University of Utrecht European Severe Storms Laboratory Master Thesis Tornado Outbreaks in Europe Author: Supervisors: Lars Tijssen Pieter Groenemeijer Aarnout van Delden August 18, 2015 Contents 1 Introduction 3 1.1 History of Tornado Research . 3 1.2 Research Goals . 4 1.3 Overview . 5 2 Data 6 2.1 European Severe Weather Database and tornado data . 6 2.2 Reanalysis Data . 7 2.2.1 ERA-Interim . 7 2.2.2 CFSR . 7 2.3 Convective Parameters . 8 2.3.1 Mixed Layer CAPE . 8 2.3.2 Mixed Layer Lifting Condensation Level . 9 2.3.3 Deep Layer Shear and Low Level Shear . 9 2.3.4 Storm Relative Helicity . 10 3 Methods 11 3.1 Coupling Tornadoes and Reanalysis Data . 11 3.2 Tornado Outbreak Definition . 13 4 Results 17 4.1 Tornado Outbreaks . 17 4.1.1 Outbreak 11 and 12: 24 and 25 June 1967 . 19 4.1.2 Outbreak 17: 23 November 1981 . 21 4.1.3 Outbreak 19: 9 June 1984 . 23 4.1.4 Outbreak 27: 18 January 2007 . 24 4.1.5 Outbreak 31: 3 August 2008 . 25 4.2 Tornado Outbreaks and Reanalysis . 27 5 Conclusion 34 6 Discussion 36 1 Abstract Tornadoes in Europe have been documented and recorded in the Eu- ropean Severe Weather Database (ESWD) which is maintained by the European Severe Storms Laboratory (ESSL). The goal of this study is to find a useful definition of a tornado outbreak and to study the environ- mental differences between tornado outbreaks and individual tornadoes. Tornado groups are created by an algorithm which dictates that if a tor- nado is located within a distance of 500 km spatially and 6 hours tempo- rally of another tornado, then these tornadoes belong to the same group. From these groups the groups which have a cumulative Fujita scale num- ber of 7 or higher are given the tornado outbreak status. This way the tornado groups which have a high number of tornadoes or several strong tornadoes are retained. Two reanalysis datasets are used in this study. The ERA-Interim reanal- ysis dataset provided by the ECMWF and the Climate Forecast System Reanalysis (CFSR) dataset provided by NCAR. Both datasets cover the 1979 - 2014 period. Several convective parameters are computed from these datasets which include mixed layer CAPE (MLCAPE), mixed layer lifting condensation level (MLLCL), deep layer shear (DLS), low level shear (LLS) and both 0-3 km and 0-1 km storm relative helicity (SRH). By taking a local maximum (minimum for LCL) of each convective param- eter in a local spatial and temporal domain, each tornado in the ESWD in the 1979 - 2014 period is assigned a value for each of the convective parameters. It is found that tornadoes inside outbreaks are associated with higher values of DLS, LLS and to a lesser extent SRH and MLCAPE compared to tornadoes outside outbreaks. In tornado environments sig- nificant differences exist between the reanalysis datasets. In the large majority of tornado occurrences the CFSR convective parameters have higher values than their ERA-Interim counterparts. 2 1 Introduction 1.1 History of Tornado Research Tornadoes have long since been reported in various places in the world. Stories about tornadoes go back to the beginning of time. The first written reports of tornado events start in the first millennium. Often these records come from stories and poems that are written by scribes or monks which is the only source of information that is available for these events. Knowledge of tornadoes around those times was practically non-existent and therefore tornadoes and encoun- ters with them were often attributed to gods, devils or other folklore. Tornadoes were most often documented when they occur near to towns/cities where people could observe them or left behind visible destruction. An example of such an event is shown in figure 1 where a tornado has destroyed the nave of the dom church in Utrecht, The Netherlands. The destruction of the church was so big that it was decided not to rebuild the church. Therefore only the tower remains to this day. Figure 1: A tornado caused the collapse of the nave of the dom church of Utrecht in 1674. The nave was never rebuild and only the tower remains to this day. Source: Het Utrechts Archief: Drawings by Herman Saftleven. Until the start of the twentieth century research on tornadoes had not gotten very far. In 1917 the famous geophysicist, meteorologist and polar researcher Alfred Wegener, who is best known for his revolutionary ideas on continental drift, wrote a book called Wind- und Wasserhosen in Europa (Wegener (1917)). 3 He was the first to discuss tornado occurrences on a European level in this book. He also noted that there should be at least 100 yearly occurrences of tornadoes throughout Europe. This has been proven to be correct (Groenemeijer and K¨uhne(2014)). Modern day research into tornadoes started in the early fifties in the USA but re- ally took flight when Ted Fujita of the University of Chicago started to research tornadoes in the sixties and seventies (Bluestein (1999)). Fujita studied tornado damage intensively during his career and introduced the Fujita-scale, where tor- nadoes are categorized into five separate levels according to the from damage estimated wind speed (Fujita et al. (1971)). He also concluded after researching tornado damage that tornadoes can consist of sub vortices and identified non- tornadic meteorological phenomena such as downbursts and microbursts. One of the inventions that really helped with tornado research was the invention of weather radar, on which the structure of thunderstorms could be made visible. Researchers in the early fifties found that some storms exhibited a small hole where the radar reflection was very low. This resulted in a storm displaying a "hook" on the radar image. This display phenomenon was coined a "hook echo" due to its hook like shape. Around the same time it was noticed that these hook echoes and tornadoes were correlated. Research in Europe started quite a bit later than in the USA. In 1997 a first attempt was made to build a climatology of tornadoes and other severe weather phenomena of the countries Germany, Austria and Switzerland by the Tor- DACH network. This database of severe weather was used to make estimates of tornado occurrence in these countries (Dotzek (2001)). In 2002 the TorDACH network was succeeded by the founding of the European Severe Storms Labo- ratory (ESSL) network and the European Severe Weather Database (ESWD) which incorporated the previous TorDACH data and other datasets in an effort to create a pan-European severe weather database. Both the TorDACH net- work and the ESSL were spearheaded by Nikolai Dotzek who both put in a lot of effort in creating these networks and doing research using these to build a climatology of tornadoes in Europe (Dotzek (2003), Bissolli et al. (2007)). Af- ter his early death in 2010 the ESSL appointed Pieter Groenemeijer as the new director. Under his supervision the ESSL grew to what it is today: A non-profit research organization with multiple researchers who work part- or full-time re- searching tornadoes and other severe weather phenomena in Europe. The latest findings on tornado climatology in Europe from the ESWD are presented in Groenemeijer and K¨uhne(2014). 1.2 Research Goals The main focus of this study will be tornadoes in the ESWD and reanalysis datasets. This study will try to answer the following questions: • Is it possible to create a workable definition of a tornado outbreak in Europe? 4 • How can the environments of European tornado outbreaks be character- ized? • Do European tornado outbreak environments differ from European single tornado environments? • Is there a difference in European tornado outbreak environments between different reanalysis datasets? • What are the differences in European tornado outbreak environments be- tween summertime and wintertime? 1.3 Overview In the next section some detailed explanation will be given pertaining the data used in this study. In this section both the tornado data from the ESWD and the fields from the reanalysis datasets will be discussed as well as the convective parameters that are calculated from these fields. In the Methods section both the coupling between tornado reports and definition of a tornado outbreak will be discussed. In the Results section both the found tornado outbreaks and the results of the convective parameter coupling will be discussed. Finally a Conclusion and Discussion are written where respectively the results from the study will be concluded and limitations of the study will be discussed. 5 2 Data 2.1 European Severe Weather Database and tornado data The European Severe Weather Database is a database containing severe weather reports from all over Europe. Users are encouraged to report as many details as possible about weather phenomena they encounter. Phenomena that are re- ported include wind damage (straight line wind damage or tornado damage), hail over 2 centimeters, high rain rates or flash flood damage and tornado sight- ings. These reports are accompanied by a time stamp, longitude/latitude details and often also include images or videos of the event. Extra information per- taining the details of the event can also be added. Reports get a quality check ranging from QC0 (where an event is unverified) to QC2 (where an event is fully verified by an expert in the field). At this moment there are more than 80000 severe weather reports (large hail, severe wind, heavy rain and tornado reports. Excluding QC0) of which more than 6500 are tornado reports. These reports also include land- and waterspouts. Most tornadoes have a strength rat- ing according to the aforementioned F (Fujita) scale which ranges from F0 (very weak) to F5 (very strong).