Quantification of Dust load in the ITD region

Bou Karam D., C. Flamant, P. Tulet, J-P. Chaboureau, A. Dabas, M. C. Todd (2009b): Estimate of Sahelian dust emissions in the Intertropical discontinuity region of the West African Monsoon, submitted to JGR.


Abstract

A three-dimensional mesoscale numerical simulation has been performed to investigate the dust emissions over Sahel associated with strong near-surface winds in the region of the West African Inter Tropical Discontinuity (ITD) during the summer, when the ITD is located over Niger and Mali around 18°N.

The study focuses on the period from 2 to 12 July 2006, in the framework of the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1. A comparison with space-borne observations from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) as well as airborne lidar observations acquired on 7 July 2006 demonstrates that the model is able to reproduce the complex vertical structure of the dynamics and aerosol field associated with the monsoon-harmattan interface.

This suggests that the model can be used reliably to analyse and quantify the dust emissions associated with the strong monsoonal surface winds blowing over the Sahelian dust sources during the rainy season. A comparison with the surface observations in Niamey between 2 and 12 July period indicates that the near surface characteristics of the monsoon to the south of the ITD are realistically reproduced over the period of the simulation.

The dust load in the ITD region simulated by the model is compared to the estimates derived from satellite observations using the method of Koren et al. [2006]. The daily mean values of dust load related to the strong winds on both side of the ITD, estimated from the simulation within the model domain (2°W-16°E, 12-28°N), are in excess of 2 Tg on some days and are found to be underestimated with respect to the observational estimates.

In the present case, the dust load associated with the strong winds south of the ITD accounts for approximately 1/3 of the total load over the entire domain on a given day, and is simulated to range between 0.5 and 0.8 Tg on average.

This indicates that the mean daily dust load estimated in the ITD region is close to the estimated daily rate of dust emission from the Bodélé depression. This study suggests that emissions driven by strong surface winds occurring on both sides of the ITD while it lies across the Sahel may contribute significantly to the total dust load over West and North Africa observed annually.

Keywords: Dust, MesoNH, airborne lidar, density current, AMMA

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.