The Origin of Severe Winds in a Tornadic Bow-Echo Storm Over Northern Switzerland
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192 MONTHLY WEATHER REVIEW VOLUME 128 The Origin of Severe Winds in a Tornadic Bow-Echo Storm over Northern Switzerland WILLI SCHMID AND HANS-HEINRICH SCHIESSER Atmospheric Science, Swiss Federal Institute of Technology, ETH, Zurich, Switzerland MARKUS FURGER Paul Scherrer Institute, Villigen, Switzerland MARIO JENNI Institute of Geography, University of Bern, Bern, Switzerland (Manuscript received 3 September 1998, in ®nal form 10 February 1999) ABSTRACT A severe bow-echo storm over northern Switzerland is investigated. Wind damage occurred along a track 15 km long and some 100 m wide. Damage data, meteorological data from a ground micronet, and Doppler radar data are analyzed. Volume-scan radar data in the direction of the approaching storm are available every 2.5 min. The storm reached a weak-evolution mode when the damage occurred. Updraft impulses followed each other in time steps of typically 5 min. The damage track can be attributed to a strong radar-observed vortex of 2±7- km diameter. The vortex developed at a shear line that was formed by the downdraft out¯ow of an earlier thunderstorm cell. Most of the damage was collocated with the strongest Doppler winds but some of the damage occurred beneath the strongest signature of azimuthal shear. A weak tornado was observed in that shear region. The two extremes in Doppler velocity, associated with the vortex and referred to as in¯ow and out¯ow velocities, are analyzed separately. Early strengthening of the vortex at 2±4-km altitude was due to an acceleration of in¯ow velocity, caused by the rising updraft impulses. Subsequent strengthening at low layers (0±2 km) could be related to acceleration of both the in¯ow and out¯ow velocities. At this stage, the diameter of the vortex decreased from about 7 to less than 2 km. The low-level intensi®cation of the vortex is attributed to vortex stretching. Later on, the vortex and in¯ow velocity at low layers weakened but the out¯ow velocity remained strong. 1. Introduction ed States in the last decades. Fujita (1978) de®ned a bow-echo as an outward bulge within a line of radar A large mesoscale convective system (MCS) passed echoes and associated the signature with downbursts. over northern Switzerland and southern Germany on 22 He pointed out that the strongest downbursts may occur July 1995. Hail, water, and wind damage were reported near the apex of the bow-echo. Two explanations for in both countries. The southern part of the system de- the formation of a bow-echo exist. A bow-echo can be veloped into an intense bow-echo, which produced a associated with a rear-in¯ow jet, caused by entraining track of severe wind damage and a weak tornadic event. of dry air into the rear ¯ank of the storm (e.g., Smull The parent thunderstorm crossed a dense ground mi- and Houze 1987). Another explanation is that the bow- cronet and was captured with a Doppler radar, producing echo evolves from a redistribution of hydrometeors due sector volume scans in time steps of 2.5 min. Detailed to a vorticity couplet circulation (Lee et al. 1992). Such information about the damage is available. The damage a circulation distorts an elongated radar echo into a bow track can be associated with a rapidly growing thun- shape. Przybylinski (1995) showed that bow-echo derstorm cell and with a vortex, observed with the radar storms exhibit a variety of features that can be associated at low layers. either with downbursts, severe straight-line winds, or Bow-echo storms have been investigated in the Unit- tornadoes. Multiple vortex circulations within bow-echo storms are documented by Funk et al. (1996a,b). The ®rst of the two studies shows that the vortices developed Corresponding author address: Dr. Willi Schmid, Institute for At- in the low levels as a cyclonic-convergent zone along mospheric Science, ETH, CH-8093 ZuÈrich, Switzerland. the bow apex and intensi®ed and deepened into the mid- E-mail: [email protected] dle levels of the storm. The second study discusses such q 2000 American Meteorological Society JANUARY 2000 SCHMID ET AL. 193 an evolution in more detail. Three vortex circulations stretching in the subsequent strenghtening of the vortex are documented. The three circulations are linked to at low layers. Conclusions are given in section 6. rapid multicellular growth on the leading convective line. It is argued that vortex stretching is one important 2. Data and procedures element for spinup of the circulations, generally similar to that found by Wakimoto and Wilson (1989). Meteorological data are available from several data Studies about European bow-echo storms are rare. sources: two networks of surface stations, two radio- Houze et al. (1993) showed that many hailstorms in sounding stations, damage data, satellite data, data from Switzerland exhibit a ``false hook'' structure, similar to the operational radar network of Switzerland, and data bow-echo storms. Three severe storms in central Swit- from the Doppler radar at the Swiss Federal Institute of zerland were analyzed in Schmid et al. (1997). Two of Technology (ETH). An overview of the investigated the storms had a bow-echo structure. The two storms area is given in Fig. 1. The radar station on Albis and were similar with respect to the simultaneous occurrence the sounding station in Payerne are operated by the of the strongest wind damage and rapidly growing ro- Swiss Meteorological Institute (SMI). The Paul Scherrer tating cells. Another storm with a possible coincidence Institute (PSI) operated a dense network of 12-m towers, between a tornado and growing cells has been analyzed measuring wind, temperature, and humidity. Twenty sta- by Martin et al. (1997). Hence, these European storms tions were distributed in an area of about 20-km radius may have similarities to the U.S. storms analyzed by (crosses in Fig. 1). Time-averaged measurements of me- Funk et al. (1996a,b). teorological parameters are stored in 2.5-min intervals. Thus, rapidly growing convective cells along a bow- Data from private station owners and from the ANETZ, echo are often a source for severe winds, for example, an automatic network operated by the SMI (stars in Fig. downbursts or microbursts, but also for tornadoes. The 1) are included in the database. These stations provide link between the growing cells and the damaging winds, additional data of pressure, wind, maximum wind gusts, however, is not understood in detail. One reason for this and precipitation with a time resolution of 10 min. is the dif®culty of documenting the full sequence of Satellite data (Meteosat) and the data of the opera- evolving thunderstorm cells and associated vortex sig- tional C-band radar network are used to document the natures. The radar data used by Funk et al. (1996a,b) evolution of the system on the mesoscale (see section and Schmid et al. (1997) have a time resolution of 5±6 3). Detailed radar measurements of the event were ob- min. This time step may lead to errors in the process tained with the C-band Doppler radar at ETH (Li et al. of identifying and tracking cells that follow each other 1995). The radar, which is at 600 m above mean sea in time intervals of typically 5 min. Radar measurements level (MSL), operates at a frequency of 5.63 GHz and with a temporal resolution of 2±3 min or less are re- has a transmitted power of 250 kW, a half-power beam- quired for a reliable analysis of these rapidly changing width of 1.658, pulse repetition frequencies (PRFs) of convective elements. High-resolution Doppler studies of 1200/800 Hz (dual-PRF mode, see, e.g., Keeler and Pas- similar phenomena exist and demonstrate the possibility sarelli 1990), and a pulse width of 0.5 ms. On 22 July to identify relevant processes in the formation of down- 1995, sector volume scans were made between 1443 bursts or tornadoes (e.g., Kessinger et al. 1988; Waki- and 1531 UTC in time steps of 2.5 min. Two volume moto and Wilson 1989; Lee et al. 1992; Roberts and scans were missed (at 1446 and 1521 UTC). The sector Wilson 1995; Wakimoto and Atkins 1996). However, boundaries were continuously adapted such that the the question remains of whether one can extrapolate the storm center was scanned by the radar. The elevation ®ndings of these studies to the small-scale features along angles were ®xed by an automated procedure. The scans bow-echoes. were made ``from top to bottom.'' The number of scans The availability of radar volume scans every 2.5 min per volume decreased from 15 to 9, depending on the makes it possible for us to investigate, for the ®rst time, distance of the approaching storm from the radar. Two the full cycle of the events that led to the damaging full PPIs at low elevation angles (38 and 208) were re- winds. An overview of the available data can be found peated in time steps of 10 min. The collected data were in section 2. In section 3, we summarize the mesome- processed following the steps given in the appendix. teorological setting and the evolution of the storm sys- Hereafter, Doppler velocities toward (away from) the tem. The damage and the radar patterns associated with radar are de®ned as positive (negative) since the storm the damage area are shown in section 4. The connection occurred in direction west±westnorthwest from the ra- between the damage and the radar-observed vortex is dar. In such a setting, the Doppler winds represent emphasized.