What Will a West African Extreme Rainfall Event Look like By 2100?

In West Africa, the effects of climate change are already being felt, with an increase in the intensity of the most extreme rainfall events observed in rain gauge and satellite datasets over the last few decades. Given the region’s high economic dependence on agriculture, and its rapidly growing urban population, it is of vital importance to understand how extreme rainfall events may change further in the future climate. West Africa receives the majority of its rainfall via Mesoscale Convective Systems (MCSs). Convection associated with the land–sea breeze circulation is also significant along the Guinea Coast. However, convection is not well-represented in coarse climate models, for example the Coupled Model Intercomparison Project (CMIP) models contain large biases. Indeed, for West Africa there is large disagreement between models over the sign of the end-of-century mean precipitation change, although all models do project an increase in extremes. Recent work has shown the benefits of high-resolution ‘convection-permitting’ models for improving the representation of the diurnal cycle, intensity, and organisation of convection. However, these models are computationally expensive, and remain a challenge to run for climate timescales. One solution is to run pseudo-global-warming (PGW) simulations, where 4D (x,y,z,t) climate ‘deltas’ from CMIP models are added to high-resolution reanalysis. The resulting dataset may be used to drive high-resolution case study model runs, providing insight into what present-day extreme events might look like if they were to occur in an end-of-century climate. Initial results from case studies performed using the ICOsahedral Nonhydrostatic (ICON) model with horizontal grid-spacings down to 3.3 km will be presented. The results include sensitivity to different temperature changes, as well as sensitivity to model resolution. Overall, the most intense rainfall rates show the largest increase under end-of-century conditions. These increases cannot be explained by Clausius-Clapeyron scaling alone, therefore the contribution of thermodynamic and dynamic processes to the change in rainfall extremes will be discussed.