Dense dust outbreak across Arabian peninsula

By Hans-Peter Roesli (Switzerland)

North to north-westerly winds between the Arabian peninsula and the Persian Gulf are called Shamal. They essentially have two seasons, the winter Shamal and the summer Shamal.

Outbreaks of winter Shamal (December to April) are linked to frontal systems, deepening over the area after having swept over the eastern Mediterranean. They may last from hours to a couple of days. Summer Shamal (May to August) is a much more persistent wind regime, driven by high pressure over eastern Africa and thermal low pressure centres over Iran-Pakistan, where Shamal is known as Sistan or 120-day winds.

In 2018 a massive and long-distance case shrouded the whole Arabian peninsula and a good part of the Arabian Sea between end of July and beginning of August, as can be seen on the animated sequence of Dust RGBs from Meteosat-8 or IODC (Figure 2). The IODC mission has a sub-satellite point at 41.5°E which covers the area particularly well.

The initial pickup of dust and sand started over the tri-border area of Iran-Kuwait-Saudi Arabia on 26 July, which then developed into a massive Haboob over the southern parts of Saudi Arabia by 27 July (Figure 1).

At a slower pace, the dust then ascended the hilly-rocky coastline of Yemen and Oman and started to spread over the Arabian Sea on 28 July (Figure 3). Faint dust veils were still visible over the north Indian Ocean in the first days of August (Figure 4).

During 29 July the eastern end of the dust outbreak got pushed against the Omani Hajar mountain range (green arrow on Figure 5). At the same time this part of the dust started to be ingested into a thermal low, outlining it well on 30 July at 06:00 UTC as shown together with the black isobars from the GFS MSLP analysis (Figure 6).

The head of this dust whirl drifted south-westward and brought dense dust for a second time to the Omani Dhofar Governorate on 31 July (green arrows on Figure 7).

The coastal strip of this region, between June and September, is persistently shrouded in low rain clouds. They form there under the influence of the south-west monsoon that induce the humid AlKhareef climate there (see previous case study Khareef in Dhofar Governorate, Southern Oman).

A sequence of three True Color RGB images from VIIRS on Suomi NPP, taken during the dusty days on 28 to 30 July, show that most of the AlKhareef clouds survived the blowing dust; the red arrow on the animated gif (Figure 8) points to the most persistent cloud over the coastal plain of Salalah, i.e. the dust got advected above the cloud tops.

This is confirmed by the midnight radiosoundings at Salalah before and during the dust events affecting it between 26 July and 1 August. The animated gif of the soundings shows a permanent monsoon inversion up to a bit less than 900hPa that tends to get only slightly shallower, but does not disappear, during the most intense passages of dust overhead.

The Khareef situation can also be clearly seen on the four-panel image (Figure 9). It shows the dust veil and Khareef cloud at local noon on 31 July as seen, clockwise, on the True Colour RGB from OCLI on Sentinel-3a, the Dust RGB from Meteosat-8 and the Natural Colour RGB from Meteosat-10 and Meteosat-8, respectively.

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