A 14‐Year Climatology of Saharan Dust Emission Mechanisms Inferred From Automatically Tracked Plumes

Abstract

The central and western Sahara is the largest source of mineral aerosols during boreal summer, but observed ground‐based data are extremely scarce and typically distant from key source regions. Knowledge of dust emission mechanisms has therefore been mostly limited to short‐term observations from a point or model approximations. To address this deficiency, dust plumes from the central and western Sahara are classified according to emission mechanism for June, July, and August of 2004–2017 using an automated inference method, which accurately tracks the timing, convective association, and geometry of plumes observed with the Spinning Enhanced Visible and Infrared Imager aboard Meteosat Second Generation satellites. From these characteristics, plumes are classified as either low‐level jet or cold pool outflow events. The extensive data set is used to generate the largest available climatology of dust emission sources and Saharan emission mechanisms. Automated inference compares well with ground‐based measurements from the Fennec Campaign (76% accuracy) as well as with an entirely manual approach (88% accuracy). Cold pool activity accounts for 82% of total observed dust and 88% at the point of emission. Dust from cold pools evolves seasonally from hot spots around the Mali‐Niger‐Algeria border triple point toward the central Sahara to the northwest, while dust from low‐level jets is organized along the axis of the northeasterly Harmattan, and dominates emission within the Tidihelt Depression of central Algeria. The widespread importance of cold pool outflows in this research supports the findings of the Fennec Campaign, but low‐level jets remain highly significant in certain isolated hot spots.

The Atmospheric Drivers of the Major Saharan Dust Storm in June 2020

“…In the cold and transition seasons, on the other hand, dust storms are usually triggered by mid‐latitude weather disturbances, in particular cut‐off lows that drift southeastwards into northwestern Africa (D. Francis et al., 2018, 2019). At more local‐scales, dust is lifted by different mechanisms such as (i) downward mixing of momentum from nighttime low‐level jets after sunrise when the boundary layer deepens (e.g., Allen & Washington, 2014; Caton Harisson et al., 2019), (ii) convective downdrafts associated with deep convective events (e.g., Heinold et al., 2013), and (iii) dust devils that occur during daytime arising from the strong heating of the surface by the Sun (e.g., Knippertz & Todd, 2012). As noted by Schepanski et al.…”

Satellite‐Derived Characteristics of Saharan Cold Pool Outflows During Boreal Summer

“…Mineral aerosols play an important role in the Earth's radiation budget on climatic and meteorological timescales, as a source of nutrients and as a pollutant (Boucher et al., 2013). Surface observations and satellite retrievals from the CWS indicate that cold pool outflows (CPOs) from deep convection are the dominant summertime meteorological dust emission mechanism (Allen et al., 2013, 2015; Caton Harrison et al., 2019); Lidar observations from the core of the central Sahara in June 2011 show that two‐thirds of emission events detected can be traced to this process (Allen et al., 2013). CPOs responsible for dust emission are often referred to as “haboobs” (Sutton, 1925).…”

“…As dust emission from the Sahara is highly variable from year to year (Mahowald et al., 2010; Prospero & Lamb, 2003; Tegen et al., 2013; Wagner et al., 2016), we use a large SEVIRI data set covering June, July and August of 2004–2017. A previous study (Caton Harrison et al., 2019) made use of the same satellite data set to infer whether observed dust plumes could be linked to CPO activity. Whereas Caton Harrison et al., (2019) aimed to account for all SEVIRI‐observed dust, the focus of this paper is a subset of these plumes which can be found at the leading edge of CPO density currents and which therefore render the CPO density current itself visible to satellite.…”

“…A previous study (Caton Harrison et al., 2019) made use of the same satellite data set to infer whether observed dust plumes could be linked to CPO activity. Whereas Caton Harrison et al., (2019) aimed to account for all SEVIRI‐observed dust, the focus of this paper is a subset of these plumes which can be found at the leading edge of CPO density currents and which therefore render the CPO density current itself visible to satellite. By contrast, a majority of plumes in Caton Harrison et al., (2019) had either no clear outflow boundary or persisted in the atmosphere long after this sharp boundary had dissipated.…”

“…Whereas Caton Harrison et al., (2019) aimed to account for all SEVIRI‐observed dust, the focus of this paper is a subset of these plumes which can be found at the leading edge of CPO density currents and which therefore render the CPO density current itself visible to satellite. By contrast, a majority of plumes in Caton Harrison et al., (2019) had either no clear outflow boundary or persisted in the atmosphere long after this sharp boundary had dissipated. In summary, this paper aims to: Derive an automated method for identifying and tracking Saharan CPOs in SEVIRI imagery Characterize the diurnal cycle, propagation speed, direction of travel and distribution of CPOs Relate patterns of CPO activity through boreal summer to the seasonality of Saharan dust …”