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

Atmospheric tides over the Pyrenees: observational study and numerical simulation

International audienceBarometric tides around the Pyrenees mountain range are analyzed by means of synoptic surface-station data recorded during one year, surface data from the Pyrenees Experiment (PYREX) and the CRA/LA Very High Frequency (VHF) wind profiler installed in the north of the range. Tides are decomposed into their diurnal and semi-diurnal components. Diurnal tides show a strong non-migrating component and are very dependent on local conditions. Semi-diurnal tides are more homogeneous and present a north-south asymmetry, also noted in the Alps. This cross-range asymmetry could be related to some interference effect caused by the mountain range in the migrating semi-diurnal tide wave. The asymmetry of the diurnal component presents a very strong seasonal variation, probably related to local diabatic effects. A three-month long simulation has been carried out with the National Center for Atmospheric Research's Weather Research and Forecasting (WRF) limited-area model to try to reproduce the tide structure. The validation of the results with wind-profiler data shows reasonable agreement with the observed diurnal tide and poorer results for the semi-diurnal component. At surface level, however, the model reproduces some of the features of the observed semi-diurnal tide, and especially the cross-range asymmetry. Copyright © 2010 Royal Meteorological Societ

Reply to 'Comments on "Comparison of rafar reflectivity and vertical velocity observed with a scannable c-band doppler radar and two UHF profilers in the lower troposphere"'

The objective of Lothon et al.'s (2002, hereafter L2002) paper was to investigate the ability of a scannable C-band Doppler meteorological radar for the documentation of the lower clear atmosphere with an emphasis on the atmospheric boundary layer (ABL). Comparison with the observations of two UHF wind profilers provided the opportunity to address the problem of the so-called downward bias of UHF vertical velocity measurements made in a fully developed and turbulent convective ABL previously put in evidence and described by Angevine (1997). Worthington's (2003) reproach of a too-narrow review of the different aspects of this problem is valid, but a thorough survey of this question was beyond the scope of the paper. The ambition of the authors was to bring some elements of discussion to this open debate—elements that tend to confirm Angevine's conclusion on the existence of UHF downward bias of instrumental origin.In the reply presented in the following section we discuss Worthington's (2003) suggestion of the possible meteorological origin of the UHF vertical velocity bias observed in the ABL. Coming back to certain arguments developed previously in L2002, the importance of which seems underestimated in Worthington's comment, and with new data analysis, we reinforce our position on a most probable instrumental origin of the UHF vertical bias

Dynamical and ozone measurements by constant volume balloons in the low troposphere during ESCOMPTE experiment

The objectives of the ESCOMPTE program concern the study of emission of the primary pollutants in industrial and urban areas, their transport and their diffusion in the atmosphere. This experiment, realized in the southeast of France on a 120km*120km, can be used to validate and to improve chemical transport numerical models. One major aim of this experiment is to follow the pollutant plume, and to investigate its thermodynamical and chemical time evolution. This is realized with constant volume balloons (CVB), operated by the Centre National d'Etudes Spatiales (CNES) and instrumented by the Laboratoire d'Aérologie (LA) and Météo France. The CVBs, located by GPS and equipped with thermodynamical and ozone sensors, fly at a constant density level in the atmospheric boundary layer. These CVBs constitute a way to validate the high resolution trajectories against in situ measurements. During the ESCOMPTE campaign which took place in June and July 2001, thirty-three CVB flights were made, fifteen of them with ozone measurements. We present (i) the experimental system including CVB, radiosonde and balloon tracking stations, (ii) the first results of the different flights, in particular the time evolution of the ozone along the CVB trajectories

Pic 2005, a field campaign to investigate low-tropospheric ozone variability in the Pyrenees

International audienceThe Pic 2005 field campaign took place from 13 June to 7 July 2005 close to the high-altitude permanent atmospheric observatory Pic-du-Midi (PDM), situated at 2875 m asl in the French Pyrenees. The experimental set-up combined in situ ground-based observations at PDM with ozone lidar measurements at two lower sites in close vicinity (600 m asl/28 km away, and 2380 m asl/500 m away). Such an experimental configuration is appropriate to address the question of the vertical layering of the chemical atmosphere in a mountain area and above the plain nearby, and how this influences measurements conducted on a mountain summit under the influence of horizontal transport at regional scale, and vertical transport at local scale. Forecast tools made it possible to plan and carry out 6 one-day Intensive Observation Periods (IOPs), mostly in anticyclonic conditions favoring local thermally induced circulations, with and without local pollution in the lower troposphere. It was thus possible to document i) ozone diurnal variations at PDM; ii) correlation between ozone measurements at PDM and their counterparts at the same altitude in the free troposphere; iii) ozone variability in the vicinity of PDM. The field campaign provided direct experimental evidence that at daytime in the encountered conditions (mostly anticyclonic), PDM failed in a large extent to be representative of the troposphere above the surrounding flat areas at similar altitude. First, ozone daily averages at PDM were found lower than their free-tropospheric counterpart. Thermally induced circulations and convection pumping clean air from the rural boundary layer can account qualitatively for ozone depletion observed at PDM during daytime. However the surface measurements do not support the hypothesis of direct lifting of near-surface air masses up to PDM. Thus, mixing with free-tropospheric air, photochemistry and surface deposition in the valleys appear to be needed ingredients to account quantitatively for the observed variations (in proportions that further studies should determine). Second, ozone variability was found to be much lower at PDM than in the free troposphere--again an indication of atmospheric mixing. In particular at daytime, the PDM observatory did not allow for detection of ozone-rich layers simultaneously visible above the plain. Beyond these first results, the data set presented here paves way to detailed studies of the IOPs

Projet de sondage de la basse troposphère par ballon captif instrumenté

National audienc

Projet de sondage de la basse troposphère par ballon captif instrumenté

National audienc

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