In contrast to the higher impact weather we have seen in the Atlantic this summer, a rather odd phenomenon was observed between the Cape Verde Islands and the Canary Islands just off the West Africa coast on Thursday, 09/29/11. During my daily perusal of the various satellite products, I noticed an unusally large area of cloud-free skies over the Eastern Atlantic on the SEVIRI Natural Color product and thought it would be interesting to highlight with the various products to identify what was causing this phenomenon. Although this area of the Atlantic is known to be associated with frequent dust outbreaks and therefore rather dry, stable conditions, stratocumulus clouds are a typical occurrence.
As you can see above, the Natural Color product enhances the coldest cloud tops in a bluish coloring, while low, warm clouds appear white. The yellow circle highlights the dry, cloud-free zone. The furthest left red circle is Ophelia and the red circle in the middle is Philippe. A white line was drawn to show the approximate location of the baroclinic zone. This baroclinic zone pressing south and east appears to be strengthening the ridge to its south (yellow circle), leading to compressional subsidence and thus, no clouds. But what else is going on here? Is there dust? Glad you asked!
Reminder: The pink coloring highlights areas of dust, smoke, or ash. The yellow circle is highlighting the cloud-free region off of Africa and as you can see, there is evidence of widespread dust in the area, but the concentration doesn’t appear to be exceptional in this imagery. The red circles and white line are the same areas identified above. The black circles are highlighting regions of higher concentrations of (what appear to be) dust plumes. These two plumes are localized with the southern plume associated with an African Easterly Wave. This product will prove very useful in the identification of dust/smoke areas that aren’t as clear on convectional imagery.
Further investigation using the SEVIRI RGB Airmass product, reveals that much more is going on in this image than the first two products alone could describe. The cloud-free area in the yellow circle looks to be comprised of a tropical/subtropical (green = warm) airmass with some dry air (reddish tint) located to the north of the Cape Verde Islands (lower portion of circle). This dry air is most likely due to the subsidence under the ridge, while deeper moisture resides near and over the Cape Verde Islands in a east-west band. Philippe (right, red circle) is surrounded by dry air in all but the southeast quadrant, most likely attributing to the weak structure in association with some mid-level shear. The magenta area to the west of Ophelia (left, red circle) is due to the boundaries of the image. The white line delineates the much cooler and drier maritime polar airmass from the warmer, though somewhat dry airmass to the south. The blue circle appears to be a stratopheric intrusion as the deeper reds depict strong subsidence, while the bluish tint gives support for a much colder airmass moving south and east. More evidence of the colder air is the broken stratocumulus clouds behind the baroclinic zone.
For contrast, I have included a visible image of the tropical East Atlantic which shows the cloud-free zone west of West Africa. The arc clouds south of the Cape Verde Islands are indicative of the dry air spreading south as the ridge is displaced by the strong North Atlantic trough.
Finally, the infrared image above shows the North Atlantic trough as one would see it with the current imagery available to forecasters. Yes, you can see the cloud structures, but the RGB images include more details that a forecaster would miss with the current imagery. A water vapor image could not be shown due to an issue with the enhancements.
I hope I have shed some new light on the possible uses of the RGB products availabe in the GOES-R Proving Ground at the Hydrometeorological Prediction Center, Ocean Prediction Center, and Satellite Analysis Branch of NESDIS.