Forecasting unpredictable storms - the Laseyer windstorm near Appenzell

The dangerous Laseyer windstorm often forces the railway company Appenzeller Bahnen to switch to bus services upon the slightest signs of a storm. During Laseyer conditions, strong easterly winds evolve in the Schwende Valley, which is also the starting point for a hike to the famous restaurant Aescher near Appenzell. The easterly, gusty winds can be so strong that even trains are lifted off the railway tracks, as happened for instance in 2007. More recently, during the storm Burglind in January 2018 a Laseyer developed in the Schwende Vallye and the fierce winds teared off a anchored container of the paragliding school Appenzell. 

Laseyer conditions occur about 10-15 times during the winter season (November to March), and the Appenzeller Bahnen switch to the secure bus service, because the road to the valley is sheltered from the strong Laseyer winds by the railway dam. Still, the switching is expensive for the Appenzeller Bahnen, it requires substantial organisational steps, and is also substantially affecting customer convenience. Therefore, the Appenzeller Bahnen wanted to improve an existing, conservative warning system that allows for less switching to bus service, while still guaranteeing customer convenience and safety.

In cooperation between ETH Zurich, MeteoSwiss and Appenzeler Bahnen, a new warning algorithm has thus been developed within an Innosuisse project to give the needed guidance to the Appenzeller Bahnen. The challenges in developing the warning system were strongly related to the unique characteristics of the Laseyer storm: it is a rather rare event; it develops in a very narrow valley (1-2 km width), which cannot be adequately be resolved by current operational NWP models; and the physical mechanisms leading to Laseyer are not well understood. Based on many years of observational data and relying on the operational ensemble NWP simulations by MeteoSwiss, the refined warning system could be developed and operationally implemented in 2021/2022. It is currently evaluated.

The project was led from IAC by Michael Sprenger. In addition to the aim of a refined warning system, the two-year Innosuisse project provided the framework for further educational activities at IAC. Extra wind measurements were implemented during 2021/22 in the Schwende Valley and also on a train of the Appenzeller Bahnen. This allowed students, under the supervision of Ulrich Krieger and Maxi Böttcher, to learn about a fascinating meteorological phenomenon, but also to see how private industry can benefit from meteorological research.