A major dust outbreak over the American Central Plains in mid-January 2021, could be clearly seen in GOES imagery.
21 January 2021
19 January 2021
By Jochen Kerkmann (EUMETSAT) and Ivan Smiljanic (CGI)
Figure 1 is the GOES-16 Dust RGB 10-minute interval animation over the Central Plains, starting at around 15:30 UTC, strong northerly winds picked up dust over eastern Colorado. The dust was carried southward across Oklahoma and Texas towards the Gulf of Mexico.
On the following day, the dust plume began to flow off the Texas coast and over the Gulf of Mexico by 06:00 UTC, and, although the plume signature began to diminish in the Dust RGB images after sunrise, it was very apparent in Geocolor (True Color) RGB imagery. The GOES-17 Geocolor RGB product (Figure 2) shows the dust cloud on 16 January 15:00 UTC (see red arrows). The GOES-17 image shows a stronger signal than GOES-16 from forward scattering at 15:00 UTC due to the sunrise in the East, while the satellite was over Pacific in the west.
Even though near-IR 1.3 μm spectral region experience strong absorption of water vapour, in the drier atmosphere the NIR1.3 channel can detect aerosols (i.e. dust) in mid and low altitudes. In this case, the dust was detected as a grey cloud (reflectance range 1-2%) traveling from Colorado towards Texas during the day on 15 January (Figure 3).
A day later the dust was already advected into the region of more moisture (Gulf of Mexico), so was only vaguely detected by the NIR1.3 channel. The dry-to-moist border, i.e. bright-to-dark grey gradient cutting though southern Texas, can be clearly seen in Figure 4.
The comparison between GOES-16 (i.e. ‘GOES-East’ at 75.2° W) and GOES -17 (i.e. ‘GOES-West’ at 137.2° W, being ‘further’ from the dust cloud ca. 15°) (Figure 4), reveals that the more slanted the viewing angle of the satellite is the less effective the NIR1.3 channel becomes for low and mid-level aerosol detection. This is opposite to what is expected from solar channels, but exactly due to water vapour absorption — the larger the path through water vapour, the damper/wetter the returning signal to the satellite is. For other standard solar channels that experience no water vapour absorption in associated spectral regions (e.g. VIS0.6 or NIR1.6), the more slanted view provides a larger path through aerosols — having a virtually bigger optical path through the aerosol cloud, providing better detection of aerosols in any vertical layer of the troposphere (e.g. Figure 2). Due to a relatively small difference in scanning angles between GOES-16 and -17 satellite, the difference seen in imagery is not very large, albeit noticeable (e.g. encircled lakes in Texas).
1-min rapid scan, DEBRA dust product with VIS0.6 overlay (RAMMB/CIRA)
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