The entrainment process between the Monsoon flow and the Saharan Air Layer in the West African Monsoon system

The West African Monsoon (WAM) has been the focus of a international program AMMA (African Monsoon Multidisciplinary Analysis) for a better understanding of its variability and processes and a better forecast of the precipitation in the Sahel. We focus on processes at meso, submeso and smaller scales that occur in the low troposphere in this region where the moist and cold monsoon flow interacts with the overlying Saharan Air Layer. We use aircraft observations from the AMMA experiment and Large Eddy Simulations (LES) to investigate the role of the PBL in the interaction between the two opposite flows. At the top of the PBL, the entrainment, which incorporates dry and warm air from the free troposphere into the PBL, is a key process. The LES, by resolving the dry tongues (coherent structures of entrainment) in the upper part of the PBL allows us to better understand this process and its relation with the shear between the two opposite flows. A first LES simulation is designed based on observations of 5 June 2006, a case under strong wind conditions during the transition period from dry conditions to the active monsoon phase. This LES is able to reproduce the boundary-layer development observed this day. From this specific case, sensitivity tests are carried out to cover a range of conditions observed during 7 other flights made in the same transition period. These tests enable us to describe more completely the entrainment zone.A previous thorough analysis of 15 flights before and after the monsoon onset revealed several results to further investigate. For instance: (i) a rapid growth of the PBL that cannot always be explained by the entrainment nor by the large-scale vertical velocity (ii) a large spectrum of entrainment rate around the typical value of 0.2, (iii) larger entrainment close to the intertropical discontinuity, and smaller entrainment in the active phase. The combination of LES and observations allows us to address some unresolved issues on entrainment, linked with the difficulty to estimate it with observations or to parameterize it in the models.