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.

ITD Diurnal Cycle

B. Pospichal, D. Bou Karam , S. Crewell, C. Flamant, A. Hünerbein, O. Bock, F. Saïd (2009): Diurnal cycle of the inter-tropical discontinuity over West Africa analysed by remote sensing and mesoscale modelling, submitted to QJRMS.


Abstract

 

The diurnal cycle of the Inter-tropical discontinuity (ITD), i.e. the interface at the ground between the moist monsoon air and the dry Harmattan air, is an important factor in the West African monsoon system. During the whole year of 2006, high resolution ground-based remote-sensing measurements have been performed in the area of Djougou, Benin which made it possible to observe the ITD and the associated sharp gradient of temperature and humidity in detail.

 

In order to extend the point measurements to a 3D view of the ITD and to enhance the knowledge of the processes around the ITD, the mesoscale atmospheric model MesoNH has been run for a 84-hours period in April 2006. In addition Meteosat infrared observations have been used to determine the ITD position and its movement. From these observations a northward propagation of the moist air front (ITD) of 8-12 m s-1 has been calculated.

 

The model turned out to match well with the observations: For example, the time of front arrival in Djougou is simulated with a maximum error of about 1 hour and the speed of the ITD is consistent with Meteosat images. This agreement suggests the use of the model to further describe processes in the lower atmosphere.

 

Keywords: AMMA, West African Monsoon, Low level jet, MesoNH

Biomass burning aerosol characterisctics over Benin

Pelon,  J., M. Mallet, A. Mariscal, P. Goloub, D. Tanré, D. Bou Karam, C. Flamant, J. Haywood, B. Pospichal, S. Victori: “Characterisation of biomass burning aerosol from microlidar observations at Djougou (Benin) during AMMA-SOP0/DABEX”, J. Geophys. Res., 113, D00C18,  doi:10.1029/2008JD009976. 

Abstract:

Microlidar observations performed at the Djougou-Nangatchori site in northern Benin, have been performed during the AMMA-SOP 0/DABEX intensive observation period of AMMA in the dry season, from mid-January to mid-February 2006. During the dry season, the Djougou area is a region where biomass burning aerosols are heavily produced from agriculture fires.

The aerosol vertical distribution is also controlled by dynamics, as the penetration of the winter monsoon flux to the north, and northern winds bring mineral dust leads to a frontal discontinuity location close to Djougou latitude. During the early dry season, the aerosol vertical distribution was observed to be structured in two layers, the lower being the boundary layer reaching altitudes up to 2 km, and an upper layer reaching 4 to 5 km.

Lidar data are used to retrieve the time evolution and vertical profile of extinction, and discuss transport processes during the period analyzed. As the monsoon flux during the dry season is steadily progressing to the north, but also moving back and forth according to shorter time scale forcings, biomass burning particles are transported from the boundary layer into the upper troposphere. This transport has a strong impact on the distribution of aerosol particles on the vertical, and extinction values larger than 0.3 km-1 have been retrieved at altitudes close to 3 km.

A particular event of biomass burning air mass outbreak associated with a synoptic forcing is studied, where satellite observations are used to discuss observations of biomass burning particles over Djougou and at the regional scale.

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.