Tornadoes [pdf]
A tornado is a violently rotating column of air that is in contact with
both a cumulonimbus cloud base and the surface of the earth. This is the first
line of the definition provided by Wikipedia.
I came into contact with tornadoes - or better tornado research - in
2002 when Nikolai Dotzek wrote me an email starting a discussion on tornado
intensity distributions. He gathered quite some national data sets on tornado
occurrences and partly also on intensity distributions. We started
investigating the various “national” intensity distributions and searched for a
common description and characteristic regional differences. We saw that all
comprehensive data sets could be described by Weibull distributions. This was
our first result. It allows characterizing local tornado intensity
distributions by two Weibull parameters (shape and dispersion). As a
consequence we expected that different regions with similar tornado intensity
distributions could be found by drawing each data set as a dot in a
shape-parameter vs. dispersion-parameter plot. We hoped to see cluster in that
plot and feared to see no structures but we found that all dots were located on
a curved line. We could not mathematically describe nor explain the line but
published the finding in order to attract others to deal with this interesting
effect (Dotzek,
Grieser, Brooks, 2003).
Bernold Feuerstein contacted us and we explained our findings to him on
a Friday afternoon. Late at the same Friday night he replied our email and
attached the mathematical description and the reason of the curve we found in
the first paper. An ad-hoc invariant was found that is common to each observed
tornado-intensity distribution regardless of the period or region of the data
source. And so we published a second paper (Feuerstein,
Dotzek, Grieser, 2005).
We now had a mathematical description of an observed phenomenon but
didn’t understand it from a physical point of view. In September 2004 I gave a
talk on that subject on the German-Austrian-Swiss Meeting on Meteorology (DACH
MT). Andreas Will happened to be in the audience and pointed us to Michael
Kurgansky, a Russian colleague, working in Chile. At the same time also Peter
Névir from Freie Universität Berlin came up with some comments on the intensity
distribution of tornadoes based on energetic assumptions. Michael Kurgansky
offered a concept for the evidence of a certain tornado intensity distribution,
which was in line with our pure statistical description of the data. Putting
all this together led to the third paper (Dotzek, Kurgansky,
Grieser, Feuerstein, Névir, 2005).
Independent of our work Peter Bissolli from the German Meteorological
Service (DWD) and a student (M. Welsch) from University Mainz started to
investigate the statistical relation between the occurrences of tornadoes in
Germany and specific weather types. They compared the number of observed
tornadoes with the number of thunderstorm days and published that “The fraction
between the number of tornadoes and thunderstorm days has a high variability
within Germany and is highest in some north-western areas of Germany (values up
to 0.63 tornadoes per thunderstorm day).”, see http://www.cosis.net/abstracts/EGU05/07957/EGU05-J-07957.pdf
.They did not realize that they divided the number of observed tornadoes within
a 24-year period by the average annual number of thunderstorm days and that
this was the reason for their obviously wrong but published results. Nikolai
Dotzek and myself helped to improve their calculations. The resulting paper is
published in Global and Planetary Change (Bissolli,
Grieser, Dotzek, Welsch, 2007).
If I was asked to give a short statement on our major findings with
respect to tornado distributions I’d say that
a)
it is wrong to distinguish between real US American tornadoes on the one
hand and European mini tornadoes on the other hand and
b)
the number of tornadoes needs not to have increased just because the
number of observed tornadoes has increased considerably.
More details on European tornadoes can be found at: http://www.torDACH.org