), a result of combining two frames from the video panorama.

3 2817 FIG. 2. The leftmost funnel in Fig. 1, shown at 0020 UTC, the time of greatest extension below cloud base. FIG. 3. The leftmost funnel in Fig. 1, shown at 0026 UTC, about 1 min prior to its dissipation.

4 2818 MONTHLY WEATHER REVIEW VOLUME Outbreak of cold air funnels over Illinois FIG. 4. Surface analysis at 0000 UTC 24 May 1988 showing pressure (solid, mb), temperature (dashed, C), winds (m s 1 ), and radar echoes. A full wind barb is 5 m s 1 and a half barb 2.5 m s 1. Radar data are standard Weather Service Radar (WSR-57) VIP reflectivity factor levels 1 and 2 (lightest shading: 37 dbz), and levels 3 and 4 (darker shading, dbz). The locations of the soundings in Fig. 6 are indicated by dark squares. The small circles denote echotop reports (km) and locations. The location of the video images appearing in Figs. 1 3 is noted. The counties outlined in the map are those where reports of cold air funnel sightings were received. structure both in its early and later life to examples presented by Cooley and Soderberg (1973). Cooley and Soderberg (1973) noted that about 70% of funnels observed were long slender ropelike formations, similar to Fig. 2, while the remaining 30% were shorter and more columnar shaped with a quite blunt or rounded lower extremity, similar to Fig. 3. The funnels observed near Champaign occurred between 2350 UTC 23 May and 0033 UTC 24 May 1988 [ central daylight time (CDT) 23 May 1988], very close to the launch time of soundings at Peoria and Salem, Illinois. The funnels had an average lifetime of about 10 min, and through most of the time that funnels were observed, at least two were present simultaneously. In this note, we present surface, upper air, and radar analyses for this event and discuss characteristics of the two soundings to illustrate the environment in which these funnels formed. We also provide some speculative discussion about the cause of the funnels. Figure 4 shows a surface chart depicting sea level pressure, surface temperature, and winds at 0000 UTC, the time the cold air funnels occurred over central Illinois. The composite radar summary superimposed on the surface chart is from 0035 UTC, 16 min after the time of the video image appearing in Fig. 1. The counties where sightings were reported are shown in the figure. At the time of the funnels, a weak 1003-mb low pressure center was located over the Missouri Arkansas border. A stationary front extended from the low northeastward across Missouri, Illinois, Indiana, and Ohio. Showers, some reported as thunderstorms, occurred in all quadrants of the low pressure center. A line of heavier showers occurred along the front, with stronger radar echoes [video integrator and processor (VIP) levels 3 and 4, dbz] in western Illinois and western Indiana. The video images appearing in Figs. 1 3 were taken near the eastern edge of the weakest radar echo (VIP levels 1 and 2, 37 dbz) from a vantage point just north of the surface front (see Fig. 4). All three images were taken looking south. The funnels were estimated to be directly along the front based on both the surface analysis in Fig. 4 and the edge of the cloud line visible south of the observer. The southern edge of the convective clouds can be seen in Figs. 1 3, with clearer skies in the distance beyond the convection. At times, towering cumulus could be seen briefly above the funnels, although most of the time the view was obscured by the lower cloud deck. The funnels all occurred along an approximate east west line south of the observer, with the easternmost funnel approximately 20 east of south and the westernmost funnel 50 west of south. The funnel located directly south was estimated to be 3 4 km from the observer. If this estimate is correct, individual funnels were separated by about 2 4 km. The funnels all drifted slowly westward. Light rain appeared to be falling to the west of the region where the funnels were observed. The radar echoes to the immediate west were apparently associated with this light rainfall (see Fig. 4). At the time of the funnel outbreak, surface winds reported by stations on either side of the front ranged from 2.5 to 5 m s 1. The author observed only weak surface winds, and no wind gusts, during the time period that the funnels occurred. During the outbreak, a cold pool of air extended across Illinois and Missouri (Fig. 4). The surface temperature along the front across Missouri and Illinois ranged from about 16 to 18 C. With the exception of cold air residing over Lake Michigan, temperatures were warmer in all states surrounding Missouri and Illinois. The relatively cool pool of air at the surface over Missouri and Illinois was associated with a cold-core closed low pressure system aloft. Figure 5 shows 850-, 500- and 300-mb charts over the region at 0000 UTC 24 May A cold core cutoff low extended from the lower atmosphere through the 300- mb level. The center of the low tilted eastward with height

5 2819 FIG. 5. Height contours (m) overlaid on radar data at (a) 850, (b) 500, and (c) 300 mb for 0000 UTC 24 May The temperature ( C), height (m), and winds (m s 1 ) are noted at each sounding site. A half barb is 2.5 m s 1, a full barb, 5 m s 1, and a flag 25 m s 1. The locations of the soundings displayed in Fig. 6 and the location of the video images in Figs. 1 3 are also noted. Radar data are standard WSR-57 VIP reflectivity factor levels 1 and 2 (lightest shading: 37 dbz), and levels 3 and 4 (darker shading, dbz). from its surface position in southern Missouri to the 300- mb level over east-central Illinois. The coldest temperatures at the 300-mb level were over eastern Illinois, and nearly coincident with the location of the funnels. Soundings taken at Peoria and Salem, just north and south of the stationary front and very close to the time of the cold air funnels, appear in Fig. 6. The Salem sounding (SLO), which was launched south of the stationary front (see Fig. 4), most likely represents source air for the convection developing along the front in the vicinity of the cold air funnels. On this sounding, a mixed layer extended from the surface to the 900-mb level. A drier stable layer was present above the mixed layer, but a small amount of lifting of the lower atmosphere would have been sufficient to trigger convection in this environment. Parcels originating in the mixed layer had the potential to rise to the tropopause; however, the convective available potential energy (CAPE) of a parcel originating in the mixed layer was only 258 J kg 1. Radar echo tops of 12.5 km were reported approximately 140 km west of the cold air funnels. This height would have been well above the tropopause based on the Salem and Peoria soundings. This suggests that mesoscale variations in CAPE may have been present across the region, due possibly to moisture and boundary layer temperature variations (Weckwerth et al. 1996), and that the CAPE in the vicinity of the front may have been higher. Other convective tops reported along the stationary front were approximately 9 km, about 1.4 km below the tropopause and consistent with the equilibrium level based on the CAPE of the Salem sounding. The vertical wind shear on the Salem sounding was extremely weak. Winds veered slightly from south-southeasterly to south-southwesterly in the lowest 200 mb and were 5 m s 1 throughout the layer. The strongest wind throughout the depth of the troposphere was only 7.7 m s 1. The center of the synoptic-scale cold-core low was directly over Salem at 400 mb. The Peoria sounding, taken north of the stationary front, had a shallow surface mixed layer and a sharp stable layer at 600 mb. Surface-based convection was less likely, considering the large convective inhibition, 56 J kg 1, and relatively small CAPE over Peoria (146 J kg 1, calculated using a mixed layer between the surface and 900 mb). Weak vertical shear was present on the Peoria sounding between the surface and 400 mb. Within this layer, the winds over Peoria veered from northeasterly to easterly, never exceeding 5 10 m s 1. Wind increased above 400 mb to a maximum of 28 m s 1 at 250 mb, the height of the tropopause. 3. Discussion Tilting of horizontal vorticity (e.g., Rotunno 1981) was very unlikely as a mechanism to create rotation due to the extremely weak vertical wind shear characterizing the lower troposphere. There also was no obvious source of midlevel rotation in this case to support the creation of funnels through a dynamic pipe effect (Trapp and Davies-Jones 1997). The fact that so many funnels occurred nearly simultaneously along the same line is suggestive of a vortex sheet breaking down due the development of horizontal shearing instability (see review in Lee and Wilhelmson 1997a). The presence of horizontal shearing instability is also supported by the fact

6 2820 MONTHLY WEATHER REVIEW VOLUME 129 Numerical simulations by Lee and Wilhelmson (2000) suggest that environments supporting nonsupercell tornadoes have CAPE exceeding 500 J kg 1 and a threshold cross-front velocity change of at least 5 10 ms 1. Although both the CAPE ( 250 J kg 1 ) and the velocity change across the stationary front ( 5 ms 1 ) were below these thresholds based on available data, it is important to note that the Lee and Wilhelmson studies focused on stronger vortices. This case therefore probably represents the lower threshold of shear and instability required to initiate misoscale vortices that can produce funnel clouds aloft. This case also provides the first evidence of the potential importance of weak horizontal shear along boundaries in generating cold air funnels. Future occurrences of cold air funnels should be examined closely to see if shear boundaries, possibly indicated by finelines of weak radar echo, can be identified in their vicinity. With the wide coverage of the Weather Surveillance Radar-1988 Doppler network, such studies should be possible when a cold air funnel outbreak occurs in the future. Acknowledgments. This work was supported by the National Science Foundation under Grant NSF ATM REFERENCES FIG. 6. Thermodynamic diagrams (Skew T logp format) showing soundings at (top) Salem, IL, and (bottom) Peoria, IL, for 24 May 1988 at 0000 UTC. Winds are in m s 1, with a half barb 2.5 m s 1, a full barb 5 m s 1, and a flag 25 m s 1. that the line along which the funnels formed coincided with the position of the horizontal shear zone along the stationary front. If vortex sheet breakdown was the source of rotation, the family of cold air funnels observed along the weak stationary front would have been dynamically similar to nonsupercell tornado families that develop along cold fronts and outflows (Lee and Wilhelmson 1997a,b). Documented cases of vortex breakdown (e.g., Wilson and Wakimoto 1989; Roberts and Wilson 1995) and numerical simulations (Lee and Wilhelmson 1997a,b) suggest that vortices formed by this mechanism extend to the surface. Unfortunately, we were unable to determine if the vortices associated with the cold air funnels extended to the surface. The visible condensation funnels extended no farther than halfway to the ground from cloud base. Surface dust swirls were not observed, although this may have been because the fields under the funnels had growing crops and probably were damp from earlier rains. Branick, M., 1993: A comprehensive glossary of weather terms for storm spotters. NOAA Tech. Memo. NWS SR-145. [Available from National Weather Service, 1200 Westheimer Drive, Norman, OK ] Cooley, J. R., 1978: Cold air funnel clouds. Mon. Wea. Rev., 106, , and M. E. Soderberg, 1973: Cold air funnel clouds. NOAA Tech. Memo. NWS CR-52, Scientific Services Division, NWS Central Region, Kansas City, MO, 29 pp. Davies-Jones, R. P., 1986: Tornado dynamics. Thunderstorm Morphology and Dynamics, E. Kessler, Ed., 2d ed., University of Oklahoma Press, pp. Doswell, C. A., III, and D. W. Burgess, 1993: Tornadoes and tornadic storms: A review of conceptual models. The Tornado: Its Structure, Dynamics, Prediction and Hazards,Geophys. Monogr., No. 79, Amer. Geophys. Union, Lee, B. D., and R. B. Wilhelmson, 1997a: The numerical simulation of non-supercell tornadogenesis. Part I: Initiation and evolution of pretornadic misocyclone circulations along a dry outflow boundary. J. Atmos. Sci., 54, , and, 1997b: The numerical simulation of non-supercell tornadogenesis. Part II: Evolution of a family of tornadoes along a weak outflow boundary. J. Atmos. Sci., 54, , and, 2000: The numerical simulation of non-supercell tornadogenesis. Part III: Parameter tests investigating the role of CAPE, vortex sheet strength, and boundary layer vertical shear. J. Atmos. Sci., 57, Leslie, L. M., 1971: The development of concentrated vortices: A numerical study. J. Fluid Mech., 48, O Hara, B. F., J. L. Adolphson, T. Reaugh, and E. Holicky, 1999: Severe weather climatology for the new NWSO northern Indiana county warning area. NOAA Tech. Service Publ. NWS CR-09, 36 pp. Roberts, R. D., and J. W. Wilson, 1995: The genesis of three non-

7 2821 supercell tornadoes observed with dual-doppler radar. Mon. Wea. Rev., 123, Rotunno, R., 1981: On the evolution of thunderstorm rotation. Mon. Wea. Rev., 109, Trapp, R. J., and R. P. Davies-Jones, 1997: Tornadogenesis with and without a dynamic pipe effect. J. Atmos. Sci., 54, Wakimoto, R., and J. W. Wilson, 1989: Non-supercell tornadoes. Mon. Wea. Rev., 117, Weckwerth, T. M., J. W. Wilson, and R. M. Wakimoto, 1996: Thermodynamic variability within the convective boundary layer due to horizontal convective rolls. Mon. Wea. Rev., 124,