Thunderstorm Tracks and Their Relationship to Orography from an Alpine Lightning Composite

A deeper understanding of orographically influenced precipitation systems is one of the primary scientific objectives of MAP. The investigation of the electrical activity of them will complement this effort. During the ‘convective season’ from May to November 1999 including the period of the MAP-SOP (Special Observing Period) from early September to mid November an Alpine Lightning Location System which interconnected existing lightning sensors from 5 Alpine countries was in operation to get the best possible coverage of the entire Alpine Region.

A recently developed tracking algorithm is used to calculate the trajectories of the thunderstorms. The method uses a simple Gaussfilter and a selection procedure to define displacement vectors of the specific convective system. The change of the filter width results in a separation of the system’s scale which allows to distinguish between the movement of a single convective cell and the whole cell complex. In principle the method is applicable to several data sources like satellite data, RADAR and lightning data. However, so far only results from lightning data will be presented.

In the paper the role of orography in relation to thunderstorms will be investigated in several respects. These includes: a) Connection of storm tracks to orography. Recent studies show a strong correlation between storm tracks and main Alpine valleys and the Alpine foreland. Are there differences between convective cells and cell complexes? b) Regions of initiation of thunderstorms triggered by mountains in the Alpine area. Since lightning data are continually available they are frequently the first indicator in space and time for the beginning of convection. Case studies will be presented for certain weather situations and c) possible enhancement of lightning frequency in the vicinity of mountain peaks.