The 7–13 March 2006 dust storm over West Africa: Generation, transport, and vertical stratification

Abstract

[1] Several studies have shown the importance of desert dust aerosols in weather forecast models. Nevertheless, desert dust has been poorly represented in such models and is the source of some prediction errors, in particular for tropical and subtropical regions. The purpose of this paper is to illustrate the formation and the three-dimensional transport of a severe dust storm which occurred in March 2006 over West Africa. An intense dust haze was transported southward over the Gulf of Guinea thereby generating an aerosol optical thickness (AOT) greater than 4 over Nigeria. The MesoNH mesoscale atmospheric model coupled with new dust parameterization schemes has been used to illustrate the three-dimensional transport of the dust plume and the vertical layering of this desert air mass above the lower atmosphere monsoon flux layer. It is modeled that more than 50 g m À2 of dust was emitted during this event from the surface by a strong Harmattan wind over the Sahel region. It is also shown that when the dust layer is located over the boundary layer, it can modify the atmospheric stability by as much as 9.5 K in terms of potential temperature in the lowest 2000 m of the atmosphere.

Impact of dust aerosols on the radiative budget, surface heat fluxes, heating rate profiles and convective activity over West Africa during March 2006

“…This work seems to confirm results previously obtained by Grini et al (2006) over the Sahel region by using MesoNH and simulations conducted by Feingold (2006), Fan et al (2008) or Wendisch et al (2008), who show that the reduction of surface incoming radiation due to aerosols produce a weaker convective atmosphere.…”

“…As an example of results, H is changing from 270 to 150 W/m 2 , for the 11 and 13 March, respectively (at noon). Although the model well captures the decrease of sensible heat fluxes at the surface, direct comparisons (not shown) are not conclusive due to the difference observed between simulated and measured dust optical depth at Djougou (Tulet et al, 2008). A second explanation could come from the resolution used (12×12 km) in our simulations, which is maybe not adapted to conduct direct comparisons.…”

“…In parallel, experimental observations obtained at Djougou (Northern Benin) allowed to complement modelling simulations. Here, we perform a case study for an intense dust event occurring during March 2006 (Slingo et al, 2006;Milton et al, 2008;Tulet et al, 2008;McFarlane et al, 2009), leading to dust aerosol optical depth (AOD) larger than 2 (at 550 nm) over a large part of West Africa.…”

Long‐range transport of Saharan dust and its radiative impact on precipitation forecast: a case study during the Convective and Orographically‐induced Precipitation Study (COPS)

“…Regarding the short-wave effect, the refractive index of the dust aerosols was assigned according to that measured over West Africa. Their values, depending on the wavelength calculated over six spectral intervals, are described in Tulet et al (2008).…”

“…Refraction indices of pure water and ice were used for cloud liquid and ice crystals respectively. For consistency with the numerical experiments, the refractive index of mineral dust was taken from Tulet et al (2008), i.e. 1.448-2.92 × 10 −3 i at 532 nm, 1.44023-1.16 × 10 −3 i at 730 and 820 nm and 1.41163-1.06 × 10 −3 i at 1064 nm.…”