The winter storm Lothar: an integrated view on Doppler radars, ground winds, and forest damage in northern Switzerland

The killer storm Lothar from 26 December 1999 devastated many regions in northern France, southern Germany and northern Switzerland. Here, we present an analysis of radar data, together with ground wind measurements and information on damage to forests. The storm was monitored by the ETH Doppler weather radar and the Doppler radar network of MeteoSwiss. Wind data are available from the ANETZ and ENET: automated ground networks operated by MeteoSwiss. Information on uprooted and broken trees in forests are obtained from aerial photographs (collected and analysed by the "Bundesamt für Umwelt, Wald und Landschaft" BUWAL and WAHO-ETH) and ground surveys (undertaken by WSL and WAHO-ETH). These data make it possible to assess the severity of the damage and, at least partly, the fall direction of the uprooted and broken trees.

The radar data reveal that the peak wind gusts were associated with a narrow cold-frontal rainband (NCFR). The rainband contained core and gap regions. The core regions produced stronger winds than the gap regions. Starting from this point, we obtained two major results:

1. The peak wind gusts occurred AFTER the passage of the frontal convergence line. Such a behaviour is atypical for NCFR's. This finding is substantiated directly by the Doppler wind fields, and indirectly by the main fall direction of the broken trees.

2. The Doppler wind maxima could be tracked along W-E oriented "strips" and attributed to W-E oriented regions where damage to forests was more severe and more frequent than further south or north.

These findings call for a detailed analysis of the internal dynamics of the precipitation core regions. One of these regions was in a favorable position for a dual-Doppler wind retrieval. The dual-Doppler analysis is presently ongoing. First results will be shown at the conference.