Dust storm over north Africa & eastern Med

By Maria Putsay, Kornel Kollath (Hungarian Meteorological Service), Jochen Kerkmann (EUMETSAT) and Jarno Schipper (ZAMG)

Dust clouds usually develop above the desert. Strong winds blow the sand into the air and it can remain airborne for a long time, thus, dust clouds can travel long distances. Sand storms originating in the Sahara can travel across the Mediterranean Sea and reach the European continent.

Dust clouds can be seen on a short-wave satellite image since the dust particles scatter the solar radiation. Unfortunately, the desert sand below the dust cloud also reflects strongly, only the blurring of the usual surface structure gives a hint of its presence. The dust cloud only becomes clearly visible on a short-wave satellite image when the wind blows it over the darker sea surface.

An example for such a dust storm can be seen near the Greek island of Crete on the high-resolution visible (HRV) image (lower images). Well structures gravity waves produced by the mountains of Crete can also be observed.

Many features can be seen better in RGB images. A frequently used RGB is the one that combines the three solar channels (NIR1.6,VIS0.8,VIS0.6, see image). In this image the dust cloud is easily distinguished from ice clouds. However, some types of water clouds (like stratiform water clouds) may show quite similar structure and colour. This can make recognition of dust clouds difficult.

Using the new channels on MSG satellites, a new approach has been developed to recognise dust clouds. This RGB composite image, the Dust RGB, does not use short-wave channels, it combines infrared channels and their differences (IR12.0–IR10.8, IR10.8–IR8.7, IR10.8). The infrared spectral features of the dust are a little different from those of water/ice clouds. Using the channel differences, the dust cloud becomes recognisable. The advantage of this method is that it works even during night-time. The dust clouds are easily separable from water or ice clouds and clearly visible, even over the desert.

The Dust RGB can be seen in the images below. Two animations are also presented. In the Dust RGB, the dust clouds appear in magenta/pink colour, the thick high-level clouds in red to brown, the thin high clouds (semitransparent ice clouds) in black, the mid-level clouds in brown-yellow, and the semitransparent mid-level clouds in green colours, while the low clouds are closer to yellow.

It should be noted that dust storms in Greece are not unusual, where such strong dust clouds are observed two or three times a year, causing extreme low visibility. Based on synoptic observations (see RGB images with Synop observations from 06:15 UTC in the Southern Aegean Sea there was significantly reduced visibility with reports of 800m visibility on Crete and only 200m on Naxos.

Additional content

Met-8 Airmass RGB composite (23 February 2006, 00:00 UTC)
Met-8 Dust RGB composite (23 February 2006, 00:00 UTC)
Met-8 Dust RGB composite (24 February 2006, 09:00 UTC)
Aqua MODIS image (23 February 2006, source: NASA Earth Observatory)