The inter-tropical discontinuity (ITD, the interface between the monsoon and the harmattan fluxes) is a key feature of the West African monsoon (WAM) system as well as its variability on daily-to-seasonal timescales. During the summer, the ITD is generaly observed to migrate northward during the so-called monsoon onset (associated with the rapid progression inland of the inter-tropical convergence zone). The arrival of the ITD over the Sahel marks the begining of the much awaited rainy season. At the regional scale, the position of the ITD is generaly controled by the location and intensity of features such as the Saharan heat low, the Libyian and St Helens anticyclones, the inter-tropical convergence zone as well as mid-latitude cyclogenesis. At the mesoscale, the surface-atmosphere interactions play a major role on the diurnal evolution of the ITD structure. Improved knowledge of these interactions and of the underpining processes are needed in order to better comprehend the gradual soil moistening associated with the northward progression of the ITD at the seasonal scale, which in turn favors the propagation of mesoscale convective systems over northern Sahel.

The African Monsoon Multidisciplinary Analysis (AMMA) is an international project to improve the knowledge and understanding of the West African monsoon (WAM). AMMA is motivated by an interest in fundamental scientific issues and by the societal need for improved prediction of the WAM and its impacts on West African nations. The AMMA Special Observing Periods (SOPs) which were carried out in the summer of 2006 were designed to fulfill one of the three overarching aims of AMMA concerned with improved knowledge of surface-ocean-atmosphere interactions (including the role of aerosols) in West Africa and the influence of WAM on the physical and chemical environment at the regional scale. The first two SOPs took place in the early monsoon (spanning from 1 June to 15 July 2006 ) were dedicated to the analysis of surface-ocean-atmosphere interactions prior to and after the monsoon onset.

In the framework of these SOPs, the structure of the ITD has observed by means of an airborne lidar –the differential absorption lidar LEANDRE 2 system onboard the SAFIRE Falcon 20- between 3 and 10 july 2006 over northern Niger. Large dust uptakes were observed to be associated with the leading edge of the monson flux and to be transported southward above the monsson flux. In this presentation, the mechanims associated with the Saharan dust uptake and transport in connection with the inter-tropical discontinuity dynamics are analysed by means of observations and numerical simulations using the mesoscale model Meso-NH.