African Monsoon Multidisciplinary Analysis (AMMA) is an international project to improve the knowledge and understanding of the West African monsoon (WAM) and its variability with an emphasis on daily-to-interannual timescales. 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 so-called monsoon onset (associated with the rapid progression inland of the inter-tropical convergence zone). They aimed at understanding the role of key WAM features such as the inter-tropical discontinuity (the interface between the monsoon and the harmattan fluxes), the saharan heat low and the african easterly jet, as well as the Saharan aerosol layer, in the onset process.

In the framework of the first sub-SOPs, several ground-based instrumented sites (Tamanrasset, Algeria, Niamey, Niger and Djougou, Benin) were deployed across West Africa which were equipped with laser remote sensing systems (lidars). In addition, two aircraft operating from Niamey, Niger and equipped with airborne lidars – the LEANDRE 2 and WIND systems onboard the SAFIRE Falcon 20 and the DLR Falcon 20, respectively- were also operated during the period bracketing the climatological onset. Flight plans were tailored so that the differential absorption lidar LEANDRE 2 and the Doppler lidar WIND could make coordinated quasi-coincident measurements of water vapor mixing ratio and horizontal wind vector profiles.

This presentation provides an overview of the survey conducted during the WAM onset period of the evolution of key WAM features over Sahel using airborne and ground-based lidar measurements as well as numerous other in situ and passive remote sensing measurements (i.e. radiosondes, dropsondes, boundary-layer pressurized ballons, sunphotometers, etc..).