The use of automatic station data for continuing the long time series (1864 to 2008) of foehn in Altdorf

Abstract

This paper is dedicated to Gian Gensler, former Chief Meteorologist of MeteoSwiss, who passed away on April 13, 2013 at the age of 92.In Altdorf, in the Uri part of the Reuss Valley, foehn observations have been recorded from 1864 until 2008 at a station whose position was changed only marginally during this time. Hence, with 145 years, this is the longest time series of foehn events in the Alpine massif.Based on the three main parameters temperature, humidity, and wind, the occurrence of foehn was originally subjectively determined at the so-called climate observation times in the morning, at noon, and in the evening. These triple observations were concentrated in monthly sums for each of the observation times. At the end of 2008, these observations were discontinued.Climatologists regret the ending of the classical observations as a serious loss for future analyses. In response to this, a method was developed by which triple observations can be extracted from newly available objective parameters. The aim is to allow a seamless continuation of the classical long time series.Today, in the time of automatic observation networks, special procedures allow the identification of foehn in 10-minute intervals; this new data series is called foehn index. This paper describes how triple observations can be extracted from foehn indices.Starting in 1955, foehn hours were determined manually from station recordings. From 1981 to 2008, foehn hours as well as the “classical” foehn observations at the three observation times were computed from automated foehn indices. In addition, a regression analysis shows the possibilities for transforming either the original observations to foehn hours or vice versa.

Keywords

• Foehn
• climatology
• Altdorf
• time series
• discontinuity