ALIDALCOM - Aspects of Lightning Data Deduced from an Alpine Lightning Composite during MAP-SOP

Lightning data gets an increased importance in meteorology during the last decade. The reasons are: a) There exist a high correlation between the lightning flash rate and other physical properties of the convective system, b) lightning data is the only data set which is continually available. This makes it to an excellent nowcasting tool, c) preferred regions of initiation, movement and dissipation of thunderstorms can be deduced from a set of lightning data. In the Mesoscale Alpine Programme (MAP) the study of orographically influenced precipitation involving deep convection is one of the primary objectives. This project proposal is devoted to this scientific objective. Lightning data will be used a) to investigate the correlation between lightning frequency and other thundercloud parameters, b) to study the feasibility of lightning data for nowcasting purposes of such events and c) to get an insight into the relationship between thunderstorm tracks and orography, orographically influenced precipitation and foehn. Aims a) - c) have been studied in some detail over flat terrain or tropical oceans. The joint international effort to measure the microphysical, dynamic and thermodynamic properties of convective systems with high advanced instruments will allow for a detailed investigation in mountainous terrain. However, most investigations in Central Europe suffer by the fact that the lightning systems have been designed for national purposes. It is another aim of this project to interconnect existing sensors in different countries for the Special Observing Period (SOP) in fall 1999 in the frame of the MAP project. Direct creation of a composite of the lightning activity over the Alpine Region from the data of the national systems is not possible due to different technologies used for the sensors. In addition no existing system covers the Alpine Region sufficiently and combination of the national lightning data would result in high uncertainty in data quality especially in this region. A much more efficient way is to interconnect the existing sensors to one combined system providing good coverage of the Alpine Region. Running the combined network for a certain period also allows to analyse the performance improvement compared to the stand alone national networks.

The main aim of the project from the scientific point of view was the detailed investigation of lightning data in the Alpine Region. This included the interconnection of the existing national lightning sensors to an temporary alpinwide network, the project aim from the technical point of view. The project aims can be broken down as follows: a. Implementation of the Alpine lightning composite for the MAP-SOP from 7 Sept. 1999 to 15 Nov. 1999 (Mesoscale Alpine Programme-Special Observing Period) b. Statistical evaluation of the lightning data c. Use of the lightning data for nowcasting (2-3 hours) d. Specific thunderstorm tracks and their relationship to orography Lightning activity shows a pronounced variability during a day. The expected peak in the lightning activity in the afternoon becomes evident but varies from month to month. From the mean curve we can fix this maximum at about 1500 UTC (+100 for LT) for the `convective` season of 1999. It is difficult to define the time of a corresponding minimum. Instead an almost constant lightning activity of 30 to 40 flashes in 10min can be observed in the area for the months June, July, August and September. It could be shown that this `background` activity stems from frontal thunderstorms. Another part of the project dealt with the connection of complex systems and orography. We used a tracking algorithm developed at our department to investigate the tracks of thunderstorm cells and convective systems. The influence of the orography became evident. Single cells show a displacement direction which is related to the winds in the middle troposphere. Larger systems show a strong connection to the northern or southern foothills of the alps. Other parameters which have been investigated in the course of the project have been e.g. the enhancement of lightning activity due to orography or the statistical investigation of polarity and current amplitudes. It was the first time that lightning data have been used for nowcasting. For this purpose we adapted the COTREC algorithm which has been originally developed for nowcasting of radar data. After filtering of the lightning data we performed a 20, 40 and 60 min forecast of the convective system. The following verification showed very clear, that a useful prediction of single cells is only possible for 20 min, for cell complexes it extends to 60 min. The limits given could be enlarged if the knowledge of the tracks of convective systems over orography could be increased. For this purpose a climatology of thunderstorm tracks has to be calculated and the forecast could be refined with this pre-knowledge of preferred tracks. This seems to be a useful application of the modules developed within the project.