Large dust swirl over Algeria

By Jochen Kerkmann (EUMETSAT) and Kornél Kolláth (Hungarian Meteorological Service)

The dust cloud was first visible at 10:00 UTC on 20 February (see Dust RGB) when the cold air from the eastern Atlantic entered Morocco and Mauritania.

At 12:00 UTC on the following day a fully developed, spectacular dust spiral appeared on the MSG images in conjunction with the low-level depression over Algeria (see Dust RGB). In the following days the dust cloud moved eastward crossing Tunesia, Libya, Egypt and the eastern Mediterranean.

It is well known that aerosols like desert dust particles can have a large impact on cloud microphysics and cloud development. Generally, the entrainment of desert dust into Cb or Ns clouds leads to the formation of small ice particles, which can be observed from satellites. This can be seen in the second image mosaic below that shows three different MSG RGB composites: the Dust RGB (left), the Day Microphysics RGB (centre) and the Day Convective Storms RGB (right).

The colour of the high-level ice clouds in the centre and right images (light orange and yellow, respectively) indicates the presence of ice particles with small effective radii (also indicated by the high reflectivity in the IR3.9 channel >10%). As small ice particles can stay up in the air without evaporating or fall out, such clouds typically preserve their shape over a much longer period of time than clouds with large (ice) particles and thus travel over long distances (not shown here).

Finally, the last image taken at 15:45 UTC on 22 February (i.e with low Sun elevation) shows interesting fine-scale structures of the dust cloud, in particular a gravity wave-like structure of the top of the dust cloud to the east and southeast of the depression.