The BLLAST field experiment: Boundary-Layer Late Afternoon and Sunset Turbulence
- 1Laboratoire d'Aérologie, University of Toulouse, CNRS, France
- 2Applied Physics Department, Barcelona Tech UPC, Barcelona, Spain
- 3CNRM-GAME (UMR3589, Météo-France and CNRS), Toulouse, France
- 4University of Utah, Salt Lake City, Utah, USA
- 5Geophysical Institute, University of Bergen, Bergen, Norway
- 6Meteorology and Air Quality Section, Wageningen University, Wageningen, the Netherlands
- 7Laboratoire de Physique et Chimie Atmosphériques, Université du Littoral Côte d'Opale, Dunkerque, France
- 8Institute of Biometeorology – National Research Council (IBIMET-CNR), Florence, Italy
- 9National Center for Atmospheric Research, Boulder, Colorado, USA
- 10Land, Air and Water Resources, UC Davis, California, USA
- 11Dpt. Geofísica y Meteorología, Universidad Complutense de Madrid, Facultad Ciencias Físicas, Madrid, Spain
- 12CIRES, University of Colorado, and NOAA ESRL, Boulder, Colorado USA
- 13SEDOO, OMP, Toulouse, France
- 14Service des Avions Français Instrumentés pour la Recherche en Environnement, CNRS-CNES-Météo-France, Francazal, France
- 15Departament de Fisica, Universitat de les Illes Balears, Palma de Mallorca, Spain
- 16Mechanical and Aerospace Engineering, University of California, San Diego, California, USA
- 17Laboratoire de Météorologie Dynamique, Ecole Polytechnique, Palaiseau, France
- 18Institut für Bio- und Geowissenschaften, Juelich, Germany
- 19PMOD-WRC, Davos Dorf, Switzerland
- 20Mediterranean Institute for Advanced Studies (UIB-CSIC), Esporles, Illes Balears, Spain
- 21University of Tübingen, Tübingen, Germany
- 22Technische Universitaet Braunschweig, Braunschweig, Germany
- 23Institut de Recherches en ENvironnement Industriel (IRENI), Dunkerque, France
- 24Institut of Space Studies of Catalonia (IEEC-UPC), Barcelona, Spain
- 25Meteorological Institute, University of Bonn, Bonn, Germany
- 26Institute of Mediterranean Agricultural and Forest Systems – National Research Council (ISAFOM-CNR), Naples, Italy
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 tethered-balloon 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.