1. Introduction
While supercell thunderstorms tend to be most prevalent in North America east of the Rocky Mountains over predominantly flat terrain, almost all regions around the world support the development of these storms, albeit on a less frequent basis. Countries such as Argentina, Australia, and Bangladesh are among regions where supercells have been known to occur on a regular basis (Held et al. 2005). Supercells also occur intermittently on the European continent. Synoptic conditions over Great Britain, the Netherlands, France, Germany, and Italy, for example, support organized convection in the form of supercellular storms several times during the warm season and to a lesser extent also during the cool season (Dessens and Snow 1989; Collier and Lilley 1994; Schiesser et al. 1995; Huntrieser et al. 1997; Hernández et al. 1998; Robert and Calas 2004). For example, the Italian Pô Valley is particularly conducive to supercell storms in the warm season having many environmental characteristics that are similar to those of the U.S. Great Plains, with such observable features as lee cyclogenesis, the presence of a dryline, high surface dewpoints originating from a warm body of water, and strong low-level speed and directional vertical wind shear related to a low-level jet (Costa et al. 2001; Giaiotti and Stel 2007).
Many studies focus largely on the severe storm environments typical of the Great Plains in the United States (Lemon and Doswell 1979; Johns and Doswell 1992; Moller et al. 1994). However, the specific supercell ingredients, and underlying physical concepts elaborated upon in these studies, are applicable to storm environments elsewhere, with regional ingredient climatologies being the primary differences. The Geneva supercell of 18 July 2005 shared many environmental and structural characteristics of other supercell events, as will be documented in the following sections.
2. Switzerland's severe weather synoptic patterns and severe weather climatology
a. Synoptic conditions favorable for severe thunderstorm development over Switzerland
Synoptic conditions most favorable for supercellular storms over France and Switzerland during the warm season usually involve either a closed upper-level low or a broad trough over the Bay of Biscay and/or the Iberian Peninsula coupled with a ridge of high pressure centered over the Sahara Desert or the Mediterranean Sea (Dessens and Snow 1989; Collier and Lilley 1994; Schiesser et al. 1995; Huntrieser et al. 1997; Hernández et al. 1998) (Fig. 1). The closed low or trough in question usually ends up ejecting a jet maximum to the east or northeast, at 500 hPa, toward the area of concern. At the surface may lay a slow-moving front usually oriented southwest to northeast or north to south over France. Back trajectories over isentropic surfaces have shown that the low-level air mass is most often of Mediterranean origin (maritime–tropical), whereas the upper-level flow at 300 hPa is oriented more westerly and is of Atlantic origin (Hernández et al. 1998). The air masses at intermediate heights (700 hPa) tend to originate over the dry and hot Iberian Peninsula, which tends to introduce a warm layer or “cap” over the moisture-rich Mediterranean air mass advected underneath. This configuration, frequently referred to as the Spanish plume (Morris 1986; Morel and Senesi 2002), is analogous to the severe weather setup over the Great Plains of the United States, whereby a low-level moisture-laden Gulf of Mexico air mass is usually topped by a drier and warmer capping air mass originating from the Mexican Plateau, which is in turn overridden by a more westerly and maritime Pacific flow (Barnes and Newton 1986; Johns and Doswell 1992).
Conceptual model of the Spanish plume synoptic configuration. (a) Surface frontal depictions with isobars and airmass characteristics showing a warm conveyor belt streaming northeastward from the Iberian Peninsula. (b) Upper-level flow depiction with heights and jet streak location associated with an approaching long-wave trough. (c) Low-level equivalent potential temperature maxima ahead of a surface cold front denoting that a warm and humid air mass is in place. (d) Typical Spanish plume sounding from Nîmes, France, on 28 Aug 2003 showing moist low levels overridden by the dry elevated mixed layer (Spanish plume) and steep midlevel lapse rates.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
The resultant vertical wind shear profile (directional and speed) described above is most often supportive of supercell thunderstorms, some of which may become tornadic. For Switzerland, it was found via Houze et al. (1993) that the very weak low-level wind speeds and resultant weaker environmental vertical shear compared to U.S. storms often tended to inhibit tornadogenesis, while the mid- and upper-level wind shear were often sufficient for storms to exhibit persistent rotational characteristics (mesocyclones). As Dessens and Snow (1989) among others brought to light, tornadogenesis in Switzerland appears to be more dependent on the presence of local topographical features, which in a minority of the cases seems to enhance the phenomena through a channeling process while in a majority of other cases seems rather to impede it through a reduction of the low-level wind shear (Bourgeat 1890a,b Gauthier 1890; Piaget 1976; Houze et al. 1993; Schiesser et al. 1995; Schmid et al. 1997). This channeling process and how it may have been influential in this particular event will be investigated further in the results section.
b. Severe thunderstorm and tornado climatology in the Alpine region
Switzerland’s large lowland valley (Swiss mittelland or Swiss plateau) and the northern Alpine foothill region have been known to support supercell development and, to a lesser extent, tornadoes (Houze et al. 1993; Schiesser et al. 1995) (Fig. 2). Based on 8 yr’s worth of data, Houze et al. (1993) found that Swiss hailstorms were equally divided between right- and left-moving storms and that this balance was most likely attributable to the nature of the terrain. Via model simulations, they were able to show that by slightly modifying the hodograph structure in the lowest 2–3 km AGL, the terrain was able to favor one storm mode over the other depending on the local orientation of mountain features and the subtle low-level wind shear profiles thereby created (Fig. 3). Hodographs with clockwise shear tended to produce splits with well-defined right-moving classic supercells and left-moving storms of an intermediate structure, while with a slight counterclockwise turning of the hodograph in the lowest 3 km, left-moving storms with a false hook appendage tended to form. This counterclockwise turning seemed to prevent cell splitting and resulted in there being two long-lived updraft regions, a stronger leading line updraft to the north and a weaker one associated with a weak vorticity center to the south (false hook). Schiesser et al. (1995) later studied 82 Swiss mesoscale convective systems (MCSs) over a 5-yr period and were able to classify them into general categories of organization, similar to Houze et al. (1990), who examined the mesoscale structure of major springtime rainstorms in Oklahoma. While only moderately and weakly classifiable storm systems were found compared to the Oklahoma study, four general categories of organization were nonetheless found for Switzerland: isolated cell complexes (CCs), groups of CCs, broken-line CCs, and continuous-line CCs. The broken-line organization was found to be the most common type, accounting for 35% of the total, and tended to cause the most hail damage. Of the 82 MCSs studied, 43 had line structure and of those 26 had a “leading line–trailing stratiform” (ll-ts) structure, which tended to be the most severe. This resulted in a 32% ll-ts count for Swiss storms compared to a 66% count for Oklahoma storms. The study found that the Swiss orography interfered with the airflow in such a way as to prevent MCSs from having enough time and space to develop to a higher degree of organization as is possible over the relatively flat terrain of Oklahoma. It was also found, with little surprise, that the instability and wind shear in the Swiss storm environment was weaker than for the Oklahoma environment (Table 1).
Geographical map of Switzerland showing locations of reported tornadoes and waterspouts between 1600 and 2010 UTC (based on work by M. Jeanneret, University of Fribourg). Red-filled circles show confirmed tornadoes, red-outlined white circles show both probable and confirmed tornadoes without intensity designation, blue-filled circles indicate confirmed waterspouts, and blue-outlined white circles are probable waterspouts. Largest red-filled circles indicate EF4 intensity and smallest indicate EF0. No EF5 tornadoes have ever been measured in Switzerland. Locations of selected Swiss cities are also noted with black asterisks.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
Mean hodographs of the Payerne sounding for all days on which the first occurring storm was a (a) right or (b) left mover. The dots at 950 and 850 hPa are based on the wind measurements from ground stations. The × marks the wind of the Payerne sounding at 850 hPa. (c) Adapted from Maddox (1976), shows the mean hodograph for 62 tornado outbreaks in the central United States. The × shows the approximate mean motion of the storms, which were generally right moving. [From Houze et al. (1993).]
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
Comparison of U.S. and Swiss hailstorm characteristics [partly based on work from Houze et al. (1993)].
With regard to tornado production, in contrast to Alfred Wegener’s (1917) estimate of “at least 100 tornadoes per year” over the European continent, it has recently been found (Dotzek 2003) that the number is closer to 170 based on observations and that the true number is most likely closer to 300 due to significant underreporting. Dotzek (2003) attributes the underreporting to several factors including the short historical tornado records of many European countries as well as the low population density and low mass-media availability in certain regions, which confirms similar results obtained in the United States. Concerning these last two factors, the number of weak tornado reports tends to suffer most. While tornadic supercell development tends to be favored in warm sectors over the continent during the warm season, low-topped supercells seem to be primarily responsible for tornado generation along the European coastal plains during the cool season, such as in France (Dessens and Snow 1989; Robert and Calas 2004). More intense surface heating over the continent in the summer and significant latent heating influences along the coast in winter seem to be responsible for this observed partition.
Out of 107 significant tornado events over France reported between 1680 and 1988, Dessens and Snow (1993) identified several regions where tornadic thunderstorms tended to cluster. Among the identified regions is a sector along the Jura Mountains considered to be a local “tornado alley,” located north of Geneva along the Swiss–French border (Gallimore and Lettau 1970) (Fig. 4). Over this time period, four significant–violent tornadoes as defined by the Fujita scale (Fujita 1971) have occurred in this region (two F3s, two F4s) and numerous other reports of narrow damage swaths have been reported over the years in the forests of this same area (Bourgeat 1890; Gauthier 1890a,b; Piaget 1976; Nuss 1986). These observations suggest that low-level wind flow modified through channeling by the mountains may provide a locally favorable wind shear environment for tornadogenesis (Nuss 1986; Tripoli and Cotton 1986; Dessens and Snow 1989, 1993; Korotky 1990; Bluestein 2000; Bosart et al. 2006).
Map depicting the location of four significant tornado (F3 and F4) occurrences in the local “tornado alley” along the Swiss–French border in the Jura Mountains. [From Dessens and Snow (1993).]
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
Though apparently less prone to tornadoes than the Jura Mountains sector described above, the Swiss Plateau is nevertheless not spared by them. Recently, on 10 August 2004 and again 1 week later on 17 August 2004, two tornadoes occurred in the rural area of Fribourg province (Fig. 5). Both tornadoes occurred in environments of strong low-level environmental winds of 10–20 m s−1 (20–40 kt, 1 kt = 0.51 m s−1), between 0 and 3 km, and in low to moderate values of lowest 100-hPa mixed-layer CAPE (MLCAPE), on the order of 800–1200 J kg−1. Though hodographs did not provide particularly high storm-relative helicity (SRH) values, local channeling effects perpendicular to the storm motion appeared to have played a certain role in tornadogenesis by locally increasing SRH values and allowing stronger streamwise low-level inflow into the storm. As we shall see shortly, this seems to have played an important role during the 18 July 2005 event as well.
Map depicting the locations of two tornadoes that struck the Fribourg Province region of the Swiss Plateau in August 2004. Tornadoes tracks are depicted by the two thick black lines. Line 1 denotes the Massonens F2 tornado of 10 Aug 2004 and line 2 denotes the Villargiroud F1 tornado of 17 Aug 2004. The tornadoes were separated by only 2 km and 1 week apart. Both tornado tracks were about 2 km long.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
3. In-depth analysis of the 18 July 2005 tornadic supercell event
We will now analyze numerical model data leading up to the event and then elaborate upon the radar, satellite, and cell-based signatures of the observed tornadic supercell on this day as well as detailing the material damages caused by the storm. Finally, we will delve more deeply into the effect wind channeling in and around mountains features may have had on tornadogenesis by analyzing surface observations and summarizing results from a numerical model reanalysis of the event.
a. Numerical model data
Already a few days ahead of time, both the 7-km regional Consortium for Small-Scale Modeling (COSMO-7) model and the global European Centre for Medium-Range Weather Forecasts (ECMWF) model were forecasting the approach from the west of a deep and well-marked upper-level trough over the Iberian Peninsula. Ahead of this trough, these models showed the advection of very warm and relatively dry air over southeastern France and western Switzerland accompanying the downstream upper-level ridge. The synoptic setup resembled a “Spanish plume” configuration with low-level warm, moist air from the Mediterranean Sea overridden by a drier and very warm midlevel air mass from the Iberian Plateau with cooler air of Atlantic origin farther aloft. The 0000 and 1200 UTC runs of 17 July and the 0000 UTC run of 18 July from both these models predicted that 850-hPa temperatures over western Switzerland were to reach 18°–20°C by 0000 UTC on 18 July, indicative of the warm elevated mixed layer from the Iberian Peninsula [UTC = central European time (CET) − 2 h during daylight savings time]. Also evident in the runs was an upper-level jet streak associated with the short-wave trough, poised just east of the long-wave trough axis by 0000 UTC 18 July and forecast to lift out to the northeast toward the north slope of the Alps by 1200 UTC 18 July before translating farther northeastward by 0000 UTC on 19 July (Fig. 6a). Both the COSMO-7 and ECMWF models forecasted 500-hPa winds in the 20–25 m s−1 [~(40–50) kt] range by 1200 UTC 18 July over southeastern France and western Switzerland as well as the presence of the jet-streak left-exit region aloft at 300 hPa. By 1800 UTC 18 July the jet structure at upper levels forecast by the COSMO-7 model (Fig. 6b) even resembled a double-jet structure (left-exit–right-entrance region) configuration, a setup known to be very favorable for strong large-scale ascent due to the coupling of the ascending portions of the direct and indirect ageostrophic circulations (Uccellini and Johnson 1979; Kocin et al. 1986). In terms of instability, MLCAPE values on the order of 600–1000 J kg−1 were forecast for the region by the COSMO-7 model for the afternoon of 18 July along with 850-hPa equivalent potential temperature values in the 328–333-K (55°–60°C) range. Based on the above considerations, the decision was made at 1000 UTC on 17 July by the lead forecasters at the MeteoSwiss regional forecast offices in Geneva and Zurich to issue a severe thunderstorm watch–outlook to the local authorities indicating possible severe thunderstorm development over western and eastern Switzerland encompassing the regions of the Jura Mountains, the Lake Geneva basin, the Swiss Plateau, and the northern Alpine foothills beginning at 1200 UTC on 18 July and valid through 0000 UTC on 19 July. This Swiss severe thunderstorm watch is comparable in the United States to a convective outlook encompassing a rather large threat region. However, the issuance of Swiss severe thunderstorm warnings, called “flash orages,” is radar based and corresponds quite well in space and time to the U.S. severe thunderstorm warnings.1
COSMO-7 model 300-hPa heights and isotachs: (a) analysis for 1200 UTC 18 Jul 2005 at time of convective initiation and (b) 6-h forecast valid 1800 UTC 18 Jul 2005 showing double-jet structure. Light gray hue denotes wind speeds of 70 kt with darker gray hues indicating increasing winds in 10-kt increments. White and black circles denote the Lake Geneva region.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
On the morning of 18 July, isolated elevated moist convection was already in progress along the northern Alpine foothills and on the Swiss Plateau, the first signs that destabilization was beginning to take place aloft with the approach of the left-exit region of the oncoming jet streak. Isolated showers and generous sunshine intervals during the morning hours over Lake Geneva allowed the surface dry-bulb and dewpoint temperatures to increase significantly to 29° and 16°C, respectively, by 1200 UTC. This aided in increasing the instability in the Lake Geneva area significantly, with MLCAPE values based on the modified 1200 UTC 18 July Payerne sounding on the order of 1000–1500 J kg−1 and most-unstable CAPE (MUCAPE) values approaching 2000 J kg−1 (Figs. 7a–c). Evident in this sounding was the fact that the boundary layer was already well mixed and that the capping lid was eroding quickly with convective inhibition (CIN) values on the decrease. Also apparent from this sounding were the stronger-than-forecast low- and upper-level winds with values close to 12–15 m s−1 [~(25–30) kt] at 850 hPa and values approaching 35 m s−1 (~70 kt) at 500 hPa, a result of the nose of the upper-level jet streak ejecting out over Switzerland from the southwest more quickly than anticipated. Given the thermodynamic ingredients in place, the abundant morning sunshine, and the strong large-scale ascent in the left-exit region of the upper-level jet streak approaching from the southwest and starting to impinge into the warm sector, severe deep moist convection appeared imminent.
COSMO-7 12-h forecast CAPE fields valid at 1200 UTC 18 Jul 2005, time of thunderstorm initiation with (a) MLCAPE and (b) MUCAPE. The cities of Lyon, Geneva, Le Bouveret, and Payerne are shown by the black, gray, white, and blue dots, respectively. (c) Modified Payerne 1200 UTC 18 Jul 2005 rawinsonde adjusted with observed surface temperature and surface dewpoint temperature.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
Between 1100 and 1200 UTC, the first significant surface-based echoes (as confirmed shortly thereafter by the well-mixed boundary layer in the 1200 UTC Payerne and Lyon soundings) began to appear on radar over the eastern portion of the Swiss Plateau as well as in the Lyon region in France. Very quickly, the eastern Swiss Plateau convection evolved into small squall-line segments heading for Zurich and Basel, while a northeast-moving isolated cell that initiated around Lyon quickly underwent a splitting process, resulting in a left- and right-moving cell pair. The hodograph based on the 1200 UTC Payerne sounding appeared rather straight, indicative that both left- and right-moving storms would theoretically be favored. However, given the fact that the storm motion was to the right of the mean flow before the cell split, that the largest instability stretched along an axis from Lyon to the Lake Geneva region, and that the instability farther north was significantly lower, a case could be made for a dominant right mover (Nielsen-Gammon 1995). This was confirmed on radar in the following hour, with the left-moving cell undergoing rapid dissipation and the right-moving storm exhibiting rapid intensification as it approached Geneva from the southwest. The first severe thunderstorm warning (or flash orage) was issued by the Geneva regional forecasting office at 1224 UTC for the city and province of Geneva warning of possible wind gusts > 20 m s−1 (75 km h−1) and the presence of hail > 2 cm.
By 1300 UTC, the right-moving storm had reached Geneva and continued exhibiting rapid intensification. The following three subsections will review the various storm attributes as measured passively by satellite and radar.
b. Satellite imagery
Beginning with MeteoSat Second Generation (MSG) 15-min satellite imagery, several signatures typically associated with severe storms were clearly identifiable, including supercells. From 1200 UTC up through 1515 UTC, an enhanced V could be seen continuously on MSG enhanced infrared imagery. The enhanced-V (cold U) signature is most often favored with severe storms evolving in environments with strong and significant vertical wind shear, environments in which supercells are favored, and where it is visible in infrared satellite imagery as a long, narrow region of colder pixels along the upshear edges of a thunderstorm anvil, advecting downshear along its long axis (McCann 1983; McGinley 1986; Melani et al. 2003; Setvák and Rabin 2003, 2005; Setvák et al. 2006). Within this enhanced V, the protrusion of the overshooting top could clearly be seen, with brightness temperatures averaging around 213 K (−60°C), but falling to as low as 208 K (−65°C) at times of peak stratospheric penetration. In addition, intermittently during this period, a central warm area (CWA) was apparent with brightness temperatures averaging around 230 K (−43°C) as the storm pulsed through several peaks in intensity. A CWA is known to be a long-lived (more than 30 min) warm wedge-shaped wake region embedded in a thunderstorm anvil and visible in infrared imagery, surrounded by long, narrow regions of colder pixels along either side (the enhanced V) extending from the upshear edge of a thunderstorm anvil, downshear along its long axis. The first peak in brightness temperatures of 207 K (66°C) occurred at 1245 UTC, approximately 15 min prior to the first tornado touchdown in Veigy, France, before warming to a local maximum of 210 K (−63°C) during tornadogenesis (Fig. 8a). The second peak in brightness temperatures occurred between 1315 and 1330 UTC, approximately 15–25 min before the second tornado touchdown in Le Bouveret, before warming to a value of 213 K (−60°C) during tornadogenesis (Fig. 8b). Following this second tornado, brightness temperatures continued to warm to a maximum value of 220 K (−53°C) between 1400 and 1445 UTC before cooling once again during the supercell’s final intensification phase.
MSG enhanced infrared satellite images of the 18 Jul 2005 supercell during its most intense phase over the Lake Geneva region showing (a) first peak in IR brightness temperatures (207 K) at 1245 UTC, 15 min prior to first tornado touchdown in Veigy (Geneva suburbs) (black circle denotes city of Geneva). (b) Second peak in IR brightness temperatures (208 K) at 1330 UTC, 15–25 min prior to second tornado touchdown in Le Bouveret. Black circle denotes town of Le Bouveret. The cold-V (enhanced V) signature, the overshooting top, and the CWA associated with the supercell can be seen in both images. Parallax shift is responsible for slight northern placement of the central storm core (coldest cloud tops) with respect to its actual position.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
This cloud-top ascent (cooling) 30–45 min prior to tornadogenesis has often been observed and has been attributed to the development of the mesocyclone, which through perturbation pressure forces results in rapid intensification of the updraft in the lower to middle levels of supercells (Adler and Fenn 1981; Moller et al. 1994). A similar relation in radar case study observations shows echo-top height increases during mesocyclone formation (Lemon et al. 1978). While a strengthening midlevel mesocyclone is not by any means in and of itself a guarantee that a tornado will form [Trapp et al. (2005) indicate that only about 20%–25% of radar-detected mesocyclones go on to produce tornadoes in the United States], a strong midlevel vortex certainly increases the probability that one could possibly form, given favorable low-level kinematics (winds) and thermodynamics (airmass characteristics) (Brandes 1978; Klemp and Rotunno 1983; Brooks et al. 1993; Markowski et al. 2002, 2003; Thompson et al. 2003, 2007; Trapp et al. 2005; Shabbott and Markowski 2006; Grzych et al. 2007). Regarding the cloud-top decrease–warming just prior to both tornadoes on 18 July 2005, this characteristic has often been observed in satellite imagery in the past as well (Adler and Fenn 1981) and corroborates with radar studies that have shown that vortex intensification and tornadogenesis are sometimes associated with updraft weakening and/or descending reflectivity cores (Lemon and Doswell 1979; Rasmussen et al. 2006; Byko et al. 2009). Dowell and Bluestein (1997) have also shown that this updraft weakening can favor massive precipitation fallout once the mass of liquid water and ice aloft can no longer be supported, thereby redistributing the charge within the storm and occasionally resulting in downdraft-induced [i.e., rear-flank downdraft (RFD) induced] tornadogenesis.
c. Radar imagery
1) Reflectivity (CAPPI)
Radar imagery has been known to aid in the identification of specific storm features associated with supercells and severe storms in general (Burgess and Lemon 1990; Burgess et al. 1993; Trapp and Mitchell 1995; Burgess et al. 2002). The radar that observed the supercell was the La Dôle radar, one of the three operational C-band Doppler radars of the MeteoSwiss radar network (Fig. 9). Numerous radar signatures were present with the 18 July 2005 supercell, indicative of its severity. Very early in its lifetime, the storm underwent a splitting process just as it exited the Lyon region in France at around 1140 UTC (Fig. 10). The much lower instability located north of the Lyon area most likely resulted in the rather rapid dissipation of the left mover in this case not to mention the fact that left-moving storms are rarely favored even with straight hodographs, such as the one that was observed on this day. The right mover, which eventually went on to strike the Lake Geneva region, exhibited deviant motion (~20° to the right of the mean 0–6-km wind based on the 1200 UTC 18 July 2005 Payerne hodograph), typical of a supercell (Browning 1964; Moller et al. 1994).
(a) View of Lake Geneva with city of Geneva on western end, town of Le Bouveret on eastern end, and location of La Dôle C-band Doppler radar. Black line denotes supercell storm core path. (b) Scan geometry of the radar situated in the Jura Mountains (altitude 1680 m) just north of Geneva, with orography in the direction of azimuth 84°. This is roughly the direction of the supercell storm motion on 18 Jul 2005. The La Dôle radar, located about 25 km north of Geneva at an altitude of 1680-km, like the two other radars in the network, performs 20 elevation volume scans between −0.3° and 40° every 5 min. The Cartesian dataset consists of 12 CAPPIs between 1- and 12-km height. [From Hering et al. (2006).]
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
Maximum composite reflectivity image depiction of the left- and right-moving supercells at 1250 UTC 18 Jul 2005, approximately 1 h 15 min after the splitting process occurred. The cell tracks are labeled in white. The left mover experienced rapid dissipation in a relatively dry environment whereas the right mover experienced rapid intensification in a much more unstable environment as it approached the city of Geneva from the southwest.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
Weak-echo region (WER) and bounded weak-echo region (BWER) radar signatures were also visible at specific times during the supercell’s life cycle. Beginning at 1300 UTC, a WER was visible in the lower-elevation constant-altitude plan-position indicator (CAPPI) scans as the storm was bearing down on Geneva. This signature, most evident in the 2–4-km elevation scans, remained visible for over an hour as the storm progressed east-northeastward, albeit with some change in morphology over time. During tornado touchdown, these WERs were most evident at 3-km height for the 1305 UTC Veigy tornado and at 5-km height for the 1345 UTC Le Bouveret tornado (Figs. 11a and 11d). Beam shielding due to topography most likely hindered a better WER visualization at lower-elevation scans for the Le Bouveret tornado. Between 1320 and 1330 UTC, a WER resembling a hook echo could clearly be observed at the 3-km CAPPI elevation, even though no evidence of any tornado touching down at this time was ever obtained (Fig. 11b). BWER signatures were clearly visible at two distinct times during the supercell’s lifetime and are visible between 6- and 9-km height. The first signature occurred between 1325 and 1330 UTC, approximately 15 min before the Le Bouveret tornado and was most discernible at the 7-km CAPPI scan height (Fig. 11c). The second, also most visible at 7-km height, occurred between 1400 and 1410 UTC as the storm was traversing the Gruyère region along the northern Alpine foothills not far from Fribourg. These intermittent appearances of the BWER signature highlight the pulsating character of the supercell’s main updraft as it went through periodic phases of intensification.
CAPPI radar reflectivity images (mm h−1) from the La Dôle radar showing (a) WER signature at 3-km elevation over Veigy at 1305 UTC, time of first tornado touchdown. The white circle surrounds the town of Veigy, just northeast of Geneva. (b) Hook echo signature at 3-km elevation associated with supercell at 1325 UTC, approximately 20 min before second tornado touchdown. (c) BWER hole signature at 7-km elevation just to the northeast of Evian, France, at 1330 UTC, 15 min before second tornado touchdown. The white circle denotes the town of Evian and the red circle denotes the town of Le Bouveret. (d) WER signature at 5-km elevation over Le Bouveret at 1345 UTC, time of second tornado touchdown. The red circle denotes the town of Le Bouveret.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
These BWER appearances correlated quite well with the times of maximum storm-top height as compared to the coldest brightness temperatures found in the enhanced MSG satellite imagery. This seems to indicate that the supercell’s updraft was the most intense during these times, most likely strengthened to some extent during the mesocyclone’s most intense phases. Unfortunately, the MeteoSwiss radar software back in 2005 did not allow for thunderstorm echo-top determinations above 12-km height, which did not allow for further in-depth temporal comparisons of echo-top height, brightness temperatures, and the presence of a BWER. However, radar case study observations in the past have shown that echo-top heights increase during mesocyclone formation and that the BWER, which implies updraft, is collocated with the mesocyclone in its early stages (Lemon et al. 1978; Lemon and Doswell 1979). The radar and satellite data for this case, albeit not complete, certainly seem to support this view, as the coldest brightness IR temperatures in the MSG satellite imagery were observed between 1315 and 1330 UTC, a time period during which the first BWER and hook echo were observed by radar. In addition, the storm’s evolution on both radar and satellite imagery during the 30 min prior to tornadogenesis in Le Bouveret closely resembled the evolution described in previous studies regarding violent tornadic storms in the United States where storm-top increase followed by either storm-top lowering and/or the presence of descending reflectivity cores lead to RFD surging just prior to tornado formation (Burgess and Lemon 1990; Burgess and Magsig 1998; Broyles et al. 2002; Burgess et al. 2002; Rasmussen et al. 2006; Byko et al. 2009).
Concerning the V-notch radar signature (a V-shaped notch in the downwind part of a thunderstorm echo resulting from divergent flow around the main storm updraft), several were discernible during the 18 July 2005 supercell’s lifetime. The most visible signature being the one on the composite plan-position indicator (PPI) view at 1220 UTC as the storm was approaching Geneva from the west-southwest (Fig. 12a). The presence of the V notch coincided well with a local maximum height of 45-dBZ reflectivity at this time (12 km), as well as a local peak in vertical integrated liquid (VIL; 67 kg m−2) and the height of the maximum reflectivity (11 km). Another V notch was discernible between 1310 and 1330 UTC during the intensifying phase of the mesocyclone that preceded cell collapse and tornado number 2 in Le Bouveret and during the time period when both the most impressive cold V shape and the CWA were observable in the MSG satellite imagery (Fig. 12b). This correlates very well with previous work that purports that the radar V notch is a manifestation of very strong updraft blocking that deviates the mid-/upper-level tropospheric flow around it and that the satellite cold V shape (enhanced-V signature) is the satellite representation of this blocking during the phase of maximum cell intensification (Setvák and Rabin 2003).
La Dôle radar reflectivity images of the supercell of 18 Jul 2005 with (a) most visible radar V notch in the composite PPI image at 1220 UTC, approximately 45 min before first tornado touchdown in the Geneva area, and (b) radar V notch at 5-km CAPPI elevation at 1315 UTC, approximately 30 min before second tornado touchdown in Le Bouveret.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
2) Base velocity PPI
Base velocity PPI data on 18 July 2005 allowed for the observation of the rotational signature associated with this supercell. At specific points during the supercell’s lifetime, this rotational signature was sufficiently strong to be qualified as a mesocyclone, based on the mesocyclone recognition guidelines set forth by the National Severe Storms Laboratory in Oklahoma (Fig. 13a). Despite missing base velocity data in the archive at the different scanning elevations during a 30-min period between 1232 and 1302 UTC, a cyclonic rotational velocity signature was nearly continuously visible during the following 70 min starting at 1307 UTC and up through 1417 UTC at a 3.5° elevation scan. This scanning height corresponds to elevations around 3 km at 20-km distance from the radar and to an elevation of around 7 km at approximately 100 km from the radar. The rotational signature had time and height continuity, as this rotation was also visible at certain times in the 5.5°, 7.5°, and 9.5° elevation scans, particularly at distances rather close to the radar when the beam was penetrating the low to midlevels of the storm. At 1307 UTC, time of the Veigy tornado touchdown, the rotational signature aloft was sufficiently strong to be considered a minimal mesocyclone with a rotational velocity (Vrot) of approximately 15 m s−1 (30 kt) corresponding to a delta V of 30 m s−1 between 3- and 5-km heights and at a distance of 22 km from the radar (Fig. 13b). At 1342 UTC, the time of the Le Bouveret tornado touchdown, the Vrot aloft was a bit stronger with 17 m s−1 or 35 kt (delta V of 35 m s−1) at 5.5-km height and at a distance of 60 km from the radar, sufficiently strong to be considered a moderate mesocyclone (Fig. 13c). Unfortunately, due to interference from nearby mountains, the outgoing part of the rotational couplet to the immediate southwest had to be estimated since it could not be sampled nor was a rotational signature visible at the elevation scans below 5.5-km height.
(a) Mesocyclone recognition guidelines from the National Severe Storms Laboratory (NSSL) in Oklahoma City, OK. (b) Mesocyclone rotational velocity 4 km above the surface at time of the first tornado touchdown in Veigy, on the outskirts of Geneva, at 1307 UTC 18 Jul 2005. The rotational velocity of 32 kt at 22-km distance from the La Dôle Doppler radar qualified it as a minimal mesocyclone based on the NSSL recognition guidelines. With respect to the La Dôle radar, green hues (turquoise arrow) and yellow hues (red arrow) denote approaching and receding velocities, respectively. (c) Mesocyclone rotational velocity 5.5 km above the surface at time of the second tornado touchdown (tornadic waterspout) over the east end of Lake Geneva at 1342 UTC, shortly before making landfall near Le Bouveret. The rotational velocity of 35 kt at 60-km distance from the La Dôle Doppler radar qualified it as a moderate mesocyclone based on the NSSL recognition guidelines. With respect to the La Dôle radar, green and yellow hues denote approaching and receding velocities, respectively. In this image, only the approaching part of the couplet (green hue denoted by blue arrow) is visible due to missing data farther south (receding velocity was assumed with a conservative estimate and is denoted with the red arrow).
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
d. Cell-based attributes
Among the suite of forecasting applications available to MeteoSwiss forecasters is the thunderstorm radar and tracking (TRT) algorithm nowcasting–extrapolation system developed by the MeteoSwiss Radar and Satellite Research Group (RASA) in Locarno-Monti (Hering et al. 2004, 2005, 2006). The algorithms embedded in this tool permit the extrapolation of thunderstorm cells in space and time out to 60 min into the future. It uses heuristic and centroid-based methods derived from the rapid developing thunderstorm (RDT) algorithm. This tool permits the superposition and visualization of various parameters pertinent to the tracking of severe convection. Among the most utilized visualizations include cloud-to-ground (CG) lightning frequency and polarity, height of the 15–45-dBZ echo top, and cell-based and grid-based VIL, as well as the maximum storm reflectivity altitude, whose evolution has already been discussed in section 3. The supercell’s CG lightning frequency evolution as the storm crossed the Lake Geneva region was particularly noteworthy. Just as several phases of intensification could be observed via satellite and radar imagery as described previously, several pulsating phases could also be identified in the CG lightning frequency over time. Particularly noteworthy was the drastic drop in CG lightning frequency observed 10–15 min prior to tornadogenesis in Le Bouveret (Fig. 14). CG lightning frequency dropped from approximately 52 strikes per 5 min (11 strikes min−1) around 1330 UTC to a value of less than 3 strikes per 5 min (1 strike min−1) as the tornado touched down at approximately 1342 UTC. Over the next hour, the CG count increased once again to around 20 strikes per 5 min (4 CG strikes min−1) but never reached the peaks observed prior to the second tornado touchdown. Significant drops in lightning frequency just prior to tornado formation have already been observed in the past over the American Great Plains and over Brazil, for example (Perez et al. 1997; Held et al. 2005; Rison et al. 2005). It has been shown by Perez et al. (1997) and from previous work that the overall maximum CG flash rate offers no predictive value for tornadogenesis since the overall CG flash rate is quite variable. However, a correlation in the Perez et al. (1997) study was found between CG lightning flash trends and tornadogenesis where two different flash rate trends were found. One trend had a peak in CG frequency 15–20 min prior to tornado formation and another had a local minimum CG flash rate coincident with tornado touchdown. These two trends were certainly present prior to and during the second tornado touchdown in Le Bouveret. Among possible physical explanations for these trends, MacGorman et al. (1989) state that an intensifying updraft may reduce CG flashes by raising negative charge centers, resulting in a decrease in the distance between the negative and positive charged regions in a thunderstorm and thus enhancing the intracloud (IC) flash rate. For the 18 July 2005 case, while a slight increase in IC flash rate was observed with the CG flash rate decrease, this IC enhancement was not significant. Cloud electrification studies have also shown that subsequent updraft weakening can favor massive precipitation fallout once the mass of liquid water and ice aloft can no longer be supported, thereby redistributing the charge within the storm and resulting in a CG polarity reversal (Seimon 1993; MacGorman and Burgess 1994; Carey and Rutledge 1998; Stolzenburg et al. 1998; Smith et al. 2000) and/or downdraft-induced (i.e., RFD induced) tornadogenesis (Dowell and Bluestein 1997). For the 18 July 2005 case, while no significant CG polarity reversal could be observed as the updraft experienced a temporary weakening phase and IR brightness temperatures began warming rapidly, the CG flash rate dropped to almost zero, followed 10–15 min later by an RFD microburst and a tornado rated as category 2 on the enhanced Fujita scale (EF2; Fig. 14). It must be noted, however, that the first tornado touchdown in Veigy at 1305 UTC was not preceded by a drop in CG lightning frequency but rather that the CG lightning frequency was at that time in the midst of a steady increase that had begun around 1130 UTC and lasted up until 1330 UTC.
Graph showing different cell-based attributes of the 18 Jul 2005 tornadic supercell, including CG lightning frequency (blue), vertical integrated liquid (kg, red), echo top (45 dBZ, green), maximum storm reflectivity (yellow), and satellite brightness temperatures (orange). The two thick black vertical lines denote times of the tornado touchdowns.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
Other cell-based attributes from the TRT algorithm tool that proved useful were the grid-based and cell-based VIL. Elevated values of VIL have been known to be correlated to hail probability and hail size. A grid-based 250-km-long VIL swath with values greater than 30 kg m−2 and of nearly 1200 km2 stretched from the city of Geneva to the town of Interlaken in the central Alpine foothills after the supercell passage. Values reached as high as 65–70 kg m−2 along certain portions of the path (Fig. 15a). The probability of hail (POH) as computed by Waldvogel et al. (1979) was also calculated and matched rather nicely with the locations of insured claims received by the Swiss Hail Insurance as well as compared to the storm path and VIL swath (Figs. 15b and 15c) .
(a) Maximum grid VIL map (1030–1700 UTC) over Lake Geneva and Alpine region on 18 Jul 2005. White squares represent locations where hail was reported at least once by the Swiss Hail Insurance. (b) As in (a), but for POH as calculated by Waldvogel et al. 1979 (from Hering et al. 2006). (c) Map showing supercell storm track (black line) and the various locations where the most significant damage occurred due to wind, hail, and tornadoes. The yellow dot represents the microburst location.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
e. The supercell’s wind-induced damage paths and hail damage
The damage survey conducted by the author near Veigy, on the outskirts of Geneva, identified a 50-m-wide, 500-m-long narrow damage path through farm fields and a small wooded area. The uprooted and snapped trees in the wooded area were aligned in an overall convergent pattern. The tree trunks had an average diameter of 1 m and were approximately 50 m tall. A small golf hut nearby was dislodged from its foundation, but luckily no trees fell on the occupied hut. The damage was attributed to a tornado and an EF1 rating (Fujita 1971; McDonald and Mehta 2006) was attributed based on the observed damage.
Based on information from different observational platforms including radar and satellite imagery, surface stations, and photograph/video footage, a conceptual model of the supercell’s gust front structure could be constructed during its tornadic phase over the east end of Lake Geneva (Fig. 16). Along the northern lake shore in Vevey, a teenager observing the storm’s core to the south from a park was able to photograph a tornado 5 km to the south over the lake in contact with the water (tornadic waterspout) just before the arrival of the forward-flank gust front at his location farther north (Fig. 17a). The roll cloud associated with the forward-flank gust front was clearly visible and most evident in video footage from a webcam located on the roof of a lakeside hotel in Montreux (Fig. 17b). Wind gusts estimated at 20–30 m s−1 (80–120 km h−1) were followed 3 min later by a brutal and very destructive hailfall lasting between 5 and 7 min. Hail damage was considerable all over the Riviera portion (northeast end) of the lake where numerous vineyards were severely damaged or completely destroyed. The accumulated hailfall drift in low spots due to rainwater runoff was knee deep in places and the subsequent melting hail exacerbated the local flooding in many places. Many building windows were shattered in Montreux by the windblown hailstones, and local structural damage due to the wind occurred as well (Figs. 18a–e). Farther south in Le Bouveret, the rear-flank gust front impinged violently and very rapidly from the west. A wind gust of 40 m s−1 (161 km h−1) was measured on the Le Bouveret pier by one of MeteoSwiss’s automatic weather stations at approximately 1340 UTC with widespread tree damage (Figs. 17c and 17d).
Conceptual model of the 18 Jul 2005 supercell’s gust front structure (black ovals) during its second tornadic phase between 1340 and 1350 UTC over the east end of Lake Geneva. Blue arrows denote the FFD and green arrows denote the RFD. The microburst location, inflow winds, and tornado track are also shown.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
Photographs of phenomena on the east end of Lake Geneva associated with the passage of the 18 Jul 2005 supercell, including (a) the tornadic waterspout taken from the Vevey lakeshore looking south toward Le Bouveret. (The photo was taken by Y. Barton with his cell phone.) (b) A Montreux hotel webcam photo of the forward-flank gust front with 80–120 km h−1 wind gusts. (c) An aerial view of the RFD-induced microburst damage inflicted on a section of forest near Le Bouveret, which had a nearby recorded wind gust of 161 km h−1. (d) Part of the narrow 200-m-wide, 2-km-long damage path that went over a campground just east of Le Bouveret resulting from the passage of the EF2 tornado.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
Hail damage on the east end of Lake Geneva after the passage of the supercell. Hailfall drifts due to runoff were knee deep in places and golf-ball-sized hail combined with the 80–120 km h−1 forward-flank gust front–downdraft ravaged vineyards and trees, and blew out many building windows in the Montreux area as shown.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
The damage survey conducted by the author on the east end of Lake Geneva identified two separate damage paths between the towns of Le Bouveret and Noville exhibiting distinctly different characteristics. An eastern zone close to a campground (Les Grangettes) that exhibited a rather narrow 200-m-wide and 2000-m-long damage path extending east-southeastward from the lakeshore to approximately 2000 m inland and a western zone along the Rhône river delta that exhibited a rather circular 400-m-diameter-wide damage swath. While both damage tracks were rated EF2, the eastern narrow track damage was strewn with tree damage oriented in various directions in an overall convergent pattern whereas the western track’s vegetation blowdown was more unidirectional and divergent in nature and oriented toward the east (Figs. 17c and 17d). It is strongly hypothesized that the eastern damage zone corresponds to the tornado track whereas the western zone’s damage was a result of a microburst induced by the supercell’s rear-flank downdraft. Indeed, in radar images at this time (1345 UTC), it can be seen that over the southern periphery of the storm, near Le Bouveret, a weak echo region signature seemed to pass just to the west and right under a low-level reflectivity feature resembling an RFD gust front (Figs. 19a and 19b). Analyzing proximity and forecast soundings, some remnant dry air from the elevated mixed layer was clearly evident, supporting the possibility of downward-directed accelerations due to evaporational cooling. Furthermore, as Fig. 13c previously showed, a moderate midlevel rotational velocity couplet (mesocyclone) was located over the eastern end of the lake very close at this time (1342 UTC), which corroborates well with the time of the tornado sighting and is consistent with the location of the narrow damage track near the campground to the east of Le Bouveret, where the WER reflectivity signature seemed to pass. The damage reports seem pertinent to the author since they helped categorize the origin of the damage and helped elucidate that both a tornado and a microburst had occurred close together, thereby reinforcing the classic supercell conceptual model (i.e., RFDs role in microburst production and in subsequently facilitating tornadogenesis in some instances) (Doswell and Burgess 1993; Markowski et al. 2002; Byko et al. 2009). It also adds further confirmation to the possibility of occurrence of such well-defined storms over the heterogeneous terrain in proximity to the Alps (Houze et al. 1993; Schmid et al. 1997).
La Dôle radar CAPPI reflectivity images at (a) 5-km height at 1345 UTC showing midlevel WER notch on southeastern periphery of the supercell moments after the tornado touched down just east of Le Bouveret. Red circle denotes town of Le Bouveret. (b) As in (a), but at 3-km height at 1345 UTC showing the bowing low-level precipitation echo (RFD) on the southeastern periphery of the supercell passing over Le Bouveret moments after the 161 km h−1 recorded microburst wind gust on the Le Bouveret pier and shortly after the tornado passage farther to the east of Le Bouveret. The red circle denotes the town of Le Bouveret.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
f. The effect of wind channeling of inflow winds on tornadogenesis based on observations and a COSMO-2 reanalysis of the event
Starting at 1320 UTC, approximately 20 min prior to the rear-flank gust front’s arrival, a steady down-valley 2–5 m s−1 (10–20 km h−1) southeast wind had been measured at the Le Bouveret SwissMetNet (Swiss mesonet) weather station. This down-valley wind orientation and speed undoubtedly aided in increasing the SRH values of the local environment (Fig. 20a) (Davies-Jones et al. 1990; Droegemeier et al. 1993; Thompson et al. 2007). Recalculated 0–1-km SRH values showed an increase from approximately 0 m2 s−2 without the wind modification to a value of around 85 m2 s−2 with the inclusion of this southeast flow at low levels (Fig. 20b). Given the location of the tornado based on the available tornado photographs, and testimonials from several eyewitnesses, it appears most probable that the tornadic waterspout first developed at the occlusion point between the forward-flank downdraft (FFD) and RFD and then moved east-southeastward in the direction of Le Bouveret. One could further speculate for this case that tornadogenesis may have been facilitated by the enhanced southeast inflow present along the storm’s southern flank as well as from the important baroclinically induced streamwise horizontal vorticity present along both powerful gust fronts (Davies-Jones 1984).
(a) The north–south portion of the Swiss Rhône valley looking north-northwest toward Lake Geneva. Evident is the wind channeling potential of the valley, channeling which was nearly perpendicular to the supercell storm motion on 18 Jul 2005. (b) Payerne proximity 1200 UTC sounding on 18 Jul 2005 with modified low-level thermodynamic variables (temperature and dewpoint) and inclusion of the low-level wind shear environment near Le Bouveret with down-valley 10-kt southeast wind just prior to second tornado touchdown.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
To elucidate some of these uncertainties, a reanalysis simulation of the event was undertaken using the operational MeteoSwiss version of the COSMO-2 nonhydrostatic mesoscale model (http://www.cosmo-model.org/content/tasks/operational/meteoSwiss/default.htm). The model operates with a 2.2-km horizontal resolution covering a 520 km × 350 km domain over the Alpine region along with 60 vertical levels. Prognostic variables include pressure perturbation, three wind components, temperature, specific humidity, and turbulent kinetic energy. In addition, precipitation processes are explicitly described using a bulk-type cloud microphysics scheme containing five prognostic hydrometeor types (rain, snow, cloud water, cloud ice, and graupel). A parameterization for deep convection is not used (explicit), but shallow convection is parameterized. The COSMO-2 forecasts are driven using boundary conditions from the regional COSMO-7 model with 6.6-km mesh size and covering central Europe, which in turn is driven using boundary conditions from the global Integrated Forecast System (IFS) model of the European Centre for Medium-Range Weather Forecasts (ECMWF). The COSMO-2 model uses a data assimilation system based on a nudging technique (Schraff 1997) for conventional observations from surface stations, radiosondes, aircraft, and wind profiler. Radar surface rainfall observations are assimilated by the latent heat nudging (LHN) scheme (Jones and Macpherson 1997) following the technique described in Leuenberger and Rossa (2007).
While it would have been ideal to utilize a storm-scale model to resolve the storm-scale details (Wicker and Wilhelmson 1995; Xue et al. 2003), the results obtained with the available COSMO-2 model were nonetheless interesting. Rather encouraging was COSMO-2’s ability to resolve the supercell’s mesocyclone circulation at 1300 UTC using the supercell detection index (SDI) as devised by Wicker et al. (2005) just as the storm was about to affect Geneva (Figs. 21a and 21c). Visible are both the rotating updraft–downdraft pair (Fig. 21a) and the cyclonic updraft signature (Fig. 21c) based on the model’s analysis. As would be expected for a model analysis using an LHN scheme, the location of the mesocyclone corresponded quite well with the location of the supercell observed by radar at that time (Fig. 21e). The same could be said regarding COSMO-2 model SDI analysis fields valid at 1400 UTC as the supercell was exiting the Lake Geneva area (Figs. 21b, 21d, and 21f). Also noteworthy was the apparent secondary rotating updraft analyzed by the model seen in Fig. 21b, which never materialized into a splitting left-moving cell on radar. Unfortunately, the author was not able to obtain COSMO-2 plots at intermediary times between 1300 and 1400 UTC, namely at around 1340 UTC, time of the tornado on the east end of the lake.
(a) COSMO-2 analysis plot of SDI1 at 1300 UTC 18 Jul 2005 showing a rotating updraft (red) and downdraft (blue) pair just north of Geneva associated with a supercell. (b) As in (a), but for 1400 UTC analysis with up–downdraft pair located just east of Lake Geneva. (c) COSMO-2 analysis plot of SDI2 at 1300 UTC 18 Jul 2005 showing cyclonic updraft (red) just north of Geneva. (d) As in (c), but for 1400 UTC showing cyclonic updraft (red) and anticyclonic updraft (blue) just east of Lake Geneva. No cell split actually occurred. (e) MeteoSwiss composite maximum reflectivity image valid at 1300 UTC 18 Jul 2005 as seen with the TRT cell extrapolation algorithm viewer. (f) As in (e), but for the 1400 UTC valid time.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
Concerning the COSMO-2 10-km wind field forecasts, while the model resolved the background gradient wind field quite well, it had a much harder time (as would be expected) in attempting to resolve the storm-scale wind field and the storm-scale–gradient wind interactions. While it appears that the latent heat nudging most likely aided to some extent (i.e., the model resolved the 1400 UTC downdraft surface winds rather well on the east end of the lake), at other times (i.e., over Geneva at 1300 UTC) the model analysis–forecast surface winds and observed SwissMetNet surface winds show more discrepancy (i.e., the COSMO-2 winds underestimated the strength of the observed postfrontal northwest winds, which were modified (strengthened) by the storm-scale interaction (i.e., the supercell’s downdraft) (Figs. 22a–d). Unfortunately, no COSMO-2 1330 UTC model plots were obtainable as it would have been interesting to see if the model could have caught the observed 5 m s−1 (10 kt) southeasterly flow into the storm as observed at the Le Bouveret SwissMetNet weather station.
The 10-m wind (m s−1) field over the Lake Geneva region from (a) COSMO-2 analysis at 1300 UTC, (b) COSMO-2 analysis at 1400 UTC, (c) SwissMetNet surface stations at 1300 UTC, and (d) SwissMetNet surface stations at 1400 UTC.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
However, excluding the two southeasterly wind observations measured at Le Bouveret up to 20 min prior to the supercell’s arrival, no other southerly or southeasterly gradient winds were detected during the hour preceding the supercell’s arrival at any of the SwissMetNet stations at various elevations in proximity to the east end of the lake nor visible on the COSMO-2 wind analyses below 2000 m. The presence of a 2000–3000-m Alpine range located to the immediate west and southwest of Lake Geneva ultimately prevents much of any gradient wind flow from ever reaching the east end of the lake with a southwesterly flow aloft (Fig. 23). Given the fact that the low-level synoptic flow was oriented southwest over western Switzerland on the afternoon of 18 July 2005 and that no southeasterly gradient wind was channeled through the Rhône Valley, it is very improbable that the observed southeasterly winds 5–20 min before the supercell’s arrival in Le Bouveret were a manifestation of the background gradient wind field. Therefore, the only feasible wind regime at the surface over the east end of the lake ahead of the supercell was east-to-southeasterly oriented inflow winds (i.e., easterly from Montreux and southeasterly from the Rhône Valley). COSMO-2 reanalysis wind plots between the surface and 3000 m available at 1400 UTC (20 min after the tornado) indeed show the absence of any type of significant south-to-southeasterly gradient wind values below 2000 m in this region, owing to the topographical effects (Fig. 24). This case apparently highlights the important role that inflow winds can play in temporarily increasing the low-level directional wind shear and helicity in heterogeneous terrain ahead of organized convection and the probable facilitating effect this had in inducing tornadogenesis on the east end of Lake Geneva. While the exact mechanism by which tornadogenesis occurred is beyond the simulating capabilities of COSMO-2, based on the conceptual model of this event constructed after surface observations, video footage, damage surveys, and available COSMO-2 wind analyses, it strongly appears that inflow winds played a nonnegligible role in feeding and strengthening the streamwise component of the baroclinically generated horizontal vorticity along the forward-flank and rear-flank gust fronts.
Google Earth depiction of the orography in proximity to the east end of Lake Geneva. Especially noteworthy is the imposing mountain barrier to the west and southwest blocking much of the southwesterly gradient wind flow up to 1500–2000-m elevation.
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
COSMO-2 horizontal wind (m s−1) analyses valid at 1400 UTC 18 Jul 2005 at the heights of (a) 800, (b) 1000, (c) 1500, (d) 2000, and (e) 3000 m. Black circle denotes east end of Lake Geneva where the second tornado and microburst occurred about 20 min prior (1340 UTC).
Citation: Weather and Forecasting 28, 6; 10.1175/WAF-D-13-00022.1
4. Conclusions
The supercell that struck the Lake Geneva region on 18 July 2005 was formidable in many respects. During its 3-h lifespan, it was responsible for golf-ball-sized hail (5-cm diameter), a hail swath of nearly 1200 km2 that damaged approximately 15 000 vehicles, and the total destruction of several vineyards with insured claims topping 70 million Euros (~$93 million). It produced two confirmed tornadoes of EF1 and EF2 strength as well as a microburst with a recorded wind gust of 40 m s−1 (161 km h−1). Miraculously, no one was killed but eight injuries were reported and material damages topped $100 million (based on November 2010 currency conversion).
The prestorm environment on 18 July 2005 was characterized on the synoptic scale by a Spanish plume configuration with an embedded double-jet structure. Both dynamic and thermodynamic conditions on the synoptic and mesoscales were typical of patterns associated with severe convection, including supercells. Various observational platforms such as satellite–radar, surface observations, and cell-based attributes derived using developed algorithms all showed distinctive signatures typically associated with supercells, such as the enhanced V, cell splitting, WER, BWER, the V notch, hook echoes, storm longevity, deviant motion, storm splitting, and radar base velocity shear signatures. Doppler radar base velocity images allowed the calculation of the rotational velocity of the shear circulation associated with the storm through a height of 2–4 km and a time frame of about 60 min, helping establish the presence of a prolonged weak mesocyclone as well as the presence of a short-lived mesocyclone of moderate intensity. Moreover, the occurrence of these pertinent features in space and time corresponds well with their expected locations based on results from previous studies found in the literature. Namely, maximum IR MSG brightness temperatures seemed to correspond to times of maximum 45-dBZ echo top as well as to the presence of a hook echo, BWER, and maximum CG lightning frequency. Additionally, a distinct drop in CG lightning frequency, 45-dBZ echo top, and IR brightness temperatures could be observed just prior to the second tornado touchdown in Le Bouveret and consistent with a number of previous studies.
Concerning tornado development, modifying the 1200 UTC Payerne proximity sounding winds at low levels using observed valley-channeled winds 5–20 min prior to the supercell’s arrival in Le Bouveret, resulted in increased directional shear and larger 0–1-km storm-relative helicity values. Based on observational data and a reanalysis simulation using the COSMO-2 model, it appears that the low-level flow responsible for augmenting the rotational potential of the local environment was based on inflow winds and not background gradient winds. The important topographical features in proximity to the east end of the lake prevented the gradient winds from reaching this region and effectively allowed the inflow wind regime to become the dominant flow ahead of the storm at low levels. It appears that these inflow winds contributed to the helicity-rich environment and most likely aided tornadogenesis to some extent in Le Bouveret, along with the vertical tilting of the baroclinically induced streamwise horizontal vorticity associated with both the very strong forward-flank and rear-flank gust fronts. Similarly to Braun and Monteverdi (1991), this event clearly seems to support the hypothesis that low-level wind channeling in and around mountain features and valleys can enhance tornadogenesis potential in supercells evolving in heterogeneous terrain. The channeled wind involved can seemingly be either of a gradient nature and/or be inflow induced.
Acknowledgments
The author would like to thank the MeteoSwiss Radar and Satellite Group (RASA) in Locarno–Monti for its help in obtaining critical radar and satellite image information, most notably Hans-Peter Roesli, Alessandro Hering, and Urs Germann. Warm thanks also go out to the MeteoSwiss model group—namely, my fellow colleague Daniel Leuenberger, who provided me with critical COSMO model data for this case. I gratefully acknowledge Yannick Barton, for providing the tornadic waterspout photo in Fig. 17a, and Yolande Zimmermann from the canton Vaud administration, for transmitting the aerial photo of the microburst damage in Fig. 17c taken by its hydrology/wildlife service. Other contributors include Morgane Jeanneret of the University of Fribourg, who provided the tornado climatology information in Fig. 2, and Christophe Salamin of MeteoSwiss, for providing me with severe thunderstorm warning data. Thanks are also extended to all of the individuals not specifically mentioned in this manuscript for their ideas and comments; to the interviewees in the field during the damage surveys; and finally, to the anonymous reviewers for their enlightening comments that further honed the quality of this article. And last but not least, I wanted to gratefully thank my employer MeteoSwiss for funding the publication of this article.
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A Swiss severe thunderstorm is defined as a thunderstorm producing wind gusts > 20 m s−1 (75 km h−1), hail > 2 cm in diameter, and rainfall rates > 25 mm h−1.
