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Discussion papers
https://doi.org/10.5194/acp-2018-1238
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1238
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 16 Jan 2019

Research article | 16 Jan 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

The sensitivity of the colour of dust in MSG-SEVIRI Desert Dust infrared composite imagery to surface and atmospheric conditions

Jamie R. Banks1, Anja Hünerbein1, Bernd Heinold1, Helen E. Brindley2, Hartwig Deneke1, and Kerstin Schepanski1 Jamie R. Banks et al.
  • 1Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, Germany
  • 2Space and Atmospheric Physics Group, and NERC National Centre for Earth Observation, Imperial College London, London, UK

Abstract. Infrared Desert Dust composite imagery taken by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI), onboard the Meteosat Second Generation (MSG) series of satellites above the equatorial East Atlantic, has been widely used for more than a decade to identify and track the presence of dust storms from and over the Sahara Desert, the Middle East, and southern Africa. Dust is characterised by distinctive pink colours in the Desert Dust false-colour imagery, however the precise colour is influenced by numerous environmental properties, such as the surface thermal emissivity and skin temperature, the atmospheric water vapour content, and the quantity and height of dust in the atmosphere. This paper is the follow-up to Banks et al. (2018), which analysed the sensitivity of the colour of the dust in the imagery to its infrared optical properties. The previous paper introduced a modelling system combining dust concentrations simulated by the aerosol transport model COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model) with radiative transfer simulations from the RTTOV (Radiative Transfer for TOVS) model, in order to simulate the SEVIRI infrared measurements and imagery. Investigating the sensitivity of the synthetic infrared imagery to the environmental properties over a six month summertime period from 2011 to 2013, it is confirmed that water vapour is a major control on the apparent colour of dust, obscuring its presence when the moisture content is high. Of the three SEVIRI channels used in the imagery (8.7, 10.8, and 12.0 μm), the channel at 10.8 μm has the highest atmospheric transmittance and is therefore the most sensitive to the surface skin temperature. A direct consequence of this sensitivity is that the background desert surface exhibits a strong diurnal cycle in colour, with light blue colours possible during the day and purple hues prevalent at night. In dusty scenes, the clearest pink colours arise from high-altitude dust in dry atmospheres. Elevated dust influences the dust colour primarily by reducing the contrast in atmospheric transmittance above the dust layer between the SEVIRI channels at 10.8 and 12.0 μm, thereby boosting red and pink colours in the imagery. Hence the higher the dust altitude, the higher the threshold column moisture needed for dust to be obscured in the imagery: for a sample of dust simulated to have an AOD at 550 nm of 2–3 at an altitude of 3–4 km, the characteristic colour of the dust may only be impaired when the total column water vapour is particularly moist (⪆ 39 mm). Meanwhile dust close to the surface (altitude < 1 km) is only likely to be apparent when the atmosphere is particularly dry and when the surface is particularly hot, requiring column moistures ⪅ 13 mm and skin temperatures ⪆ 314 K, and is highly unlikely to be apparent when the skin temperature is ⪅ 300 K. Such low altitude dust will regularly be almost invisible within the imagery, since it will usually be beneath much of the atmospheric water vapour column. It is clear that the interpretation of satellite-derived dust imagery is greatly aided by knowledge of the background environment.

Jamie R. Banks et al.
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Jamie R. Banks et al.
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Latest update: 17 Feb 2019
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Short summary
Saharan dust storms may be observed over the desert using false-colour infrared satellite imagery, in one widely used scheme dust displays characteristic pink colours. Simulating satellite imagery using a dust transport model, we confirm that water vapour is a major control on the apparent colour of dust in the false-colour imagery, and that dust displays its deepest colours when it is at a high altitude and when the atmosphere is dry. Water vapour can obscure the presence of low altitude dust.
Saharan dust storms may be observed over the desert using false-colour infrared satellite...
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