A newly identified mechanism for dust emission over West Africa


Bou Karam, D., C. Flamant, P. Knippertz, O. Reitebuch, J. Pelon, M. Chong, A. Dabas (2008): Dust emissions over the Sahel associated with the West African Monsoon inter-tropical discontinuity region: a representative case study, Q. J. R. Meteorol. Soc. 134: 621–634.

Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/qj.244

Abstract:

Near dawn airborne lidar and dropsonde observations acquired on 7 July 2006, during the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1, were used to investigate dust mobilisation, lifting and transport in the inter-tropical discontinuity (ITD) region over western Niger.

Atmospheric reflectivity data from the LEANDRE 2 lidar system enabled us to analyse the structure of dust plumes in the context of wind and thermodynamic information provided by the WIND lidar system and dropsondes.

Dust mobilisation was mainly observed in two locations: (a) Within the monsoon flow as the result of the passage of a density current issued from a mesoscale convective system over southwest Niger. (b) At the leading edge of the monsoon flow where the near-surface winds and turbulence were strong, because the monsoon flow was behaving as an intrusive density current. The circulation in the head of the monsoon density current lifted the mobilized dust towards the wake, along an isentropic surface. Away from the leading edge, some of the mobilized dust was observed to mix across the monsoon-harmattan interface, due to the existence of mechanical shear above the monsoon layer, and to become available for long-range transport by the harmattan.

Because dust sources are widespread over the Sahel and presumably active on many days when the ITD is located in this region during summer, dust emissions associated with the described mechanism may influence the radiation budget over West Africa.

Key words: Monsoon, Harmattan, Density current, AMMA, Airborne lidars, Dropsondes, ECMWF analyses.

Main goal

I’m interested in evaluating the radiative impact of aerosol dust on atmospheric dynamics which is an important component of the climate system but is still poorly quantified.

To reach my goals, I use an observations-modelling coupled aproach.

In term of modelling my mean is the regional atmospheric model MesoNH coupled with a dust emission model (the DEAD model) together with the ECWMF analyses. In term of observations, I use the measurements collected during two field campaigns: the BoDEx campaign 2005 and the SOP2a1 2006 of the AMMA project where airborne LIDAR and ground based measurements of meteorological variables were performed. Satellite observations are also investigated in my studies such as CALIPSO , OMI , MODIS and SEVIRI data. For more details, please check the other sections on this website.