Interactions between downslope flows and a developing cold-air pool

A numerical model has been used to characterize the development of a region of enhanced cooling in an alpine valley with a width of order (Formula presented.) km, under decoupled stable conditions. The region of enhanced cooling develops largely as a region of relatively dry air which partitions the valley atmosphere dynamics into two volumes, with airflow partially trapped within the valley by a developing elevated inversion. Complex interactions between the region of enhanced cooling and the downslope flows are quantified. The cooling within the region of enhanced cooling and the elevated inversion is almost equally partitioned between radiative and dynamic effects. By the end of the simulation, the different valley atmospheric regions approach a state of thermal equilibrium with one another, though this cannot be said of the valley atmosphere and its external environment.Peer reviewe

Pollutant dispersion in a developing valley cold-air pool

Pollutants are trapped and accumulate within cold-air pools, thereby affecting air quality. A numerical model is used to quantify the role of cold-air-pooling processes in the dispersion of air pollution in a developing cold-air pool within an alpine valley under decoupled stable conditions. Results indicate that the negatively buoyant downslope flows transport and mix pollutants into the valley to depths that depend on the temperature deficit of the flow and the ambient temperature structure inside the valley. Along the slopes, pollutants are generally entrained above the cold-air pool and detrained within the cold-air pool, largely above the ground-based inversion layer. The ability of the cold-air pool to dilute pollutants is quantified. The analysis shows that the downslope flows fill the valley with air from above, which is then largely trapped within the cold-air pool, and that dilution depends on where the pollutants are emitted with respect to the positions of the top of the ground-based inversion layer and cold-air pool, and on the slope wind speeds. Over the lower part of the slopes, the cold-air-pool-averaged concentrations are proportional to the slope wind speeds where the pollutants are emitted, and diminish as the cold-air pool deepens. Pollutants emitted within the ground-based inversion layer are largely trapped there. Pollutants emitted farther up the slopes detrain within the cold-air pool above the ground-based inversion layer, although some fraction, increasing with distance from the top of the slopes, penetrates into the ground-based inversion layer.Peer reviewe

Biogéographie de Madagascar = Biogeography of Madagascar

La fréquence du fonctionnement photosynthétique de type CAM (Métabolisme Acide des Crassulacées) parmi les Orchidées malgaches a été étudiée par analyse de composition isotopique pour le carbone (delta 13C) des tissus. Dans les forêts chaudes de basse altitude aussi bien que dans les forêts de montagnes plus fraîches, environ 50% des orchidées épiphytes fonctionnent comme des plantes CAM. Par contre, toutes les espèces terrestres dans ces habitats montrent une photosynthèse de type C3. Ces résultats sont en faveur de l'hypothèse selon laquelle le CAM, mode photosynthétique assurant aux plantes une économie d'eau, procure un avantage écologique aux espèces vivant en situation épiphytique. Parmi les orchidées épiphytes CAM se trouvent l'espèce très connue #Angraecum sesquipedale et toutes les orchidées dites "aphylles". Le fonctionnement CAM a aussi été détecté chez les orchidées poussant sur les sols superficiels des inselbergs secs, par exemple #Angraecum sororium et #A. eburneum. Les espèces de #Bulbophyllum poussant sur les mêmes sites fonctionnent "en C3", probablement parce qu'elles sont capables de stocker de l'eau dans leurs pseudo-bulbes et ainsi ne dépendent pas d'une adaptation de leur type de photosynthèse. (Résumé d'auteur