Extreme Precipitation in the West African Cities of Dakar and Ouagadougou—Atmospheric Dynamics and Implications for Flood Risk Assessments

Two extreme, high-impact events of heavy rainfall and severe floods in West African urban areas (Ouagadougou on 01 September 2009, Dakar on 26 August 2012) are investigated with respect to their atmospheric causes and statistical return periods. In terms of the synoptic-convective dynamics, the Ouagadougou case is truly extraordinary. A succession of two slow-moving African Easterly Waves (AEWs) caused record-breaking values of tropospheric moisture. The second AEW, one of the strongest in recent decades, provided the synoptic forcing for the nighttime genesis of Mesoscale Convective Systems (MCSs). Ouagadougou was hit by two MCSs within six hours, as the strong convergence and rotation in the AEW-related vortex allowed a swift moisture refuelling. An AEW was also instrumental in the overnight development of MCSs in the Dakar case, but neither the AEW vortex nor the tropospheric moisture content was as exceptional as in the Ouagadougou case.

Tropical Rainfall Measuring Mission (TRMM) 3B42 precipitation data show some promise to estimate centennial Return Values (RVs) using the "peak-over-threshold" approach with a Generalized Pareto Distribution fit, although indications for errors in estimating extreme rainfall over the arid Sahel are found. In contrast, the PERSIANN-CDR (Precipitation Estimation from Remotely Sensed Information using Artificial Neural Network - Climate Data Record) dataset seems less suitable for this purpose despite the longer record. Notably, the Ouagadougou event demonstrates that highly unusual dynamical developments can create extremes well outside of RV estimates from century-long rainfall observations. Future research will investigate whether such developments may become more frequent in a warmer climate.