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We present a new characterisation of the three‐dimensional (3D) distribution of dust over the Sahara during summer, exemplified for June 2011. Our approach, called AEROIASI, is based on the innovative retrieval of vertical profiles of the dust extinction coefficient from daily cloud‐free hyperspectral Infrared Atmospheric Sounder Interferometer (IASI) satellite observations. AEROIASI observations clearly agree with other widely used measurements (from lidar and radiometers). The 3D characterisation is focused on the dust maximum in June 2011, located in the central Sahara (17–23°N, 1–7°E) and linked to the major atmospheric dynamical drivers associated with the West African Monsoon (WAM) system. AEROIASI shows the near‐surface dust load to be dominated by five major emission events occurring every 3–4 days. These all occur when the study region is under the influence of northward bursts of the WAM and convection‐related cold pools, likely associated with orographic forcing by the Aïr Mountains. During the earliest (June 10) and the dustiest (June 17) cases, northward advection of moisture over the hotspot is favoured by the superposition of cyclonic circulations related to an extratropical disturbance northwest of the Sahara and to the Saharan heat low over Mauritania, respectively. Convection over the hotspot also triggers wave‐like disturbances that travel westwards. The three dustiest events are characterised by elongated dust fronts moving northwards, with a leading edge spanning 200–300 km horizontally and extending from the surface up to 2 km of altitude. Further south, the dust layer progressively elevates to 3.5 km along the slanted isentropes at the interface of the monsoon and the harmattan, increasingly losing contact with the ground. When northerlies blow over the study region, elevated dust layers at 3–5 km are observed, which are transported southwards within the Saharan air layer and westwards along the northern edge of the African easterly jet (after June 13). [ABSTRACT FROM AUTHOR]

The aerosol direct and indirect effects are studied over west Africa in the summer of 2016 using the coupled WRF-CHIMERE regional model including aerosol–cloud interaction parameterization. First, a reference simulation is performed and compared with observations acquired during the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) field campaign which took place in June and July 2016. Sensitivity experiments are also designed to gain insights into the impact of the aerosols dominating the atmospheric composition in southern west Africa (one simulation with halved anthropogenic emissions and one with halved mineral dust emissions). The most important effect of aerosol–cloud interactions is found for the mineral dust scenario, and it is shown that halving the emissions of mineral dust decreases the 2 m temperature by 0.5 K and the boundary layer height by 25 m on a monthly average (July 2016) and over the Saharan region. The presence of dust aerosols also increases (decreases) the shortwave (longwave) radiation at the surface by 25 W m -2. It is also shown that the decrease of anthropogenic emissions along the coast has an impact on the mineral dust load over west Africa by increasing their emissions in the Saharan region. It is due to a mechanism where particulate matter concentrations are decreased along the coast, imposing a latitudinal shift of the monsoonal precipitation and, in turn, an increase of the surface wind speed over arid areas, inducing more mineral dust emissions. [ABSTRACT FROM AUTHOR]

In the framework of the Fennec international programme, a field campaign was conducted in June 2011 over the western Sahara. It led to the first observational data set ever obtained that documents the dynamics, thermodynamics and composition of the Saharan atmospheric boundary layer (SABL) under the influence of the heat low. In support to the aircraft operation, four dust forecasts were run daily at low and high resolutions with convection-parameterizing and convection-permitting models, respectively. The unique airborne and ground-based data sets allowed the first ever intercomparison of dust forecasts over the western Sahara. At monthly scale, large aerosol optical depths (AODs) were forecast over the Sahara, a feature observed by satellite retrievals but with different magnitudes. The AOD intensity was correctly predicted by the high-resolution models, while it was underestimated by the low-resolution models. This was partly because of the generation of strong near-surface wind associated with thunderstorm-related density currents that could only be reproduced by models representing convection explicitly. Such models yield emissions mainly in the afternoon that dominate the total emission over the western fringes of the Adrar des Iforas and the Aïr Mountains in the high-resolution forecasts. Over the western Sahara, where the harmattan contributes up to 80% of dust emission, all the models were successful in forecasting the deep wellmixed SABL. Some of them, however, missed the large nearsurface dust concentration generated by density currents and low-level winds. This feature, observed repeatedly by the airborne lidar, was partly forecast by one high-resolution model only. [ABSTRACT FROM AUTHOR]

This article is concerned with a dust-raising cold pool over the Sahara desert that occurred on 3-5 August 2006. Both the quantity of the uplifted dust and its spatio-temporal evolution are examined using satellite observations and a numerical simulation. The dust emission during this event was initiated by a mesoscale cold pool emanating from mesoscale convective systems (MCSs) that developed over northern Niger and Mali on 3 August. This event is one of several exceptional northward surges of the West African Monsoon (WAM) during the 2006 wet season. We examine the propagation of the cold pool and associated dust lofting using high temporal resolution false-colour dust product images from the Meteosat Second Generation Spinning Enhanced Visible and InfraRed Imager (MSG-SEVIRI). Observations from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) are used to characterize the vertical structure of the dust cloud as it spreads over the Sahara and across the Atlantic coast. The European Centre for Medium-range Weather Forecasts African Monsoon Multidisciplinary Analysis (ECMWF-AMMA) special reanalysis was used to describe the synoptic conditions that accompanied this event. Furthermore, a numerical simulation using the mesoscale model MesoNH was performed to estimate the emissions and the westward transport of dust during this event. MODIS aerosol optical depth (AOD) satellite imagery has then been used to track the dust plume across the Atlantic Ocean to Barbados, where comparisons are made with the local dust record there. The dusty cold pool covered southern Algeria and a large part of northern Mali and western Niger attaining a total area close to 2 × 106 km2. It extended over 2-3 km in altitude and had an AOD on the order of 1.5 and an estimated total dust load of about 1.5 Tg on average. Following daytime heating, the dusty cold pool and associated northward surge of moisture favoured the development of new convection and additional precipitation over the Sahara. The northward extension of the dusty cold pool was accompanied by a collapse of the Saharan heat-low, a characteristic feature of monsoon surges. A model-estimated quantity of 0.4 Tg of the dust produced during this event was subjected to westward transport toward the Atlantic Ocean after being mixed upward in the thickening boundary layer by the daytime heating over the Sahara to altitudes as high as 5-6 km. The arrival of the dust plume in Barbados in the Caribbean Sea 9 days after its departure from the west coast of Africa was characterized by a peak in dust concentration of 48.5 µg m−3. [ABSTRACT FROM AUTHOR]

Over the Sahara in summer, the activity of the Saharan thermal low pressure system (SHL), which is linked to the West-African monsoon dynamics and the mid-latitude circulation, is modulated by dust concentration and water-vapor transport. In this context, the role of clouds over western Sahara remains under-investigated. Using Meteosat-Second-Generation geostationary satellite data, for the first time the variability of cloud occurrence over Sahara by type in summer, at diurnal, daily and intra-seasonal time scales for the 2008–2014 period is documented. Using European Center for Medium-range Weather Forecasting (ECMWF) Reanalysis (ERA) Interim (ERAI) reanalysis, cloud cover occurrences are characterized in terms of regional circulation patterns and moisture balance. We show that, over West-Sahara and Hoggar, mid-top clouds are the most frequent cloud-type in summer. Their summit reaches between 500 hPa and 400 hPa and lies just above the top of the Saharan Atmospheric Boundary Layer (SABL). During the rest of the year, high-top clouds are the most frequent. The variations in the spatial distribution of mid-top cloud occurrence coincide with the seasonal displacement and strengthening of the SHL and, in the mid-troposphere, of the Saharan anticyclone. Mid-top clouds occur most frequently when, at large scale, mass and humidity converge in the lower SABL due to heating on an extensive surface, and diverge in the upper SABL. Their diurnal cycle, with minimal frequency around 10 UTC and maximum in the evening, is consistent with the diurnal development of the Saharan Convective-Boundary-Layer. The frequency of high cloud increases when anticyclonic circulations at mid-level and upper-level retreat to the southeast and upper-level trough from mid-latitudes can penetrate more southwards. [ABSTRACT FROM AUTHOR]

The Sahara is the world's largest source of mineral dust in the atmosphere, which is a key player of the Earth system. Desert dust layers impact the radiative budget, atmospheric stability, cloud properties, chemical balances and air quality depending on the altitude at which they are transported. Over the Sahara, the three-dimensional (3D) distribution of dust is closely linked to dust uplifting mechanisms and its redistribution in the vertical, driven by the main atmospheric dynamical actors in the region. A better understanding of these mechanisms is very important for assessing the 3D budget of Saharan dust, which is currently highly uncertain. However, observations of the 3D distribution of desert dust are highly scarce and particularly over the Sahara where they exclusively rely on satellite remote sensing. Indeed, the two-dimensional (2D) distribution of dust is standardly observed: either its horizontal distribution in terms of aerosol optical depth from e.g. MODIS, OMI, IASI measurements and also in this last case mean dust layer altitudes or latitudinal transects of vertical profiles of dust from the CALIOP lidar. In order to overcome this limitation, we have developed a so-called AEROIASI new satellite approach for retrieving the 3D distribution of desert dust from thermal infrared spectra measured by IASI. This method uses full radiative transfer calculations from the KOPRA model and an auto adaptive Tikhonov-Philips-type regularization for deriving vertical profiles of dust extinction coefficient at 10 μm for most cloud-free IASI pixels, both over land (including bright desert surfaces) and ocean. The dust vertical distribution derived from AEROIASI agrees remarkably well with along track transects of CALIOP space-borne lidar vertical profiles as well as with aerosol optical depth derived from AERONET sun photometer measurements over West Africa.We use AEROIASI to investigate the role of the main dynamic actors in the vertical redistribution of dust over the Sahara during June 2011. We find a major role of the African easterly waves and cold air intrusions as those from the West African monsoon, affecting directly or indirectly the vertical distribution and propagation of major dust plumes. Cold pools generated by mesoscale convective systems are as well a major mechanism for uplifting dust over the Sahel and Southern Sahara. In the present study, we will present the full 3D distribution of dust observed by AEROIASI and the link with these major dynamic actors of the region. [ABSTRACT FROM AUTHOR]

In June 2020, a major dust outbreak occurred in the Sahara that impacted the tropical Atlantic Ocean. In this study, the dust load and radiative forcing of the dust plumes on both the atmosphere and ocean surface is investigated by means of observations and modelling. We estimated dust loadings in excess of 8 Tg over the eastern tropical Atlantic, comparable to those observed over the desert during major Saharan dust storms. The dust induced an up to 1.1 K net warming of the ocean surface and a 1.8K warming of the air temperature (i.e., two to three times the respective climatological standard deviations), with a +14 W m−2 (∼28% of the mean value) increase in the surface net radiation flux at night. As the dust plumes extended all the way to the Caribbean, it is possible that this historical dust event helped fuel the record-breaking 2020 Atlantic hurricane season. The Saharan dust storm in June 2020 over the Atlantic Ocean heading to the Caribbean. Image Credit: NOAA. Dust is an important component of the Earth's atmosphere, with a wide range of impacts ranging from human health to effects on the ocean and climate. In fact, airborne dust aerosols modify the energy budget of the Earth's surface by interacting with the solar radiation in different ways where they absorb in the ultraviolet and longwave and reflect in the shortwave. In June 2020, massive amounts of dust were lifted from the Sahara, the major dust source region in the world, and transported all the way into the Americas across the tropical Atlantic Ocean. During this event, large dust loads, in excess of 8 Tg, covered the tropical Atlantic Ocean and induced a radiative forcing at the surface. The energy budget of the atmosphere and the ocean surface was impacted during this event resulting in a sustained warming during night and a cooling during day. The resulting effect of dust during this event was an increase in the air temperature and the sea surface temperature associated with an increase in the longwave radiation especially during night due to the re-emission of radiation towards the surface by the dust clouds. This warming may have contributed to the very active tropical storms' season in summer 2020. [Display omitted] • June 2020 Saharan dust storm associated with highest-on-record aerosol optical depths. • The estimated dust loads exceeded 8 Tg over the eastern tropical Atlantic. • The dust event caused a net warming of the ocean surface by up to 1.1 K. • Dust may have contributed to the extremely active 2020 Atlantic hurricane season. [ABSTRACT FROM AUTHOR]