Abstract
Abstract. Satellite imagery of atmospheric mineral dust is sensitive to the optical properties of the dust, governed by the mineral refractive indices, particle size, and particle shape. In infrared channels the imagery is also sensitive to the dust layer height and to the surface and atmospheric environment. Simulations of mineral dust in infrared Desert Dust imagery from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) have been performed, using the COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model) dust transport model and the Radiative Transfer for TOVS (RTTOV) program, in order to investigate the sensitivity of the imagery to assumed dust properties. This paper introduces the technique and performs initial validation and comparisons with SEVIRI measurements over North Africa for daytime hours during 6 months covering June and July of 2011–2013. Using T-matrix scattering theory and assuming the dust particles to be spherical or spheroidal, wavelength- and size-dependent dust extinction values are calculated for a number of different dust refractive index databases, along with several values of the particle aspect ratio, denoting the particle shape. The consequences for the infrared extinction values of both the particle shape and the particle orientation are explored: this analysis shows that as the particle asphericity increases, the extinctions increase if the particles are aligned horizontally, and decrease if they are aligned vertically. Randomly oriented spheroidal particles have very similar infrared extinction properties as spherical particles, whereas the horizontally and vertically aligned particles can be considered to be the upper and lower bounds on the extinction values. Inputting these values into COSMO-MUSCAT-RTTOV, it is found that spherical particles do not appear to be sufficient to describe fully the resultant colour of the dust in the infrared imagery. Comparisons of SEVIRI and simulation colours indicate that of the dust types tested, the dust refractive index dataset produced by Volz (1973) shows the most similarity in the colour response to dust in the SEVIRI imagery, although the simulations have a smaller range of colour than do the observations. It is also found that the thermal imagery is most sensitive to intermediately sized particles (radii between 0.9 and 2.6 µm): larger particles are present in too small a concentration in the simulations, as well as with insufficient contrast in extinction between wavelength channels, to have much ability to perturb the resultant colour in the SEVIRI dust imagery.
Highlights
For over a decade since they were first launched in 2002, the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instruments onboard the Meteosat Second Generation (MSG) series of satellites (Schmetz et al, 2002) have provided a wealth of high temporal resolution information on the meteorology, climate, and the surface environment of Africa, Europe, the Middle East, and the Atlantic
It is more informative instead to consider comparisons between the SEVIRI observations and retrievals and the Radiative Transfer for TOVS (RTTOV) simulations when they are under the same conditions, which implies comparing points sub-divided within binned variable values such as the dust aerosol optical depth (AOD) and surface properties, as well as the dust type and shape
A new imaging and analysis tool has been established in order to simulate SEVIRI Desert Dust RGB imagery over the Sahara in the infrared, by combining three-dimensional dust concentration output from the COSMO-MUSCAT regional aerosol transport model with radiative transfer simulations using the RTTOV program
Summary
For over a decade since they were first launched in 2002, the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instruments onboard the Meteosat Second Generation (MSG) series of satellites (Schmetz et al, 2002) have provided a wealth of high temporal resolution information on the meteorology, climate, and the surface environment of Africa, Europe, the Middle East, and the Atlantic. Known as the “Pink Dust” scheme, atmospheric mineral dust in this rendering scheme is characterized by a distinctive pink colour, in contrast with a light blue sandy desert background surface during the daytime This description of dust in the imagery is a substantial simplification of the overall picture, which is complicated by the characteristics of the background surface (thermal emissivity and surface skin temperature), the atmospheric state (temperature and water vapour), and the precise characteristics of the dust itself (optical properties and size/vertical distribution). 5 we take a look at the colour output with respect to dust loading and optical properties, and with reference to measurements and retrievals by SEVIRI This comparison with the satellite observations provides an assessment of the accuracy and appli-. In a follow-up paper, we will further investigate the relationships between dust AOD, optical properties and height, surface, and atmospheric properties, and the complex interplay of their effects on the resultant colour
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