Dynamics and Model Representation of Two Contrasting Extreme Precipitation Events in the Sahel

Episodes of heavy monsoonal rainfall have become more common in the Sahel in recent decades. Reliable forecasts of such cases would allow a better warning of the affected population. In this study, two extreme precipitation events causing flooding in two cities in Mali, on 08 August 2012 in San (127 mm) and on 25 August 2019 in Kenieba (126 mm), are investigated with respect to rainfall structures, dynamical forcings, and the ability of the ICOsahedral Nonhydrostatic (ICON) model to represent their evolution. Two sets of experiments with convective parameterization enabled (PARAM) and disabled (EXPLC), both at 6.5 km grid spacing, are conducted for each case. The simulations are evaluated with ERA5 reanalysis data and the satellite-based precipitation data IMERG, using amongst others the Fractional Skill Score (FSS) and the Structure-Amplitude-Location (SAL) score for spatial verification of the rainfall fields. In both cases, organized convective systems developed in association with a westward propagating cyclonic vortex but differ in their environmental setting. While the San case likely featured an African easterly wave (AEW), the Kenieba case occurred within an unusual wet environment extending northward to the southern fringes of the Sahara with less dynamical forcing. Consistent with previous studies, EXPLC captures the AEW-related dynamics and the rainfall distribution in the San case better than PARAM. In the Kenieba case, however, EXPLC struggles to represent the convective organization by producing too many scattered rainfall systems, while PARAM converts the abundant moisture into excessive rainfall through the moist vortex. The results stress the continued need for more research into how to best represent the complex convective dynamics leading to extreme Sahelian rainfall in numerical models, especially in terms of better assessing the changes in extreme rainfall due to climate change.