Abstract. Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state.These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere.Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso-or synoptic scale conditions.Ground-based measurements combined with tetheredballoon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period.Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations.

The authors give an overview of the diurnal cycle of the low troposphere during 2006 at two different sites, Niamey (Niger) and Nangatchori (Benin). This study is partly based on the first observations of UHF wind profilers ever made in West Africa in the context of the African Monsoon Multidisciplinary Analysis (AMMA) project. Also used are the radiosoundings made in Niamey and ground station observations at Nangatchori, which allow for the study of the impact of the dynamics on the water vapor cycle and the turbulence observed at the ground.Profiler measurements revealed a very consistent year-round nocturnal low-level jet maximal around 0500 UTC and centered at 400-m above the ground, with wind speed around 15 m s Ϫ1 . This jet comes either from the northeast during the dry season or from the southwest during the wet season, in relation with the position of the intertropical discontinuity. The radiosoundings made in Niamey highlight both the role of the nocturnal jet in bringing water vapor from the south during the night when the intertropical discontinuity has reached the vicinity of the considered area at the end of the dry season and the role of the daytime planetary boundary layer in mixing this water vapor within a larger depth of the troposphere. The planetary boundary layer processes play a large role in the diurnal cycle of the position of the intertropical discontinuity itself.The observations of turbulence made at the ground in Nangatchori showed that the best signature of the nocturnal jet close to surface can be seen in the turbulent kinetic energy and skewness of the air vertical velocity, rather than on the mean wind itself. They reveal the downward transport of momentum from the jet core aloft to the surface.

The UBL/CLU (urban boundary layer/couche limite urbaine) observation and modelling campaign is a side-project of the regional photochemistry campaign ESCOMPTE. UBL/CLU focuses on the dynamics and thermodynamics of the urban boundary layer of Marseille, on the Mediterranean coast of France. The objective of UBL/CLU is to document the four-dimensional structure of the urban boundary layer and its relation to the heat and moisture exchanges between the urban canopy and the atmosphere during periods of low wind conditions, from June 4 to July 16, 2001. The project took advantage of the comprehensive observational set-up of the ESCOMPTE campaign over the Berre-Marseille area, especially the ground-based remote sensing, airborne measurements, and the intensive documentation of the regional meteorology. Additional instrumentation was installed as part of UBL/CLU. Analysis objectives focus on (i) validation of several energy balance computational schemes such as LUMPS, TEB and SM2-U, (ii) ground truth and urban canopy signatures suitable for the estimation of urban albedos and aerodynamic surface temperatures from satellite data, (iii) high resolution mapping of urban land cover, land-use and aerodynamic parameters used in UBL models, and (iv) testing the ability of high resolution atmospheric models to simulate the structure of the UBL during land and sea breezes, and the related transport and diffusion of pollutants over different districts of the city. This paper presents initial results from such analyses and details of the overall experimental set-up.Ã