Heavy precipitation does not affect the different regions of Switzerland evenly, and differs from one season to the other. Here we present the spatial distribution of heavy precipitation in Switzerland and its seasonal occurrence.
Climatology of heavy precipitation
Spatial distribution of heavy precipitation
The mean of the largest precipitation amounts each year over a given period provides some information on the geographical distribution of heavy precipitation. It is shown in the following figure in mm for 1 and 5 day precipitation (period 1961-2014) and for 1 hour and 10 minute precipitation (period 1981-2014).
For all precipitation accumulation durations, the Ticino stands out as an area affected by very heavy precipitation. At the other end of the scale, the largest amounts reached in the inner Alpine valleys are very modest, especially in the Rhone and the Inn valleys.
North of the Rhone and Rhine valleys, the spatial distribution of precipitation depends on the duration of accumulation. For durations exceeding a day, the northern Alpine rim (Prealps) and the Jura mountains experience heavier precipitation than the Plateau, thus highlighting topographical features. This pattern reflects the frequency of lightning strikes, and is consistent with the fact that thunderstorms are likely to contribute substantially to 1 day, but also to 5 day precipitation maxima.
At very short durations, on the other hand, the signature of the northern Alps is not visible: very heavy hourly or 10 minute precipitation does not depend on topography. (Note that this is not the case for average hourly or 10 minute precipitation, which displays the largest values over the northern Alpine rim and the Jura mountains, not shown). The reasons for this are unclear, but it is possible that while the generation of thunderstorms is more frequent along the northern Alpine rim than in the plain below, they are transported northwards or northeastwards by the large-scale flow and reach full maturity over the Plateau.
Seasonal variations
The following slide-show displays the percentage of years in which the yearly maxima falls in a given season (winter: December-January-February; spring: March-April-May; summer: June-July-August; autumn: September-October-November).
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Seasonal occurrence 5-day precipitation
Proportion of years in the period 1961-2014 in which the yearly maximum 5-day precipitation falls in winter, spring, summer, and autumn. Where the dot is grey, not a single year had its maximum in the given season.
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Seasonal occurrence 1-day precipitation
Proportion of years in the period 1961-2014 in which the yearly maximum 1-day precipitation falls in winter, spring, summer, and autumn. Where the dot is grey, not a single year had its maximum in the given season.
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Seasonal occurrence 1-hour precipitation
Proportion of years in the period 1981-2014 in which the yearly maximum 1-hour precipitation falls in winter, spring, summer, and autumn. Where the dot is grey, not a single year had its maximum in the given season.
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Seasonal occurrence 10-minute precipitation
Proportion of years in the period 1981-2014 in which the yearly maximum 10-minute precipitation falls in winter, spring, summer, and autumn. Where the dot is grey, not a single year had its maximum in the given season.
The seasonal distribution of heavy precipitation for durations above one day puts in evidence the processes involved, and the interaction of the Alps with large-scale circulation in the different seasons. The Ticino, subject to the heaviest precipitation in Switzerland, seems to experience most of its yearly maxima in autumn. This corresponds to a situation of southerly flow, when moist air picked up over the Mediterranean impinges upon the southern flank of the Alps. Over the northern Alpine rim, the vast majority of yearly maxima occur in summer, can usually be attributed to thunderstorms in connection with westerly flow, but also with northwesterly flow and the persistent transport of moisture towards the Alps.
For short precipitation durations, the yearly maximum hardly ever occurs in winter in most of Switzerland, except, occasionally, in the inner Alpine valleys. As thunderstorms produce the heaviest short-term precipitation, it is no surprise that most hourly or 10 minute yearly maxima are recorded in the summer months.
Related publications
Extreme weather events, such as the flood episode in Switzerland in August 2005, deeply affect our society through their adverse impact on human life, infrastructure and economy. The assessment of their return periods and values is crucial for accurate planning of infrastructure and prevention measures. It is done by means of the statistical tools of Extreme Value Theory, which assumes stationarity of the precipitation process. So far, only yearly return levels are computed operationally, yet monthly values are desirable for many applications but still not as reliable as necessary.
Extreme value analyses of a large number of relatively short time series are in increasing demand in environmental sciences and design. Here, we present an automated procedure for the peaks-over-threshold (POT) approach to extreme value theory and use it to provide a climatology of extreme hourly precipitation in Switzerland. The POT approach fits the generalized Pareto distribution (GPD) to independent exceedances above some high threshold. To guarantee independence, the time series is pruned: exceedances separated by less than a fixed interval called the run parameter are considered a cluster, and all but the cluster maxima are discarded. We propose the automation of an existing graphical method for joint selection of threshold and run parameter. Hourly precipitation is analyzed at 59 stations of the MeteoSwiss observational network over the period 1981–2010. The four seasons are considered separately. When necessary, a simple detrending is applied. Results suggest that unnecessarily large run parameters have adverse effects on the estimation of the GPD parameters. The proposed method yields mean cluster sizes that reflect the seasonal and geographical variation of lag dependence of hourly precipitation. The climatology, as represented by the return level maps and Alpine cross-section, mirror known aspects of the Swiss climate. Unlike for daily precipitation, summer thunderstorm tracks are visible in the seasonal frequency of events, rather than in the amplitude of rare events.
Das Charakteristikum der Statistik der Starkniederschläge in der Schweiz 1901-1996 ist die signifikante Häufigkeitszunahme im letzten Viertel dieses Jahrhunderts. Die Zunahme betrifft auf der Alpennordseite und in den inneren Alpen die Niederschläge >= 70 mm / Tag, auf der Alpensüdseite (Sopraceneri) diejenigen >= 100 und 150 mm / Tag.
Es werden die Häufigkeiten der Starkniederschläge in der Schweiz pro Jahr und pro Jahrzehnt sowie der Jahresgang auf der Alpensüdseite dargestellt. Ferner werden die Starkniederschläge mit dem Jahresmittel des Niederschlags, den grössten Tagesmengen und mit den Hochwassern früherer Jahrhunderte verglichen. Das letzte Viertel dieses Jahrhunderts ist ausser durch die Zunahme der Starkniederschläge auch durch zahlreiche Trockenperioden auf der Alpensüdseite gekennzeichnet. Zum Schluss werden Erklärungsmöglichkeiten für die Zunahme der Starkniederschläge diskutiert.
Der Bericht informiert über 2256 Schadenereignisse aus der Zeit von 563 bis 1988. Er beschränkt sich auf Schäden, die durch Gewitter, Dauerregen und Schneeschmelze verursacht wurden.
Im ersten Teil werden Methoden und Kriterien erläutert. Ausmass, Ursache, Art und jahreszeitliche Verteilung der Schadenfälle werden statistisch ausgewertet. In chronologischen Jahrhundert-Überblicken werden Klima, extreme Wetterjahre und katastrophale Ereignisse beschrieben. Der zweite Teil schildert in chronologischer Reihenfolge 261 schwere Unwetterereignisse. Schadenkarten und Bilder sowie ein umfangreiches Literaturverzeichnis ergänzen die Arbeit. Hinweise über Schadenkosten, Prognosen und Massnahmen runden den Bericht ab.
Further reading
- Frei C, Schär C (1998) A precipitation climatology of the Alps from high resolution rain-gauge observations. Int J Climatol 18:873–900
- Frei C, Schmidli J (2006) Das Niederschlagsklima der Alpen: Wo sich Extreme nahe kommen. promet 32:61–67
- Isotta et al. 2013. The climate of daily precipitation in the Alps: development and analysis of a high-resolution grid dataset from pan-Alpine rain-gauge data. International Journal of Climatology, Vol. 34, Issue 5, pp 1657–1675.
- Meteorologische Ereignisanalyse des Hochwassers 8. bis 9. August 2007. Arbeitsbericht MeteoSchweiz Nr. 222 (PDF, 32 Seiten, 2,8 MB, Deutsch)
- Möglichkeit und Grenzen der Niederschlagsanalyse zum Hochwasser 2005. Arbeitsbericht MeteoSchweiz Nr. 221 (PFD, 23 Seiten, 1,7 MB, Deutsch)
- Eine Länderübergreifende Niederschlagsanalyse zum August Hochwasser 2005, Ergänzung zum Arbeitsbericht 211. Arbeitsbericht MeteoSchweiz Nr. 213 (PDF, 12 Seiten, 2,9 MB, Deutsch).
- Starkniederschlagsereignis August 2005. Arbeitsbericht MeteoSchweiz Nr. 211 (PDF, 64 Seiten, 6,5 MB, Deutsch)
- Bezzola G.R. und Hegg C. (Ed.) 2007. Ereignisanalyse Hochwasser 2005, Teil 1 - Prozesse, Schäden und erste Einordnung. Bundesamt für Umwelt BAFU, Eidgenössische Forschungsanstalt WSL, Umwelt-Wissen Nr. 0707. 215pp.
- Bezzola G.R., Ruf W. (Ed.), 2009. Ereignisanalyse Hochwasser August 2007. Analyse der Meteo- und Abflussvorhersagen; vertiefte Analyse der Hochwasserregulierung der Jurarandgewässer. Umwelt-Wissen Nr. 0927. Bundesamt für Umwelt, Bern. 209pp.
- OcCC, 2003, Extremereignisse und Klimaänderung , insb. Kapitel 2.5 Starkniederschläge