Development and verification of a real time stochastic 1 precipitation nowcasting system for urban hydrology in 2 Belgium
10 The Short Term Ensemble Prediction System (STEPS) is implemented in real time at the 11 Royal Meteorological Institute (RMI) of Belgium. Th e main idea behind STEPS is to quantify 12 the forecast uncertainty by adding stochastic pertu rbations to the deterministic Lagrangian 13 extrapolation of radar images. The stochastic pertur ba ions are designed to account for the 14 unpredictable precipitation growth and decay proces ses and to reproduce the dynamic scaling 15 of precipitation fields, i.e. the observation that l rge scale rainfall structures are more 16 persistent and predictable than small scale convecti ve cells. This paper presents the 17 development, adaptation and verification of the syst em STEPS for Belgium (STEPS BE) 18 STEPS BE provides in real time 20 member ensemble prec ipitation nowcasts at 1 km and 5 19 min resolution up to 2 hours lead time using a 4 C ba nd radar composite as input. In the 20 context of the PLURISK project, STEPS forecasts wer e generated to be used as input in sewer 21 system hydraulic models for nowcasting urban inundati ons in the cities of Ghent and Leuven. 22 Comprehensive forecast verification was performed in order to detect systematic biases over 23 the given urban areas and to analyze the reliabilit y of probabilistic forecasts for a set of case 24 studies in 2013 and 2014. The forecast biases over the cities of Leuven and Ghent were found 25 to be small, which is encouraging for future integra tion of STEPS nowcasts into the hydraulic 26 models. Probabilistic forecasts of exceeding 0.5 mm hr 1 are reliable up to 60 90 min lead 27 time, while the ones of exceeding 5.0 mm hr 1 are only reliable up to 30 min. The STEPS 28 ensembles are slightly under dispersive and represen t only 75 90% of the forecast errors. 29 Deleted: meteo.be