Atmospheric Boundary Layer (ABL) characteristics can be highly complex; the links between spatial and temporal variability of ABL meteorological quantities and existing land use patterns are still poorly understood due to the non-linearity of air-land interaction processes. This study describes the results from Monin Obukhov similarity theory and statistical analysis of meteorological observations collected by a network of ten Automatic Weather Stations (AWSs). The stations were in operation in the Highveld Priority Area (HPA) of the Republic of South Africa during 2008 – 2010. The spatial distribution of stability regimes as presented by both bulk Richardson number (BRN) and Obukhov length (L) indicates that HPA is dominated by strong stability regime. The momentum and heat fluxes show no significant spatial variation between stations. Statistical analysis revealed localization, enhancement and homogenization in the inter-station variability of observed meteorological quantities (temperature, relative humidity and wind speed) over diurnal and seasonal cycles. Enhancement of the meteorological spatial variability was found on a broad range of scales from 20 to 50 km during morning hours and in the dry winter season. These spatial scales are comparable to scales of observed land use heterogeneity, which suggests links between atmospheric variability and land use patterns through excitation of horizontal meso-scale circulations. Convective motions homogenized and synchronized meteorological variability during afternoon hours in the winter seasons, and during large parts of the day during the moist summer season. The analysis also revealed that turbulent convection overwhelms horizontal meso-scale circulations in the study area during extensive parts of the annual cycle / Dissertation (MSc)--University of Pretoria, 2013. / Geography, Geoinformatics and Meteorology / Unrestricted

Ce travail de recherche concerne l’interaction entre une couche limite atmosphérique et une canopée (représentant un couvert forestier). J’ai étudié le problème complexe de production et d’évolution de grosses structures turbulentes au dessus de couverts homogènes et hétérogènes, moyennement denses. J’ai abordé ce problème en mettant en œuvre les outils de la simulation numérique des grosses structures (LES) et du calcul haute performance (HPC). Les résultats numériques obtenus, reproduisent correctement les principales caractéristiques de cet écoulement, telles qu’elles sont rapportées dans la littérature : la formation d’une première génération de structures cohérentes en rouleaux, orientées transversalement par rapport à la direction de l’écoulement principal, puis la réorganisation et la déformation de ces structures qui évoluent vers une forme en fer à cheval. Les résultats obtenus au dessus d’un couvert discontinu (représentant une clairière ou une coupure de combustible dans une forêt), ont été comparés avec des données expérimentales collectées dans une soufflerie. Ceux-ci confirment l’existence d’un niveau élevé de turbulence au sein même du couvert végétal à une distance égale à 8 fois la hauteur de canopée. Cette zone, (appelée « Enhance Gust Zone » dans la littérature), est par ailleurs caractérisée par l’existence d’un pic local du facteur de dissymétrie (« skewness factor »).Le transport d’un contaminant passif émis par le feuillage a été également étudié, dans deux configurations, en supposant que la concentration à la surface du feuillage pouvait être considérée comme constante (source infinie) ou variable (source finie) en fonction du temps. Les résultats montrent un impact significatif de cette hypothèse sur la dynamique et le niveau des concentrations relevées dans l’atmosphère. / The work is dedicated to the investigation of the interaction between an Atmospheric Boundary Layer and a canopy (representing a forest cover). We have focused our attention to the complex problem of the generation and transformation of turbulent vortices over homogeneous, heterogeneous and sparse canopy. This problem has been studied using Large Eddy Simulation (LES) approach and High Performance Computing (HPC) technique.The numerical results reproduced correctly all the main characteristics of this flow, as reported in the literature: the formation of a first generation of coherent structures aligned transversally with the wind flow direction, the reorganization and the deformation of these vortex tubes into horse-shoe structures. The results obtained with the introduction of a discontinuity in the canopy (reproducing a clearing or a fuel break in a forest) are compared with the experimental data collected in a wind tunnel. In this case, the results confirmed the existence of a strong turbulence activity inside the canopy at a distance equal to 8 times the height of the canopy, referenced in the literature as the Enhance Gust Zone (EGZ) characterized by a local peak of the skewness factor. Then, the process of passive scalar transport from a forest canopy into a clear atmosphere is studied for two cases, i.e., when the concentration held by the forest canopy is either constant or variable. While this difference has little influence on the concentration patterns, results show that it has an important influence on the concentration magnitude as well as on the dynamics of the total concentration in the atmosphere.

Turbulent collapse and recovery are both observed to occur abruptly in the atmospheric stable boundary layer (SBL). The understanding and predictability of turbulent recovery remains limited, reducing numerical weather prediction accuracy. Previous studies have shown that regime occupation is the result of the net effect of highly variable processes, from turbulent to synoptic scales, making stochastic methods a compelling approach. Idealized stable boundary layer models have shown that under some circumstances, regimes can be related to the stable branches of model equilibria, and an additional unstable equilibrium is predicted. This work seeks to determine the extent to which the SBL regime occupation can be explained using a one-dimensional stochastic differential equation (SDE). The drift and diffusion coefficients of the SDE of an input time series are approximated from the statistics of its averaged time tendencies. These approximated coefficients are fit using Gaussian Process Regression. Probabilistic estimates of the system's equilibrium points are then found and used to create an empirical bifurcation diagram without making any prior assumptions on the dynamical form of the system. This data driven bifurcation diagram is compared to modelled predictions. The analysis is repeated on several meteorological towers around the world to assess the influence of local meteorological settings. This work provides empirical insights into the nature of regime dynamics and the extent to which the SBL displays hysteresis. / Graduate

京都大学 / 0048 / 新制・論文博士 / 博士(理学) / 乙第12913号 / 論理博第1549号 / 新制||理||1591(附属図書館) / 32123 / 名古屋大学大学院理学研究科 / (主査)准教授 林 泰一, 教授 石川 裕彦, 教授 余田 成男 / 学位規則第4条第2項該当 / Doctor of Science / Kyoto University / DGAM

The following work represents the research attempt of Dr. Frasier's group to develop a FMCW wind profiler for atmospheric boundary layer studies. The hardware development and integration are described in detail.

This thesis studies the implementation of an Explicit Algebraic Reynolds-Stress Model(EARSM) for Atmospheric Boundary Layer (ABL) in an open source ComputationalFluid Dynamics (CFD) software, OpenFOAM, following the guidance provided by thewind company ENERCON that aims to make use of this novel model to improvesites’ wind-field predictions. After carefully implementing the model in OpenFOAM,the EARSM implementation is verified and validated by testing it with a stratifiedCouette flow case. The former was done by feeding mean flow properties, takenfrom OpenFOAM, in a python tool containing the full EARSM system of equationsand constants, and comparing the resulting flux profiles with the ones extracted bythe OpenFOAM simulations. Subsequently, the latter was done by comparing theprofiles of the two universal functions used by Monin-Obukhov Similarity Theory(MOST) for mean velocity and temperature to the results obtained by Želi et al. intheir study of the EARSM applied to a single column ABL, in “Modelling of stably-stratified, convective and transitional atmospheric boundary layers using the explicitalgebraic Reynolds-stress model” (2021). The verification of the model showed minordifferences between the flux profiles from the python tool and OpenFOAM thus, themodel’s implementation was deemed verified, while the validation step showed nodifference in the unstable and neutral stratification cases, but a significant discrepancyfor stably stratified flow. Nonetheless, the reason behind the inconsistency is believedto be related to the choice of boundary conditions thus, the model’s implementationitself is considered validated. Finally, the comparison between the EARSM and the k − ε model showed thatthe former is able to capture the physics of the flow properties where the latter failsto. In particular, the diagonal momentum fluxes resulting from the EARSM reflectthe observed behaviour of being different from each other, becoming isotropic withaltitude in the case of unstable stratification, and having magnitude u′u′ > v′v′ > w′w′ for stably stratified flows. On the other hand, the eddy viscosity assumption used bythe k − ε model computes the diagonal momentum fluxes as being equal to each other.Moreover, the EARSM captures more than one non-zero heat flux component in theCouette flow case, which has been observed to be the case in literature, while the eddydiffusivity assumption used by the k − ε model only accounts for one non-zero heat fluxcomponent.

This thesis deals with Large Eddy Simulations (LES) of the Atmospheric Boundary Layer (ABL), focusing on studying the resolution dependence of turbulent passive scalar transport within the layer. The ABL is the lowest part of the atmosphere, where humans live and conduct most of their daily activities. Here, a scalar was injected at four different heights in a mixed shear- and convective-driven ABL, which was simulated using the Spectral Element Method (SEM) code Nek5000. The statistics of the four scalars were analysed and their resolution dependence was studied and compared to that of non-scalar quantities. No significant resolution dependence was found with regards to non-scalar quantities, while scalar quantities show a rather strong dependence on resolution especially in the first quarter of the simulation. Negative concentration values are found within the layer and some approaches to solve the problem are proposed. Statistics alone provide an accurate description of the general ABL behaviour, but are found to be insufficient to capture the dynamics of the scalar injection, which ought to be analysed with more advanced methods (e.g. modal decomposition). The structures arising within the layer are also analysed, and further work regarding the study of scalar fronts is suggested.

Die Arktis erwärmt sich aus noch nicht vollständig geklärten Gründen drei- bis viermal schneller als der Rest der Erde. Wolken, die den Energiehaushalt der Oberfläche und den vertikalen Transport von Wärme und Feuchtigkeit über dem Meereis signifikant beeinflussen, werden durch die oft begrenzte Verfügbarkeit von tröpfchenbildenden Aerosolpartikeln beeinflusst. Diese Aerosol-Wolken-Wechselwirkungen sind schwer zu erfassen, da die untere Troposphäre zumeist komplex geschichtet ist. In dieser Doktorarbeit werden drei neue wissenschaftliche Veröffentlichungen vorgestellt, die unternommenen wurden, um die vertikale Aerosolverteilung über dem arktischen Meereis mit einem Fesselballon zu untersuchen. Im ersten Schritt wurde eine neue Aerosolmessplattform, genannt CAMP, für Fesselballoneinsätze konzipiert. CAMP beinhaltet vier mobile Instrumente in einem Gehäuse, das vor Umwelteinflüssen schützt, zur Messung der Mikrophysik von Aerosolpartikeln. Die Sensoren wurden gründlich kalibriert und charakterisiert und die Leistung der Plattform in Feldtests bewertet. Im zweiten Schritt wurden während einer Forschungsexpedition in der zentralen Arktis Fesselballonmessungen von einer Eisscholle aus durchgeführt. Neben CAMP wurden vier weitere Instrumentenpakete mit dem Ballon geflogen, um die atmosphärische Grenzschicht zu charakterisieren. Die gewonnenen Daten wurden validiert und der wissenschaftlichen Gemeinschaft frei zur Verfügung gestellt. Im dritten Schritt wurden vierunddreißig Aerosolprofile analysiert und die Auswirkungen des Luftmassenursprungs und der Troposphärenstruktur auf die vertikale Aerosolverteilung bewertet. Die Ergebnisse der Studie zeigten, dass die Aerosolpartikel oberhalb der Grenzschicht für Wolken-Wechselwirkung von wesentlicher Bedeutung sind. Eine Analyse der Kopplung zwischen Wolke und Oberfläche zeigte deutlich, dass der vertikale Transport von Aerosolen von der Oberfläche zur Wolkenbasis in entkoppelten Wolkenfällen gehemmt war. Sekundäre Partikelbildung nach dem Transport von Vorläuferdämpfen von südlich des Eisrandes führte zu hohen Konzentrationen kleinerer Partikel oberhalb der Grenzschicht. In einem anderen Fall unterstützten hohe Mengen größerer Partikel die Bildung einer dichten Nebelschicht nach dem Ferntransport. Diese Arbeit hat gezeigt, dass es möglich ist, qualitativ hochwertige Aerosolmessungen mit Fesselballons in einer abgelegenen Region und unter schwierigen Umweltbedingungen durchzuführen. Die gewonnenen Daten und die bereitgestellten Analysen ermöglichen neue Einblicke in die vertikale Aerosolverteilung über dem Meereis. Zusammenfassend lässt sich sagen, dass diese Arbeit dazu beiträgt, unser Verständnis von Aerosol-Wolken-Wechselwirkungen über dem Arktischen Meereis zu erweitern.:Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Aerosol particles in the Arctic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 The lower troposphere above the Arctic sea ice . . . . . . . . . . . . . . . . . . . . . 4 1.3 Aerosol measurements with tethered balloons . . . . . . . . . . . . . . . . . . . . . . 6 1.4 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1 The cubic aerosol measurement platform (CAMP) . . . . . . . . . . . . . . . . . . . 9 2.1.1 Platform design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1.2 Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.1.3 First field deployments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2 Tethered balloon measurements above the Arctic sea ice . . . . . . . . . . . . . . . . 12 2.2.1 The MOSAiC expedition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2.2 Tethered balloon operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2.3 Deployed instrument packages . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2.4 Data validation and availability . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.1 Back trajectories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.2 Cloud borders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.3.3 Inversion detection algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.3.4 Tropospheric structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.1 First publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2 Second publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.3 Third publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4 Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.1 First publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.2 Second publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.3 Third publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 4.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 List of Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i A Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i A.1 Publications included in the Doctoral Thesis and Author’s contributions . . . . . . . i A.2 Contributions to other publications as co-author during the PhD . . . . . . . . . . . iii A.3 Colophon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv / The Arctic is warming three to four times faster than the rest of the Earth for reasons that are not yet fully understood. Clouds, which significantly affect the surface energy budget and the vertical transport of heat and moisture above sea ice, are influenced by the often limited availability of droplet-forming aerosol particles. However, aerosol-cloud interactions are challenging to assess due to the commonly complex structured lower troposphere. This doctoral thesis presents three novel scientific publications that detail the steps taken to investigate the vertical aerosol distribution above the Arctic sea ice with a tethered balloon. In the first step, the new cubic aerosol measurement platform (CAMP) was designed for tethered balloon deployments. CAMP contains four mobile instruments in an environmentally robust housing for measuring aerosol particle microphysics. The sensors were thoroughly calibrated and characterized, and the platform performance was evaluated in field tests. Secondly, tethered balloon measurements were performed from an ice floe during a research expedition into the central Arctic. CAMP and four other instrument packages were deployed with the balloon to characterize the atmospheric boundary layer. The obtained data were validated and made freely available to the scientific community. Lastly, thirty-four aerosol profiles were analyzed, and the impact of the air mass origin and the lower tropospheric structure on the vertical aerosol distribution was evaluated. The study results showed that the aerosol particles above the boundary layer are essential for interactions with low-level clouds. An analysis of the cloud-surface coupling state clearly demonstrated inhibited vertical transport of aerosols from the surface to the cloud base in decoupled cloud cases. Secondary particle formation initiated by low-level transport of precursor vapors from south of the ice edge caused high concentrations of smaller particles above the boundary layer. In another case, high amounts of larger particles supported the formation of a dense fog layer after long-range transport. This thesis demonstrated the feasibility of providing high-quality aerosol measurements with tethered balloons from a remote region under challenging environmental conditions. The obtained data and the provided analysis enable novel insights into the vertical aerosol distribution above the sea ice. In conclusion, this work contributes to expanding our understanding of aerosol-cloud interactions in the Arctic.:Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Aerosol particles in the Arctic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 The lower troposphere above the Arctic sea ice . . . . . . . . . . . . . . . . . . . . . 4 1.3 Aerosol measurements with tethered balloons . . . . . . . . . . . . . . . . . . . . . . 6 1.4 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1 The cubic aerosol measurement platform (CAMP) . . . . . . . . . . . . . . . . . . . 9 2.1.1 Platform design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1.2 Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.1.3 First field deployments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2 Tethered balloon measurements above the Arctic sea ice . . . . . . . . . . . . . . . . 12 2.2.1 The MOSAiC expedition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2.2 Tethered balloon operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2.3 Deployed instrument packages . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2.4 Data validation and availability . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.1 Back trajectories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.2 Cloud borders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.3.3 Inversion detection algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.3.4 Tropospheric structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.1 First publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2 Second publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.3 Third publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4 Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.1 First publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.2 Second publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.3 Third publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 4.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 List of Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i A Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i A.1 Publications included in the Doctoral Thesis and Author’s contributions . . . . . . . i A.2 Contributions to other publications as co-author during the PhD . . . . . . . . . . . iii A.3 Colophon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Diese Arbeit verwendet in situ Turbulenzmessungen aus Fesselballonbeobachtungen, um Methoden zur Ableitung der Höhe der bodennahen turbulenten Mischungsschicht (surface mixing layer, SML) in der arktischen atmosphärischen Grenzschicht zu bewerten. Die SML ist der unterste Teil der Atmosphäre, der turbulent ist und in dem selbst unter stabilen Bedingungen in der Arktis vertikale Vermischung und Transport von Wärme und Impuls stattfinden. Es werden zwei typische Zustände der Grenzschicht beobachtet: wolkenlos mit Temperaturinversionen am Boden und bewölkt mit Inversionen in der Höhe. Je nach Zustand variiert die vertikale Ausdehnung der SML erheblich. Die genaue Bestimmung der SML-Höhe ist entscheidend, da kleine Ungenauigkeiten einen erheblichen Einfluss auf die oft geringen SML-Höhen haben. Während der Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) Expedition wurden im Winter und Frühjahr vertikal hoch aufgelöste Profilmessungen mit einem Fesselballon durchgeführt, die vom Meereis bis in mehrere hundert Meter Höhe reichen. Diese Profile eignen sich für die Bestimmung einer Referenz-SML-Höhe, mit der andere häufig verwendete Methoden wie die auf der Bulk-Richardson-Zahl oder die auf bodennahen Austauschströmen basierenden Methoden bewertet werden können. Der kritische Wert von 0,12, der mithilfe der Turbulenzdaten bestimmt wird, ermöglicht eine genauere Abschätzung der SML-Höhen mit der Bulk-Richardson Methode. Dieser Ansatz wird auf Radiosondenaufstiege der MOSAiC-Expedition angewandt, um die Analyse zu erweitern. Außerdem werden die beiden typischen Zustände der arktischen Grenzschicht im Winter analysiert und der Übergang von bewölkten zu wolkenlosen Bedingungen anhand eines Messbeispiels untersucht. Diese Arbeit unterstreicht das Potential der Fesselballon-Turbulenzmessungen und wie diese Daten Einblicke in die vertikale Struktur der arktischen Grenzschicht geben. Die Messungen tragen zum Verständnis der Stabilität, der vertikalen Verteilung der Turbulenz und des Einflusses von Wolken auf die Grenzschicht bei, deren vertikale Schichtung wesentlich für Rückkopplungsprozesse ist, die zu einer verstärkten Erwärmung der Arktis beitragen.:Contents 1 Introduction 1 2 Fundamentals 7 2.1 Atmospheric boundary layer (ABL) 7 2.1.1 General characteristics 7 2.1.2 Vertical stability 10 2.1.3 Surface mixing layer (SML) 12 2.1.4 Cloudy conditions 13 2.2 Special features in the Arctic 14 2.3 Bulk Richardson number 16 2.4 Monin-Obukhov similarity theory 18 2.5 Turbulent flows 19 2.5.1 Eddies 19 2.5.2 Turbulent kinetic energy 22 2.5.3 Inertial subrange and energy dissipation rate 26 3 Balloon-borne turbulence measurements 29 3.1 MOSAiC expedition 29 3.2 Tethered balloon-borne measurements 31 3.2.1 Balloon operation 31 3.2.2 Vaisala tethersonde 34 3.2.3 High-resolution wind and temperature measurements 35 3.2.4 Calibration of the hot-wire anemometer 37 3.2.5 Derivation of energy dissipation rates 41 4 Surface mixing layer height estimates 47 4.1 In situ turbulence method 47 4.2 Bulk Richardson number method 48 4.2.1 Profiles of bulk Richardson number 48 4.2.2 Determining the critical Bulk Richardson number 50 4.3 SML height estimates based on a mean critical Richardson number 52 4.4 Surface flux-based method 54 5 Two typical states of the Arctic ABL 61 5.1 Observations in cloudless and cloudy conditions 61 5.2 Vertical mean and turbulent structure of the Arctic ABL 63 5.3 Transition from cloudy to cloudless states 66 6 Summary and outlook 71 Appendix 75 List of Figures 77 List of Acronyms 80 List of Symbols 83 References 86 / This thesis uses in situ turbulence measurements from tethered balloon observations to evaluate methods for deriving the surface mixing layer (SML) height in the Arctic atmospheric boundary layer (ABL). The SML is the lowermost part of the atmosphere, which is turbulent and experiences vertical mixing and transport of heat and momentum even under stable conditions in the Arctic. Two typical states of the ABL are observed: cloudless conditions with surface-based temperature inversions and cloudy conditions with elevated inversions. Depending on the state, the SML’s vertical extent varies significantly. Accurately determining the SML height is crucial because small inaccuracies in altitude are significant to the often shallow SML heights. Vertically highly resolved in situ profile measurements were obtained with a tethered balloon during the year-long Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in winter and spring, reaching from the sea ice to several hundred meters. These profiles are well suited to derive a reference SML height, which is used to evaluate other commonly used methods, such as the bulk Richardson number or surface flux-based method. A critical value of 0.12 is derived, leading to more accurate SML height estimates. This approach is applied to radiosonde ascents from the MOSAiC expedition to extend the analysis and bridge observational gaps without tethered balloon-borne profiles. Further, the two typical states of the Arctic ABL in winter are analyzed, and the transition from cloudy to cloudless conditions is studied based on a measurement example. This work demonstrates the unique potential of the tethered balloon turbulence measurements and how these profiles provide insights into the vertical structure of the Arctic ABL. These measurements contribute to understanding the stability, the vertical distribution of turbulence, and the influence of clouds on the ABL. The ABL’s vertical stratification is essential for feedback processes, which contribute mainly to Arctic amplification.:Contents 1 Introduction 1 2 Fundamentals 7 2.1 Atmospheric boundary layer (ABL) 7 2.1.1 General characteristics 7 2.1.2 Vertical stability 10 2.1.3 Surface mixing layer (SML) 12 2.1.4 Cloudy conditions 13 2.2 Special features in the Arctic 14 2.3 Bulk Richardson number 16 2.4 Monin-Obukhov similarity theory 18 2.5 Turbulent flows 19 2.5.1 Eddies 19 2.5.2 Turbulent kinetic energy 22 2.5.3 Inertial subrange and energy dissipation rate 26 3 Balloon-borne turbulence measurements 29 3.1 MOSAiC expedition 29 3.2 Tethered balloon-borne measurements 31 3.2.1 Balloon operation 31 3.2.2 Vaisala tethersonde 34 3.2.3 High-resolution wind and temperature measurements 35 3.2.4 Calibration of the hot-wire anemometer 37 3.2.5 Derivation of energy dissipation rates 41 4 Surface mixing layer height estimates 47 4.1 In situ turbulence method 47 4.2 Bulk Richardson number method 48 4.2.1 Profiles of bulk Richardson number 48 4.2.2 Determining the critical Bulk Richardson number 50 4.3 SML height estimates based on a mean critical Richardson number 52 4.4 Surface flux-based method 54 5 Two typical states of the Arctic ABL 61 5.1 Observations in cloudless and cloudy conditions 61 5.2 Vertical mean and turbulent structure of the Arctic ABL 63 5.3 Transition from cloudy to cloudless states 66 6 Summary and outlook 71 Appendix 75 List of Figures 77 List of Acronyms 80 List of Symbols 83 References 86

La modélisation par simulation des grandes échelles (Large-Eddy Simulation - LES) des processus physiques régissant la couche limite atmosphérique (CLA) demeure complexe de part la difficulté des modèles à capter l'évolution de la turbulence entre différentes conditions de stratification. De ce fait, l'étude LES du cycle diurne complet de la CLA comprenant des situations convectives la journée et des conditions stables la nuit est très peu documenté. La simulation de la couche limite stable où la turbulence est faible, intermittente et qui est caractérisée par des structures turbulentes de petite taille est tout particulièrement compliquée. En conséquence, la capacité de la LES à bien reproduire les conditions météorologiques de la CLA, notamment en situation stable, est étudiée à l'aide du code de mécanique des fluides développé par EDF R&D, Code_Saturne. Dans une première étude, le modèle LES est validé sur un cas de couche limite convective quasi stationnaire sur terrain homogène. L'influence des modèles sous-maille de Smagorinsky, Germano-Lilly, Wong-Lilly et WALE (Wall-Adapting Local Eddy-viscosity) ainsi que la sensibilité aux méthodes de paramétrisation sur les champs moyens, les flux et les variances est discutées. Dans une seconde étude le cycle diurne complet de la CLA pendant la campagne de mesure Wangara est modélisé. L'écart aux mesures étant faible le jour, ce travail se concentre sur les difficultés rencontrées la nuit à bien modéliser la couche limite stable. L'impact de différents modèles sous-maille ainsi que la sensibilité au coefficient de Smagorinsky ont été analysés. Par l'intermédiaire d'un couplage radiatif réalisé en LES, les répercussions du rayonnement infrarouge et solaire sur le jet de basse couche nocturne et le gradient thermique près de la surface sont exposées. De plus l'adaptation de la résolution du domaine à l'intensité de la turbulence et la forte stabilité atmosphérique durant l'expérience Wangara sont commentées. Enfin un examen des oscillations numériques inhérentes à Code_Saturne est réalisé afin d'en limiter les effets / Large-eddy simulation (LES) of the physical processes in the atmospheric boundary layer (ABL) remains a complex subject. LES models have difficulties to capture the evolution of the turbulence in different conditions of stratification. Consequently, LES of the whole diurnal cycle of the ABL including convetive situations in daytime and stable situations in the night time is seldom documented. The simulation of the stable atmospheric boundary layer which is characterized by small eddies and by weak and sporadic turbulence is espacialy difficult. Therefore The LES ability to well reproduce real meteorological conditions, particularly in stable situations, is studied with the CFD code developed by EDF R&D, Code_Saturne. The first study consist in validate LES on a quasi-steady state convective case with homogeneous terrain. The influence of the subgrid-scale models (Smagorinsky model, Germano-Lilly model, Wong-Lilly model and Wall-Adapting Local Eddy-viscosity model) and the sensitivity to the parametrization method on the mean fields, flux and variances are discussed.In a second study, the diurnal cycle of the ABL during Wangara experiment is simulated. The deviation from the measurement is weak during the day, so this work is focused on the difficulties met during the night to simulate the stable atmospheric boundary layer. The impact of the different subgrid-scale models and the sensitivity to the Smagorinsky constant are been analysed. By coupling radiative forcing with LES, the consequences of infra-red and solar radiation on the nocturnal low level jet and on thermal gradient, close to the surface, are exposed. More, enhancement of the domain resolution to the turbulence intensity and the strong atmospheric stability during the Wangara experiment are analysed. Finally, a study of the numerical oscillations inherent to Code_Saturne is realized in order to decrease their effects