KOSTRA Method

Heavy rainfall index (SRI)

The present Heavy Rainfall Index (SRI) assigns various precipitation levels to a 12-stage heavy rainfall scale. The heavy rainfall index was created site-specifically for 12 different duration levels. Depending on the duration level, a given amount of precipitation leads to a different heavy rainfall index.

The classification of precipitation levels into individual heavy rainfall categories is based on the method of Schmitt et al. (2018). For the heavy rainfall categories from 1 to 7, precipitation with a certain probability of recurrence is used. The precipitation amounts of the heavy rainfall categories from 8 to 12 are derived from the precipitation heights of category 7 multiplied by an increase factor that is constant for all duration levels.

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

Data basis SRI

The assignment of precipitation heights to the heavy rainfall scale is based on area-wide available design precipitations, in each case for each of the duration levels under consideration. In this case, the design precipitation from KOSTRA-DWD-2020 was used as the base data. However, as the KOSTRA-DWD-2020 data are not available for each of the duration levels or required return intervals considered in the SRI, the corresponding values were derived using the available KOSTRA extreme value & correction parameters on the ordinary KOSTRA-DWD-2020 grid and then conservatively distributed to the spatially higher-resolution grid of the SRI (~ 2km x 2km).

Event data

The heavy rainfall events listed in this web application were extracted from the radar climatology data (RADKLIM) of the German Weather Service (DWD). This data is available throughout Germany in a grid of approx. 1km x 1km. All events that occurred in the period from 2001 to 2021 were taken into account for the listing. You can find more information about the DWD's RADKLIM product here .

In order to compensate for the systematic underreporting of heavy rainfall events in the radar data compared to station measurements, the events were corrected using a statistical method. This procedure is described in Hänsler & Weiler (2020) .

In order to intersect the RADKLIM data with the coarser SRI tiles (approx. 2km x 2km), the original 5-minute data from the grid points that intersect with the respective SRI tile were averaged according to their area fraction.

In general, only events that correspond to an SRI ≥ 1 are displayed. Furthermore, the maximum number of events shown was limited to 15. If fewer than 15 events with an SRI ≥ 1 were observed in the considered period, correspondingly fewer events are shown.

In order to prevent the majority of the events listed from being caused by the same precipitation event (same event, but different duration level), the additional criterion that a maximum of 4 duration levels per event are shown in the table was introduced.