Method to classify summer heavy rainfall events

Data basis

This web application is based on homogenized and quality-controlled precipitation time series from nationwide radar measurements. The underlying data period extends from 2006 to 2024, with only data from May to September (extended summer period) being used. The precipitation radar data was processed and provided in the AVOSS project by Kachelmann GmbH and is described in Weiler et al. (2019).

Classification of heavy rainfall events using the Heavy Rainfall Index (HRI, german: SRI)

The Heavy Rainfall Index (HRI) can be used to categorize precipitation events on a 12-level heavy rainfall scale based on the precipitation amount and the duration of the event. Precipitation amounts are assigned to individual heavy rainfall categories using the method developed by Schmitt et al. (2018). For heavy rainfall categories 1 to 7, precipitation with a certain return probability (see data basis for HRI classes) is used for this purpose. The precipitation amounts for heavy rainfall categories 8 to 12 are derived from the precipitation levels for heavy rainfall category 7 using an increase factor that is constant for all duration levels.

In general, precipitation events in heavy rainfall categories 1 and 2 can be classified as normal, events with HRI 3 to 5 as intense and events with HRI 6 and 7 as exceptional heavy rainfall events. Events in category 8 and above can be categorized as extreme heavy rainfall events.

The heavy rainfall index was created for 12 different duration levels specific to each location. Depending on the duration level, a given amount of precipitation leads to a specific HRI. If the amount of precipitation for an event remains constant over several duration levels, longer events tend to have a lower HRI.

Data basis for the HRI classes

The assignment of precipitation levels to a heavy rain scale is based on area-wide design precipitation values for each of the considered duration levels. Since it is essential that the design precipitation database matches the data to be classified, the design precipitation values in this case were derived on the basis of homogenised radar time series.

However, since design precipitation with return periods of up to 100-yearly events is required for the HRI, the radar time series had to be extended using a statistical approach. To this end, we applied a method for each grid cell that is described in Hänsler et al. (2022)using Baden-Württemberg as an example.

From the extended radar time series, design precipitations can now be derived for each grid cell of the precipitation radar data for different event durations. In the present case, the design precipitation was derived for 12 different duration levels (5 minutes to 6 hours) and processed nationwide in the original spatial resolution of the radar data for each radar grid cell.

Event data

The heavy rainfall events listed in this web application were derived in the same way as the design rainfall from the radar climatology developed in AVOSS. The data is available for all of Germany in a grid of approx. 0.7 km x 0.7 km.

The application takes into account all heavy rainfall events that occurred between May and September in the period from 2006 to 2024. However, only events corresponding to an HRI ≥ 1 are generally displayed.

Furthermore, the maximum number of events displayed has been limited to 15. If fewer than 15 events with an HRI ≥ 1 were observed in the period under review, fewer events are displayed accordingly.

Events are selected solely on the basis of HRI values and across all duration levels. To prevent the majority of the events listed from being attributable to the same precipitation event (same event, but different duration level), an additional criterion was introduced whereby the same event may only be included a maximum of three times in the final events, with only one short (5, 10, 15 & 30 minutes), medium (45, 60 & 90 minutes) and long (120, 180, 240, 300 & 360 minutes) event durations are taken into account.