Abstract
The structure of the Saharan air layer over Niger and Benin during a major springtime dust event from the Bodele region and Sudan is investigated using airborne lidar and dropsonde measurements. Aircraft operations were conducted on 13 and 14 June 2006, within the framework of the African Monsoon Multidisciplinary Analysis Special Observing Period. Complementary ground-based and satellite observations, as well as European Centre for Medium-range Weather Forecasts analyses are used to investigate the regional aspects of emission, transport and deposition in the period from 9 to 14 June 2006, to provide a framework for the interpretation of the airborne measurements. The study details the transport patterns of dust from eastern Saharan sources towards the southwest in the springtime, and highlights the role of the intertropical discontinuity and the Darfur mountains in injecting the aerosols from Bodele and Sudan, respectively, over the monsoon flow, and in the African easterly jet (AEJ) region. It also illustrates the impact of the daily variability of the emissions in the source regions on the dust load advected westward by the AEJ and observed over Benin. The impact of a mesoscale convective system (MCS) on the dust load and vertical distribution observed over Benin and southern Niger was also investigated, nearly 12 hours after its passage over Benin. The only discernible impact on the dust distribution is observed to be associated with widespread subsidence in the wake of the MCS, over northern Benin and Niger. Wet scavenging related to convective or stratiform rain could not be observed, as processed air masses were replaced by fresh dust transported in the upper Saharan air layer by the AEJ. Over southern Benin, the dust distribution appeared to be mostly controlled by processes affecting the planetary boundary layer upstream, i.e. over Nigeria or Chad. Because springtime dust from remote eastern sources (such as observed in this case) is mainly transported into the AEJ, it could impact on the radiation budget in the AEJ region, thereby possibly modifying the West African weather at the synoptic scale.
Highlights
The impact of dust on the climate system and the large uncertainties associated with the role of dust in the Earth’s radiation budget is widely recognized (e.g. IPCC, 2008)
Tompkins et al (2005) have highlighted how improvements in aerosol representation in the European Centre for Medium-Range Weather Forecasts (ECMWF) model significantly improves the 5-day forecasts for the West African monsoon. They have shown that a more realistic representation of the threedimensional fields of dust led to improved representation of the structure and position of the African easterly jet (AEJ), a central feature of the West African weather playing an important role in the organisation and propagation of mesoscale convective systems (MCSs) and in the transport of dust across the African continent
The linear Pearson correlation coefficient between the averaged 925 hPa winds extracted from the ECMWF analyses at 0600 UTC and the Moderate-resolution Imaging Spectroradiometer (MODIS) Deep Blue derived AODs around midday the same day is equal to 0.87, 0.89 and 0.94, over the Bodele, west Sudan and east Sudan regions, respectively
Summary
The impact of dust on the climate system and the large uncertainties associated with the role of dust in the Earth’s radiation budget is widely recognized (e.g. IPCC, 2008). It is a period when the dust load over Sahelian and sub-Sahelian countries exhibits the largest temporal and spatial variability, both in the horizontal and the vertical This is related to: (i) the presence of the intertropical discontinuity (ITD) over the Sahel which has been shown to favour the uplift of dust during the night-time when the leading edge of the monsoon flow exhibits the characteristics of a density current (Bou Karam et al, 2008), (ii) the occurrence of the first MCSs over Sahel (south of the ITD) and their associated cold pools which offer an efficient mechanism for dust lifting and injection to altitudes favourable for long-range transport, before the growing vegetation rapidly inhibits local dust emission (e.g. Sterk, 2002; Flamant et al, 2007; Marsham et al, 2008), and (iii) the strong harmattan winds to the north of the ITD which can generate large uplift of dust from the North African sources in the form of discrete outbreaks (Knippertz, 2008).
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