Transport of dust particles from the Bodele region to the monsoon layer. Case study of the 9-14 june 2006 period

Within the framework of the AMMA field campaign, state-of-the-art aerosol in situ instrumentation has been deployed on the French ATR-42 research aircraft. In particular, measurements taken on-board allowed us to study aerosol concentrations, detailed size distributions, and the chemical and hygroscopic properties of aerosols. This present study is based on two different flight missions (SOP1) over source regions in Niger and along the meridian Niamey/Cotonou. The first aim of this study is to improve the aerosol size distribution to take into account over West Africa during the long range transport of dust from its emission to its sedimentation. The second objective is to quantify the sedimentation process of the coarse and accumulation modes of mineral dust aerosols from the sahelian layer to the boundary layer.

Based on the Alfaro and Gomes 2001 (AG01) parameterisation and on ATR-42 measurements, a new scheme of dust size distribution has been constructed by summing 3 modes (a coarse mode derived from AG01, a finer mode derived from observations and a mode in-between common to observations and AG01). This multimodal log-normal distribution has been developed and implemented to modelize an intense African dust event that has been observed over Niger from 9 to14 June 2006. The results indicate that the present modelled emission and transport yield a mass and number concentration distribution as well as an aerosol optical thickness much closer to observations.

During their transport, dust particles are subject to a variety of processes like sedimentation or buoyant mixing that may lead to the presence of dust particles in the lower layer of the atmosphere. In particular, a breeze circulation due to vegetation boundaries have been observed and highlighted by the simulation results. Moreover, evidence of links between area of dust sedimentation and breeze localization exist in the simulation results as well as in the observations. Then, the concentration of dust particles transported in the Saharan Air Layer (SAL) and retrieved in the Monsoon Layer (ML) will be quantified.