Unstationary aspects of foehn in a large valley part I: Operational setup, scientific objectives and analysis of the cases during the special observing period of the MAP subprogramme FORM

The Rhine valley, which stretches from the main Alpine crest to the Lake of Constance, was chosen as the target area to study unstationary aspects of foehn during the Special Observing Period (SOP) of the Mesoscale Alpine Programme (MAP). This large valley is up to 10 km wide and has some of the highest foehn frequencies in the European Alps. The MAP subprogram FORM (FOehn in the Rhine valley during MAP) was designed to investigate various aspects of the foehn including the interaction of foehn flow with the boundary layer and the processes that remove the cold air pool. The subprogram was also focused on improving the understanding and forecasting of foehn-related phenomena such as waves and turbulence. A large number of in-situ and remote sensing observing systems were deployed to take measurements during the field phase of MAP. Among them were about 50 surface stations, up to 9 radiosonde stations, 2 wind profilers, 4 Doppler sodars, 2 scintillometers, 1 scanning and 1 backscatter lidar and different research aircraft. This paper gives an overview of the objectives of FORM, describes the target area and its instrumentation, and provides a detailed synoptic description of the 12 foehn cases observed during the MAP SOP.