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1. Introduction
Foehn flows strongly affect many Alpine valleys, for example, the Rhine valley and the Reuss valley in Switzerland and the Wipp/Inn valley in Austria. The foehn characteristics, especially the strong gusty winds, make a reliable prediction very desirable (see Richner and Hächler 2013 for a recent review). However, the socioeconomic impacts of foehns are not limited to extreme wind gusts. For example, foehns also affect air quality (ozone concentrations and aerosol loadings), snow melting, agriculture, fire outbreaks, cable car operation (e.g., case study by Burri et al. 1999), and wind-induced waves on lakes (Graf et al. 2013). Hence, foehns are an important aspect of operational weather forecasting and nowcasting.
Foehn prediction and nowcasting can be accomplished using several different approaches. At the Swiss national meteorological service (MeteoSwiss; www.meteoswiss.ch), a probabilistic method is applied that originally was introduced by Widmer (1966) and later refined and simplified for operational needs by Courvoisier and Gutermann (1971). This method relies on two pressure gradients across the Alps and also incorporates a pressure tendency. Based on these components it defines a seasonally varying threshold that has to be surpassed to indicate a foehn. The resulting index, called the Widmer foehn index, works rather reliably up to a time window of 36 h (Richner and Hächler 2013). While the Widmer index is designed for forecasting foehn events, the approach developed by Dürr (2008) is designed for foehn nowcasting. Dürr (2008) uses several meteorological parameters characteristic of foehns (wind speed and direction, relative humidity, and potential temperature difference relative to an Alpine crest station) and derives with a simple threshold-based approach the occurrence or absence of foehn conditions at individual valley stations. This objective algorithm is operationally applied to 10-min observational data and provides real-time information about foehns at about 30 measurement sites across Switzerland. An interesting aspect of Dürr’s (2008) algorithm is its application to long observational time series. For instance, in Switzerland 10-min observations have been available since 1981 for many measurement sites, hence allowing one to compile a 30-yr climatology of foehn occurrence at these stations and to compare them to a long “reference” foehn time series at the Swiss station Altdorf (Richner et al. 2014; Gutermann et al. 2012). Another statistical approach was recently applied by...