An evaluation of the convection‐permitting ensemble COSMO‐E for three contrasting precipitation events in Switzerland

Abstract

Specific aspects of the performance of COSMO‐E, a newly operational convection‐permitting ensemble prediction system run by the Swiss national weather service, are evaluated for three contrasting precipitation events and compared with its driving model, the ECMWF ensemble (EC ENS). The events include locally triggered air‐mass convection on four consecutive days (CONV), a complex flood‐producing rainfall episode (LSF1), and a summertime cold‐frontal precipitation event (LSF2). Investigating the precipitation forecasts reveals that COSMO‐E outperforms EC ENS in all cases in terms of precipitation pattern, whereas both ensembles fail in predicting the correct domain‐averaged precipitation for LSF1 and struggle with the correct timing of the daily precipitation onset for CONV. For LSF2, EC ENS produces an almost perfect time series of domain‐averaged precipitation, but this seemingly excellent forecast fails in predicting the spatial distribution. Spread–error relationships also yield a rather complex picture. The higher resolution of COSMO‐E leads to increased spread and reduced underdispersion for near‐surface variables. However, in the free troposphere the two ensembles perform similarly, and the agreement of spread and error varies between cases, variables and levels. For both events with large‐scale advection, underdispersion occurs for mid‐tropospheric relative humidity near fronts, whose propagation is overconfident in EC ENS, while for the convective event COSMO‐E clearly shows larger spread than EC ENS. Because frontal propagation is largely determined by lateral boundary conditions, this underdispersion also occurs for COSMO‐E. In summary, this study confirms the benefit of a convection‐permitting model for many aspects of ensemble forecasting, and illustrates the challenge of robustly assessing the quality of an ensemble forecast for hydrometeorologically relevant events.