Exceptional integrated vapour transport toward orography: an important precursor to severe floods in Switzerland

Floods in mountainous regions like the Swiss Alps are still challenging to forecast because they result from very diverse hydrometeorological processes. A better understanding of large-scale flood precursors is therefore important. In this study the synoptic situations leading to 14 high-impact floods in Switzerland between 1987 and 2011 are analysed using the ERA-Interim dataset. In a first step, the flood-related synoptic flow situations are classified into four categories: (i) atmospheric rivers (ARs) with northwesterly flow associated with floods in northwest Switzerland; (ii) ARs with west to souhwesterly flow associated with floods in northern Switzerland; (iii) pivoting potential vorticity (PV) cut-offs associated with floods in northeast Switzerland and (iv) PV streamers associated with floods in southern Switzerland. The strong link between ARs and floods in northern Switzerland is demonstrated for the first time. The four synoptic categories are associated with very different flow patterns over Switzerland, but all categories correspond to intense integrated vapour transport (IVT) directed perpendicular to orography. In a second step, the episodes of intense IVT related to the 14 floods are compared to the local climatology in terms of IVT amplitude, direction and duration. Ten of the 14 flood events correspond to exceptionally intense IVT perpendicular to orography. Despite the enormous complexity of the involved hydrometeorological processes, applying four simple thresholds of IVT toward orography at particular grid points of ERA-Interim allows one to distinguish ten flood events from all non-flood situations with only six non-events captured and four missed flood events. The close relationship between large-scale IVT and highly destructive local flood events shown here motivates the use of IVT information for medium-range flood warning systems.