Spatial coherency of extreme weather events in Germany and Switzerland

Abstract

With a simple statistical approach, the spatial representativeness of extreme events in daily meteorological surface observations from Germany and Switzerland is analysed. A percentile-based definition of extremes is employed, and the frequency of simultaneously occurring extreme events at different stations is used as a measure for their spatial coherency. The largest representativeness is diagnosed for daily temperature extremes, i.e. hot and cold days, which very often occur simultaneously at many stations. On the contrary, precipitation extremes usually are restricted to local or regional scales. The spatial coherency of wind gust maxima is intermediate between the other extremes and depends on station location and terrain complexity. For cold extremes, station altitude has a strong influence on the spatial coherency of the events. It is argued that the spatial representativeness of extreme weather events directly relates to the importance of a coherent large-scale atmospheric forcing, compared to more local factors, for triggering these events. In particular, large-scale advection is the most important driver of temperature extremes. Strong wind gusts are often caused by midlatitude cyclones over the North Sea. The results from this study can also be used to assess the spatial representativeness of extreme weather reconstructions from proxy data in paleoclimate studies.

Characterising the relationship between weather extremes in Europe and synoptic circulation features

“…This larger coherence of the synoptic-scale forcing for wind gust compared to precipitation extremes (for which the patterns off c vary more between different target locations, see again Sect. 3.1) is also linked to a higher spatial coherence of the actual wind extreme events (Pfahl and Wernli, 2012b). This indicates that wind storms simultaneously affect relatively large regions in central Europe.…”

“…Nevertheless, the same blocking may affect cold extremes at very different locations in Europe. This large-scale character of cold extremes corresponds to the large spatial coherence of the events found by Pfahl and Wernli (2012b). Extremely cold winter conditions often occur simultaneously in many regions on a continental scale (for an example, see Cattiaux et al, 2010).…”

“…Comparing the anomaly patterns related to the different categories of extreme events shows that these categories have very different spatial characteristics, which also correspond to the spatial coherence of the extreme events (cf. Pfahl and Wernli, 2012b). While cold extremes are linked to a common large-scale anomaly signal, the synoptic forcing mechanisms for precipitation extremes are strongly variable and depend significantly on the location of the extreme event.…”

Intense and Extreme Wind Speeds Observed by Anemometer and Seismic Networks: An Eastern U.S. Case Study

“…The spatial coherence of extreme wind events is quantified by selecting a reference station and event threshold to identify the occurrence of ''events,'' and then observations from other stations are examined to determine whether they also exceeded that threshold within a given time period of the event (Pfahl and Wernli 2012;Ricciardulli and Sardeshmukh 2002). This approach quantifies the degree to which the occurrence of extreme values at two or more sites is coherent in time and thus is fundamentally different from spatial correlation analyses that test the degree of coherency of the entire time series.…”

An empirical method for estimating probability density functions of gridded daily minimum and maximum temperature

“…As daily temperature extremes are often recorded simultaneously at many stations (Pfahl and Wernli, 2012;Wulfmeyer and Henning-Müller, 2006), it is convenient to use the high-resolution meteorological information provided by the whole surface network through a statistical interpolation procedure.…”