Impact of horizontal resolution and model time step on European precipitation extremes in the OpenIFS 43r3 atmospheric model

Abstract. Events of extreme precipitation pose a hazard to many parts of Europe but are typically not well represented in climate models. Here, we evaluate daily extreme precipitation over Europe during 1982–2019 in observations (GPCC), reanalysis (ERA5), and a set of atmosphere-only simulations at low (100 km), medium (50 km), and high (25 km) horizontal resolution and also at different time steps (i.e., 60, 30, and 15 min) using low resolution (100 km) with identical vertical resolutions using OpenIFS (version 43r3). We find that both OpenIFS simulations and reanalysis underestimate the rates of extreme precipitation compared to observations. The biases are largest for the lowest resolution (100 km) and decrease with higher-horizontal-resolution (50 and 25 km) simulations in all seasons. The sensitivity to horizontal resolution is particularly high in mountain regions (such as the Alps, Scandinavia, Iberian Peninsula), likely linked to the sensitivity of vertical velocity to the representation of topography. The sensitivity of precipitation to model resolution increases dramatically with increasing percentiles, with modest biases in the 70th–80th percentile range and large biases above the 99th percentile range. We also find that precipitation above the 99th percentile mostly consists of large-scale precipitation (∼ 80 %) in winter, while in summer it is mostly large-scale precipitation in northern Europe (∼ 70 %) and convective precipitation in southern Europe (∼ 70 %). Convective precipitation is more sensitive to model time step than to horizontal resolution. Large-scale precipitation increases significantly with both higher horizontal resolution and a shorter model time step.