Real-time extremity assessment of rainfall observations and SINFONY forecasts for small hydrological catchments

Heavy rainfall caused by convection over small catchments represents a major challenge in flood forecasting. In Germany, each federal state runs its own flood forecasting center providing operational forecasts obtained from hydrological models. This system works well for large and medium-sized catchments. However, hydrological response happens fast for small catchments and operational models do not perform well. In many cases, these models only use hourly rainfall observations and NWP forecasts but do not take into account rainfall nowcasts.

We aim at supporting German flood forecasting agencies in a co-design approach. Specifically, we intend to provide a novel post-processing product containing information about the extremity of catchment-specific areal rainfall. First, a nationwide catchment delineation is performed for each pixel of a 50 m x 50 m grid using a digital elevation model, while catchments smaller than 10 km² and greater than 1000 km² are discarded. Next, we perform an upscaling of catchment information to a 1 km x 1 km grid. Finally, we compute areal rainfall for each pixel using the underlying catchment geometry from rain gauge adjusted radar observations as well as seamless rainfall forecasts resulting from the SINFONY project.

We consider various accumulation durations and perform a recalculation of areal rainfall for all catchments using a 20-year dataset of radar-derived rainfall. A partial series of areal rainfall accumulation results for each catchment and we fit extreme value distributions in order to provide information about the expected statistical return period of rainfall events and their potential hydrological impact in real-time. Extreme value statistics with time series of only 20 years is not sufficient to obtain reliable return periods for very rare events. In order overcome this limitation, we include long-term rain gauge statistics and combine them with areal rainfall recalculations in a reasonable manner.

The resulting spatial distribution of areal rainfall with corresponding return periods based on an exhaustive collection of catchments appears to be a useful visualization technique to identify small catchments affected by heavy rainfall and can be computed in real-time using SINFONY products. In order to illustrate the benefit of this approach, we will show a comprehensive case study for the catastrophic flooding event on 14^th of July 2021.