The Dry Conveyor Belt (DCB) is usually free of clouds in the mid and higher levels, so barely traceable on solar imagery, but in February 2020 it could be seen due to dust transport.
By Ivan Smiljanic (CGI)
Dust lifted over Mauritania and Senegal travelled in the low level easterly flow, below the Warm Conveyor Belt (WCB), towards Cape Verde islands. From there it was injected into the Dry Conveyor Belt (DCB), that had a prevailing northerly flow, revealing its position and dynamics.
Figure 1: Dust-induced DCB seen with GOES-16 ABI Near infrared 2.25 channel, 25 Feb 08:10–20:00 UTC. Dry areas indicated by yellow shades
Usually the DCB is detected by observing infrared water vapour channel imagery, looking at the dry bands that stretch towards the centre of a cyclonic system (yellow shades in the Figure 3. However, on this occasion the cloud-free atmosphere had dust as a tracer of DCB, revealing the flow of the grey, wavy hue that flowed over lower clouds from Cape Verde towards the north.
This flow was slowly rising, and eventually, started to condensate into high-reaching cloudiness, near the centre of an associated cyclone (Figure 1).
The WCB that contained moist air was flowing east of the DCB, turning eventually towards the north east. It had plenty of associated cloudiness that obscured much of the dust that travelled from Africa westwards in the lower layers.
In the Dust RGB product this dust even less visible, due to a fact that moisture has exactly the opposite effect on the channel difference IR12.0–IR10.8 (red component of this RGB), responsible for the dust/moisture signal. For (low-level) moisture this difference is nominally negative, but for dust it is slightly positive. However, dust induced into the DCB had typical magenta shades (Figure 2).
Figure 3: DCB seen trough Water Vapour imagery, 25 Feb 13:00–23:45 UTC. Dry areas indicated by yellow shades
Looking at the vertical cross-sections (geographical positions indicated by the white lines in the WV imagery in Figures 4 and 5) one can mentally visualise the Dry and Warm Conveyor Belt, looking into temperature and moisture fields.
In Figure 4 the dry intrusion of the upper-level air is seen through slanted isolines of potential temperature, and through the moisture field, which also indicated dry air presence in the mid and upper levels of troposphere, along the DCB.
The vertical cross-section orthogonal to the flow of associated Conveyor Belts (Figure 5) confirms the dry-wet contrast in the mid and higher troposphere between DCB and WCB, respectively.
As noted above, the Brightness Temperature Difference (BTD) of two 'window' infrared channels is normally used to detect the dust signal in the atmosphere (namely BTD IR12.0 - IR10.8, in the case of SEVIRI instrument channels). This difference is used in the Dust RGB, and it contributes to detection of dust, but also thin cirrus and low level humidity, during both day and night.
Near-IR and visible channels (jointly known as 'solar channels'), among other aerosols, can also detect dust very well - but these depend on the light from the Sun, i.e. can be only used during daylight (plus detection depends on the Sun's angle).
In addition to existing solar channels from MSG SEVIRI instrument, the FCI imager onboard the next generation of Meteosat satellites (MTG) will have other wavelength regions of detection — all (except NIR0.91) already present on the GOES ABI imager. Hence, the ABI instrument is used for solar channel comparison (with additional IR window channel as a reference for infrared appearance) in Figures 8–10. Note that the reflectance range (for solar channels) was enhanced to best capture the dust signal, infrared channels show range of brightness temperatures in Kelvin degrees.
Figure 7 shows a reference dust product for that particular moment, in which dust takes different magenta hues in the RGB.
Figures 8–10 are a comparison between following GOES-16 ABI (as proxy for MTG FCI) visible and near-infrared channels: VIS0.47, VIS0.64, VIS0.86, NIR1.38, NIR1.61, NIR2.25 and NIR3.9. Infrared channel IR10.3 added as an infrared reference channel.