The role of moist convection in the West African Monsoon system
Moist convection is integral to the West African monsoon (WAM) system, but until recently computational limits have made analysis of its role very challenging. Now, recent advances in computing, combined with new observations from this highly data-sparse region, have now provided many new insights. Moist convection provides most rainfall in the WAM region, with many organised mesoscale systems over the Sahel, and less organised convection closer to the coast. Multi-day continental-scale simulations of the WAM that explicitly resolve moist convection, show how representing convection explicitly allows greater latent and radiative heating further north, with latent heating later in the day. This weakens the Sahel-Sahara pressure gradient and the resultant flow of moisture into the Sahara, delaying its diurnal cycle and changing interactions between the monsoon and dry boundary-layer convection. This changes the water budget of the whole WAM system. Furthermore, in explicit runs, cold storm outflows provide a significant component of the monsoon flux. These insights provide new understanding of long-standing biases in operational global models. ... mehrThe results demonstrate that improved parameterisations of convection that better capture storm structures, their diurnal cycle and rainfall intensities will therefore substantially improve predictions of the WAM. These model-based insights are supported by analysis of recent field campaign data. These show that the representation of cold-pools is the major cause of biases in global forecasts for the summertime Sahara, with subsequent impacts on the radiative budget, and that the role of cold-pools over the Sahel changes with the seasonal evolution of the monsoon and the energy available to such downdraughts. Finally, we demonstrate how large systematic disagreements between meteorological analyses of the WAM are consistent with errors caused by moist convection, demonstrating the need for both improved representations of convection and the need for improved routine observations from the region.