How shrinkage of Lake Chad affects the local climate

Convective-permitting ensemble simulations are used to understand how areal changes of Lake Chad affect precipitation. Shrinkage of Lake Chad from about 25,000 km(2) to its current size (similar to 1300 km(2)) increases rainfall over the lake basin in the afternoon. Reduced precipitation over a large lake is associated with a shallow planetary boundary layer, which decreases the likelihood of triggering convection by turbulence and weakens the vertical moisture mixing. As a result, the large lake does not act as a moisture source for afternoon convection, but lessens the low to mid-level humidity leading to a more stable daytime environment. Daytime lake breezes also act to inhibit convection over the large lake by inducing low-level moisture divergence and subsidence. During the recession to a small lake, Lake Chad is partially replaced by wetlands which also reduce local afternoon rainfall but not to the same extent as the large lake. This effect is primarily associated with anomalous subsidence and cooler surface temperatures over the wetlands than over the adjacent land during the day. Boundary layer heights over the wetlands are much higher than over the large lake, accounting for the wetlands' weaker effect in suppressing daytime rainfall. Loss of Lake Chad and adjacent wetlands impacts the local and mesoscale climate through modification of the spatial area influenced by daytime lake/wetland breezes. The rainfall response to the circulation perturbation of the lake/wetlands can be overlooked in models with cumulus convective parameterization.

Automated summary

The areal changes of Lake Chad have an impact on precipitation, with the shrinkage of the lake increasing rainfall in the afternoon over the lake basin. Reduced precipitation over a large lake is due to decreased convection caused by a shallow planetary boundary layer. Additionally, loss of Lake Chad and adjacent wetlands affects the local climate and circulation patterns.