Catastrophic mid July 2021 Floods in Belgium and Germany Through the Eyes of the A-LAEF System

Extreme weather events pose a challenge in a changing climate. From 13th to 15th July 2021, an almost stationary cyclone resided over Belgium and parts of Germany (Rhineland Palatinate and North Rhine Westphalia regions). The system produced persistent heavy rain and showers that yield a record amount of rainfall on several locations and different time intervals (1, 6, 24 and 72 hours). In the following days, the system moved to Austria where it caused heavy precipitation and intense showers in the following days. Here we study the physical processes that contributed to the extreme weather event and determined the precipitation intensity by the means of a limited area mesoscale ALARO (Termonia et al. 2018) ensemble forecasting system (A-LAEF). A-LAEF provides operational short range probabilistic forecast on 4.8 km resolution using advanced multi scale ALARO physics package that is designed to perform at scales from 10 down to at least 2 km. The ensemble uses perturbations in initial and lateral boundary conditions as well as model perturbations through a combination of ALARO multi-physics with different setups of radiation, microphysics, turbulence, shallow and deep convection with the stochastic perturbation of physics tendencies. The initial conditions are perturbed through ensemble surface data assimilation (Bellus et al. 2016) and upper air spectral blending. The ensemble system operational forecast performed well, the precipitation duration and intensity were predicted well, including the small scale features, and with high probabilities. This allows us to identify which physical processes as well as which initial conditions contributed the most to the event intensity. Large amount of water was advected from the Atlantic ocean and evaporated from warmer than usual North Sea, but the soaked soil also contributed as a source of moisture, that intensified and prolonged the precipitation event. Precipitation organized into high resolution features where the most intense downpour occurred, but the fine differences depended on the choices made in physics.