Areal extremes from a different perspective: rainfall as 2D and 3D connected objects.

Using the German weather radar data for the last 20 years with a high spatial and temporal resolution, the occurrence of rainfall extremes was analysed. By extracting and examining connected rainfall areas, several research questions were investigated: (1) How many extremes occur in a given area independent of their location? (2) To what extent is their occurrence in space a random and to what extent a structured process? (3) How are the connected volumes behaving in space and time? (4) How does the areal extent relate to event duration, rainfall volume, and discharge volume? The first two research questions were investigated for all of Germany, the last two by analysing rainfall and run-off data in several small and medium size headwater catchments in southern and western Germany.

The results show that the occurrence of events in space is related to their areal extent; there are regions where the frequency of occurrence of large spatially distributed events is greater than that of smaller ones. Moreover, there are interesting relationships between the spatial extent of an event, the event duration, and the event rainfall volumes. For high discharge values, not only does the rainfall intensity matter but also the event duration and spatial distribution of rainfall within a catchment. Many discharge peaks are not necessarily caused by high-intensity events (hourly or daily maxima) but by the accumulation of rainfall cells in space and time.