Meteosat-8 was well positioned to observe dust storms over the Sahara, the Arabian desert and the Gobi desert in March 2021.
21 April 2021
24 March 2021
By Jochen Kerkmann (EUMETSAT), HansPeter Roesli (Switzerland), Scott Lindstrom (CIMSS)
Each year in March, large, synoptic-scale dust outbreaks can be observed over the main dust source areas of the world — the Sahara, the Arabian desert and the Gobi desert.
March is the peak month of large dust outbreaks on the Northern Hemisphere. Often, the dust outbreaks last several days and cover vast areas of Northern Africa or Asia (so-called continental-scale dust outbreaks). Good examples of these are the Sahara dust outbreaks from March 2004, from March 2006, and from March 2010.
The typical synoptic pattern that causes these large dust outbreaks over northern Africa is a high pressure area over northern Africa (strengthened by cold air outbreaks from Europe), that presses against a low pressure area over the southern Sahel area (i.e. the ITCZ, which during the month of March starts to move back northwards), and strong Harmattan winds in between. Minor dust outbreaks (not continental-scale) can be caused by upper-level lows (cut-off lows or COL) or by cold fronts crossing the Sahara. In most cases, numerical models (dust forecasting models) can handle such situations quite well.
Meteosat-8, at 41.5 degrees East, is ideally positioned to observe all major deserts of the Northern Hemisphere: the Sahara in Africa, the Syrian and the Arabian deserts in the Middle East, and the Karakum, Kavir, Taklamakan and Gobi deserts in Asia — known collectively as the dust belt. Probably 80-90% of all dust outbreaks occur in this area and can be observed by Meteosat-8.
Quite often, simultaneous dust outbreaks occur in various places of the dust belt. A good example for such a situation was 12-15 March 2021, when dust clouds were observed west of Africa, over Mali/Algeria, Saudi Arabia, Iran and China (Figure 1).
The dust over Mali/Algeria was caused by a deep, cut-off Low over Algeria; the dust over the Middle East by a rapidly moving upper-level trough over western Asia (Figure 2).
Figure 3 shows a 24-hour animation of the dust swirl over western Africa, related to the stationary cut-off low over Algeria. Most of this dust is low-level dust seen by the darker magenta colour during the night hours. Smaller dust oubreaks can be seen over Libya, Niger, Chad and Sudan.
The dust outbreak over the Middle East started on 12 March at around 04:00 UTC (Figures 4 and 5). Large amounts of dust were lifted behind a cold front (thick post-frontal dust, darker magenta colour) that crossed the entire Middle East. Later during the day, additional dust was lifted in the pre-frontal area. The latter appears with a different, more bright magenta colour.
Both, the thick post-frontal dust and the thinner pre-frontal dust can be seen in the SNPP VIIRS True Color RGB image (Figure 6, left) taken in the afternoon (SNPP equator local crossing time of about 13:30 h, aka PM orbit). The intense dust/sandstorm swept over many parts of Saudi Arabia, downing streetlights and electricity poles, severely limiting visibility, and forcing authorities to urge people to stay home, especially those with respiratory problems.
The considerable temperature contrast created by the thick low-level dust over RubAlKhali can be seen by comparing the Meteosat-8 Dust RGB and infrared imagery (Figure 7).
As the upper-level trough and the related cold front moved eastward, so did the dust outbreak. Towards the end of the animations in Figures 4 and 5, new dust can be seen over eastern Iran and over Afghanistan. Parts of this dust got lifted into the uppermost troposphere and into the cirrus clouds of the baroclinc system (Figure 8).
According to Michael Fromm et al. in the paper Dust‐infused baroclinic cyclone storm clouds: The evidence, meteorology, and some implications, these dust‐infused baroclinic storms (DIBS, yellow in the Convection RGB) exhibit peculiar cirrus cloud top reflected and emitted radiance from the UV through thermal IR, involving positive UV absorbing aerosol index, muted visible reflectivity, visible cumuliform texture, and systematically intense visible lidar backscatter on a synoptic scale.
Around the same time as the dust clouds were over the Middle East, Beijing, China, was hit by the worst sandstorm in a decade. The BBC reported that pollution levels were 160 times the recommended limit. The dust and sand that spread over the city originated in Mongolia, and the dust RGB animation below, shows dust appearing on 14 March and spreading rapidly southeast behind a propagating extratropical cyclone. Multiple mid-layer clouds somewhat interfere with the dust/sand detection (bright red/magenta in the dust RGB shown), but the origins in Mongolia after 06:00 UTC on 14 March, and the quick spread south by 18:00 UTC on 15 March are apparent.
Sandstorm hits Beijing China (CIMSS Blog)
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Meteosat-8 observes the dust belt: 11-22 March Dust RGB animation (three-hourly time steps)
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