Process-based assessment of the impact of reduced turbulent mixing on Congo Basin precipitation in the RCA4 Regional Climate Model

In regions featuring strong convective activity (such as the Congo Basin, CB), turbulent mixing in the planetary boundary layer strongly affects the water budget. In this study, we use a process-based evaluation to assess the performance of the Rossby Centre Regional Climate Model (RCM) RCA4 in simulating the September–November CB rainfall, under conditions of strong and weak turbulent mixing. To this regard, results from two different versions of model are analysed: the version used in the COordinated Regional climate Downscaling EXperiment framework (RCA4-v1), and a modified version (RCA4-v4), in which turbulent mixing is reduced. Experiments are driven with boundary conditions from the ERA-Interim reanalysis. Results show that RCA4-v4 improves the CB rainfall climatology compared to RCA4-v1. This result is further related to the models’ different representations of the relevant driving mechanisms and processes. The model version with a reduced turbulent mixing (RCA4-v4) shifts less moisture from the lower troposphere towards the free troposphere. As the shallow convective mixing is reduced (owing to the reduction of the turbulent mixing), lower layers are moistened, and low level cloud fraction increases over Equatorial Africa. This increase is stronger over the West Equatorial African (WEA) coast than over the CB. The result is that surface solar radiation decreases more over the WEA coast than over the CB, which would result in a lower surface temperature over WEA coast than over the CB. An enhanced pressure gradient between the WEA and the CB is created as a result, thus enhancing the Congo low level cell, and low level westerlies (LLWs). LLWs are faster, meaning that more moisture flows through the CB Cell, is uplifted in eastern up-branch, and enters African Easterly Jets (AEJs), which, in turn, are intensified due to the increase in the surface temperature gradient. Intensification of the CB cell and mesoscale convective systems (MCSs) is the cause of the higher rainfall and is what improves the CB rainfall climatology in RCA4-v4. In addition, the increase in rainfall causes an increase in soil moisture in RCA4-v4 in both the north and south of the CB. Higher soil moisture does not affect evaporation in the north as soils are already saturated in RCA4-v1. However, the increase in rainfall increases soil moisture in the south in RCA4-v4, which increases evaporation as soils were initially unsaturated. This higher evaporation is exported out of the basin towards Southern Africa, does not recirculate through the Cell, and does not therefore contribute to further improving the rainfall bias over the Congo. Our results show that reducing turbulent mixing results in a better representation of the dynamics of the climate system over the CB and, in turn, improved precipitation.