Summer Precipitation Intensity-Duration-Area-Frequency Patterns in Complex Terrain using Radar Data

Extreme precipitation in complex Alpine terrain exhibits pronounced spatial and temporal variability, challenging the reliable estimation of design-relevant return levels. Rain-gauge networks provide accurate point measurements but are often sparsely distributed and located in accessible valley floors, with few instruments on steep slopes or exposed crests where wind-induced undercatch is substantial. This limits their ability to capture localised extremes and fine-scale spatial variability. Weather radar offers the necessary coverage and resolution, yet radar archives are typically short and subject to various uncertainties. The Simplified Metastatistical Extreme Value (SMEV) framework offers a solution by enabling robust inference of rare extremes from short and error-prone datasets.

We analyse summer (JJA) rainfall extremes in Switzerland to derive intensity–duration–area–frequency (IDAF) relationships across multiple spatial and temporal scales, using nine years (2016–2024) of 1-km²/5-min dual-polarisation radar data from the MeteoSwiss C-band network. Return levels for durations from 30 min to 24 h and areas from 1 to 500 km² are estimated for return periods of 2 to 100 years using the SMEV framework. The extension of the SMEV to the areal scale was first developed by Rosin et al. (2024) for the eastern Mediterranean. We adapt and apply it here to the complex, heterogeneous Alpine topography of Switzerland. To reduce sampling noise inherent to the short radar archive, we spatially smooth the Weibull shape parameter, preserving coherent physical gradients while suppressing pixel-scale artefacts. Radar-derived SMEV return levels show strong regional agreement with SMEV estimates from 60 long-term (≥30 yr) gauges.

Rainfall extremes across Switzerland exhibit strong dependence on both spatial and temporal aggregation, affected by orography and location. Short-duration, small-area extremes display sharp, topographically anchored maxima over the Jura, Pre-Alps, and southern Alpine slopes, and persistent minima across the Plateau and inner-Alpine valleys. With increasing duration and area, small-scale peaks are progressively smoothed and broad-scale maxima emerge. The southern Alps remain the most prominent hotspot across all scales. Derived IDAF relationships display pronounced spatial differences at sub-hour scales and increasing spatial coherence for 12–24 h events, with pronounced regional differences.

Case studies of recent significant flooding events demonstrate how hydrological impacts depend on the spatio-temporal characteristics of rainfall. For each event, return levels were computed across all duration–area combinations using the IDAF framework, enabling a direct assessment of how 'extreme' the event was at different hydrologically relevant scales. Events that are highly extreme at short durations and small areas trigger flash floods and debris flows, reflecting the rapid response of steep Alpine basins. Conversely, events most extreme at long durations and large spatial scales, even when short-duration intensities are unremarkable, cause more widespread river flooding, elevated lake levels, and prolonged saturation. These results highlight the importance of evaluating extremes across multiple scales, rather than relying solely on point-scale intensities.

Overall, our findings highlight the value of combining short-record high-resolution radar precipitation fields with the SMEV framework to obtain a scale-aware extreme-rainfall climatology. The resulting multi-scale return-level maps and IDAF relationships provide improved information for flood-hazard assessment and infrastructure design.