The planetary boundary layer plays a vital role in the earth system through its controls on the transfer of heat, momentum, humidity, and trace gases between the surface and the atmosphere. The transition that occurs in late afternoon, from the mixed convective boundary layer to a residual layer overlying a stably stratified surface layer in late afternoon raises several scientific issues that are still poorly understood. For example, there is still a need to relate the characteristics of the turbulence energy decay and its vertical structure with the boundary-layer processes and forcings. Also, the evolution of the characteristic length scales of turbulence remains unclear, and needs more observations and understanding.

The BLLAST project has gathered a group of research scientists from several countries in Europe and the US to work together with the purpose of increasing our knowledge of the late afternoon turbulence processes in order to improve the representation of the diurnal cycle in the numerical weather prediction and global models. The BLLAST project emerged in 2009, and lead to a field campaign that was conducted from 14 June to 8 July 2011 in southern France. The field campaign consisted of a range of integrated instrument platforms including: full-size aircraft, Remotely Piloted Airplane Systems (RPAS), remote sensing instruments, radiosoundings, tethered balloons, surface flux stations, and various meteorological towers deployed over different heterogeneous surfaces. These instruments measured the differences in the vertical structure and evolution of the late afternoon transition among a mosaic of vegetated surfaces. This special issue is an expose of current BLLAST studies that include: the analysis of field data, Large Eddy Simulations (LES) and mesoscale simulations. These issue aims to improve our understanding of processes, develop new parameterizations, and evaluate forecast models during this transitional period.