Shaking Things Up—What Triggers Atmospheric Convection in the West African Sahel?

Bhattacharya, A., Pacific Northwest National Laboratory

Atmospheric Thermodynamics and Vertical Structures

Cloud Life Cycle

Couvreux F, C Rio, F Guichard, M Lothon, G Canut, D Bouniol, and A Gounou. 2012. "Initiation of daytime local convection in a semi-arid region analysed with high-resolution simulations and AMMA observations." Quarterly Journal of the Royal Meteorological Society, 138(662), 10.1002/qj.903.

An African sunset shimmers softly through the hazy skies over Niamey. Photo courtesy of Tony Slingo.

An African sunset shimmers softly through the hazy skies over Niamey. Photo courtesy of Tony Slingo.

French scientists suggest heating of the Earth’s surface plays a major role in semi-arid regions compared to the tropics.

In the western part of North Africa, just south of the Sahara, year-to-year failure of rainfall over the past several decades led to one of the most severe droughts the last century witnessed. In a new paper published in the Quarterly Journal of the Royal Meteorological Society, a team of French scientists has come up with an idea about how atmospheric convection cells—the starting point of cloud formation and rainfall—develop and evolve over semi-arid regions of the world, the African Sahel in particular, where drought-induced famine has left millions dead, diseased, hungry, and homeless over the course of the past sixty years.

Akin to the phenomenon of boiling water, convection cells build up in the lowest part of the atmosphere that is in contact with the Earth, though more slowly and at a far lower temperature. Based on research primarily from the tropics, scientists now have some understanding of how these convection cells start and evolve. Similar data from the drier parts of the world, however, are lacking. Climatologists have thus extended their understanding of atmospheric convection in the tropics to the drier regions of the world, but needless to say, not very successfully.

To nail down the causes and triggers of the West African Monsoon, one of the most naturally unpredictable weather systems, an international team of scientists started a joint venture, the African Monsoon Multidisciplinary Analyses, or AMMA, in the year 2002. As part of this effort, in 2006, the U.S. Department of Energy’s ARM Climate Research Facility deployed one of its two mobile research facilities to Niamey, the capital city of Niger and the largest city of the West African Sahel. Using a range of instruments, radars, meteorological stations, and balloon launches, scientists tracked the development of convective cells in the atmosphere through the summer and rainy season of that year in Niamey. July 10, 2006, was one such day.

It was still a few days before the arrival of the monsoon. In the morning, the team detected cold and moist air, common for this period of the year and associated with monsoon weather coming in from the Gulf of Guinea. Usually cold and moist air is a harbinger of large convection in the tropics and extra-tropics. Nevertheless, here at the south of the world’s largest desert, convection cells, even small ones, were still rare. As the day progressed, a drama unfolded.

Like every day, as the earth heated up through the morning, the team noticed small dry convection cells starting in the lowest level of the atmosphere, in contact with the earth. By midday, the boundary layer—the atmospheric layer where these dry convective cells develop— grew up to form clouds with cloud base at about 1.5 kilometers. Those cumulus clouds continued to grow and by late afternoon, 3:30 pm local time, a deep convective cell started about 15 kilometers away from the observation facility. By 4:30 PM, the cell had passed over the site, and the mobile facility had captured in detail the sequence of events.

Compiling such high quality data, day in and day out, led the team to some preliminary understanding of how atmospheric cells develop in the semi-arid Sahel: i.e., the cells form not just from cold air pools like in the tropics, but by gradual heating of the lowest level of the atmosphere (in contact with the earth).

Once these small cells are set up though, the progression to larger convection cell and cloud formation follows a similar routine as those in the tropics. Although far from complete, these measurements “allow an evaluation of various aspects of a high-resolution simulation against observations,” wrote Couvreux, the lead author of the paper and a research scientist at the National Center of Meteorological Research (CNRM-GAME) in Toulouse, France.