Impact of soil-vegetation-atmosphere interactions on the spatial rainfall distribution in the Central Sahel

In a Regional Climate Model (RCM) the interactions between the land surface and the atmosphere are described by a Soil-Vegetation-Atmosphere-Transfer Model (SVAT). In the presented study two SVATs of different complexity (TERRA-ML and VEG3D) are coupled to the RCM COSMO-CLM (CCLM) to investigate the impact of different representations of soil-vegetation-atmosphere interactions on the West African Monsoon (WAM) system. In contrast to TERRA-ML, VEG3D comprises a more detailed description of the land-atmosphere coupling by including a vegetation layer in its structural design, changing the treatment of radiation and turbulent fluxes. With these two different model systems (CCLM-TERRA-ML and CCLM-VEG3D) climate simulations are performed for West Africa and analyzed. The study reveals that the simulated spatial distribution of rainfall in the Sahel region is substantially affected by the chosen SVAT. Compared to CCLM-TERRA-ML, the application of CCLM-VEG3D results in higher near surface temperatures in the Sahel region during the rainy season. This implies a southward expansion of the Saharian heat-low. Consequently, the mean position of the African Easterly Jet (AEJ) is also shifted to the south, leading to a southward displacement of tracks for Mesoscale Convective Systems (MCS), developing in connection with the AEJ. As a result, less precipitation is produced in the Sahel region, increasing the agreement with observations. These analyses indicate that soil-vegetation-atmosphere interactions impact the West African Monsoon system and highlight the benefit of using a more complex SVAT to simulate its dynamics.