Harmattan, Saharan heat low, and West African monsoon circulation: modulations on the Saharan dust outflow towards the North Atlantic

Abstract

Abstract. The outflow of dust from the northern African continent towards the North Atlantic is stimulated by the atmospheric circulation over North Africa, which modulates the spatio-temporal distribution of dust source activation and consequently the entrainment of mineral dust into the boundary layer, as well as the transport of dust out of the source regions. The atmospheric circulation over the North African dust source regions, predominantly the Sahara and the Sahel, is characterized by three major circulation regimes: (1) the harmattan (trade winds), (2) the Saharan heat low (SHL), and (3) the West African monsoon circulation. The strength of the individual regimes controls the Saharan dust outflow by affecting the spatio-temporal distribution of dust emission, transport pathways, and deposition fluxes.This study aims at investigating the atmospheric circulation pattern over North Africa with regard to its role favouring dust emission and dust export towards the tropical North Atlantic. The focus of the study is on summer 2013 (June to August), during which the SALTRACE (Saharan Aerosol Long-range TRansport and Aerosol-Cloud interaction Experiment) field campaign also took place. It involves satellite observations by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) flying on board the geostationary Meteosat Second Generation (MSG) satellite, which are analysed and used to infer a data set of active dust sources. The spatio-temporal distribution of dust source activation frequencies (DSAFs) allows for linking the diurnal cycle of dust source activations to dominant meteorological controls on dust emission. In summer, Saharan dust source activations clearly differ from dust source activations over the Sahel regarding the time of day when dust emission begins. The Sahara is dominated by morning dust source activations predominantly driven by the breakdown of the nocturnal low-level jet. In contrast, dust source activations in the Sahel are predominantly activated during the second half of the day, when downdrafts associated with deep moist convection are the major atmospheric driver. Complementary to the satellite-based analysis on dust source activations and implications from their diurnal cycle, simulations on atmosphere and dust life cycle were performed using the mesoscale atmosphere–dust model system COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model). Fields from this simulation were analysed regarding the variability of the harmattan, the Saharan heat low, and the monsoon circulation as well as their impact on the variability of the Saharan dust outflow towards the North Atlantic. This study illustrates the complexity of the interaction among the three major circulation regimes and their modulation of the North African dust outflow. Enhanced westward dust fluxes frequently appear following a phase characterized by a deep SHL. Ultimately, findings from this study contribute to the quantification of the interannual variability of the atmospheric dust burden.

North African mineral dust sources: new insights from a combined analysis based on 3D dust aerosols distributions, surface winds and ancillary soil parameters

“…5), south of the Tibesti mountains. It is active throughout the year with a maximum during the winter and a minimum during the summer, which agrees well with the seasonal cycle reported in the literature (Schepanski et al, 2007(Schepanski et al, , 2009(Schepanski et al, , 2012(Schepanski et al, , 2017Crouvi et al, 2012;Allen et al, 2013). Our analysis highlights further a high fraction of days with plausible dust emission during the winter located west of the Bodélé depression towards the Aïr Mountains in east Niger.…”

The influence of dust optical properties on the colour of simulated MSG-SEVIRI Desert Dust infrared imagery

“…For over a decade since they were first launched in 2002, the Spinning Enhanced Visible and InfraRed Imager (SE-VIRI) instruments onboard the Meteosat Second Generation (MSG) series of satellites (Schmetz et al, 2002) have provided a wealth of high temporal resolution information on the meteorology, climate, and the surface environment of Africa, Europe, the Middle East, and the Atlantic. In geostationary orbit above the Gulf of Guinea, SEVIRI is particularly well-placed to observe the deserts of the Sahara and the Arabian Peninsula, providing detailed information on atmospheric Desert Dust from and over these desert regions (e.g.…”

“…2 Satellite observations and modelling strategy 2.1 SEVIRI and "Desert Dust" RGB imagery Operated by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and positioned on the MSG series of satellites in geostationary orbit above the equatorial east Atlantic (0 • N, currently at 0 • E), the SE-VIRI instruments provide measurements of the Earth's surface and atmosphere every 15 min in three visible or nearinfrared channels, eight infrared channels, and one high spatial resolution broadband visible channel (Schmetz et al, 2002). The 11 standard channels have spatial sampling rates at nadir of ∼ 3 km.…”

“…The brightness temperature T B is derived by inverting the Planck function (e.g. Schmetz et al, 2002):…”

The influence of dust optical properties on the colour of simulated MSG-SEVIRI Desert Dust imagery

“…Atmospheric and hydrological conditions simulated by COSMO together with soil characteristics such as soil texture and soil particle size distribution (Tegen et al, 2002), vegetation cover expressed by the leaf area index (LAI), sub-grid scale orography and surface roughness length, determine the actual dust flux for each grid cell. The model setup has already been tested and validated against observations as presented in detail by Schepanski et al (2016Schepanski et al ( , 2017.…”