Expedition crossing the Empty Quarter

By HansPeter Roesli (Switzerland)

The goal of the crossing was to emulate the first such undertaking by British explorer Bertram Thomas in 1930 (see Crossing the Empty Quarter website). The Empty Quarter is the largest sand desert on Earth — covering a 1,150 mile land mass that is larger than France, Germany and Spain combined. It's also one of the most inhospitable parts of the world.

Weather wise during the expedition, as well as many warm sunny days (highest maximum temperature 34.8°C, generally 30°C or below) and some cold nights (lowest minimum temperature 0.4°C), the expedition's blog noted episodes of morning fog, blowing dust and even an important downpour.

Morning fog/low cloud

Towards the end of the crossing, from 18 January onward, and having arrived in the border region between Saudi Arabia and Qatar, the expedition encountered episodes of thick early-morning fog and, according to the blog, 'heavy dew ... soaking sleeping bags and any gear left outside'. On 20 January  the blog reported that 'the Sun emerged through the low cloud only at lunch time'.

On the Meteosat-10 Dust RGB (Figure 1) the fog situation is shown for 19 January 04:00 UTC. A large fog patch (red arrows) expanded from Qatar and the Emirates deep into Rub AlKhali and also engulfed the recent expedition track.

Blowing dust

The blog recorded blowing dust on 3, 10-11 and 24-25 January. As shown on the Meteosat-10 Dust RGB on 3 January 09:00 UTC (Figure 2) stiff northerly winds lifted dust along the Gulf coast of Saudi Arabia.

The dust was then driven into the Empty Quarter under mid-level clouds. At the same time the dusty airmass pushed back a deck of low cloud.

The blowing dust on 24-25 January was very hard to identify in the satellite imagery — a faint veil moving south emerged, but only in an animated sequence of Dust RGBs (not shown).

The dust storm of 10-11 January is discussed below.

Downpour

On 11 January the blog noted "second sandstorm of the journey... strong northerly wind kicked up yesterday [10 January] afternoon, and grew... to about 20-25 knots as the afternoon and evening progressed... sand was flying everywhere".

As shown in the Meteosat-10 Dust RGB at 14:00 UTC (Figure 3) the camp of the night 10/11 January (red dot) was already behind the dust front, marked by the red arrows.

The blog continued "... awoke at dawn [11 January] to a leaden sky with the wind still strong, and with very dark clouds coming our way... walking proved a struggle into the still strong wind, and the menacing black clouds brought our first proper rain... clothes were soaking within minutes, and the camels proved very nervous... turning in cycles and bellowing... stayed... for 20 minutes... the rains today are the first there have been for several years... despite this the well [Turaiga waterhole] was full of water.".

The animated sequence of Meteosat-10 Dust RGB, 10 January 12:00 UTC to 12 January 02:00 UTC shows the lifted dust moving in from north. At the same time, along the Asir Mountains on the Red Sea coast, afternoon convection had set in, offshoots of which were then transported into the desert area, increasing  the humidity in the middle layers of the troposphere.

Also, according to the GFS analyses (Figure 4), a tongue of higher lifted-index values moved in from the north .

While debatable, the combination of these features might be at the root of the downpour that, completely unexpectedly, soaked camels and men. The animated gif of the skew-T profile of the GFS analyses of 11 January at 12:00 UTC (Figure 5) suggests a cloud base at around 3km height, and the cloud-top height product from Meteosat-10 (Figure 6) shows cloud tops around 6km (yellow colour), i.e. with a thickness of 3km the cloud deck was sufficiently thick to produce precipitation.

Actually, as hinted by the lifted index, there was embedded convection. This is suggested by the cumulus tops that were scattered across the cloud deck at 375m spatial resolution of VIIRS data.

Additional content

Crossing the Empty Quarter documentary premieres today (Oman Observer)