Sharp Dust Boundary over northern Africa

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A sharp dust boundary formed in March 2018, after the clash of a dry and dusty Saharan air mass with a moist tropical air mass.

Date & Time
29 March 2018 00:00–21:00 UTC
Satellites
Meteosat-10, Terra, Aqua
Instruments
SEVIRI, MODIS
Channels/Products
Dust RGB, True Color RGB

By Ivan Smiljanic (SISYS)

A dry, dusty air mass from the Sahara, advected towards the south, confronted a tropical air mass from the outflow of a ITCZ convective system over southern Sudan and Chad.

An irregular frontal line formed in that area around 08:00 UTC on 29 March, defined by local orography.

Along the formed front, dusty air from the north was pushed against the moist air from the south, forming a relatively sharp boundary. This boundary is clearly seen in the high resolution True Color RGB imagery (Figure 1).

Image comparison
Aqua MODIS True Color image, 29 March 11:30 UTC Terra MODIS True Color image, 29 March 08:25
Figure 1: Comparison of Aqua and Terra MODIS True Color RGB images, 29 March 08:25 UTC and 11:30 UTC

The collision of two the air masses can be clearly seen on the animated Meteosat-10 Dust RGB imagery (Figure 2). This moment is captured through the apparent slowing down of the advection of the dusty air on its way towards the south. After the collision and the short halt to the advection, the piled-up, dusty air recovers the initial speed southwards.


Figure 2: Meteosat-10 Dust RGB animation, 29 March 00:00 UTC–21:00 UTC

Looking at the higher resolution imagery in Figure 2, it can be easily assessed that between 08:25 UTC and 11:30 UTC (the two sequential MODIS overpasses over the area shown in Figure 1) the sharp dust boundary only moved about 40 km upstream of the White Nile.

The evidence for the northward movement of the tropical air mass, is the lifted ‘dust puff’ in eastern Sudan, close to border with Eritrea, with its northward movement.

Also, the fact that the two air masses collided is evident from the formed convergence zone reflected through a wavy cumulus field ahead of the dusty front (Figure 3) and the convective activity along the frontal line (Figure 4).

Figure 3
 
Figure 3: Cumulus field generated by instability between tropical and airmasses - Met-10 Dust RGB, 29 March 14:00 UTC

Figure 4: Meteosat-10 Dust RGB animation (zoomed in), 29 March 10:15 UTC–21:00 UTC
 
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