Researchers from North Carolina State University have described rapid and dramatic clearing of low cloud cover off the southwest coast of Africa. This newly observed phenomenon could help climatologists understand how clouds affect Earth's heating and cooling.
While reviewing many years of satellite data from NASA and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), NC State atmospheric scientist Sandra Yuter and her team noticed abrupt clearings of cloud cover off the coasts of Namibia and Angola.
"Large areas of low cloud are a normal feature over the ocean in the subtropics west of continents," Yuter says. "What is unusual in this case is that the cloud erosion occurs rapidly along an organized line hundreds of miles long, like a sun shade being pulled away. The lines can move west for a day or more and clear out a cloud area more than twice that of California."
The rapid cloud removal events happen throughout the year and peak in May - occurring on over half the days of the month. Yuter and her team noted that the wind direction at cloud level was often perpendicular to that of the clearing. This shows the cloud is not pushed away by the wind. The clearings start off the coast around midnight and continue through the night and the following day, suggesting that solar heating does not cause these events. "This kind of cloud erosion has never been documented before," Yuter says. "How it happens is still a mystery, although we theorize that atmospheric gravity waves are the most likely mechanism.
"These waves cause up-and-down motions in the atmosphere, akin to a boat bobbing on the ocean. We hypothesize that offshore winds from Africa interact with stable air over the ocean to make these waves, which then move through the cloud field and promote mixing and cloud evaporation."
Yuter believes that this discovery will improve scientists' understanding of climate processes. "We were very surprised by what we found," she says. "This is a way of changing large cloud areas that has not been documented before. Scientists are interested in anything that changes the area of low clouds over the ocean because these clouds reflect sunlight and cool the Earth.
"Understanding how this happens will help us predict what might happen to clouds in a warming climate," Yuter adds. "It's about the balance of warming and cooling effects - and this is one of the pieces that make up that balance."
The research appears in Science, and was supported by the National Science Foundation (AGS grants 1656237 and 1656314) and the U.S. Department of Energy (grants DE-SC0006701, DE-SC0006736 and DE-SC0016522). Yuter, Distinguished Professor of Marine, Earth and Atmospheric Sciences, is both first and corresponding author of the work. NC State senior research scholar Matthew Miller, former NC State graduate student John Hader, and David Mechem, associate professor in the Department of Geography and Atmospheric Science at the University of Kansas, contributed to the work.
Note to editors: An abstract of the paper follows.
"Abrupt cloud clearing of marine stratocumulus in the subtropical southeast Atlantic"
Authors: Sandra Yuter, John Hader, Matthew Miller, North Carolina State University; David Mechem, University of Kansas
We document rapid and dramatic clearings of large portions of the subtropical marine low cloud deck that have implications for the global radiation balance and climate sensitivity. Over the southeast Atlantic, large areas of stratocumulus are rapidly eroded yielding partial or complete clearing along sharp transitions 100s to 1000s of km in length that move westward at 8-12 m s-1 and travel as far as 1000+ km from the African coast. The westward-moving cloudiness reductions have an annual peak in occurrence in April-May-June. The cloud erosion boundaries reduce cloud at ?10 km scale in less than 15 min, move approximately perpendicular to the mean flow, and are often accompanied by small-scale wave features. Observations suggest that the cloud erosion is caused by atmospheric gravity waves.