| CLIMATE - Deciphering the timing of the monsoons
Pour percer les secrets des caprices de la mousson, pas d’autre solution que d'ausculter ce phénomène en détail. C’est pourquoi les scientifiques sont allés placer leurs capteurs dernier cri au coeur même du terrain. Plusieurs stations réparties sur le sol africain sont ainsi dédiées aux échanges continent/atmosphère.
Sitting opposite a machine called the Cloud Condensation Nuclei counter, Laurent Gomes, researcher at the CNRS in France, explains that this apparatus makes it possible for us to distinguish which of the particles suspended in the air are liable to form condensation nuclei. “Some particles such as soot are termed hydrophiles. In other words, they are good at condensing water vapour. Conversely, mineral particles are more often hydrophobes. To complicate the situation, these properties may develop during their journey in the atmosphere. If a mineral particle is in contact with salt molecules emitted from sprays, it will gradually become a hydrophile. This appliance takes dust particles present in the air and sends them into a chamber where we can expect the formation of water vapour. All we have to do then is count the drops that form on the sample. At present, of 10,000 particles in suspension, 1,700 are hydrophiles.”
On Earth as it is in Heaven
At the same time, one of the aircraft in the AMMA fleet is equipped with the same apparatus, measuring the condensation capacity of aerosols at a defined altitude. This allows for the properties of dust particles circulating on the ground to be differentiated from those in the upper layers of the atmosphere. The presence of five fully equipped planes, probing the centres of tropical storms as well as the heights of the troposphere, is one of the project’s strong points.
500 km away, near the village of Komakoukou in Niger, we find Jean-Louis Rajot’s laboratory, which is entirely dedicated to the study of aerosols, or all solid and liquid particles suspended in the air. These particles are collected, sorted by diameter, counted and chemically analysed. One of the objectives is to understand the proportion of dust of local origin compared to that coming from elsewhere. Researchers have placed instruments in cultivated fields and on fallow land in order to carry out differential measures, with the experiment showing that fallow land suffers hardly any erosion, while cultivated land areas concentrate this phenomenon. Another piece of apparatus collects raindrops in a sequential manner – the last drops justify special analysis, as they are the ones most likely to contain materials that make condensation possible.
Understanding the water cycle
Several slopes, some of them intensely cultivated and others that are not, are being monitored together and compared so as to be able to deduce the impacts of agricultural activities. The nature and density of the vegetation cover form part of the measured parameters. Estimates must be made of the amount of infiltrated water, the amount transpired by plants, the amount that evaporates naturally and the amount reaching rivers.
The initial results (obtained by a chemical comparison of water from rivers and groundwater) seem to indicate that while the water table in this region is situated at a depth of around fifteen metres, a much shallower water table of less than two metres is established during the rainy season. This temporary layer feeds flows of water, while the deep and permanent layer is accessible to trees – but not to cultivated land. If this hypothesis is confirmed, the replacement of the forest by cultivated land, which draws on the superficial layer, could result in a reduction in the portion that feeds into rivers. This type of information is particularly useful for modelling the future of the water cycle in the region. Analogue stations have been set up in Mali and Niger in order to make similar assessments based on different climate and soil conditions.
A human adventure
Conducting a programme on the scale of those carried out by the AMMA is no mean achievement in poor countries with a hostile climate. But the adventure has a human focus, as well as a scientific one. Luc Redelsperger is happy to explain: “When atmospheric specialists understood that their data could be used to combat meningitis, and not only to improve understanding of the subtleties of the climate, their enthusiasm was palpable”.
Even though AMMA scientists launch balloons in torrential rain, burn under the sun while measuring dust, interview hundreds of patients in clinics, or fly for hours in planes transformed into furnaces, they all seem eager – whatever their temperament, nationality or discipline – to contribute toward developing a continent that is too often abandoned and forgotten.