The Laseyer Rotor - Dynamics & Climatology: A detailed case study using large-eddy simulations and a climatological analysis of local weather station data
Creators: name: Egloff, Lukas
Publisher: name: ETH Zurich
Year: 2009
Subjects: Laseyer; wind storm; meteorology
info:eu-repo/classification/ddc/550
Earth sciences

description: In this thesis the phenomenon of the Laseyer winds - strong wind gusts in a narrow Alpine valley in North-East Switzerland - is examined by means of real-case numerical simulations with varying grid resolutions for the date of a train derailment in Wasserauen in January 2007. The hereby employed models are the COSMO-2 weather prediction model and the Advanced Regional Prediction System (ARPS). On a second track, measurements of local weather stations are analysed and compared to themodel results, thus assessing the quality of the numerical simulation of the event. Additionally, several years of weather station measurements are objectively analysed for similar Laseyer winds and a simple climatological pattern is established. So far no study has looked into the formation of rotors of small spatial scale over highly complex topography as can be found in the narrow valley in the Alpstein mountain range. 
The model simulations show the presence of an extended layer of stable air over the valley for the date of the accident. This stable layer may have facilitated high near surface wind speeds through constructive interference. High-resolution simulation runs exhibit north-westerly flow above the valley and strong vertical motion within, with areas of strong up- and downdrafts confined to the flanks of the down- and upstream ridgeline respectively. High horizontal wind velocities on the valley floor were modelled. The highest surface wind speeds of 38 m/s were found deep in the valley and lower maximum velocities around the location of the accident (24 m/s). The simulated flow was mainly directed towards the embouchure with hardly any cross-valley wind component. Trajectory analysis disclosed that air parcels can enter the valley from above near the valley head and in the following can generate down-valley flow on the valley floor. 
The analysis of the weather stations measurements revealed that the strong winds causing the train accident were not excited by a typical Laseyer event characterised by strong cross-valley wind gusts. Instead, the observations showed strong flow into the valley on the whole morning, marking the event as rare but not unique. The observations further disclosed that strong wind gusts with an easterly component in the valley are most likely triggered by westerly winds aloft. The angle window for incident ambient wind favourable for the occurrence of strong easterly wind gusts in the valley is surprisingly narrow, it ranges from 265 to 295 degree azimuth. The measurements affirm the account of local residents stating that Laseyer episodes predominantly occur in the winter season; ambient westerly wind aloft going along with strong easterly wind gusts in the valley were recorded almost exclusively between November and March. Neither the simulations nor the observations show strong wind components across the railway line in Wasserauen for the time of the accident and thus are inherently limited in their capability to explain the derailment. Further studies are desirable to detect additional key players for the formation of the strong wind gusts.