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A Preliminary Investigation of Supercell Longevity

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A Preliminary Investigation of Supercell Longevity 16.6 A PRELIMINARY INVESTIGATION OF SUPERCELL LONGEVITY Matthew J. Bunkers*, Jeffrey S. Johnson, Jason M. Grzywacz, Lee J. Czepyha, and Brian A. Klimowski NOAA/NWS Weather Forecast Office, Rapid City, South Dakota 1. INTRODUCTION∗ because it was unclear whether or not the supercell persisted for > 4 h before it evolved into a bow echo. Therefore, if there Supercells [as originally termed by Browning (1962)] was any doubt whether or not a supercell’s lifetime exceeded 4 arguably represent the most organized and longest-lived form h, the case was discarded, even though the supercell may of isolated deep moist convection. Their longevity derives from have become a bow echo with a total storm lifetime (supercell both a persistent rotating updraft that enhances vertical motion, plus bow echo) much greater than 4 h. and an updraft−downdraft configuration that generally prevents The primary source of supercells came from radar data precipitation from falling through the updraft (Browning 1977; that were archived by the authors from previous studies across Weisman and Klemp 1986)—both of which, in turn, are the northern High Plains (NHP) (Klimowski and Hjelmfelt 1998; dependent on the vertical wind shear. Bunkers et al. 2000). These data were reviewed for both long- Not surprisingly, part of the supercell definition includes and short-lived supercells, and they have been updated with persistence, namely, that the storm’s circulation persists for at supercell events collected from 2000 to 2002. Furthermore, least 30−45 minutes (Moller et al. 1994). In some cases, a radar data were also collected (as described in the next supercell may persist in a quasi-steady manner for several paragraph) for long-lived events across the United States that hours, and in extreme cases, the lifetime of a supercell may were observed in near-realtime. The average lifetime of all extend well beyond 4−5 hours (e.g., Paul 1973; Browning and supercells was used to determine if an event was short-lived. Foote 1976; Warner 1976). These long-lived supercells can For example, if three supercells occurred near a sounding with produce significant severe weather along the majority of their lifetimes of 1.0, 1.5, and 2.5 h, this was included as a short- paths, and on occasion, they generate a devastating lived supercell event since the average lifetime of all storms combination of large hail and damaging winds (e.g., Klimowski was 1.67 h (this occurred on three occasions). Additionally, et al. 1998). left-moving supercells were not considered for the short-lived Since these longest-lived supercells are relatively rare, cases. anticipating them in advance presents a perplexing forecasting Lastly, the historical severe weather report database was challenge. Although modeling studies have shown a potential interrogated [using the software of Hart and Janish (1999)] to relationship between vertical wind shear and supercell graphically examine spatial and temporal characteristics of longevity (Weisman and Klemp 1982; Droegemeier et al. 1993, severe storm reports from 1996 to 2000 across the United 1996; Gilmore and Wicker 1998; Bluestein and Weisman States. If there was a consistent progression of relatively 2000), observational documentation of this is generally lacking. isolated reports on a given day (suggestive of a long-lived Furthermore, other modeling studies have shown that the lifted supercell), 2-km resolution 0.5-degree mosaic radar data for condensation level (LCL) height may be related to supercell the United States were then acquired from the Cooperative longevity (e.g., McCaul and Cohen 2000), but again little Program for Operational Meteorology, Education and Training observational documentation exists to support or refute this (COMET) archive of the Weather Surveillance Radar-1988 claim. Other factors, such as boundaries and storm mergers, Doppler (WSR-88D). These data were subsequently examined may also act to either enhance or limit the lifetime of a for the presence of long-lived supercells, and some short-lived supercell. No solid relationships have emerged from the above supercells were also acquired. For the archived radar data in studies, which leaves unsolved the problem of why some which velocity data were not available (mainly the 2-km mosaic supercells persist for several hours. data), the determination of a supercell was based either on In the current study, we present preliminary findings from prior documentation (see previous paragraph), or else well- an investigation of supercells that persisted in a quasi-steady known reflectivity characteristics (i.e., hook echo, inflow manner for greater than 4 h. Specific attention was given to notches, tight reflectivity gradient on inflow side, deviant sounding parameters as well as the large-scale environment. motion). For comparison purposes, we also examined shorter-lived Observed soundings were obtained for both the long- supercells to determine if the environmental signals were the and short-lived supercell events, with the requirement that the same or different from their longer-lived counterparts. radiosonde was released ahead of, or along, the supercell’s path, and that it was within 4 h of the event. For the soundings 2. DATA AND METHODS that were furthest in space/time from the supercell event, surface data in the vicinity of the storm were used to modify the Several data sources were utilized in order to gather sounding. Due to the unavailability of observed soundings in samples of our arbitrarily defined long-lived (> 4 h) and short- some instances (i.e., the storm was not close in time/space to a lived (< 2 h) supercells. [Those supercell events which had sounding site), Rapid Update Cycle (RUC) analysis soundings mean lifetimes between 2 and 4 h were not the focus of this were utilized for four of the supercell cases. Finally, synoptic- study.] First, a literature search produced five cases of long- scale maps at either 1200 or 0000 UTC were analyzed to lived supercells (Paul 1973; Browning and Foote 1976; Warner determine the presence of fronts, instability axes, and lines of 1976; Wade 1982; Pence and Peters 2000). We required clear forcing; however, only a portion of these data have been graphical and/or written evidence of supercells that persisted analyzed at the time of this writing. for > 4 h in these documents. For example, the Lahoma, OK supercell (Conway et al. 1996) was not considered long-lived 3. RESULTS 3.1 Observations of the long-lived supercells * Corresponding author address: Matthew J. Bunkers, Based on the preliminary data, most of the long-lived Natl. Wea. Serv., 300 E. Signal Dr., Rapid City, SD supercells tracked to the east or southeast (Fig. 1). There was 57701-3800; e-mail: [email protected]. a bias toward a southeast direction over the NHP (where most of the cases have been collected), although the data are too of the supercell’s lifetime was signaled by a transition into a limited to assert that this is a common feature of long-lived bow echo (Moller et al. 1994), which may have persisted for a supercells in general. The fact that there were only six storms few hours thereafter. The remaining long-lived supercells lost with a northerly component to their motion, and none with a their identity after merging with other thunderstorms. northeast to north component, is intriguing. No long-lived supercells have been discovered in the mountainous western 3.2 Observations of the short-lived supercells United States, but some have occurred as far north as Alberta and Saskatchewan. Also of interest, there were no long-lived By way of contrast, the short-lived supercells were left-moving supercells in the dataset (although there were a few associated with groups of storms much more frequently than noted with lifespans of 2−3 h). the long-lived supercells (about 60% of the time). It may be that their relatively close proximity to other storms potentially limited their lifetime through adjacent storm interactions. The short-lived supercells consisted mostly of CL/HP storms, but identification of the HP storms tended to be more obvious than in the long-lived supercell dataset. Just because a supercell is short-lived does not mean that the severe weather threat is diminished. In about one-half of these cases, the supercells were observed to evolve into a squall line or bow echo complex, often producing widespread severe winds. Additionally, there was a tendency for the short- lived supercells to be more numerous at any given time and location (when compared to the long-lived supercells). Their tornado production, however, was lower than for their long-lived supercell counterparts, with 34% of the events resulting in tornadoes (only 3% were significant tornadoes). The demise of the short-lived supercells was equally split between (i) an evolution to an other convective mode (i.e., squall line or bow echo), and (ii) a gradual weakening or Fig. 1. Storm tracks for supercells that persisted for > 4 h. The dissipation. Adjacent cell interactions played a significant role tracks were plotted by following the centroid of the storm on the in the supercell transitions to other convective modes. Only 0.5-degree reflectivity images. The tip of the arrow indicates about 10% of the short-lived supercells merged with other where the supercell ended. storms without evolving further (and thus losing their structure). It is important to note that both the long- and short-lived An examination of the radar data revealed that most supercell datasets are biased toward the NHP, where most of (88%) of the long-lived supercells were quite isolated, and on the archived radar data were readily available. The above observations are subject to change as more data are analyzed several occasions, there were no other storms within 100−200 across the rest of the United States.
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