Dust

Dust

Introduction

1. Importance and impact

Atmospheric mineral dust is mainly produced by aeolian (wind‐driven) erosion acting in arid and semi‐arid areas. North African desert areas are the world’s most important dust sources. Mineral dust emitted on local and regional scales can represent risks and nuisances for the exposed populations living close to the source areas. Dust storms strongly reduce the visibility in the atmosphere, causing traffic problems in regions with frequent dust events. Frequent exposure to high dust concentrations occurring during dust storms has also been related to health problems like silicosis and is suspected of playing a role in the spread of infections and outbreaks of meningitis during the dry season in northern Africa (Morman and Plumlee, 2014).

During their transport in the atmosphere, mineral dust particles have an impact on the Earth’s radiative
budget by absorbing and scattering incoming solar and outgoing terrestrial radiation. Mineral dust can be involved in heterogeneous and multiphase atmospheric chemistry, affecting photo‐oxidant concentrations and the composition of precipitation. It can also contribute to biogeochemical cycles of nutrients like iron (Fe) and phosphorus (P) in deposition areas. The correct description of the spatial and temporal variability of emission, transport and deposition of dust is a prerequisite to estimate and forecast atmospheric dust concentrations and their impacts.

2. The Barcelona Dust Forecast

The MISVA site is not specifically dedicated to dust monitoring and forecasting. For this purpose it is advisable to use the produ!cts of the Barcelona Dust Forecast Center https://dust.aemet.es. Forecast bulletins and warnings are emitted by the WMO SDS-WAS Regional Centre for North Africa, the Middle East and Europe and distributed on its website https://sds-was.aemet.es/. Alert levels are determined on the basis of multi-model ensemble forecasts produced by the SDS-WAS https://sds-was.aemet.es/forecast-products/dust-forecasts/ensemble-forecast.

In addition to the SDS-WAS, the MISVA website provides meteorological situation analysis and other products that can help to understand the context of dust events and to forecast them. The remainder of this article summarises the main characteristics of desert dust over West Africa. A more detailed description is provided in Chapter 5 of the Handbook dedicated to Desert Aerosols and their Forecasting.

Caractéristiques

1. Sources

Desert aerosol emissions involve non-linear processes driven by meteorological conditions and surface properties (Marticorena 2014). Emission occurs in two phases as shown in the figure below (Laurent 2005).

  1. When the surface wind U* exceeds the erosion threshold U*t (~7 ms-1), the Saltation process allows soil particles with a diameter greater than 80 µm to be torn off and transported horizontally (Flux G).
  2. As they fall back, their collision releases finer particles constituting the desert aerosol. This is the sand-blasting process that generates a vertical flux F of dust.

Emission is therefore a complex threshold process depending on surface conditions (soil size and texture, roughness, humidity…). Above the erosion threshold U*t the horizontal flux G is proportional to the power 3 of the surface wind speed U*.

Schematic diagram of dust production. The vertical flux is produced during the ‘sandblasting’ process, when the saltating grains (horizontal flux) hit the surface and break the interparticle bonds linking dust particles together or to the surface. Source: Figure 5.1 of the Handbook.

The main dust source regions are in the southern Sahara and the northern Sahel. As highlighted in the figure below, there is disagreement on the exact location of the main sources according to the different products and analytical techniques used for their determination. The areas most common to many products are shown in grey and numbered PSANAF-1 to -6. PSANAF-2 and -3 are the most relevant for Senegambia, while PSANAF-5 affects Nigeria and neighbouring countries. The other main dust sources PSANFA-1, -4 and -6 are too remote to affect West Africa significantly, except for extraordinarily long and large-scale dust events.

The different lines and colours in the figure opposite represent analyses by different authors using different data and techniques (see legend), illustrating the challenge of identifying dust sources with good accuracy.

The most important regions are shaded: PSANAF-1: chotts area in Tunisia and northern Algeria; PSANAF-2: foothills of the Atlas Mountains (PSANAF-2a) and western coastal region (PSANAF-2b; Western Sahara, western Mauritania); PSANAF-3: Mali-Algeria border; PSANAF-4: central Libya; PSANAF-5: Bodélé depression (western Chad); PSANAF-6: southern Egypt, northern Sudan. Other notable source regions are the southern parts of the Azawagh, a dry basin in northwest Niger/northeast Mali (pink dotted line), and the Kaouar region in northern Niger.

Overview of Potential dust Source Areas in Northern hemispheric AFrica (PSANAF) based on a combined analysis by Formenti et al. (2011). Source : Figure 5.17 of the Handbook.

2. Transport

Atmospheric processes from synoptic, regional and local scales down to the turbulence scale can generate desert aerosol emissions and force their atmospheric mixing. The transport of these aerosols from and over the surface and into the atmosphere is controlled by the characteristics of the planetary boundary layer (PBL; see Chapter 4 of the Handbook).

The dominant transport directions of Saharan aerosols are: westward over the North Atlantic to South or North America, northward across the Mediterranean to southern Europe, and eastward over the eastern Mediterranean to the Middle East. Stuut et al. (2005) note the effect of the seasonal swing of the Intertropical Convergence Zone (ITCZ), from 19°N in the boreal summer to 5°S in winter, on the origin and directions of dust transport (Figure below). In summer, the desert aerosol layers undergo strong vertical mixing over the Sahel and southern Sahara.

Seasonal variation in latitudinal position of the ITCZ and its consequences for atmospheric conditions over northern Africa. Arrows indicate direction of trade winds; the extension of the dry and often dusty Harmattan air is marked by light grey shading over land and by a darker grey line over the ocean. December, January and February (winter) and June, July and August (summer) are respectively shown on left and right figures. Source : Figure 5.2 of the Handbook.

Over the ocean, Saharan aerosol layers are transported in summer above the trade wind inversion layer (up to 5-7 km above sea level) (Figure opposite). In winter, vertical mixing is weaker and occurs further south, so most of the desert aerosols are transported in the trade wind layer below 1.5-3 km.

Schematic diagram of vertical and horizontal dust export from North Africa toward the tropical East Atlantic for Northern Hemisphere winter (a, c) and summer (b, d). Source : Figure 5.3 of the Handbook.

3. Climatology

Observations made by the weather stations, such as horizontal visibility and weather conditions, allow climatologies to be established. The study by N’Tchayi Mbourou et al. (1997) shows a marked diurnal cycle of horizontal visibility reduction in the Sahel. Typically, the maximum reduction in horizontal visibility is observed between late morning and early afternoon, due to the development of the convective boundary layer. For Saharan stations, the diurnal cycle is less pronounced, but visibility reductions are frequently reached at the same time. The seasonal variation in the occurrence of horizontal visibility reduction varies according to the precipitation regime and thus according to the distance from the equator. In general, the dust storm season lasts longer as latitude increases: south of the Sahel (below 15°N), dust storms are observed from October to April/May, with a maximum in December/January. Further north, the duration of the dust storm season extends to sometimes cover the whole year. In the Sahara, the frequency of days when visibility is reduced by dust is highest in summer, while in the southern Sahara it is more common in the dry season. For Saharan and sub-Saharan stations, dust plumes generated by mesoscale convective systems (haboobs) are also observed in the rainy season.

Satellite observations provide better spatial coverage but over shorter periods. The figure below shows the total atmospheric aerosol loading, expressed as Aerosol Optical Thickness (AOT), from observations of the Multi-angle Imaging SpectroRadiometer (MISR) sensor in polar orbit on the Terra platform. The AOT represents the integrated aerosol content on the atmospheric column. Thus high AOTs do not necessarily correspond to high ground concentrations or large reductions in horizontal visibility. In addition, other aerosols, such as carbonaceous aerosols produced by biomass burning contribute to the AOT, especially over the Gulf of Guinea during the fire season (November to March). Two seasons can be clearly distinguished:

  1. From October to February, the aerosol content is at its minimum over northern Africa, despite a net increase over two regions: the Bodélé low – Ténéré region (Chad and Niger) on the one hand, and northwest Mali on the other.
  2. From March to May, the aerosol loading increases, forming a belt from the Red Sea to the Atlantic, centred between 15° and 20°N, which persists until August and diminishes in September.

Areas of high AOT are related not only to emissions, but also to atmospheric transport conditions, and to dust accumulation favoured by certain topographic structures (Schepanski et al., 2012).

Multi‐angle Imaging SpectroRadiometer (MISR) AOT monthly means for 2001–2011. Note that during November to March soot contributes significantly to the AOT over the Gulf of Guinea region. Source : Figure 5.11 of the Handbook.

Typical situations

1. Harmattan Dust Haze (HDH)

Dust emissions are generally associated with an intensification of the subtropical high pressure system, which results in an increase of the south-north pressure gradient over the region concerned. The figure below shows two frequent synoptic patterns associated with harmattan dry fog (HDH) events, explaining about 70% of the dust episodes occurring in the Sahel during the dry season.

  1. Either a strengthening of the Libyan High (eastern and central parts of West Africa),
  2. or an intensification/extension of the Azores High towards the northeast (northwest Africa).

Case studies CS05 and CS06 on the bilingual handbook case study (umr-cnrm.fr) illustrate and analyse these two configurations. These situations are sometimes preceded by the west-to-east passage of a mid-latitude low pressure system across North Africa. High pressure sometimes rises rapidly behind the cold front, which can penetrate far into the interior of the African continent. Unusual pressure distributions over northern Africa with intense episodes of HDH are often accompanied by large amplitude waves in the subtropical jet stream around the 250 hPa level.

Typical situations for the raising of dust during HDH periods. (a) Intensified Libyan High; (b) intensified/southeastward shifted Azores High accompanied by the passage of an extratropical trough across northern Africa. The dashed lines show typical isobars spaced every 5 hPa, with the 1015 hPa highlighted in bold. The arrows mark the regions of strongest 10 m winds indicating typical wind directions. The grey areas contain active dust sources. Situation (a) will typically precede an HDH episode in the eastern and central parts of West Africa, while situation (b) is more typical of HDH in the western parts. Source : Figure 5.18 of the Handbook.

2. Impact of the nocturnal Low-Level Jet

The formation of nocturnal low-level jets (NLLJs; Stensrud 1996; see section 4.1.3.2 of the Handbook) is an important phenomenon that occurs in clear skies, in dry regions with strong large-scale pressure gradients and in all seasons. These jets are dynamically related to the decoupling of the layer above the radiation inversion from the surface friction during the night. During the stabilisation of the CLP after sunset, the wind in this layer performs an inertial oscillation around the geostrophic wind with a period of about 28 hours at 25°N, which can lead to a super-geostrophic flow (faster than the geostrophic wind) around sunrise. The surface mixing of the NLLJ momentum during the morning development of the CLP leads to a peak of wind gusts near the surface and the emission of dust during this period (Washington et al., 2006; Schepanski et al., 2009).

The figure opposite shows the impact of the NLLJ on dust emission and transport in the planetary boundary layer.

  • It is a major source of dust, particularly in the Bodélé depression.
  • It contributes to the diurnal dust cycle with a low visibility peak in the morning.
  • Forecasting models have difficulty representing the gusts associated with vertical transport of NLLJ to the surface and therefore underestimate this source of emission.
Schematic depiction of transport and mixing of dust associated with a nocturnal LLJ. Source : Figure 5.20 of the Handbook.

3. Convective dust storms (haboobs)

Convective dust storms (haboobs) occur in the Sahel from the pre-monsoon period to the monsoon peak, from April to August. They are related to the descent and spreading at the surface of colder air generated by rain evaporation, often in connection with mesoscale convective systems such as squall lines (see Convection – Misva (aeris-data.fr) and Chapter 3 of the Handbook – section 3.1.3.2).

Meteosat satellite imagery is the best way to track the convection that can potentially lead to dust storms. The forecaster should be aware that a large part of the dust storms in the Sahel will be masked by the shield of cirrus clouds around the convective core (Figure opposite), while the part of the density current that extends over the Sahara can be more easily spotted. After the collapse of the associated convective system, the dust particles already suspended in the atmosphere can be tracked over considerable distances downstream. Haze events associated with the advection of dust from haboobs can be predicted in this way.

Large convective dust storm covering parts of the Sahel and Sahara on 9 June 2010, 1700 UTC. The bright pink dust over the Sahara is from a previous convective development, while the darker purple dust is from the convective system to the east and south. Note that clouds conceal the southern extension of the cold pool into the Sahel, as is often the case. Source : Figure 5.21 of the Handbook.

Products

MISVA website

  • Indication of the main dust areas on the WASA produced at 06 and 18UTC by CISMF forecasters under the name Anasyg (Météo-France)

Satellite Products

  • MeteoSat in particular the RGB colour composition Dust, but tricky to interpret as the channels used are different between day and night (see section 5.2.2.3 Observations satellitaires of the Handbook).

Barcelona Dust Forecast Center https://dust.aemet.es

References

Handbook

  • Chapter 5 Dust pages 175-203
  • Chapter 9 Section 9.1.4.4.3 Dust RGB : page 337

Illustrations and case studies

  • Handbook 
    • Figure 5.6 Large‐scale dust outbreak on 3 March 2004 as seen the the Dust RGB of MSG: page 180
    • Figure 5.16 Dust storm on 21 February 2004 as seen the the Dust RGB of MSG: page 191
    • Figure 5.21 Large convective dust storm (Haboob) on 9 June 2010 as seen the the Dust RGB of MSG:  page 199
  • Bilingual website handbook case study (umr-cnrm.fr)
    • CS05: 15-18 March 2012 –  Dust Storm driven by the Libya High pressure
    • CS06: 4-7 March 2012 – Dust Storm driven by the Azores High pressure

Articles

Laurent B. 2005. Simulation des émissions d’aérosols désertiques à l’échelle continentale: Analyse climatologique des émissions du nord-est de l’Asie et du nord de l’Afrique. Thesis, University Paris 12, Créteil, 225 pp.

Lothon M, Saïd F, Lohou F, Campistron B. 2008. Observation of the diurnal cycle in the low troposphere of West Africa. Mon. Weather Rev. 136: 3477-3500.

Marticorena B. 2014. Dust production mechanisms. Chapter 5 in: Knippertz P, Stuut J-B (eds.) Mineral Dust – A Key Player in the Earth System, Springer Netherlands, pp. 93-120.

Morman SA, Plumlee GS. 2014. Dust and human health. In: Knippertz P, Stuut J-B (eds.) Mineral Dust – A Key Player in the Earth System, Springer Netherlands, pp. 385-409.

N’Tchayi Mbourou GNT, Bertrand JJ, Nicholson SE. 1997. The diurnal and seasonal cycles of wind-borne dust over Africa north of the equator. J. Appl. Meteorol. 36: 868-882.

Schepanski K, Tegen I, Todd MC, et al. 2009a. Meteorological processes forcing Saharan dust emission inferred from MSG-SEVIRI observations of subdaily dust source activation and numerical models. J. Geophys. Res. 114: D10201. doi: 10.1029/2008JD010325.

Schepanski K, Tegen I, Macke A. 2012. Comparison of satellite based observations of Saharan dust source areas. Rem. Sens. Env. 123: 90-97. doi: 10.1016/j.rse.2012.03.019.

Stensrud DJ. 1996. Importance of low-level jets to climate: A review. J. Climate. 9: 1698-1711. doi: 10.1175/1520-0442(1996)009<1698:IOLLJT>2.0.CO;2.

Stuut J-B, Zabel M, Ratmeyer V, et al. 2005. Provenance of present-day aeolian dust collected off NW Africa. J. Geophys. Res. 110: D04202. doi: 10.1029/2004JD005161.

Washington R, Todd MC, Engelstaedter S, et al. 2006. Dust and the low-level circulation over the Bodélé Depression, Chad: Observations from BoDEx 2005. J. Geophys. Res. 111: D03201. doi:10.1029/2005JD006502.

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