Synoptic-scale flow structures associated with extreme precipitation events in northern Switzerland

This article presents a climatological analysis of upper-level synoptic-scale flow structures and low-level moisture signatures associated with extreme precipitation events in north-eastern (NE) and north-western (NW) Switzerland. Extreme daily precipitation events between 1961 and 2010 are identified using a gridded observation-based precipitation data set. ERA-Interim and ERA-40 fields are used to characterize the flow. Large-scale flow configurations over Europe that are common to extreme precipitation events are identified by a K-means clustering approach. For each area, five large-scale flow classes are defined. These flow classes include zonally oriented flows over western Europe accompanied by a ridge over the east Atlantic, amplified upper-level troughs over western Europe, strong westerly or south-westerly flows, wave-breaking events and a broad trough over the Mediterranean. For each class, the main forcing mechanism behind the ascent of moist air, the magnitude of the total precipitable water and the seasonal distribution are presented. The way in which the location and structure of the defined upper-level classes affect the local distribution of precipitation is analysed. Moreover, we analyse the directions of the moisture flux that interacts with the topography and results in extreme precipitation events. The flow conditions range from advective situations in which large amounts of moisture are transported towards the orographic obstacle to flow situations in which heavy precipitation occurs on the lee side of the Alps and ascent is mostly driven by large-scale forcing. The co-occurrence of events between NE and NW Switzerland is presented. Approximately 30% of the events occurred in both areas on the same day or differed in timing by 1 day.