Processus turbulents dans la couche limite atmosphérique pendant la transition d'après-midi

Cette thèse porte sur l'étude de la transition d'après-midi (TA) de la couche limite atmosphérique (CLA). La TA est une période complexe du cycle diurne, de par son manque de stationnarité et parce qu'elle est régie par un ensemble de forçages qui faiblissent et dont les rôles respectifs changent par rapport à ce qu'ils ont pu être durant la période convective. Il résulte une moins bonne compréhension de cette période que des régimes quasi-stationnaires convectifs, neutres ou stables. La TA pourrait conditionner l'établissement du brouillard nocturne, influencer le développement de la couche limite du lendemain et être une phase clé du cycle diurne pour la ventilation des espèces en trace vers l'atmosphère libre. Par conséquent, de meilleures description et compréhension de la TA pourraient améliorer les modèles météorologiques et de qualité de l'air. Les principaux objectifs de la thèse ont été de mieux comprendre comment la turbulence décroît pendant la TA en mettant l'accent sur l'évolution de la structure verticale de la turbulence lorsque les transferts d'énergie en surface diminuent progressivement, et sur le rôle que joue la surface à ce moment particulier du cycle diurne. Le projet international BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) a servi de cadre à cette thèse. La campagne de mesures qui s'est déroulée en été 2011 sur le Plateau de Lannemezan, au nord de la chaîne Pyrénéenne, a fourni les nombreuses observations utilisées dans cette thèse. Une approche complémentaire aux observations est l'utilisation d'un modèle atmosphérique permettant de résoudre explicitement les échelles de la turbulence (LES). Cette approche a permis d'améliorer la compréhension du devenir des définitions et des lois d'échelles, de l'évolution des caractéristiques turbulentes et du rôle des différents processus de couche limite pendant la TA. L'ensemble des moyens expérimentaux a permis d'établir une vue d'ensemble des TA de BLLAST, en fonction des différentes conditions météorologiques et des types d'évolution de la structure moyenne et turbulente de la CLA: il existe une grande variabilité de la durée des TA en fonction du type de surface et des conditions météorologiques. L'évolution des caractéristiques turbulentes pendant la TA a été étudiée en surface et in situ grâce aux observations aéroportées, ainsi qu'avec une LES. Nous avons relevé la bonne capacité de la LES à reproduire les observations de la turbulence au cours de l'après-midi. En se basant sur une analyse spectrale de la vitesse verticale du vent, il a été mis en évidence que la TA est constituée de deux périodes nommées "Early Afternoon" et "Late Afternoon". La "Early Afternoon" est caractérisée par une diminution lente de l'énergie cinétique turbulente (ECT) dans la CLA sans que cela affecte les caractéristiques fondamentales de la turbulence. Durant la "Late Afternoon", l'ECT diminue fortement, accompagnée de modifications de la répartition de l'énergie sur les différentes échelles des tourbillons et surtout de la façon dont l'énergie cascade vers la dissipation. C'est aussi durant cette période que les échelles de normalisation classiquement utilisées en régime convectif ne sont plus valides. Ces modifications sont plus marquées et apparaissent plus tôt dans la partie supérieure de la CLA. La compréhension de l'évolution des caractéristiques turbulentes pendant la TA a ensuite été approfondie à l'aide de plusieurs tests de sensibilité. L'évolution de la turbulence est sensible au cisaillement, celui-ci retardant le processus de décroissance de turbulence et d'évolution des échelles. On suspecte le processus d'entraînement au sommet de la CLA d'être à l'origine de l'augmentation des échelles intégrales. Les structures cohérentes jouent également un rôle important sur la modification des spectres de la turbulence par rapport à la théorie de Kolmogorov. Enfin, l'anisotropie peut également modifier la pente spectrale du domaine inertiel. L'heure d'initialisation d'une simulation joue également un rôle. La période de six fois l'échelle de temps convective généralement supposée être suffisante pour reproduire la turbulence dans un modèle atmosphérique ne permet pas de créer les plus grandes échelles, ce qui modifie la décroissance de la turbulence pendant la TA.This thesis focuses on the afternoon transition (AT) of the atmospheric boundary layer (ABL). This transitional period is complex, because it is non-stationary and most of the forcings, though smaller than during the previous convective period, may come into play. Thus, this transitional period is less understood than the quasi-stationary convective, neutral or stable regimes. Yet, the AT could impact on the nocturnal fog set up, influence the boundary-layer development on the following day and play a crucial role in the transport and dispersion of pollutants and trace species towards the free troposphere. Therefore, a better understanding of the AT could improve the meteorological models. One of the main objectives of the thesis is to improve the knowledge of the decay of turbulence during the AT, when the surface energy transfers are gradually decreasing. We especially put emphasis on the evolution of the ABL turbulence vertical structure. This work is in the core of the BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) international project. A field campaign took place in summer 2011 in France, on the northern side of the Pyrenean foothills (at "Plateau de Lannemezan"), providing numerous observations used in the thesis. In addition, we also used a Large-Eddy Simulation (LES) with which the turbulent scales can be explicitly resolved. Thanks to this approach, the evolution during the AT of the scale definitions, scaling laws, turbulence characteristics and of the role of the boundary layer processes are now better understood. Thanks to the set of observations, an overview of the BLLAST ATs has been done, according to the various meteorological conditions as well as the structure and evolution of the mean and turbulence structure of the ABLs. A large variability of the AT duration was observed, depending on the surface characteristics and atmospheric conditions. The evolution of the turbulent characteristics during the AT has been studied at the surface and higher in the ABL by means of aircraft measurements and LES data. The study points out the LES ability to reproduce the turbulence evolution throughout the afternoon. A spectral analysis of the vertical wind from LES data, airborne and surface measurements reveals that the afternoon transition can be divided in two periods : a first period, called "Early Afternoon Period", during which the turbulence kinetic energy (TKE) decays with a slow rate, with no significant change in the turbulence characteristics, and a second period, called "Late Afternoon Period", characterized by a larger TKE decay rate and a change of its spectral shape, translating as an evolution of eddy size distribution and energy cascade from low to high wavenumbers. Moreover, the midday convective scaling laws are not valid anymore during this latter period. These changes occur first in the upper part of the ABL. The higher in the PBL, the stronger the spectra shape changes. Some sensitivity analyses have also been performed to improve the understanding of the evolution of the turbulence characteristics during the AT. The evolution of turbulence is sensitive to wind shear, which delays the decay and the scale evolution. Entrainement is suspected to increase the integral scales and anisotropy to play a role in the change of the inertial spectral slope. Coherent structures have shown to have a strong impact on the spectra, and make them significantly depart from the Kolmogorov theory. The starting time of the simulation also plays a role in the evolution of the turbulence characteristics. Though it is often assumed in the literature that a period of time equal to six times the convective time scale is long enough to generate a fully built turbulence field in meteorological models, we found that the largest scales need more time to be created, which also impacts the decay of turbulence during the AT

Role of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer

Observations, mixed-layer theory and the Dutch Large-Eddy Simulation model (DALES) are used to analyze the dynamics of the boundary layer during an intensive operational period (1 July 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence campaign. Continuous measurements made by remote sensing and in situ instruments in combination with radio soundings, and measurements done by remotely piloted aircraft systems and two manned aircrafts probed the vertical structure and the temporal evolution of the boundary layer during the campaign. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the models runs are inspired by some of these observations.; The research focuses on the role played by the residual layer during the morning transition and by the large-scale subsidence on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night in the development of the boundary layer at the morning. DALES numerical experiments including the residual layer are capable of modeling the observed sudden increase of the boundary-layer depth during the morning transition and the subsequent evolution of the boundary layer. These simulations show a large increase of the entrainment buoyancy flux when the residual layer is incorporated into the mixed layer. We also examine how the inclusion of the residual layer above a shallow convective boundary layer modifies the turbulent kinetic energy budget.; Large-scale subsidence mainly acts when the boundary layer is fully developed, and, for the studied day, it is necessary to be considered to reproduce the afternoon observations.; Finally, we also investigate how carbon dioxide (CO2) mixing ratio stored the previous night in the residual layer plays a fundamental role in the evolution of the CO2 mixing ratio during the following day.Postprint (published version

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

Turbulent processes in the boundary layer during the afternoon transition

Cette thèse porte sur l'étude de la transition d'après-midi (TA) de la couche limite atmosphérique (CLA). La TA est une période complexe du cycle diurne, de par son manque de stationnarité et parce qu'elle est régie par un ensemble de forçages qui faiblissent et dont les rôles respectifs changent par rapport à ce qu'ils ont pu être durant la période convective. Il résulte une moins bonne compréhension de cette période que des régimes quasi-stationnaires convectifs, neutres ou stables. La TA pourrait conditionner l'établissement du brouillard nocturne, influencer le développement de la couche limite du lendemain et être une phase clé du cycle diurne pour la ventilation des espèces en trace vers l'atmosphère libre. Par conséquent, de meilleures description et compréhension de la TA pourraient améliorer les modèles météorologiques et de qualité de l'air. Les principaux objectifs de la thèse ont été de mieux comprendre comment la turbulence décroît pendant la TA en mettant l'accent sur l'évolution de la structure verticale de la turbulence lorsque les transferts d'énergie en surface diminuent progressivement, et sur le rôle que joue la surface à ce moment particulier du cycle diurne. Le projet international BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) a servi de cadre à cette thèse. La campagne de mesures qui s'est déroulée en été 2011 sur le Plateau de Lannemezan, au nord de la chaîne Pyrénéenne, a fourni les nombreuses observations utilisées dans cette thèse. Une approche complémentaire aux observations est l'utilisation d'un modèle atmosphérique permettant de résoudre explicitement les échelles de la turbulence (LES).This thesis focuses on the afternoon transition (AT) of the atmospheric boundary layer (ABL). This transitional period is complex, because it is non-stationary and most of the forcings, though smaller than during the previous convective period, may come into play. Thus, this transitional period is less understood than the quasi-stationary convective, neutral or stable regimes. Yet, the AT could impact on the nocturnal fog set up, influence the boundary-layer development on the following day and play a crucial role in the transport and dispersion of pollutants and trace species towards the free troposphere. Therefore, a better understanding of the AT could improve the meteorological models. One of the main objectives of the thesis is to improve the knowledge of the decay of turbulence during the AT, when the surface energy transfers are gradually decreasing. We especially put emphasis on the evolution of the ABL turbulence vertical structure. This work is in the core of the BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) international project. A field campaign took place in summer 2011 in France, on the northern side of the Pyrenean foothills (at "Plateau de Lannemezan"), providing numerous observations used in the thesis. In addition, we also used a Large-Eddy Simulation (LES) with which the turbulent scales can be explicitly resolved. Thanks to this approach, the evolution during the AT of the scale definitions, scaling laws, turbulence characteristics and of the role of the boundary layer processes are now better understood. Thanks to the set of observations, an overview of the BLLAST ATs has been done, according to the various meteorological conditions as well as the structure and evolution of the mean and turbulence structure of the ABLs. A large variability of the AT duration was observed, depending on the surface characteristics and atmospheric conditions. The evolution of the turbulent characteristics during the AT has been studied at the surface and higher in the ABL by means of aircraft measurements and LES data. The study points out the LES ability to reproduce the turbulence evolution throughout the afternoon

Evolution of the turbulence during the afternoon transition of the convective boundary layer: a spectral analysis

The transition from a well-mixed convective boundary layer to a residual layer overlying a stabilized nocturnal layer raises several issues, which remain difficult to address from both modeling and observational perspectives. The well mixed convective boundary layer is mainly forced by buoyancy, with fully developed turbulence. The daily decrease of the surface buoyancy flux leads to the decay of the turbulence kinetic energy, and a possible change of the structure of the turbulence before it reaches the stable regime, with more anisotropy and intermittency. It is important to better understand these processes, as it can impact on the dispersion of tracers in the atmosphere, and on the development of the nocturnal and daytime boundary layers of the following days. The turbulence decay has been studied with laboratory experiments (Monin and Yaglom, 1975, Cole and Fernando 1998), numerical studies (Nieuwstadt and Brost 1986, Sorbjan 1997) and observations (Fitzarrald et al 2004, Grant 1997). The decay is related to the decrease of the surface buoyancy flux, but with complexity gained with shear-driven boundary layers (Pino et al 2006, Goulart et al 2003), which slow the decay. The evolution of the structure of turbulence during its decay remains unclear, with results on the evolution of characteristic scales that vary with the considered approaches and conditions. An observational and numerical study is proposed to address this related issue, based on the BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) dataset. BLLAST aims at better understanding the turbulent processes of the late afternoon transition, and leans on a field experiment that took place in summer 2011 in South-West France. A well-documented cloud-free weak wind day is considered here to analyze in details the evolution of the turbulence along the day, from midday to sunset. In particular, the decay is studied as a function of height, and the evolution of the turbulence kinetic energy is described with a thorough analysis of the turbulence spectra. The case study combines observations of the mean structure by radiosondes and remote sensing, and observations of the turbulence with surface stations, tethered balloon and aircraft. It is the base of a complementary idealized numerical study with a large eddy simulation. From both observations and numerical simulations, the turbulence is described with the characteristics of the spectral energy density, especially the typical turbulence lengthscales and the sharpness of the transition from energy-containing eddies to the inertial subrange. An analytical model proposed by Kristensen and Lenschow (1988) for homogeneous non-isotropic turbulence is used to approximate the observed and LES-modeled spectra and estimate their characteristics. Our analysis enables us to (1) test the capability of the large eddy simulation to represent the turbulence structure of the afternoon decay, (2) evaluate the evolution of integral scales, turbulent kinetic energy, shape of the spectra in both observations and numerical simulation, and as a function of height, (3) further understand the potential decoupling with height that occurs when the surface fluxes get too weak to maintain fully developed turbulence within the entire depth of the convective boundary layer, (4) put into evidence the limitations of the spectral approach for this critical period of the planetary boundary layer diurnal cycle.Peer Reviewe

Evolution of the turbulence during the afternoon transition of the convective boundary layer: a spectral analysis

The transition from a well-mixed convective boundary layer to a residual layer overlying a stabilized nocturnal layer raises several issues, which remain difficult to address from both modeling and observational perspectives. The well mixed convective boundary layer is mainly forced by buoyancy, with fully developed turbulence. The daily decrease of the surface buoyancy flux leads to the decay of the turbulence kinetic energy, and a possible change of the structure of the turbulence before it reaches the stable regime, with more anisotropy and intermittency. It is important to better understand these processes, as it can impact on the dispersion of tracers in the atmosphere, and on the development of the nocturnal and daytime boundary layers of the following days. The turbulence decay has been studied with laboratory experiments (Monin and Yaglom, 1975, Cole and Fernando 1998), numerical studies (Nieuwstadt and Brost 1986, Sorbjan 1997) and observations (Fitzarrald et al 2004, Grant 1997). The decay is related to the decrease of the surface buoyancy flux, but with complexity gained with shear-driven boundary layers (Pino et al 2006, Goulart et al 2003), which slow the decay. The evolution of the structure of turbulence during its decay remains unclear, with results on the evolution of characteristic scales that vary with the considered approaches and conditions. An observational and numerical study is proposed to address this related issue, based on the BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) dataset. BLLAST aims at better understanding the turbulent processes of the late afternoon transition, and leans on a field experiment that took place in summer 2011 in South-West France. A well-documented cloud-free weak wind day is considered here to analyze in details the evolution of the turbulence along the day, from midday to sunset. In particular, the decay is studied as a function of height, and the evolution of the turbulence kinetic energy is described with a thorough analysis of the turbulence spectra. The case study combines observations of the mean structure by radiosondes and remote sensing, and observations of the turbulence with surface stations, tethered balloon and aircraft. It is the base of a complementary idealized numerical study with a large eddy simulation. From both observations and numerical simulations, the turbulence is described with the characteristics of the spectral energy density, especially the typical turbulence lengthscales and the sharpness of the transition from energy-containing eddies to the inertial subrange. An analytical model proposed by Kristensen and Lenschow (1988) for homogeneous non-isotropic turbulence is used to approximate the observed and LES-modeled spectra and estimate their characteristics. Our analysis enables us to (1) test the capability of the large eddy simulation to represent the turbulence structure of the afternoon decay, (2) evaluate the evolution of integral scales, turbulent kinetic energy, shape of the spectra in both observations and numerical simulation, and as a function of height, (3) further understand the potential decoupling with height that occurs when the surface fluxes get too weak to maintain fully developed turbulence within the entire depth of the convective boundary layer, (4) put into evidence the limitations of the spectral approach for this critical period of the planetary boundary layer diurnal cycle.Peer ReviewedPostprint (published version

Role of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer

Observations, mixed-layer theory and the Dutch Large-Eddy Simulation model (DALES) are used to analyze the dynamics of the boundary layer during an intensive operational period (1 July 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence campaign. Continuous measurements made by remote sensing and in situ instruments in combination with radio soundings, and measurements done by remotely piloted aircraft systems and two manned aircrafts probed the vertical structure and the temporal evolution of the boundary layer during the campaign. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the models runs are inspired by some of these observations.; The research focuses on the role played by the residual layer during the morning transition and by the large-scale subsidence on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night in the development of the boundary layer at the morning. DALES numerical experiments including the residual layer are capable of modeling the observed sudden increase of the boundary-layer depth during the morning transition and the subsequent evolution of the boundary layer. These simulations show a large increase of the entrainment buoyancy flux when the residual layer is incorporated into the mixed layer. We also examine how the inclusion of the residual layer above a shallow convective boundary layer modifies the turbulent kinetic energy budget.; Large-scale subsidence mainly acts when the boundary layer is fully developed, and, for the studied day, it is necessary to be considered to reproduce the afternoon observations.; Finally, we also investigate how carbon dioxide (CO2) mixing ratio stored the previous night in the residual layer plays a fundamental role in the evolution of the CO2 mixing ratio during the following day

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