Atmos. Res. 56, 203-211. (Special Issue - Eurotornado 2000)


Tornado climatology of Austria

A.M. Holzer

Weather Forecasting Department, Austrian Broadcasting Corporation (ORF) Vienna, HI1 Wetterredaktion, Argentinierstrasse 30a, A-1041 Wien, Austria

Fax: ++43 1 50101 18530;

 

Abstract

After several decades of stood still a revised tornado climatology for Austria is presented. Tornadoes seldom form in the alpine areas, but near the eastern flanks of the Alps favourable conditions for tornado genesis are found. Whereas in the alpine regions less than 0.3 tornadoes per 10000 km˛ and year touch down (averaged for provinces or major parts of a province), we can count 0.9 in the greater Graz area, 1.0 in the greater Linz area and 1.2 tornadoes per 10000 km˛ and year in the greater Vienna area, suggesting the existence of so called tornado alleys. The overall average for Austria is 0,3 tornadoes per 10000 km˛ and year.

The database consists of 89 tornadoes, one landspout and six waterspouts, with a total of 96 events. The seasonal peak is in July with a maximum probability of tornadoes in the late afternoon and early evening hours. Every fifth tornado occurs in the hour after 5 p.m. The maximum intensity determined for a tornado in Austria was T7 on the TORRO-Scale (F3 on the Fujita-Scale), the most common intensity is T2 on the TORRO-Scale (F1 on the Fujita-Scale).

 

  1. Introduction
  2. Alfred Wegener already reported in his classic book "Wind- und Wasserhosen in Europa" several tornadoes in Austria and its former crown countries (Wegener, 1917). Two devastating tornado outbreaks not only challenged scientists in the first third of the 20th century, but also were some of the worst ever recorded on the territory of modern Austria. Databases (Pühringer, 1973) reach back to 1910. In 1916 a tornado caused enormous damage in Wiener Neustadt, leaving 32 people dead and 328 wounded (Dörr, 1917). After some work on this case and its archive material it was possible to determine this tornado as an F3 event on the Fujita scale (Fujita, 1981) or T7 tornado on the TORRO scale (Meaden, 1976).

    In 1927 two tornadoes went through the eastern parts of Styria, leaving back a 35 km track of devastation. Thanks to the fine work of Wegener (1928) both events were easily classified as F3 or T6 tornadoes. This historical outbreak has a counterpart in southern Styria in 1998, when two tornadoes, classified as F2 and F3 (T4-5 and T5) caused damage of about 20 million Euro (Holzer, 1998). These strong tornadoes are the most prominent ones and also outline the areas most often hit by severe tornadic storms in Austria.

    This paper overviews not only the prominent examples for tornado formation in Austria, it describes the state of knowledge of intensity and distribution in space and time. Local topographic conditions are held responsible for the formation of "tornado alleys" (Dotzek et al., 1998) in some parts of the country. The long term goal is to provide a reliable data set for Austria in order to support scientific and economic interests and also the exchange of information on the Central European scale within the TorDACH group (a network for tornado research in the countries of Germany (D), Austria (A) and Switzerland (CH)) and other organisations. Then possible consequences of the findings are to be discussed.

    The paper is organised as follows. In Sec. 2 the data acquisition methods are presented, Sec. 3 outlines tornado intensities and their distribution in space and time. Sec. 4 and 5 present discussion and conclusions.

     

     

     

     

     

  3. Acquisition of data
  4. The data set is very inhomogeneous (Figure 1). Although Pühringer (1973) tried to establish a reliable data set, we have only fragmental records for the 20th century. Whereas we know of 71 tornadoes in the years from 1946 to 1971, that is an average of 27 tornadoes per decade, we only know of 9 events before and of only 12 tornadoes after that lapse of time. For Pühringer reliable records as weather service reports were available from 1946. Before that year very few records are found. The first tornado report for the territory of modern Austria dates back to 1910 (Wegener, 1916). A second data gap is suspect after the work of Pühringer, from 1972 to 1993, when not more than 5 tornadoes appear in the data base, only allowing an average of 2.3 events per decade. Since 1998 new efforts have been made within TorDACH to get information on as much as possible tornado events. 7 tornadoes were found in the period from 1994 to 1999. No event in the years of 1996, 1997 and 1999, but as much as 3 tornadoes and one funnel cloud sighting in 1998. Some additional historical tornado records were discovered during this work. Taking in account all these difficulties in the data set, only the period from 1946 to 1971 (Pühringer, 1973) is seen as reliable and representative enough for some parts of the statistics. The whole TorDACH-database for Austria consists of 89 tornadoes, one landspout and six waterspouts, with a total of 96 events, and one additional funnel cloud event.

    Figure 1

    Number of tornadoes per single year

     

  5. Intensities and distribution in space and time
  6. Taking the whole database in account, the most frequent intensity observed is T2 with 8 counters or 28 per cent, followed by T3 and T4 tornadoes, reaching 17 per cent each (Figure 2). 10 per cent of all events are T0 but also T5, and 7 per cent each were T1 and T6 tornadoes. One event (equal 3 per cent) was rated as a T7, the Wiener Neustadt event of 1916. Wind speeds of 82 to 93 m s-1 (or about 315 km h-1) are therefore the maximum values proved for Austria. The more summarising F-scale values are given in Table 1. The Austrian distribution is especially for the weaker values far away from the mean distribution figures proposed by Brooks et al. (2000). This fact will be discussed later.

    F-scale values

    Number of events

    Per cent of total events

    F0

    5

    17

    F1

    13

    45

    F2

    7

    24

    F3

    4

    14

    F4

    0

    0

    F5

    0

    0

    Table 1

    F-scale values of rated tornadoes in Austria

     

     

    Figure 2

    T-scale values of rated tornadoes in Austria

     

    Figure 3

    Tornadoes by hour in Austria

     

    The distribution by month (Figure 4) shows the maximum in July with 28 out of 96 events, followed by August with 20, June with 15 and May with 13 tornadic storms. Only in January and October no tornado events have occurred so far. The maximum probability for tornado formation is found in the late afternoon and early evening hours. Every fifth tornado was counted in the hour after 5 p.m. (Figure 3).

    Figure 4

    Tornadoes by month in Austria

     

    Taking the most representative data set of Pühringer (1973) as basis, we can find an average of 2.7 (with a standard deviation of 1.8) tornadoes per year in Austria. Record years were 1952, 1953 and 1961 with 6 tornadoes each, followed by 1954 and 1971 with 5 tornadoes each year. Whereas in the alpine regions less than 0.3 tornadoes per 10000 km˛ and year are counted (averaged for provinces or major parts of a province), we can count 0.9 in the greater Graz area, 1.0 in the greater Linz area and 1.2 tornadoes per 10000 km˛ and year in the greater Vienna area (Figure 5). The overall average for Austria (area: 83855 km˛) is 0,3 tornadoes per 10000 km˛ and year. That number seems to be slightly higher than in the neighbouring country Germany (Dotzek et al., 1999; Dotzek, 2000), but only if we behold the data set of Pühringer (1973) representative.

    Some districts in the Austrian "tornado alleys" even have an average of 3 tornadoes per 10000 km˛ and year, very similar to the mean values calculated for Florida (NWS, 1993), but the areas in question are quite small then and the statistical errors may though be large. One reason for local accumulations in Austria is the topography. The Alps act as a barrier to low level cold air masses from the Northwest, while from the Southeast a low level inflow of moist and warm air from the Mediterranean can sustain for some time. These favourable conditions also reflect in extraordinary high probabilities of hailstorms, especially in eastern Styria (Müller, 1974).

     

    Figure 5

    "Tornado alleys" in Austria (regions ³ 0.3 tornadoes a-1 10-4 km-2)

     

    Due to high values of low level wind shear there seem to be good conditions for the formation of supercells where mountainous and flat terrain get in contact (Dotzek et al., 1998). Similar conditions are found in and near the Danube river valley. Although it seems that different synoptic settings are responsible for the accumulation of tornado events in the southeastern and northern parts of Austria. Whereas tornadoes in Styria seem to often occur in the environment of quasi stationary or welling cold fronts lying over the eastern Alps, tornadoes in the River Danube Valley preferable seem to occur in the environment of prefrontal squall lines (van Delden, 2000) or mesoscale convective systems (MCS) approaching from the western sector, similar to the preferred environment for severe hailstorms in this area (Kurz, 2000). These are only preliminary results and based on few events analysed during the last several years within TorDACH. Therefore the sample at this time is relatively small. Investigation on the whole historical data set should be made. The statistical accumulations also may partly be produced by higher observing and reporting probabilities in densely populated areas.

     

     

  7. Discussion
  8. Future goals are the harmonisation of the European tornado data sets, especially data exchanges with countries neighbouring Austrian tornado regions, like Germany, Slovenia, the Czech Republic (Munzar, 2000), Slovakia, Italy (Bechini et al., 2000), Switzerland (Schmid et al., 2000) or Hungary (Geresdi, 2000). This would be necessary to establish a reliable central European map of tornado densities, not limited by dozens of national borders. First steps were already made by Dotzek et al. (2000) who proposed a "tornado code" for European tornado records.

    For the Austrian data set it should be cleared up within the next years, whether the high tornado probabilities in some regions, often densely populated areas of eastern Austria, are real or only statistical errors because of the still narrow data set. The number of tornadoes in the time before and after the data set of Pühringer (1973) seems to be much to small whereas the number of tornado events given in the data set of Pühringer seems to overestimate in some periods, maybe produced by misinterpretation of microburst damages. But if the high numbers would be reaffirmed, effort should be made in forecasting Austrian tornadoes and the installation of a warning system should be considered, at least for the most affected towns and urban areas. It is to say that if the named values do not overestimate, there is a serious threat to towns like Vienna, Graz or Wiener Neustadt. Vienna had already been affected four times since the end of World War II, fortunately by relatively weak storms. Both towns, Graz and Wiener Neustadt were hit five times directly or in the immediate vicinity by a tornado in the last 90 years.

    Compared with the work of Brooks (2000) and its statistical extrapolation the number of cases we know from may be even underestimating, especially the weak cases. The Austrian distribution appears to be very similar to the one of the US in the 1920s, when most of the weaker tornadoes were missed. Taking in account these findings together with the number of F2 cases seen as the most reliable part of the data set, we would end up with at least three times as much tornadoes as we know from at the moment (Tab. 1). This would for some small parts of Austria result in tornado densities comparable with those of the US plains. What it makes difficult to compare such statistical relationships is the very poor understanding and knowledge of tornado formation in Austria. Only a very few cases were studied by means like doppler radar yet. We do not exactly know at this time, whether most Austrian tornadoes do form in a supercell environment or not. Only official weather office counting and research could help to establish a more realistic data set with deeper understanding of tornado formation over highly complex terrain like the Alps and its surroundings.

     

  9. Conclusions

Historical works on tornadoes and recent events in Austria were collected and studied in order to set up a reliable data set and to allow an estimate of the real number and distribution of tornadoes. The main results are:

(1) Historical records of fatal cases already suggest reasonable tornado activity.

(2) Based on the most reliable data, 0.3 tornadoes per 10000 km˛ and year occur in Austria.

(3) Small regions have much higher tornado densities, due to topographical effects.

(4) On average 2.7 tornadoes are counted per year in Austria.

(5) Statistical considerations propose still higher numbers of weak cases.

(6) Highest tornado frequencies are in July.

(7) Daytime peak is in the late afternoon hours together with the thunderstorm maximum.

In future transnational efforts should be made to improve the knowledge on European tornadoes and the conditions favourable for their formation over complex topography in order to understand the often sharply defined regions with high tornado activity.

 

Acknowledgements

The work reported here was done under the roof of the TorDACH organisation (c/o Dr. Nikolai Dotzek, Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, D-82234 Wessling, http://www.op.dlr.de/~pa4p/TorDACH.html), a network for tornado research in the countries of Germany (D), Austria (A) and Switzerland (CH). The latest tornado reports for Austria can be found under http://www.tornados.at.

 

 

 

References

Bechini, R., 2000. The June 4th 1999 severe weather episode in San Quirino, Italy: a tornado event? Abstracts, European Tornadoes and Severe Storms Conference, 1 – 4 February 2000, Toulouse, France.

Brooks, H., Doswell, Ch.A.III, 2000. Some aspects of the international climatology of tornadoes by damage classification. Atmos. Res., this volume.

Dörr J. N., 1917. Die Windhose von Wiener Neustadt am 10. Juli 1916. Meteorologische Zeitschrift 43, 1, 1-14.

Dotzek, N., 2000. Tornadoes in Germany. Atmos. Res., this volume.

Dotzek, N., Hannesen R., Beheng K. D, Peterson R. E., 1998. Tornadoes in Germany, Austria and Switzerland. Proc. 19th Conference on Severe Local Storms, Minneapolis, 93-96.

Dotzek, N., Hannesen R, Beheng K. D., 1999. Tornados in Deutschland. Proc. Sympos. "Naturkatastrophen in Mittelgebirgsregionen". Univ. Karlsruhe, 12-13.

Dotzek, N., Berz G., Rauch E., Peterson R. E, 2000. Die Bedeutung von Johannes P. Letzmanns "Richtlinien zur Erforschung von Tromben, Tornados, Wasserhosen und Kleintromben" für die heutige Tornadoforschung. To appear in Meteor. Z., http://www.op.dlr.de/~pa4p/papers.html.

Fujita, T. T., 1981. Tornadoes and downbursts in the context of generalized planetary scales. J. Atmos. Sci. 38, 1511-1534.

Geresdi, I., Horvath, A., 2000. Severe convective storms and associated phenomena in Hungary. Abstracts, European Tornadoes and Severe Storms Conference, 1 – 4 February 2000, Toulouse, France.

Holzer, A. M., 1998. Untersuchung und Dokumentation von zwei Tornados in der südlichen Steiermark im Sommer 1998. Wetter und Leben, 50. Jg., 2, 103-114.

Kurz, M., 2000. The Munich hailstorm from 12 July 1984 – a spectacular convective event in Mid Europe. Abstracts, European Tornadoes and Severe Storms Conference, 1 – 4 February 2000, Toulouse, France.

Meaden, G. T., 1976. Tornadoes in Britain: their intensities and distribution in space and time. J. Meteorol. 1, 242-251.

Munzar, J., 2000. Historical tornadoes on the territory of Czech Republic before 1990. Abstracts, European Tornadoes and Severe Storms Conference, 1 – 4 February 2000, Toulouse, France.

Müller, W., 1974. Hagelschadenhäufigkeit in der Oststeiermark im Zeitraum 1967-1972. Wetter und Leben, 220-233.

NWS, 1993. US National Weather Service - NSWC, NOAA, Tornado Report, Tornado Statistics 1950 – 1991.

Pühringer, A., 1973. Windhosen in Österreich, Wetter und Leben 25, 15-22.

Schmid, W., Schiesser, H. H. and Walker, A., 2000. The Alps – a generator of tornadoes? Abstracts, European Tornadoes and Severe Storms Conference, 1 – 4 February 2000, Toulouse, France.

Van Delden, A.J., 2000. The synoptic setting of a thundery low and associated prefrontal squall line in Western Europe. Abstracts, European Tornadoes and Severe Storms Conference, 1 – 4 February 2000, Toulouse, France.

Wegener, A., 1916. Windhose im Mürztal, Das Wetter 33, 91.

Wegener, A., 1917. Wind- und Wasserhosen in Europa, Vieweg, Braunschweig.

Wegener, A., 1928. Die Windhose in der Oststeiermark vom 23. September 1927, Meteorologische Zeitschrift 45, 2, 41-49.