Using synoptic regimes to diagnose resolution effects on extreme precipitation in high-resolution climate simulations over complex terrain and coastal regions

Extreme precipitation represents a major hazard in Europe, particularly in regions with complex terrain and strong land-sea contrasts such as the Alps and the Mediterranean. Here, we investigate synoptic-scale circulation patterns associated with extreme precipitation using simulations from the nextGEMS (Next Generation Earth Modelling Systems) high-resolution climate experiments at different horizontal resolutions.

Synoptic regimes associated with extreme precipitation events over Italy and surrounding regions are identified using Gaussian Mixture Model clustering applied to daily standardised anomalies of mean sea-level pressure and 500 hPa geopotential height. The resulting classification of days with extreme precipitation is used to analyse the occurrence and characteristics of precipitation extremes under different large-scale flow situations and to assess how these relationships depend on model resolution and the representation of deep convection.

The analysis reveals that dominant weather regimes are consistently represented across simulations with horizontal grid spacings between 4 and 9 km, indicating a robust representation of synoptic-scale variability. However, substantial differences emerge in the representation of regime-conditioned precipitation extremes. Higher-resolution simulations more realistically capture the intensity and spatial structure of extreme precipitation, particularly over complex Alpine terrain and along Mediterranean coastlines. These improvements are primarily associated with a limited number of circulation regimes characterised by strong moisture transport and favourable flow configurations for orographic lifting. Consequently, resolution-dependent differences in extreme precipitation are not uniform across all synoptic situations but are confined to specific large-scale regimes.

These findings highlight the value of combining circulation-regime diagnostics with kilometre-scale climate simulations to better understand the multi-scale processes controlling regional precipitation extremes and their sensitivity to model resolution and convective parameterizations.