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.

在具有强对流活动的区域（例如刚果盆地，CB），行星边界层中的湍流混合强烈影响水的收支。在这项研究中，我们使用基于过程的评估来评估强湍流和弱湍流混合条件下罗斯比中心区域气候模型（RCM）RCA4在模拟9月至11月CB降雨方面的性能。为此，分析了来自两个不同版本的模型的结果：协调区域气候降尺度试验框架（RCA4-v1）中使用的版本，以及减少了湍流混合的修改版本（RCA4-v4）。实验是根据ERA-Interim重新分析的边界条件进行的。结果表明，与RCA4-v1相比，RCA4-v4改善了CB降雨气候。该结果还与模型对相关驱动机制和过程的不同表示有关。减少湍流混合的模型版本（RCA4-v4）将较少的水分从较低的对流层转移到自由对流层。随着浅层对流混合的减少（由于湍流混合的减少），下层被润湿，低空云量在赤道非洲上方增加。在西赤道非洲（WEA）海岸，这种增长比在CB上更大。结果是，在WEA海岸上的表面太阳辐射比在CB上减少更多，这将导致在WEA海岸上的表面温度比在CB上更低。结果，在WEA和CB之间产生了增强的压力梯度，从而增强了刚果低层电解槽，和低层西风（LLW）。低空走水速度更快，这意味着更多的水分流过CB电池，在东部上升支流中被抬高，并进入非洲东风急流（AEJ），而非洲东风急流（AEJs）由于地表温度梯度的增加而加剧。CB细胞和中尺度对流系统（MCSs）的增强是降雨增加的原因，并且是改善RCA4-v4中CB降雨气候的原因。另外，降雨的增加导致了CB北部和南部RCA4-v4地区土壤水分的增加。由于RCA4-v1中的土壤已经饱和，较高的土壤湿度不会影响北方的蒸发。但是，降雨的增加增加了RCA4-v4南部地区的土壤湿度，这增加了蒸发，因为土壤最初是不饱和的。这种较高的蒸发量从盆地向外流向南部非洲，没有通过该单元再循环，因此也无助于进一步改善刚果的降雨偏向。我们的结果表明，减少湍流混合可更好地表示CB上气候系统的动态，进而改善降水。