Marie Lothon

Studying the Afternoon Transition of the Planetary Boundary Layer

The planetary boundary layer is the part of the atmosphere that interacts directly with the Earths surface on a time scale of a few hours or less. In daytime, solar heating of the surface can generate buoyant turbulent eddies that efficiently mix the air through a depth of more than a kilometer. This convective boundary layer (CBL) is a conduit for trace gases such as water vapor and carbon dioxide that are emitted or absorbed by the surface (and surface vegetation) to be transported into or out of the layer nearest the surface. n nThe CBL has been extensively observed and relatively successfully modeled. But the early morning transition—when the CBL emerges from the nocturnal boundary layer—and the late afternoon transition—when the CBL decays to an intermittently turbulent “residual layer” overlying a shallower, stably stratified boundary layer—are difficult to observe and model due to turbulence intermittency and anisotropy, horizontal heterogeneity, and rapid time changes. Even the definition of the boundary layer during these transitional periods is fuzzy; there is no consensus on what criteria to use and no simple scaling laws, as there are for the CBL, that apply during these transitions.

Interactions entre surface et convection au Sahel

Strong interactions and feedbacks between nsurface processes and deep nconvection occur in the Sahel. They ntake place over a wide range of scales. nThey are found to enhance surfaceatmosphere nenergy exchanges during nthe monsoon and to generate a large nvariability of surface sensible and nlatent heat fluxes in time and space.A npositive feedback is observed between nthis variability, or more accurately soil nmoisture heterogeneities, and the frequency nof initiation of convective systems. nThis feedback operates at fine nscale, on the order of a few tens of kilometres. nThe underlying mechanisms nand their modelling are discussed.