Springtime dust outbreaks
21-26 April and 4-5 May 2022
During late April and early May 2022 numerous dust outbreaks affected the Mediterranean basin area. During this event dust originated from different sources.
05 May 2023
09 May 2022
By Djordje Gencic and Ivan Smiljanic (Exostaff), Federico Fierli (EUMETSAT) and Miguel-Angel Martinez (AEMET)
Dust in April
The Mediterranean region was affected by a persistent dust intrusion event between 21 and 26 April. The event was forecast by the system implemented in ECMWF CAMS and SDS-WAS from BSC. The dust is transported in Mediterranean from different source regions with different upflift and transport mechanisms.
Two phases of the dust uplift
Phase 1
The first phase occurred in western Mediterranean, having originated from a massive dust uplift in central Sahara.
On 20 April a vast area dust is uplifted is visible above central Algeria. The following day 'fresher' dust plumes from localised convection spots are visible as stripes, feeding a large area of dust. The dust material is transported directly north-east and is visible as plume attending central Mediterranean.
Figure 1 shows the evolution of the strong and widespread dust outbreak on 21 April, in association with strong south-westerly winds across parts of Morocco, Algeria, and Tunisia.
A day later, the same low-pressure system causing the dust outbreak in northern Africa propagated east, bringing high concentrations of dust to the central Mediterranean and Balkan peninsula.
Dust and Natural Colour RGB comparison
Figure 2: Comparison of Meteosat-11 Dust and Natural Colour RGB for central Mediterranean/Balkans dust scene, 22 April 04:45 UTC
Figure 2 shows the comparison of the Dust RGB and Natural Colour RGB for the aforementioned scene at 04:45 UTC, 22 April. A relatively pronounced band of pink, stretching from Libya into southern Italy and Balkans, identifies the dust layer. The comparison shows the importance of using both the visible and near-infrared RGB imagery whenever possible for tracking dust, since the Natural Colour RGB helps confirm the presence of dust with high confidence. This is especially true early in the morning or late in the evening, when the dust will be even more pronounced in the imagery due to low sun angle (also seen in comparison figure).
Sometimes winds become high due to fronts and cyclones, and sometimes they will be primarily caused by severe thunderstorms. One such example can be seen in Figure 3, where a strong thunderstorm system propagated eastwards across the Middle East, while in its rear there is a strong outflow picking up and transporting dust westwards.
The surface pressure field plotted in this figure shows thunderstorms were developing inside a low pressure system, plus the dust expansion (edges of the pink-violet veil) almost exactly matched the strong wind direction change, proving that this event was quite accurately captured by the ECMWF model.
Phase 2
The second phase of uplift occurred in the Middle East. The uplift images in Figure 4 show the intense convection on 23 and 24 April and the generation of a dust plume (light brown over sea) extending westward from the deep convection region over the Middle East (seen as light blue clouds).
Intense convective uplift (referred to as a haboob in the Sahara regions) is an efficient pathway for dust suspension and further transport. Figure 4 captures both the transport of dust associated to the cyclone over the central Mediterranean, and the thunderstorm-driven dust event (haboob) in the Middle East.
The whole dust outbreak associated with thunderstorm clouds developed inland on the Arabian peninsula on 23 April. Throughout the remainder of the day it spread radially and propagated westwards on the leading edge of strong thunderstorm outflow, reaching the Mediterranean Sea in early hours of 24 April.
The westward dust propagation stopped early on 24 April as the thunderstorm cleared further east, and the fewer clouds allowed for a clearer view in the Suomi-NPP True Color RGB (Figure 5).
Natural Color and Cloud Phase comparison
Figure 6: Comparison of SNPP Natural Color RGB and Cloud Phase RGB for two dust events on 24 April 11:40 UTC.
The comparison of the other Suomi-NPP VIIRS RGBs (Figure 6) — the Natural Color RGB and Cloud Phase RGB — highlights the advantage of the True Color RGB in this context because it better detects the dust particles and layers, which sometimes go unnoticed even in a Dust RGB. This is because the shorter wavelengths in the visible spectrum are more sensitive to dust, especially across poorly reflective surface such as the ocean. The Natural Color and Cloud Phase RGBs feature two components in the near-IR region of the spectra which are less sensitive to smaller dust particles. The best illustration of this is a rather thin veil of dust, seen roughly in between islands of Crete and Cyprus, visible clearly in True Colour RGB (Figure 5), but not in other two near-IR-based RGBs (Figure 6).
The overall aerosol burden can be also calculated using the absorbing aerosol index (AAI) from the polarisation measurements of the GOME-2 instrument on the Metop-B and -C satellites (Figure 7). The AAI observes in the ultraviolet spectral range, so is less shielded by the clouds and is sensitive to more intense aerosol burden, such as dust. This helps identify the widespread dust layer over the southern coasts of the Mediterranean.
Dust in May
Less than two weeks after the dust in April, another similar set of dust events occurred, with almost the same atmospheric dynamics, over the same region. A warm conveyor belt (cyclone) picked up dust over Algeria and transported it to Europe, and at the same time thunderstorms in the Middle East were blowing the dust in the region.
It is interesting to note is that dust in the Middle East was initially only lifted through a low level circulation (Syria-Iraq border) associate with a low pressure area. This circulation would be otherwise invisible, if not for lifted dust particles. A low pressure disturbance later enhanced the thunderstorms in the region, that in return pushed the dust towards the south east through a cold pool advection (Figure 8).
Several consecutive overpasses of different polar-orbiting satellites (Terra, S-NPP, Aqua and NOAA-20), within a few hours, allowed for looped True Colour RGB imagery in the Middle East region (Figure 9). Such animations also be possible after the launch of the MTG-I satellite.
Figures 10 and 11 are photos of the DIBS cloud related to advection of dust from north Africa to central Europe taken from the flight Frankfurt → Vienna → Zagreb, from the mid and high troposphere (max. 10.6km) on the morning of 6 May. Figure 12 shows the satellite view on the dust-infused cloud system around the time of flight.
