Long-range dust outbreak over Africa and Greece

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In late March 2018 a cold outbreak drove Saharan dust from the Atlas mountains down to the south-west coast of Africa and on to Greece.

Date & Time
20 March 2018 12:00 UTC–26 March 12:00 UTC
Meteosat-10 & 11, Sentinel-3
Dust RGB, Natural Colour RGB, Night Cloud Microphysics RGB, False Colour RGB

By Hans Peter Roesli (Switzerland) and Charalampos Karvelis (Hellenic National Meteorological Service)

Towards the end of March 2018 exceptionally frequent and vast dust outbreaks occurred over the Sahara, spreading dust across southern Europe, into the tropical belt of Africa, and over the Atlantic.

The long image sequence of Dust RGBs from Meteosat-11 from 20 March 12:00 UTC to 26 March 12:00 UTC gives an overview over the whole event (Figure 1).

Figure 1: Meteosat-11 Dust RGB animation, 20 March 12:00 UTC–26 March 12:00 UTC.
Download animation (MP4, 10 MB)

Unlike the usual mostly Harmattan or convective-driven March events, this dust season was dominated by cold-air outbreaks from low pressure areas crossing the Mediterranean.

On 20 March a cold front from one of these lows crossed the Atlas mountain range and lifted dust from the Moroccan-Algerian Sahara. Usually, in events like this, the lifted dust remains limited to the northern parts of the Sahara and may be driven east-north-eastward over the Mediterranean and beyond (see Previous Case Studies below)

In more exceptional cases the dust moves much more south down to the Sahel belt, where it then gets blocked, with its western branch fanning out into the Atlantic (see previous Case Studies below).

Figure 2
Figure 2: Meteosat-11 Dust RGB, 21 March 00:00 UTC (left panel) and 22 March 00:00 UTC (right panel)

In this case, on 21 March at an average speed of 45 km/h, the dust veil travelled down to the western Sahel (Figure 2). From there it spread even further south, although more slowly, into the tropical belt and the African west coast.

Figure 3
Figure 3: Sentinel-3 SLSTR, 23 March 10:45 UTC

From the west coast part of it was blown off over the Atlantic, as shown on 23 March in the false colour RGB (bands IR3.74/NIR1.61/VIS0.659) from the SLSTR instrument on Sentinel-3A (Figure 3).

This can also be seen on  the animated sequence of Natural Colour RGBs from Meteosat-11 from 22 March 08:00 UTC–25 March 18:00 UTC (Figure 4), and in four daily VIIRS True Color RGBs in the NASA Earth Observatory case study Bits of the Sahara on the Move.

Figure 4: Meteosat-11 Natural Colour RGB animation, 22 March 08:00 UTC–25 March 18:00 UTC.
Download animation (MP4, 3 MB)

The arrival of the dust-laden air in the Sahel between 21 and 23 March was well documented by the midnight radio soundings from Tamanrasset in Mali and Niamey in Niger. Both soundings showed a significant fall in temperature and a pick up of northerly winds below 700hPa.

On 22 March, simultaneously with the southward travelling dust fan over the Sahara, pre-frontal winds lifted dust over Libya and drove it out into the central-eastern Mediterranean. On the right panel of Figure 2 two parallel tongues of dust can be identified. The right-hand one shows the classical pink colour of the Dust RGB, pointing to dense dust extending vertically well above the boundary layer. The left-hand one, of a deep blue colour, lacks contrast and points to thin dust trapped in the maritime boundary layer.

This low-level dust appears much better in cyan colouring in the Night Cloud Microphysics RGB (Figure 5). On the same night, the dust front over the Sahara also appeared in cyan.

Figure 5
Figure 5: Meteosat-11 Night Cloud Microphysics RGB, 22 March 00:00 UTC, with colour reading of the cyan dust cloud

For an in-depth explanation of the cyan colour see the EUMeTrain Colour Interpretation Guide.

Dust over Greece

Figure 3
Figure 6: Meteosat-10 Natural Colour RGB, 22 March 13:30 UTC

It was one of the largest events of dust advection to Greece from North Africa. Dust levels in the atmosphere were extremely high in the southern part of the country, especially in the Crete Island and Cyclades (see Figure 6).

In fact, the African dust covered the entire country (including Athens) and concentrations were the highest in the last ten years. Forecasters were able to use satellite imagery as an aid in issuing warnings.

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