Dust lifted over Sahara by the ITCZ convective activity was transported over the Atlantic and seen by the Meteosat-11 and GOES-16 satellites.
By Ivan Smiljanic (SCISYS)
The Inter Tropical Convergence Zone (ITCZ) reaches its northernmost position during the summer months, following the Sun's illumination shift towards the northern hemisphere.
The associated tropical convection belt moves very closely to the Sahara, and strong convection episodes with associated gust fronts of vast proportions trigger the dust lifting from Sahara on a daily basis. As a consequence, an almost continuous flow of dust occurs over Atlantic, and even more to the west, driven by the easterly wind circulation (trade winds) in the tropical belt.
Such transport of dust was observed during one week in June (24–30 June), using satellite data from both Meteosat-11 (Figure 1) and GOES-16 (Figure 2). The main ‘transport route’ of Saharan dust goes over the Atlantic Ocean which is a very good overlap region for comparison between the two satellites.
Figure 1: Meteosat-11 Dust RGB animation, 24 June 00:00 UTC–01 July 00:00 UTC
Figure 2: GOES-16 GeoColor RGB animation, 24 June 11:00 UTC–30 June 11:00 UTC
When comparing the Dust RGB product from two satellites (Figure 3) it is worth noting the following:
- Dust is better detected at the high viewing angles for a particular satellite (towards the edge of observable disc), namely in the west part of the domain for the Meteosat-11 satellite, i.e. east part for GOES-16. This is due to the fact that the dust layer is effectively thicker for slant viewing angles. This effect can be also observed looking at the thin cirrus clouds, for instance cirrus band over the Dominican Republic.
- In general, dust (pink shades) and the low level moisture (deeper blue shades over cloud-free areas) is better detected by the Meteosat-11 SEVIRI instrument. The difference should come from the selection of channels for the red beam of the Dust RGB. It appears that the Brightness Temperature Difference (BTG) for selected SEVIRI channels (BTD 12.0–10.8 µm) provides better sensitivity for dust and ice clouds than for selected ABI channels (BTD 12.3–10.3 µm).
- The ABI Dust RGB product shows more information on the top of the thick convective clouds (orange shades), whereas with SEVIRI all of the thick high clouds have the similar red shades. This is highly related to characteristics of the 8.4 µm channel used for the green component with ABI instrument.
- Some of the very thin water clouds that take deep blue shades (similar to thin cirrus clouds), when observed with slant viewing angels, become yellow, revealing their true phase (cloud patch towards western Brazil and off the Ivory Coast).
This is also observable with a zoomed-in view over West Africa, where the resolution between the two satellites (nominally 2 km for ABI infrared channels and 3 km for SEVIRI) becomes more comparable due to a slant viewing angle of GOES-16 satellite (Figure 4).
If the two satellites would start to drift from the present positions (Meteosat-11 at 0° and GOES-R at 75.2°W) towards each other with the same speeds over the equator they would be at 37.6 °W. Around that region viewing angles of two satellite should be the same and, hence, this region is good for comparison between two satellites (Figure 5).
Looking at the GOES-16 Natural Colour RGB at 10:00 UTC (Figure 6) it appears that the dust, seen in grey shades, covers the Atlantic Ocean over the whole observed domain. Only by comparing this image to one from 08:00 UTC one can understand that the similar grey shades are the effect of sun glint in the top half of domain.
Comparison of the Natural Colour RGB product between the two satellites proves the fact that the forward scattering and the slant viewing angle through optically thin features in the atmosphere provides a better detection of such. Optically thin dust is in this case is hardly observable through Meteosat-11 imagery (Figure 7) but at the same time very apparent in GOES-16 imagery.
Figure 8 provides a view over the area half way between Africa and South America (centred around 11.5°N, 32.5°W).
From the GOES-16 Natural Colour RGB it appears that the low stratocumulus field is covered in dust, hence dimmer colours, compared to the higher patch of clouds in the western part of the observed area. Comparing the GOES imagery with the Meteosat-11 Dust RGB product it is obvious that this higher cloud patch is actually optically thinner then the stratocumulus cloud field around it (the reflectivity range for solar channels, e.g. RGB product, for both satellites is exactly the same - 0–40%). However, GOES-16 image appears much brighter during sunrise period because of the dominant forward scattering (versus more backward scattering for Meteosat 11).
Previous case studies
Dust from Africa travels to Brazil (25 June 2014)
Dust travelled from western Africa to the Lesser Antilles (1 Aug 2013)
Dust outbreak from Western Africa to the Lesser Antilles (25 June 2009)
Large sand and dust clouds over the Eastern Atlantic (9 March 1998)