Environmental controls on African rainfall variability

Local African communities heavily depend on rain-fed agriculture, which are vulnerable to the impacts of extreme weather and climate variability/change. However, challenges remain in predicting African rainfall. For instance, climate models with cumulus convective parametrization lack the ability to explicitly simulate mesoscale convective systems that produce much of the total rainfall over West Africa. This study aims to improve our understanding of African rainfall variability, using state-of-the-art rainfall observations and convective-permitting (CP) ensemble simulations.

This work is organized into three research projects. In the first project, we evaluate the influence of Walker circulations on East African Rainfall. Three Walker circulations are identified near East Africa, namely, the East African, the Indian Ocean, and the Congo Basin Walker Circulations. Less (more) precipitation occurs over equatorial East Africa during October – December with a strong (weak) Indian Ocean Walker Circulation. The rainfall anomalies are associated with anomalous mid-level divergence over equatorial East Africa that is likely induced by horizontal moist static energy advection.

In the second project, we investigate how the observed shrinkage of Lake Chad since the 1970s affects the local climate using CP simulations. A smaller lake area enhances the local precipitation by limiting the spatial extent of daytime lake breezes, which would otherwise induce low-level subsidence/divergence and suppress local afternoon rainfall. In addition, a shallow planetary boundary layer over the large lake weakens turbulent vertical mixing, reducing low- to mid-level humidity and the likelihood of convection triggering compared to the small lake.

In the third project, we analyze the roles of thermodynamic and dynamic controls on intense storms over the West African Sahel with climate change. Future-Warming simulations are run with initial/boundary conditions prescribed by the current-climate data plus CMIP6 multi-model mean anomalies. The 99th (99.9th) 24-hr precipitation increases from 42 (77) mm in the current climate to 59 (104) mm in the future over the analysis region (12°N-18°N, 9°W-20°E). Increased low-tropospheric moisture (gradient) is more closely associated with this rainfall intensification than enhanced low- to mid- level vertical wind shear. Both moisture and shear conditions are related to the storm area and propagating speed that affect local rainfall accumulation.