Weather patterns and hydro-climatological precursors of extreme floods in Switzerland since 1868

Abstract

The generation of 24 extreme floods in large catchments of the central Alps is analyzed from instrumental and documentary data, newly digitized observations of precipitation (DigiHom) and 20 th Century Reanalysis (20CR) data. Extreme floods are determined by the 95 th percentile of differences between an annual flood and a defined contemporary flood. For a selection of six events between 1868 and 1910, we describe preconditioning elements such as precipitation, temperature, and snow cover anomalies. Specific weather patterns are assessed through a subjective analysis of three-dimensional atmospheric circulation. A focus is placed on synoptic-scale features including mid-tropospheric ascent, low-level moisture transport, propagation of cyclones, and temperature anomalies. We propose a hydro-meteorological classification of all 24 investigated events according to flood-generating weather conditions. Key elements of the upper-level synoptic-scale flow are summarized by five types: (i) pivoting cutoff lows, (ii) elongated cutoff lows, (iii) elongated troughs, (iv) waves (with a kink), and (v) approximately zonal flow over the Alpine region. We found that the most extreme floods (as above, but ! 98 th percentile) in Switzerland since 1868 were caused by the interaction of severe hydro-climatologic conditions with a flood-inducing weather situation. The 20CR data provide plausible synoptic-scale meteorological patterns leading to heavy precipitation. The proposed catalogue of weather patterns and hydro-climatologic precursors can give direction when anticipating the possibility of severe floods in the Alpine region.

Climatology of Vb cyclones, physical mechanisms and their impact on extreme precipitation over Central Europe

“…In the case of the WPEs, we conclude that the Alpine inflow takes place at the southern or southeastern side of the Alps, which is supported by the mostly southerly located precipitation amounts. These features are similar to the "Canarian Trough" described by Stucki et al (2012) in association with past extreme floods in Switzerland. These cyclones are strongly influenced by a low over Brittany and thus show a southwesterly flow (Stucki et al, 2012).…”

“…Thus, most of the precipitation falls on the northern to northeastern side of the Alps. Similar situations (pivoting cut-off) have been found by Stucki et al (2012) in the context of past extreme floods in Switzerland. Hereby the cutoff low is located over the Adriatic Sea and is near-stationary due to blocking surface highs, located over western and eastern Europe.…”

Dynamical Downscaling and Loss Modeling for the Reconstruction of Historical Weather Extremes and Their Impacts: A Severe Foehn Storm in 1925

“…The foehn storm of 15 February 1925 ranks among the most hazardous windstorms in Switzerland since the mid-nineteenth century (Stucki et al 2014). Substantial documentation is available on the associated damage (Fig.…”

“…The 20CR has proven to be reliable for analyzing synoptic-scale, midlatitude weather systems Stucki et al 2012;Trigo et al 2014). Going one step further, Michaelis and Lackmann (2013) In the reanalysis, a deep surface low is located over Great Britain and surface air pressure increases toward southeastern Europe at 0000 UTC on 15 February 1925 (Fig.…”

Flood triggering in Switzerland: the role of daily to monthly preceding precipitation

“…They however also found that, although flash floods are more frequent after wet antecedent conditions in central Europe, they primarily occur following dry conditions in the Mediterranean region and show no dependence on the antecedent conditions in the Alpine-Mediterranean region. For large Swiss lakes and streams, Stucki et al (2012) underline the importance of high soil saturation due to excessive water supply by enhanced melt and precipitation over several months for the generation of historical floods.…”