Large dust fan (Spanish Dancer) over west Africa

By Jochen Kerkmann (EUMETSAT)

March seems to be the peak month for large-scale dust outbreak caused by cyclonic activity and cold fronts in northwest Africa. Similar to the famous cases of 3 March 2004 (see previous case studies below) and 9 March 2007 (see below), the cold air fanned out across the Sahara, diverging greatly over subtropic regions, giving the dust front the form of a Spanish fan (see Figure 1).

As seen in the animation (Figure 2), the colour of the dust front varied throughout the day, from bright magenta (daytime, beginning of the animation) to dark magenta (night-time). This indicates that this was thin dust clouds at low elevations. Thick dust clouds have a dark magenta colour (day and night); thin dust clouds at high elevations do not show such colour variations, but appear with a red to bright magenta colour.

Interestingly, the dust outbreak over west Africa, and adjacent Atlantic Ocean (including the Canary Islands), was observed from three different angles by three multi-spectral instruments on three different satellites: SEVIRI on Meteosat-11 (nearly NADIR view), SEVIRI on Meteosat-8 (view from east), and ABI on GOES-16 (view from west, see Figure 3).

West Africa is probably the best observed area of the world, as regards geostationary weather satellite observations. Note that the dust cloud is best seen (best contrast) in the GOES-16 Dust RGB, where the dust cloud appears on the very limb of the image.

Finally, along the frontal zone, some dust was lifted to higher levels, reaching the level of the cirrus cloud shield of the cyclone over Algeria. This can be seen in the Meteosat-11 Convection RGB (Figure 4) where the upper-level cirrus shield appeared as a yellow colour (small ice particles). This is typical for 'dusty cirrus' clouds, also called DIBS clouds as a result of the paper Dust-infused baroclinic cyclone storm clouds: the evidence, meteorology, and implications, by Mike Fromm and others.

It is known that dust aerosols have a large impact on cloud microphysics, such as cloud phase and cloud particle size, and on cloud lifetime (dusty cirrus live longer than normal water or ice clouds). In general, clouds that have ingested dust glaciate more quickly, forming a large number of very small ice particles. These have a higher reflectance than large ice particles, in the 'microphysical' channels (near-IR 1.6 and IR 3.9 micrometer), so they appear as bright yellow in the Convection RGB. An example of this is given in the High levels of air pollution in parts of western Europe case from April 2014.

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