Colour-changing dust cloud vortex

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Dust in northern Iraq shows different colours before and after circulation.

Colour-changing dust cloud vortex
Date & Time
01 February 2017 00:00 UTC–02 February 12:00 UTC
Dust Composite

By Vesa Nietosvaara, Jose Prieto and Jochen Kerkmann (EUMETSAT)

Dust presents different hues on satellite channel composites. The reasons are not yet clear.

The Meteosat-8 Dust composite with the ECMWF 10m-wind and mean sea level pressure overlaid, Figure 1 (top right, click to expand) and infrared composite animation,  01 February 2017 00:00 UTC–02 February 12:00 UTC (MP4, 10 MB) indicate the presence of a weak circulation in the Middle East area, close to northern Iraq.

The wind associated with the cyclonic circulation on eastern Syria raises dust south of it, as shown in the infrared animation around 10:00 UTC on 1 February.

Over the northwestern part of Iraq, the dust joins the circulation along the south flank (red and pink on the left hand side of Figure 2), but it comes out mostly from the cyclone's west flank with a peach hue near Iraq's eastern border. An influence of the ground in the coloration is not evident. The time of the day plays no role, since the same hues show day and night.

Image comparison
Image from 1 Feb, 14:30 UTC Image from 2 Feb, 06:00 UTC
Figure 2: Comparison of two different moments in the evolution of the dust area.

Both Meteosat-8 and 10 see the same colours, so the viewing angle through a thicker humidity layer is not a sufficient explanation. Differences are only around 1 K between the measurements by the two satellites, so slant effects are not impacting colour hue either.

Orange (or peach) colour results from higher green, which is a larger positive Brightness Temperature Difference (BTD) between channels at 10.8µm and 8.7µm. In cloud free areas bigger BTD comes from higher water vapour, whereas in cloudy areas this BTD is affected by 1) dust height, 2) dust concentration and 3) dust properties, mainly particle size.

If the reason for peach hues in this case would be dust particle size, one could imagine coalescence in the upward circulation, followed by drying up close to the circulation boundaries and the creation of dry bigger dust particles, producing the peach tone. That is suggested by the fact that volcano eruptions (made of big particles) show often in peach in the window infrared composite.

Figure 3 is analysis for all pixels inside the red square on the left Meteosat-10 IR composite image. The graph on the right hand side compares 10.8 µm–8.7 µm differences (vertical axis) and values at 12 µm (horizontal). The peach coloured pixels (exit of a conveyor belt) show a BTD 10.8 µm–8.7 µm which is around one kelvin higher than the magenta-red pixels at the southern part of the conveyor belt. Height or size effect?

   Figure 3: Meteosat-10 IR composite, 2 February 00:00 UTC and BTD graph.

This Meteosat-10 Dust RGB with ECMWF 2 m temperature lines, shows similar temperatures close to the ground in both pink and peach areas of dust contamination.

Any alternative suggestions for an explanation of this colour shift are welcome, please email our Helpdesk.

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