In 2015, lightning observations were carried out in the Alps to explore cloud-to-ground discharges. On the one hand, special attention was paid to the higher percentage of negative single stroke discharges and, on the other hand, the determination of the lightning parameters in mountainous conditions in general was of interest.
Based on the measured data and the correlation with ALDIS lightning location data for the Alpine region of Austria, new answers to scientific questions can be found.
The measurements for this research work are carried out using a mobile measurement system consisting of a modern high speed video camera combined with an electric field measurement system (VFRS), which is designed for on-site measurements. Data of the VFRS are so-called "ground truth" data. These data are currently the only way to determine the type and properties of cloud-to-ground lightning that are independent of lightning location. This can be evaluated with the recorded video information with high time resolution. The mobile VFRS enables the observation of all cloud-to-ground flashes in regional thunderstorms and at different locations in the Alps. This is a great advantage compared to other, conventional methods, such as measurements on instrumented towers or mountain spike-initiated lightning discharges, which are confined to a certain area and most often have special types of cloud-to-ground discharges. VFRS data represent typical cloud-to-ground lightning events that occur during a thunderstorm and are therefore scientifically significant in collecting information about the characteristics of atmospheric discharges.
Figure 1: VFRS on-site setup: (1) Flat-plate antenna and optical transmission system, (2) High speed camera, (3) GPS antenna, (4) Measurement vehicle, (5) Power generator
The VFRS consists of two main components, an E-field system to measure transient electric fields and a camera system to record atmospheric discharges in the form of high-speed video. Both components, the E-field system and the camera, provide time-synchronized measurements (GPS-time). This allows the correlation of VFRS data with other data (for example, lightning location system data). In addition, the VFRS is easily portable (in a car), operable from the car (safety for the operator during a thunderstorm), independent of a power supply and protected from weather conditions. Figure 1 shows the structure of the VFRS on site with the components outside the car.
The main goal of this research project is the collection of VFRS ground truth data during thunderstorm activities in the warm season in the Alps. In order to explore cloud-to-ground lightning behavior, observatories must meet certain criteria.
Generally, observatories need to have small electromagnetic interference and provide a wide field of view. Neither installations, such as electric fences or overhead lines, nor structures that restrict visibility, should be present at the observatory.
The second project-specific criterion is the visibility of exposed, mountainous structures and objects at a distance of approximately 10 km to 40 km from the observation site. Such objects and structures can be:
Towers and antennas
Other exposed structures
Figure 2: Observation locations and areas of investigation in the Austrian Alps
Between May and August of 2015, 2017 and 2018, measurements were carried out using the VFRS in large areas of the Austrian Alps. Within this period, data from 51 thunderstorms were recorded on 51 thunderstorm days. Figure 2 shows the measuring points (black) and the regions in which it was possible to measure lightning activity.
The extensive raw data is recorded systematically and must be processed for each individual case. This is one of the most time-consuming steps due to the large amount of data.
By applying defined quality criteria for the acquired measurement data (such as visibility conditions, visibility of all individual discharges, quality of the electrical and optical measurement results), numerous data records have already been assessed and stored successfully.
Based on the current state of data analysis, a total of 532 negative lightning strokes were analyzed.
Studies regarding lightning parameters and sequential discharges, as well insight into the phenomenon of the high percentage of negative lightning discharges, consisting of only a partial flash, are of great interest. Further investigations are planned on the basis of the existing results from previous measurements in the next years, in order to increase the database and thus to continuously compare the results. These results will contribute to the estimation of the hazard potential and the sustainable improvement of lightning protection systems.
C. VERGEINER, S. PACK, W. SCHULZ, G. DIENDORFER: "Negative Cloud-to-Ground Lightning in the Alpine Region: A new Approach", CIGRE C4 International Colloquium on EMC, Lightning and Power Quality Considerations for Renewable Energy Systems, Curitiba, Brazil, 2016.
C. VERGEINER, W. SCHULZ, S. PACK: "On the Performance of the Austrian Lightning Detection and Information System (ALDIS)", 11. Höfler's Days, Portorož, Slovenia, 2013.
W. SCHULZ, B. LACKENBAUER, H. PICHLER, G. DIENDORFER: "LLS Data and Correlated Continuous E-Field Measurements", VIII International Symposium on Lightning Protection (SIPDA), Sao Paulo, Brazil, 2005.