Dust emissions over the Sahel associated with the West African monsoon intertropical discontinuity region: A representative case‐study

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 mobilization, lifting and transport in the intertropical 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 mobilization was mainly observed in two locations: (a) within the monsoon flow as the result of the passage of a density current originating from a mesoscale convective system over southwest Niger, and (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. Behind and 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. The dust thus becomes 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.

[1]  Konrad Kandler,et al.  Dust mobilization due to density currents in the Atlas region: Observations from the Saharan Mineral Dust Experiment 2006 field campaign , 2007 .

[2]  Richard Washington,et al.  Atmospheric controls on the annual cycle of North African dust , 2007 .

[3]  Didier Bruneau,et al.  Wind infrared Doppler lidar instrument , 2001 .

[4]  R. Goff.  Vertical Structure of Thunderstorm Outflows , 1976 .

[5]  Paul Linden,et al.  A laboratory study of the velocity structure in an intrusive gravity current , 2000, Journal of Fluid Mechanics.

[6]  Michael Manga,et al.  Gravity Currents in the Environment and the Laboratory, by John E. Simpson , 1999 .

[7]  Geert Sterk,et al.  Causes, consequences and control of wind erosion in Sahelian Africa: a review , 2003 .

[8]  Pierre H. Flamant,et al.  Experimental Validation of Wind Profiling Performed by the Airborne 10-μm Heterodyne Doppler Lidar WIND , 2001 .

[9]  Didier Tanré,et al.  Satellite Climatology of Saharan Dust Outbreaks: Method and Preliminary Results , 1992 .

[10]  Richard Washington,et al.  Regional Model Simulations of the Bodélé Low-Level Jet of Northern Chad during the Bodélé Dust Experiment (BoDEx 2005) , 2008 .

[11]  L. Horowitz,et al.  Impact of air pollution on wet deposition of mineral dust aerosols , 2004 .

[12]  B. Grisogono,et al.  The critical bulk Richardson number in urban areas: verification and application in a numerical weather prediction model , 2006 .

[13]  J. Houghton,et al.  Climate change 2001 : the scientific basis , 2001 .

[14]  D Bruneau,et al.  Airborne Lidar LEANDRE II for Water-Vapor Profiling in the Troposphere. I. System description. , 2001, Applied optics.

[15]  A. Evan,et al.  Analysis of winter dust activity off the coast of West Africa using a new 24-year over-water advanced very high resolution radiometer satellite dust climatology , 2006 .

[16]  K. Schepanski,et al.  A new Saharan dust source activation frequency map derived from MSG‐SEVIRI IR‐channels , 2007 .

[17]  P. Linden,et al.  The sea breeze and gravity‐current frontogenesis , 1993 .

[18]  C. Thorncroft,et al.  African easterly jet in the ECMWF integrated forecast system : 4 DVar analysis , 2004 .

[19]  Jacques Pelon,et al.  Airborne observations of the impact of a convective system on the planetary boundary layer thermodynamics and aerosol distribution in the inter‐tropical discontinuity region of the West African Monsoon , 2007 .

[20]  C. Thorncroft,et al.  African Monsoon Multidisciplinary Analysis: An International Research Project and Field Campaign , 2006 .

[21]  P. Alpert,et al.  A jet stream associated heavy dust storm in the western Mediterranean , 1993 .

[22]  Charles S. Zender,et al.  Impact of Desert Dust Radiative Forcing on Sahel Precipitation , 2005 .

[23]  S. H. Melfi,et al.  Structure of an Internal Bore and Dissipating Gravity Current as Revealed by Raman Lidar , 1991 .

[24]  K. Hamilton.  Gravity Currents in the Environment and the Laboratory , 1998 .

[25]  Jacques Pelon,et al.  Seasonal evolution of the West African heat low: a climatological perspective , 2009 .

[26]  Qin Xu.  Density Currents in Shear Flows-A Two-Fluid Model , 1992 .

[27]  O. Torres,et al.  ENVIRONMENTAL CHARACTERIZATION OF GLOBAL SOURCES OF ATMOSPHERIC SOIL DUST IDENTIFIED WITH THE NIMBUS 7 TOTAL OZONE MAPPING SPECTROMETER (TOMS) ABSORBING AEROSOL PRODUCT , 2002 .

[28]  Corinne Le Quéré,et al.  Climate Change 2013: The Physical Science Basis , 2013 .

[29]  Jean-Jacques Morcrette,et al.  Influence of aerosol climatology on forecasts of the African Easterly Jet , 2005 .

[30]  Richard Washington,et al.  Atmospheric controls on mineral dust emission from the Bodélé Depression, Chad: The role of the low level jet , 2005 .

[31]  C. Thorncroft,et al.  The African easterly jet in the ECMWF Integrated Forecast System: 4D‐Var analysis , 2005 .

[32]  Peter F. Patel-Schneider,et al.  DLP System Description , 1998, Description Logics.

[33]  Andreas H. Fink,et al.  Synoptic and dynamic aspects of an extreme springtime Saharan dust outbreak , 2006 .