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Journal Article: ID no. (ISBN etc.):  0148-0227 BibTeX citation key:  BouKaram2009b
Bou Karam, D., Flamant, C., Tulet, P., Todd, M. C., Pelon, J. & Earle, W. (2009) Dry cyclogenesis and dust mobilization in the Inter Tropical Discontinuity of the West African Monsoon: a case study. IN Journal of Geophysical Research - Atmospheres, 114. D05115.
Added by: Cyrille Flamant 2009-01-18 00:00:28    Last Edited by: Fanny Lefebvre 2011-01-21 11:10:40
Categories: Atmospheric processes
Keywords: Aerosol
Creators: Bou Karam, Earle, Flamant, Pelon, Todd, Tulet
Collection: Journal of Geophysical Research - Atmospheres
Bibliographies: cnrm, Prior150410

Peer reviewed
Number of views:  1222
Popularity index:  64.66%
Maturity index:  accepted

Three-dimensional mesoscale numerical simulations were performed over Niger in order to investigate dry cyclogenesis in the West African Inter Tropical Discontinuity (ITD) during the summer, when it is located over the Sahel. The implications of dry cyclogenesis on dust emission and transport over West Africa are also addressed using the model results, together with space-borne observations from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) and the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP). The study focuses on the case of 7-8 July 2006, during the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1. Model results show the formation of three dry cyclones in the ITD during a 24-hour period. Simulations are used to investigate the formation and the development of one of these cyclones over Niger in the lee of the Hoggar-Aïr Mountains. They show the development of the vortex to be associated with (a) strong horizontal shear and low-level convergence existing along the monsoon shearline and (b) enhanced northeasterly winds associated with orographic blocking of air masses from the Mediterranean Sea. The dry cyclone was apparent between 0700 and 1300 UTC in the simulation and it was approximately 400 km wide and 1500 m deep. Potential vorticity in the centre of vortex reached nearly 6 PVU at the end of the cyclogenesis period (1000 UTC). The role of the orography on cyclogenesis along the ITD was evaluated through model simulations without orography. The comparison of the characteristics of the vortex in the simulations with and without orography suggests that the orography plays a secondary but still important role in the formation of the cyclone. Orography and related flow splitting tend to create low-level jets in the lee of the Hoggar and Aïr mountains which, in turn, create conditions favorable for the onset of a better defined and more intense vortex in the ITD region. Moreover, orography blocking appears to favor the occurrence of a longer-lived cyclone. Furthermore, model results suggested that strong surface winds (~11 m s-1) enhanced by the intensification of the vortex led to the emission of dust mass fluxes as large as 3 μg m-2 s-1. The mobilized dust was mixed upward to a height of 4-5 km to be made available for long-range transport. This study suggests that the occurrence of dry vortices in the ITD region may contribute significantly to the total dust activity over West Africa during summer. The distribution of dust over the Sahara-Sahel may be affected over areas and at time scales much larger than those associated with the cyclone itself.
Added by: Cyrille Flamant    Last Edited by: Fanny Lefebvre