Articles | Volume 10, issue 1
Model evaluation paper
03 Jan 2017
Model evaluation paper |  | 03 Jan 2017

On the forecast skill of a convection-permitting ensemble

Theresa Schellander-Gorgas, Yong Wang, Florian Meier, Florian Weidle, Christoph Wittmann, and Alexander Kann

Abstract. The 2.5 km convection-permitting (CP) ensemble AROME-EPS (Applications of Research to Operations at Mesoscale – Ensemble Prediction System) is evaluated by comparison with the regional 11 km ensemble ALADIN-LAEF (Aire Limitée Adaption dynamique Développement InterNational – Limited Area Ensemble Forecasting) to show whether a benefit is provided by a CP EPS. The evaluation focuses on the abilities of the ensembles to quantitatively predict precipitation during a 3-month convective summer period over areas consisting of mountains and lowlands. The statistical verification uses surface observations and 1 km  ×  1 km precipitation analyses, and the verification scores involve state-of-the-art statistical measures for deterministic and probabilistic forecasts as well as novel spatial verification methods. The results show that the convection-permitting ensemble with higher-resolution AROME-EPS outperforms its mesoscale counterpart ALADIN-LAEF for precipitation forecasts. The positive impact is larger for the mountainous areas than for the lowlands. In particular, the diurnal precipitation cycle is improved in AROME-EPS, which leads to a significant improvement of scores at the concerned times of day (up to approximately one-third of the scored verification measure). Moreover, there are advantages for higher precipitation thresholds at small spatial scales, which are due to the improved simulation of the spatial structure of precipitation.

Short summary
Ensemble forecasting offers a useful method to simulate the uncertainty of a numerical forecast model for each individual forecast run. This study compares ALADIN-LAEF, a 16-member ensemble with a resolution of 11 km that combines several perturbation methods, with AROME-EPS, which downscales the members of ALADIN-LAEF to 2.5 km resolution. The verification shows that there are benefits of a higher-resolution ensemble, especially for highly localized precipitation and for mountainous terrain.