Nikolai Dotzek

Tornadoes in Germany

Abstract A climatology of tornadoes in Germany using the TorDACH database up to the year 2000 is presented. The total number of tornadoes is 517 and the number of reports has increased substantially after 1870. Tornado activity is at maximum in July, and at minimum from November to February. The daily distribution peaks in the afternoon and early evening with a weak secondary morning maximum due to waterspout outbreaks. In general, both daily and seasonal trends follow the variation of thunderstorm activity. The highest F -scale rating so far is F4 and in the range from F1 to F4 a nearly lin–log distribution of tornado intensity is found. The low number of reported F0 tornadoes indicates that perhaps only every third tornado in Germany is reported. Also, the geographical distribution shows that many cases from east Germany are missing. However, three different regions of typical tornado activity can be inferred: the northern coastal region, the hilly terrain of mid and southern Germany, and a zone influenced by Mediterranean air in summer and an orogenic low-level wind shear in the south–west. For the latter, an example of a tornado alley is given. From a conservative estimate of tornadic activity in Germany, a number of four to seven tornadoes per year and a recurrence density of about 0.1 to 0.2 a −1 10 −4 km −2 is deduced, in accordance with earlier work. However, both extrapolation based on statistical arguments and the more detailed records of recent years suggest values a factor of three to four higher.

An updated estimate of tornado occurrence in Europe

Abstract Results from a survey on average tornadic activity in Europe conducted among the participants of the European Conference on Severe Storms (ECSS) 2002 are presented. Compared to Alfred Wegeners estimate of “at least 100 tornadoes per year in Europe” the present survey shows a total of 329±12 tornadoes over land and water per year based on observations, and more than twice as many cases (697±36) for an estimate of the expected true climatological number, accounting for present underreporting in many European countries. Traditionally, European tornado numbers include waterspouts. For comparison to the current number of 1170 observed tornadoes over land per year in the USA, the European numbers are 169±9 per year based on observations, and 304±25 based on estimates. As European severe weather research is rapidly developing, one can expect less underreporting in the future, leading to an augmented database for upcoming surveys like the present one.

Statistical modeling of tornado intensity distributions

We address the issue to determine an appropriate general functional shape of observed tornado intensity distributions. Recently, it was suggested that in the limit of long and large tornado records, exponential distributions over all positive Fujita or TORRO scale classes would result. Yet, our analysis shows that even for large databases observations contradict the validity of exponential distributions for weak (F0) and violent (F5) tornadoes. We show that observed tornado intensities can be much better described by Weibull distributions, for which an exponential remains a special case. Weibull fits in either v or F scale reproduce the observations significantly better than exponentials. In addition, we suggest to apply the original definition of negative intensity scales down to F-2 and T-4 (corresponding to v=0ms � 1 ) at least for climatological analyses. Weibull distributions allow for an improved risk assessment of violent tornadoes up to F6, and better estimates of total tornado occurrence, degree of underreporting and existence of subcritical tornadic circulations below damaging intensity. Therefore, our results are relevant for climatologists and risk assessment managers alike. D 2003 Elsevier B.V. All rights reserved.

Assessing a tornado climatology from global tornado intensity distributions

Recent work demonstrated that the shape of tornado intensity distributions from various regions worldwide is well described by Weibull functions. This statistical modeling revealed a strong correlation between the fit parameters c for shape and b for scale regardless of the data source. In the present work it is shown that the quality of the Weibull fits is optimized if only tornado reports of F1 and higher intensity are used and that the c–b correlation does indeed reflect a universal feature of the observed tornado intensity distributions. For regions with likely supercell tornado dominance, this feature is the number ratio of F4 to F3 tornado reports R(F4/F3) 0.238. The c–b diagram for the Weibull shape and scale parameters is used as a climatological chart, which allows different types of tornado climatology to be distinguished, presumably arising from supercell versus nonsupercell tornadogenesis. Assuming temporal invariance of the climatology and using a detection efficiency function for tornado observations, a stationary climatological probability distribution from large tornado records (U.S. decadal data 195099) is extracted. This can be used for risk assessment, comparative studies on tornado intensity distributions worldwide, and estimates of the degree of underreporting for areas with poor databases. For the 1990s U.S. data, a likely tornado underreporting of the weak events (F0, F1) by a factor of 2 can be diagnosed, as well as asymptotic climatological c,b values of c 1.79 and b 2.13, to which a convergence in the 1950–99 U.S. decadal data is verified.

Climate Extremes and Society: The spatial distribution of severe convective storms and an analysis of their secular changes

Severe convective storms are responsible for billions of US dollars in damage each year around the world. They form an important part of the climate system by redistributing heat, moisture, and trace gases, as well as producing large quantities of precipitation. Reporting of severe convection varies from country to country, however, so that determining their distribution from the reports alone is difficult, at best. Evidence does exist that the intensity of some events, particularly tornadoes, follows similar distributions in different locations, making it possible to build statistical models of occurrence. Remotely-sensed observations provide some insight, but the relationship between the observable parameters and the actual events of interest limits the quality of the estimate. Another approach is to use observations of the larger-scale environments. As stated, the relationship between the observation and the event limits the estimate, but global coverage is possible. Time series of the favorable environments can also be developed from such data. In order to improve the estimates, the most pressing need is better observational data of events. Very few countries have formal systems for collection of severe thunderstorm reports. A new effort from a consortium of researchers in Europe to develop a continental-wide database offers the possibility of a significant improvement in data in that part of the world.

Lightning evolution related to radar-derived microphysics in the 21 July 1998 EULINOX supercell storm

Abstract Results of a combined analysis of data from a C-band polarimetric Doppler radar and a 3D VHF interferometric lightning mapping system, as obtained during the European Lightning Nitrogen Oxides project (EULINOX) field campaign, are presented. For 21 July 1998, the lightning data from a supercell thunderstorm weakly indicate a tendency for a bi-level vertical distribution of lightning VHF emissions around the −15°C and −30°C temperature levels. Also, in some parts of the clouds, evidence is found for the presence of a lower positive charge center near the freezing level. However, where strong vertical motions prevail, VHF emissions are not organized in horizontal layers but in oblique or vertical regions. Correlation of VHF signals with radar quantities shows that in the growing storm, peak VHF activity is low and related to reflectivity factors around 30 dBZ, while after the mature stage, the peak VHF activity is about three times larger. The highest density of VHF signals is now found near reflectivity factors of 45 dBZ. A polarimetric hydrometeor classification indicates that during storm development, most lightning activity occurs where graupel and, secondarily, snow and small dry hail are present. In the decaying phase of the supercell hailstorm, however, most lightning VHF emissions stem from the region with hail and heavy rain. Furthermore, while the VHF signal frequency per cubic kilometer in the graupel and rain regions remains nearly constant throughout the supercell life cycle, the signal frequency in the hail region rises during storm decay.

Instantaneous fluctuations of temperature and moisture in the upper troposphere and tropopause region. Part 1: Probability densities and their variability

Using nine years (1995--2003) of MOZAIC temperature and humidity data, we analyse the statistics of instantaneous fluctuations of temperature, relative and absolute humidity with various spatial resolutions. We determine the probability density functions (p.d.f.s), their low order moments up to kurtosis, and study how these quantities vary with spatial resolution and with the background mean relative humidity. Seasonal and geographical variations are considered. Bivariate distributions of joint fluctuations of temperature and relative humidity are presented as well. These investigations are thought to provide an observational basis for the validation of statistical cloud schemes for large--scale models. For instance, we can show that temperature fluctuations cannot be neglected in such a scheme and that the bivariate distributions of simultaneous fluctuations of temperature and humidity have to be taken into account.

Waterspouts over the North and Baltic Seas: Observations and climatology, prediction and reporting

On 25 August 2005, three waterspouts were observed close to the research platform FINO1, 45 km off the German North Sea coast and situated in a prospected large offshore wind park area. We analyse this case in relation to the synoptic setting of a waterspout day over the Baltic Sea and compare it to the waterspout climatology of the German Bight and the western and south-western parts of the Baltic Sea. The waterspout hazard to offshore wind parks is assessed as about 1 waterspout per year and per 10 000 km 2 . So, should current scenarios for future wind park development materialise, their large total area may experience a waterspout every other year. The prediction of such events is investigated in view of concepts recently proposed for Mediterranean and western North Sea waterspout forecasting. Reporting issues influencing the climatology encompass both a weekend low in reported events, as well as a bias toward ship routes and main SYNOP times in ship reports. The latter may be mitigated by more reports from yachtsmen.

Comments on “Proposed Conceptual Taxonomy for Proper Identification and Classification of Tornado Events”

Agee and Jones (Agee and Jones 2009, hereafter AJ09) have introduced a tornado classification scheme that they propose be adopted by the National Oceanic and Atmospheric Administration (NOAA) in order to improve the U.S. tornado database and aid climatological analyses and detection of climate change impacts on tornado occurrence. AJ09’s classification scheme identifies tornadoes as being associated with a supercell (type I), a quasilinear convective system (QLCS; type II), or neither a supercell nor a QLCS (type III). Fifteen tornado subclassifications (Ia–Ic, IIa–IIf, and IIIa–IIIf) are included as well. We appreciate AJ09’s attempt to refine U.S. tornado recording, but we are skeptical that their proposal will improve the U.S. tornado database. Although there are well-known limitations with the historical and even contemporary U.S. tornado database (e.g., Verbout et al. 2006; Doswell et al. 2009), our opinion is that there are major problems with AJ09’s proposed tornado classification scheme. The aspect of the proposed classification system with which we are most uneasy is the attempt to identify dynamical differences between tornado types, particularly the subclassifications. For example, AJ09’s scheme considers (see their Table 1) whether or not a vortex sheet is present, whether vorticity is tilted by a downdraft, the degree to which horizontal vorticity is augmented by baroclinity, and the amount of stretching of preexisting boundary layer vertical vorticity. Although it would be wonderful to be able to record the dynamical circumstances behind every tornado, there are grave limitations in our ability to ascertain the dynamics responsible for tornadogenesis using operational data only [e.g., Weather Surveillance Radar-1988 Doppler (WSR88D), aviation routine weather reports (METARs), and satellite]. One cannot even compute vorticity from such datasets, let alone evaluate its forcings. Assigning dynamical cause and effect is not always straightforward even when field experiment data are obtained, and such datasets are extremely rare. In 2009, for example, such a dataset was obtained for only one of the O(1000) tornadoes occurring on average in the United States each year. Classification problems (classification as types I, II, or III, let alone subclassification) would be posed by supercells that are embedded within QLCSs (AJ09 state on p. 616 that tornadoes developing in such situations would be type II, but there is no apparent dynamical basis for this choice) and supercells that produce landspouts (how will one assess whether preexisting vorticity was amplified by stretching alone, or whether a downdraft was responsible for the development of circulation at the surface?). Moreover, even tornadoes such as waterspouts/landspouts are often associated with mesocyclone radar signatures once the rotation that is amplified

Instantaneous fluctuations of temperature and moisture in the upper troposphere and tropopause region. Part 2: Structure functions and intermittency.

Probability density functions (p.d.f.s) in MOZAIC instantaneous fluctuations of temperature, relative and absolute humidity with various spatial resolutions had shown signs of intermittency. This appears to have originated either from turbulence or certain physical processes (e.g. gravity waves, deep convection) that bring air masses of different origin close together or from air mass boundaries within the averaging volumes, at which the thermodynamical properties show a discontinuity and are nearly independent on both sides of the boundary. In the present paper, we derive a stochastic model for the observed p.d.f.s in the tropopause region and compare the p.d.f.s of the fluctuations in the atmosphere to small-scale Rayleigh-Benard convection p.d.f.s with similar shapes. In both cases, intermittency originates from rare events of large amplitude superposed unto a Gaussian background. The peculiar ogival shape of some of the distributions results from the sampling procedure, and the ogival character becomes more pronounced with increasing spatial resolution. Analysis of structure functions up to order three reveals that theMOZAIC data represent an intermediate state between large-scale two-dimensional and small-scale three-dimensional inertial range turbulence. This can be visualised by the scale interaction of the typical length of MOZAIC flight legs and the sampling rate (and hence also spacing). While flight legs belong to and exceed the synoptic scale, the events causing intermittency have a scale roughly corresponding to the sampling scale which is clearly sub-synoptic. Our results are directly relevant for the development of stochastic cloud-microphysical schemes in general circulation and weather forecast models.