A statistical analysis of the lightning activity for Southern Germany is presented. The data were provided by the Bayernwerke AG, which has been operating a lightning mapping system (LPATS) since 1992. This network covers the south of Germany (7-15\xfb E, 46.5-51.5\xfb N) and detects cloud-to-ground lightning strokes with an overall efficiency of 70 %. The positioning accuracy is about 500 m. The south of Germany has a mean annual number of 30 thunderstorm days, the highest in Germany.
In the observation range a total number of about 700,000 lightning strokes per year was detected. This corresponds to a mean lightning frequency during the summer months May-August of 4.5 per minute. In the following the lightning data are examined in their spatial and temporal structure.
The monthly sums of lightning events for the years 1992 and 1993 are displayed in Fig.1. It is notable that about 95% of all lightning events were observed in the 4 summer months May-August. Looking at the daily sums in the same figure, it is obvious that lightning is very unevenly distributed in time and highly concentrated on single days. Thus, in 1993 the 17 days with more than 15,000 lightning events detected contributed to 57 % of the year sum. In addition, the days with high lightning activity are clustered in groups of few days.
Figure 1. Daly and monthly sums of lightning strokes.
This `fractal' behavior of the lightning sum time series reflects the characteristics of the thunderstorm activity and causes the great variability in the lightning number for selected time periods. The monthly sums in Fig.1 illustrate this statement.
Analyzing the occurence of lightning during the day, we observe the maximum of the lightning frequency in the afternoon hours between 1500-1900 local time and the minimum in the morning hours (700-1100 local time). A secondary maximum, which is clearly seperated from the afternoon maximum exists at 2100 local time. This `nighttime' intensification of thunderstorm activity is also known from other observations.
Fig. 2 shows the mean annual density of lightning strokes in the south of Germany. Maximum stroke densities of 10 yr-1 km -2 are observed over the mountain regions and in the pre-Alps while the mean density is about 2.7 yr-1 km-2 .
The pattern of the spatial lightning distribution is highly correlated with the orographic features. Enhanced thunderstorm and thus lightning activity is observed where orographic lifting favours the initialization of thunderstorms. On the other hand, in the mountains' lee the lightning density is reduced.
Figure 2. Lightning stroke density 1992-1994.
It is interesting to note that if the number of lightning strokes inside 2 km x 2 km cells is considered, only 2% of these cells are empty. Thus, at any point we can observe with a probability of about 98% at least one lightning stroke per year within a radius of 1 km.
The temporal sequences of lightning positions form spatial patterns which reflect the size and propagation of thunderstorms. Besides small isolated patterns, long traces are observed which can extend over a few hundred kilometers (resp. 4-5 hours) with a width of only 20 km.
The value of lightning statistics is manifold. They are used for
the assessment of thunderstorm risks and damages. In climatologic
research, lightning statistics can monitor the regional thunderstorm
activity, which may change as a result of global warming. Lightning
data can also help to verify the convective parametrization in
model calculations, at least in a statistical sense. Besides the
use in statistics, lightning data, if available in real time,
can serve for nowcasting and warning purposes.
MAP Data Centre - April '05 - MAP WebMaster