The BLLAST field experiment: Boundary-Layer late afternoon and sunset turbulence

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.publishedVersio

GABLS4 intercomparison of snow models at Dome C in Antarctica

International audienceThe Antarctic plateau, characterized by cold and dry weather conditions with very little precipitation, is mostly covered by snow at the surface. This paper describes an intercomparison of snow models, of varying complexity, used for numerical weather prediction or academic research. The results of offline numerical simulations, carried out during 15 d in 2009, on a single site on the Antarctic plateau, show that the simplest models are able to reproduce the surface temperature as well as the most complex models provided that their surface parameters are well chosen. Furthermore, it is shown that the diversity of the surface parameters of the models strongly impacts the numerical simulations, in particular the temporal variability of the surface temperature and the components of the surface energy balance. The models tend to overestimate the surface temperature by 2–5 K at night and underestimate it by 2 K during the day. The observed and simulated turbulent latent heat fluxes are small, of the order of a few W m−2, with a tendency to underestimate, while the sensible heat fluxes are in general too intense at night as well as during the day. The surface temperature errors are consistent with too large a magnitude of sensible heat fluxes during the day and night. Finally, it is shown that the most complex multilayer models are able to reproduce well the propagation of the daily diurnal wave, and that the snow temperature profiles in the snowpack are very close to the measurements carried out on site

Elucidating physiology of plant mediated exchange processes using airborne hyperspectral reflectance measurements in synopsis with eddy covariance data

International audienceThe Carbo Europe Regional Experiment Strategy (CERES), conducted between May and June 2005, aims to determine a spatially resolved regional balance of carbon dioxide fluxes using different methodological approaches bundled within an international framework. In this study we elucidated several meteorological and physiological parameters determining plant mediated exchange processes using airborne hyperspectral reflectance measurements in synopsis with meteorological information like eddy flux covariance data. We used an airborne hyperspectral system to record spatial and temporal transects of vegetated areas surrounding flux tower sites in the Bordeaux / Landes Region (France). Alongside hyperspectral measurements a comprehensive range of meteorological and surface flux parameters were measured at a flux tower site within the same time frame. Unsupervised data analysis using cluster analysis was performed on predefined spectral wavelength windows. Additionally multiblock principal component analysis was used as explanatory data driven tool to reveal underlying data structures and to elucidate potential interdependencies between airborne hyperspectral reflectance data and meteorological ground measurements. In detail temporal, physiological changes in relevant ground information like temperature, humidity, vegetation mediated CO2 flux, latent heat flux or photosynthetic light use efficiency on the one hand and selected regions of the hyperspectral signatures, such as reflectance in the region of 531nm, which determines variations in the photochemical reflectance index (PRI) or in the spectral regions of chlorophyll fluorescence were examined. Unsupervised cluster analysis revealed coupled dependencies between changes in the derivate spectra in the range of 720 to 740 nm with gross photosynthetic uptake rate, global radiation and time of day. Multiblock principal component analysis revealed that first derivative reflectance in wavelengths from 500 to 540 nm and between 680 and 750 nm had higher loading values in respect to observed ground variables global radiation and gross photosynthetic uptake rate respectively. The significance of these findings is discussed

The BLLAST field experiment: Boundary-Layer late afternoon and sunset turbulence

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

Evaluation of the lake model FLake over a coastal lagoon during the THAUMEX field campaign

The THAUMEX measurement campaign, carried out during the summer of 2011 in Thau, a coastal lagoon in southern France, focused on episodes of marine breezes. During the campaign, three intensive observation periods (IOPs) were conducted and a large amount of data were collected. Subsequently, standalone modelling using the FLake lake model was used, first to assess the surface temperature and the surface energy balance, and second to determine the energy budget of the water column at the measurement site. Surface fluxes were validated against in situ measurements, and it was determined that heat exchanges are dominated by evaporation. We also demonstrated that the model was sensitive to the light extinction coefficient at Thau, due to its shallowness and clarity nature. A heat balance was calculated, and the inclusion of a radiative temperature has improved it, especially by reducing the nocturnal evaporation. The FLake lake model was then evaluated in three-dimensional numerical simulations performed with the Meso-NH mesoscale model, in order to assess the changing structure of the boundary layer above the lagoon during the IOPs more accurately. We highlighted the first time ever when Meso-NH and FLake were coupled and proved the ability of the coupled system to forecast a complex phenomenon but also the importance of the use of the FLake model was pointed out. We demonstrated the impact of the lagoon and more precisely the Lido, a sandy strip of land between the lagoon and the Mediterranean Sea, on the vertical distribution of turbulent kinetic energy, evidence of the turbulence induced by the breeze. This study showed the complementarities between standalone and coupled simulations