A modeling study is undertaken to better understand the physics of katabatic flows. This study is divided into three topics; a comparison between a large eddy simulation (LES) and a mesoscale model of katabatic flows, a sensitivity study of katabatic flows to various physical parameters, and an investigation into the effect of subgrid scale terrain features on katabatic flow models. In the first topic, a comparison between LES, and a mesoscale model, ARPS, of katabatic flows is made to better quantify the accuracy of subgrid parametenzation in ARPS. It is shown that, although the modeled flows agree on a number of parameters, the LES model produces a lower and faster jet than that of ARPS, and also cools more near the surface. The momentum budgets of the two models agree well with each other. The ARPS model has a higher amount of TKE than the LES model, due to an overproduction by shear in the ARPS subgrid parameterizations. The second portion of this thesis represents a sensitivity study of katabatic flows to various physical parameters. The depth and strength of katabatic flows are shown to vary with surface heat fluxes, slope angle, and ambient stratification. Katabatic flows are shown to grow in depth and magnitude as slope angle increases, due to an increase in entrainment of overlying ambient air. The ratio of advection to mixing is shown to collapse to a near universal value regardless of surface heat fluxes. With increasing ambient stratification, entrainment in katabatic flows becomes small and the momentum equation is reduced to a two-way balance between buoyancy and drag. In this case, the heat flux of entrained air into the katabatic flow approaches that of the surface cooling, and the flow ceases to grow in the down-slope direction. Finally, predictions for bulk velocity and buoyancy strength scales are developed as a function of slope angle and surface heat fluxes. The last portion of this study focuses on the effect of subgrid scale terrain features on katabatic flows. It is shown that in areas of inadequate terrain resolution, the effect of the terrain smoothing routine in ARPS is to increase the slope height in areas of concave mountains. The concept of energy conversion in katabatic flows is introduced, and it is shown that the effect of raising terrain is to assign parcels more buoyant potential energy than they would otherwise have, and thus over-predict the magnitude of katabatic flows. Finally, an investigation into the effect of changing upper slope angle on katabatic flows over combined slopes is made. It is concluded that a combined slope cannot be predicted using a linear combination of simple slopes, since the transition portion of the slope results in a turbulent hydraulic jump with enhanced mixing. The magnitude of mixing in the turbulent hydraulic jump in combined slopes is shown to depend on the difference between upper and lower slope angle. / Graduation date: 2004

No description available.

Thesis (PhD)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT:This work is concerned with the numerical simulation of the aeolian snow transportation process (drifting or wind blown snow) and especially the snow deposition and erosion phenomenon (snow drift). The research work is interested in modelling the atmospheric boundary layer wind flow and its associated snow drifting processes around threedimensional obstacles by means of computational fluid dynamics (CFD). A modelling method is required to predict and evaluate the snow drifting phenomenon surrounding the SANAE IV research station in Antarctica. This station is of an elevated design to ensure that wind blown snow may travel around the structure relatively undisturbed and without deposition near the structure. This design is partly successful but localised drifts are formed especially leeward of the interconnecting structures that join the main building sections together. The theoretical and numerical description to describe the turbulent transport of the two-phase mixture of air and snow particles is investigated. This theory is subsequently employed to describe the snow deposition and erosion process and two models are developed to determine the deposition flux onto the snow surface. These models presented and discussed are a threshold based approach and a conservative based approach. The first model is dependent on a threshold shear velocity to determine the onset of either erosion or deposition. The second model determines the deposition or erosion flux based on the conservation of the snow mass transport in the near surface control volume. A numerical scheme that evaluates the snow deposition flux at the surface and forces a temporal surface adaptation during the simulation is established and implemented in a commercial CFD software code by means of user subroutines. Various test cases for which observed snow drift data are available are numerically modelled to validate the snow drift schemes presented in this work. These tests include the wind driven snow accumulation around a three-dimensional cube, around two adjacent three-dimensional cubes and near a typical porous snow fence. The results indicate that both methods can predict realistic snow drifts for a variety of wind flow conditions but also show that the conservative approach is superior to the threshold based approach in describing the snow drift process around obstacles. This model allows drifts to form not only in areas of low flow velocities but also under high shear conditions. The theoretical investigation and the development and validation of the conservatively based snow drift scheme shows that drift formation depends strongly on the near surface flow divergence and secondary flow structures. To resolve the snow drift formation under a variety of flow conditions a three-dimensional field solution is required to determine velocity and snow concentration gradients and include the effects of near surface convective and turbulent entrainment. The model is applied to numerically simulate and predict snow drifting around the SANAE IV base for a moderate as well as a high wind speed event. The predicted snow drift around the base agrees favourably with the observed drifts at the station. Further numerical simulations are carried out to evaluate the effects a few design modifications may have on the snow deposition. These results suggest that a simple baffle plate installation near the bottom of the interconnecting link structures may minimise the snow accumulation leeward of that area. This study shows that to achieve realistic numerical snow drift predictions around, on or near obstacles, a conservative based snow drift scheme should be considered using some form of temporal terrain adaptation strategy. Only then does one include a sufficient level of important flow effects such as deposition along near surface boundaries of strong flow divergence which plays as an important role as vertical settling and entrainment in determining deposition rates. / AFRIKAANSE OPSOMMING:Hierdie studie behels die numeriese simulasie van windgedrewe sneeubeweging asook die daarmee gepaardgaande sneeu neerslag en erosie eienskappe. Die navorsing het verder belang in die berekening van die atmosferiese grenslaag vloei en die simulasie van sneeu neerslag naby drie-dimensionele strukture deur gebruik te maak van berekeningsvloeimeganika (BVM). ‘n Berekeningsmetodiek is nodig om die eienskappe van die sneeu neerslag rondom die SANAE IV navorsingsstasie in Antarktika te voorspel en te evalueer. Die bogrondse struktuur is spesifiek so ontwerp om te verseker dat wind gedrewe sneeu hoofsaaklik onversteurd verby die struktuur kan beweeg sonder neerslag teenaan die struktuur. Die ontwerp is grotendeels suksesvol alhoewel sneeu neerslag wel lokaal plaasvind, wind af vanaf die aansluitings strukture tussen die hoof geboue. Die teoretiese en numeriese beskrywing van die twee-fase lug- en sneeumengsel beweging word ondersoek en gebruik om die sneeu neerslag en erosie einskappe te beskryf. Twee modelle wat hierdie verskynsel beskryf word beskryf en bespreek naamlik ‘n drumpel gebaseerde benadering en ‘n konserwatief gebaseerde benadering. Die eerste model is afhanklik van ‘n drumpel skuifsnelheid om die aanvang van of erosie of neerslag te bereken. Die tweede model bereken die neerslag eerder gebaseer op die behoud van die sneeu massa vloei in die kontrole volume naby aan die oppervlak. ‘n Numeriese metode is ontwikkel en geimplementeer in ‘n kommersiële BVM sagteware pakket deur van gebruikerssubroetines gebruik te maak. Die ontwikkelde kode evalueer die sneeu neerslag vloed by die oppervlak en forseer ‘n tydafhanklike oppervlak aanpassing gedurende die simulasie. Die sneeu neerslag metode wat beskryf word in hierdie studie word ge-evalueer teen verskeie toetsgevalle waarvoor daar waargenome sneeu neerslag resultate beskikbaar is. Hierdie toetse sluit in die wind gedrewe sneeu neerslag rondom ‘n drie-dimensionele kubus, rondom twee naby geleë kubusse en naby ‘n tipiese poruese sneeu heining. Die resultate dui aan dat beide die metodes realistiese sneeu neerslag voorspel vir verskeie wind toestande. Die studie wys ook dat die konserwatief gebaseerde benadering vir die beskrywing van die sneeu neerslag proses meer akkuraat is as die drumpel gebaseerde benadering aangesien die neerslagvoorspel kan word nie net alleenlik in gebiede met lae vloeisnelhede nie, maar ook in gebiede waar hoë skuifsnelhede teenwoordig is. Die teoretiese ondersoek, ontwikkeling en toepassing van die konserwatief gebaseerde model dui daarop dat die neerslag afhanklik is van die divergensie van die vloeiveld asook van die sekondêre vloei patrone naby die oppervlak. Ten einde die sneeu neerslag vir verskeie toestande op te los is dit nodig om snelheids- en sneeukonsentrasie gradiënte te kan bereken in ‘n drie-dimensionele vloei veld om sodoende die invloed van naby-oppervlak konveksie en turbulente verspreiding in ag te neem. Die metode word toegepas deur die sneeu neerslag rondom die SANAE IV navorsingsstasie te voorspel vir ‘n gematigde asook ‘n hoë wind snelheid toestand. Die sneeu neerslag voorspelling stem gunstig ooreen met die waargenome neerslag by die struktuur. Verdere numeriese simulasies is uitgevoer om die invloed van ontwerpsverandering op die neerslag te evalueer. Uit hierdie resultate blyk dit dat ‘n eenvoudige plaat struktuur onder die aansluitingsstrukture die sneeu neerslag wind af mag verminder. Hierdie navorsingsstudie dui daarop dat ‘n tydafhanklike terrein aanpassing strategie saam met die konserwatiewe neerslag model noodsaaklik is ten einde realistiese resultate te behaal vir die sneeu opbou rondom of naby strukture. Sodoende word genoegsame vlakke van belangrike vloei verskynsels, soos die invloed van vloei divergensie, in ag geneem wat net so ‘n belangrik rol in neerslag speel soos vertikale afsetting.

<p>In this thesis various descriptions of the near surface atmospheric flow over a high latitude glacier is used in an effort to increase our understanding of the basic flow dynamics there.</p><p>Through their contribution to sea-level change, mountain glaciers play a significant role in Earth’s climate system. Properties of the near surface atmospheric flow are important for understanding glacier response to climate change.</p><p>Here, the near surface atmospheric flow is studied from several perspectives including the effects of both rotation and slope. Rotation is an important aspect of most atmospheric flows and its significance for mesoscale flows have gained recognition over the last years. Similarly, the very stable boundary layer (VSBL) has lately gained interest. Within a VSBL over sloping terrain katabatic flow is known to be usual and persistent. For the present thesis a combination of numerical and simple analytical models as well as observations from the Vatnajökull glacier on Iceland have been used. The models have continuously been compared to available observations. Three different approaches have been used: linear wave modeling, analytic modeling of katabatic flow and of the Ekman layer, and numerical simulations of the katabatic flow using a state of the art mesoscale model. The analytic models for the katabatic flow and the Ekman layer used in this thesis both utilizes the WKB method to allow the eddy diffusivity to vary with height. This considerably improves the results of the models. Among other findings it is concluded that: a large part of the flow can be explained by linear theory, that good results can be obtained for surface energy flux using simple models, and that the very simple analytic models for the katabatic flow and the Ekman layer can perform adequately if the restraint of constant eddy diffusivity is relieved.</p>

In this thesis various descriptions of the near surface atmospheric flow over a high latitude glacier is used in an effort to increase our understanding of the basic flow dynamics there. Through their contribution to sea-level change, mountain glaciers play a significant role in Earth’s climate system. Properties of the near surface atmospheric flow are important for understanding glacier response to climate change. Here, the near surface atmospheric flow is studied from several perspectives including the effects of both rotation and slope. Rotation is an important aspect of most atmospheric flows and its significance for mesoscale flows have gained recognition over the last years. Similarly, the very stable boundary layer (VSBL) has lately gained interest. Within a VSBL over sloping terrain katabatic flow is known to be usual and persistent. For the present thesis a combination of numerical and simple analytical models as well as observations from the Vatnajökull glacier on Iceland have been used. The models have continuously been compared to available observations. Three different approaches have been used: linear wave modeling, analytic modeling of katabatic flow and of the Ekman layer, and numerical simulations of the katabatic flow using a state of the art mesoscale model. The analytic models for the katabatic flow and the Ekman layer used in this thesis both utilizes the WKB method to allow the eddy diffusivity to vary with height. This considerably improves the results of the models. Among other findings it is concluded that: a large part of the flow can be explained by linear theory, that good results can be obtained for surface energy flux using simple models, and that the very simple analytic models for the katabatic flow and the Ekman layer can perform adequately if the restraint of constant eddy diffusivity is relieved.

La couche-limite atmosphérique turbulente stable, particulièrement en zone de relief, n'est pas totalement comprise. Elle est, donc, mal représentée par les modèles atmosphériques. En présence de pente et d'un refroidissement du sol, l'augmentation locale de masse volumique génère un écoulement catabatique. En région de montagne, le maximum de vent est généralement enregistré à une hauteur (z_j) de 1-10 m. Le jet de paroi engendre un changement de signe du flux de qdm ainsi qu'une variabilité du flux de chaleur sensible proche du sol. Ces variabilités de flux turbulents contredisent l'applicabilité de la théorie des similitudes de Monin-Obukhov (TSMO), pourtant utilisée de manière universelle dans les modèles atmosphériques. Si la TSMO est discutable pour les cas très stables, c'est en présence de pente qu'elle n'est naturellement plus valide puisqu'elle néglige le couplage entre les équations de vitesse et de température. Il est donc impossible de représenter correctement un écoulement catabatique (z_j O(1m)) par un modèle utilisant la TSMO et avec une résolution verticale de l'ordre de la hauteur du maximum de vent. L'objectif du travail de thèse est d'apporter une contribution dans la compréhension et la modélisation de ces écoulements.Afin de compléter les observations peu nombreuses, une campagne de mesure a été réalisée sur une pente raide (20-40 deg) : la pente ouest du Grand Colon (chaîne de Belledonne, Alpes). Les analyses spectrales témoignent de la sensibilité de l'écoulement local aux perturbations externes, même faibles. Les caractéristiques turbulentes classiques sont observées à haute-fréquence alors que des comportements moins standards sont observés aux fréquences intermédiaires ou basses et expliqués par la présence de perturbations turbulentes d'énergie du même ordre de grandeur que l'injection locale. Les cospectres montrent un comportement propre aux écoulements catabatiques: recouvrement progressif selon z des corrélations croisées <0 et >0. La TSMO est mise en défaut pour l'écoulement observé et une solution alternative est utilisée pour estimer les flux en surface, permettant une bonne description de la vitesse de frottement.Le modèle 1D de surface de ISBA (Météo-France) est modifié pour répondre à la modélisation des écoulements catabatiques. Dans un premier temps, le modèle est validé sur un cas standard: en comparaison avec un modèle de Prandtl adapté. Dans un second temps, les données in-situ sont modélisées, d'abord en fournissant des profils de diffusivités effectives puis en utilisant un modèle modifié de turbulence d'ordre 1.5. Les modélisations 1D représentent correctement les champs moyens de vitesse et température mais montrent cependant des comportements trop diffusifs. Le modèle de longueur de mélange est principalement remis en cause, y compris en utilisant des paramétrisations adaptées.Des simulations LES 3D réalistes (Meso-NH, Météo-France) sont effectuées à haute résolution pour représenter le cas d'étude. Ces modélisations représentent finement les variabilités spatiales de l'écoulement catabatique. Cependant, des biais sont engendrés principalement par l'utilisation de la TSMO en condition aux limites de surface. Malgré la forte résolution spatiale, l'utilisation de la TSMO repousse à seulement z=2 m la perception des termes sources de l'écoulement catabatique par le modèle, alors que la source de l'écoulement atteint son maximum précisément en surface. Les modèles analytiques d'écoulement catabatique (de type Prandtl, qui pourraient aisément être intégrés en conditions aux limites) nécessitent de connaître "a priori" les profils de diffusivité. Ceci implique l'utilisation d'un modèle de turbulence. Le couplage du modèle 1D de surface (précédemment modifié et validé "off-line") est donc proposé pour répondre au manque de description de la physique par les CaL classiques de surface. Le travail préliminaire du couplage est présenté et des solutions sont proposées en perspective. / The stable atmospheric boundary layer, particularly in complex terrain, is not yet fully understood and it is thus still inadequately modelled. A surface cooling of a sloping terrain generates katabatic wind due to local density increase. This flow behaves as a wall-bounded turbulent jet, often simply modelled by a local balance between the buoyancy force and the turbulent friction. In mountainous regions, the wind maximum is typically observed at a height (z_j) of 1-10 m above the ground. The wall-bounded jet is responsible for a momentum-flux sign change and a heat-flux variability close to the ground. Those turbulent-flux variabilities are fully conflicting with the aplicability of the Monin-Obukhov similarity theory (MOST), which is nevertheless universally used in the atmospheric models to provide the surface boundary condition. If the MOST is already questionable for the very stable cases, it is obviously not valide over sloping surfaces because it neglectes the coupling of the wind and temperature equations, which constitutes the katabatic source. Hence, it is not possible to adequatly model a katabatic flow (z_j O(1m)) using the MOST, especially with a vertical resolution of the order of magnitude of z_j. The aim of the this PhD work is thus to improve the current understanding and modelling capacity of the katabatic winds.Since data sets of turbulent-katabatic-flow measurements are still scarce, a new field campain was carried out on a steep slope (20-40 deg): the west face of the Grand-Colon mountain (Belledonne ridge, French Alps). The experimental setup was mainly composed of a 6m mast with four sonic-anemometer levels (1, 2, 4 and 6m) to measure the turbulence on both sides of the katabatic jet. The spectral analysis shows the hight sensitivity of the local flow to external perturbations, even when these are weak. The hight-frequency subrange shows a classical behaviour (energy-injection frequency, inertial subrange), but the spectra of the intermediate and low-frequency subranges are less typical: turbulent perturbations with an energy of the order of magnitude of the local injection are present. A specific cospectra behaviour of the katabatic flows is shown: negative and positive cross-correlations overlap gradually, increasing z. The MOST fails in representing the observed flow and a surface-flux alternative estimation is succesfully used to describe the friction vellocity.The 1D surface model of ISBA (Météo-France) is modified to model katabatic flows. The model is firstly validated with a standard calibrated Prandtl model (with variable eddy difusivity). Secondly, the field data are modelled both with a prescribed effective diffusivity (from data) and using the 1.5-order turbulence scheme. The mean velocity and temperature fields are well reproduced, but it appears that the model is over-diffusive (which generates excessive fluxes), even when an adapted mixing-length is used.Realistic 3D LES simulations (Meso-NH, Météo-France) are computed with high resolution to model the field data. Spatial flow variabilities over sloping terrain are finely represented, but are biased, mainly due to the using of MOST for the surface boundary counditions. The using of MOST shifts the start of the katabatic source detection by the atmospheric model to a height of 2 m, while the katabatic source reaches its maximum at the surface. Analytical katabatic models (of the Prandtl type, which could be easily used to feed surface boundary counditions) need an "apriori" definition of the eddy and heat diffusivities. Currently, the general definition of these diffusivities is only possible by the use of turbulent models that include closures. The coupling of the previously-presented 1D surface model (validated off-line) is suggested to overcome the lack of physics description in the classic surface boundary counditions. Preliminary work on this coupling is developed and perspective solutions are proposed.

No description available.

La surface enneigée du continent Antarctique, sauf pour quelques heures les après-midi d'été, se refroidit constamment radiativement. Il en résulte une stratification stable persistante de la couche limite atmosphérique qui alimente un écoulement catabatique le long des pentes qui descendent du plateau vers l'océan. Les inversions de températures et les vitesses de vents associées sont extrêmes l'hiver où une inversion moyenne de 25°C sur le plateau et des vitesses dépassant les 200 km/h sur la côte sont régulièrement observées. L'été, les inversions restent très marquées la nuit, mais le réchauffement de la surface par le soleil conduit au développement de couches convectives l'après midi. Des replats et des pentes immenses et vides, inlassablement recouverts de neige : l'Antarctique est un laboratoire unique pour étudier les transitions entre les régimes turbulents, et surtout la turbulence dans les couches limites stables et catabatiques. Des processus délicats à étudier, puisque très sensibles aux hétérogénéités de la surface. Ce travail de thèse documente trois cas d'école estivaux typiques : le cycle diurne sur le plateau Antarctique, la génération d'un écoulement catabatique local, et la couche limite soumise à un forçage catabatique. Ces trois situations ont été explorées avec des observations in-situ. Pour deux d'entre elles, les observations ont nourri et ont été complétées par des simulations avec le modèle atmosphérique Méso-NH. Le premier cas s'intéresse au cycle diurne au Dôme~C. Le Dôme~C, sur le plateau Antarctique est une zone plate et homogène éloignée des perturbations océaniques. Depuis quelques années, une tour de 45 m échantillonne la couche limite. L'été, un cycle diurne marqué est observé en température et en vent avec un jet de basse couche surgéostrophique la nuit. Une période de deux jours, représentative du reste de l'été, a été sélectionnée, pour la construction du cas d'intercomparaison GABLS4, préparé en collaboration avec Météo-France. Les simulations uni-colonnes menées avec le modèle Méso-NH ont montré la nécessité d'adapter le schéma de turbulence afin qu'il puisse reproduire à la fois les inversions de température et l'intensité de la turbulence mesurées. Le deuxième cas d'école examine un écoulement catabatique généré localement, au coucher du soleil, observé sur une pente de 600 par 300 m en Terre Adélie. Certaines caractéristiques de la turbulence, en particulier l'anisotropie, ont été explorées à l'aide de simulations à fine échelle (LES). Le troisième cas s'intéresse à la couche limite mélangée typique des zones côtières soumises à un vent intense. Ce vent d'origine catabatique, a dévalé les 1000 km de pente en amont. En remobilisant la neige, il interagit avec le mélange turbulent. Le travail s'est intéressé dans ce troisième cas à l'impact du transport de neige sur l'humidité de l'air et au calcul des flux turbulents à partir des profils de température, vent et humidité. / Except during a few summer afternoon hours, the snow-covered surface of Antarctica is constantly cooling because of radiative processes. This results in a stable, persisting stratification of the atmospheric boundary layer that feeds katabatic winds along the slopes descending from the Plateau to the Ocean. Temperature inversions and wind speeds both peak during the winter, with inversions regularly reaching 25 degrees (C) over the Plateau and winds exceeding 200,km/h along the coast. In the summer, significant inversions remain at night but solar heating leads to the formation of convective layers near the surface in the afternoon. With berms and large, empty slopes constantly covered with snow, Antarctica is a unique and perfect laboratory for the study of transitions between turbulent regimes and of the turbulence within stable and katabatic boundary layers. The investigation of these processes is usually made difficult by their sensitivity to heterogeneities at the surface. This thesis work documents three typical "text-book" summer cases: the diurnal cycle on the Antarctic Plateau, the generation of a local katabatic wind and the katabatic forcing of the boundary layer. The investigation of these three cases uses in-situ data. For two of these cases, the observational data has fed and been completed with some Meso-NH model simulation outputs. The first case focusses on the diurnal cycle at Dome C. On the Antarctic Plateau, Dome C is a flat, homogeneous area far from oceanic perturbations. Since a few years, a 45 meters tower samples the boundary layer there. In the summer, the diurnal cycle there is characterized by clean signals in both temperature and winds, with a nocturnal low-level jet within the boundary layer. A two-days data set representative of the rest of the summer has been selected for analysis and is used in the GABLS4 comparison study prepared in collaboration with Meteo France. Single-column simulations have been run for this comparison work launched in June. The second case examines a local katabatic flow generated at sunset over a 600 by 300 meters slope in Terre Adelie. Characteristics of the turbulence of this flow, in particular, its anisotropy, are investigated using small-scale model simulations. A measuring station has been deployed in order to prepare and evaluate these simulations. The third case is concerned with boundary layers typical of coastal areas with strong winds of katabatic origins, which have flown over 1000 km-long slopes towards the sea. By moving around the snow at the surface, these winds interact with turbulent mixing processes. For this final case, the work is interested in the impact of blowing snow on atmospheric moisture and with the calculation of turbulent fluxes based on temperature, wind and humidity profiles.

La capa límit atmosfèrica és l'àrea directament influenciada per la presència de la superfície de la terra i la seva alçada és d'uns centenars de metres a uns pocs quilòmetres. Durant el vespre, el refredament radiatiu estratifica establement l'aire prop del sòl i es forma el que es coneix com a Capa Límit Estable (CLE). D'avui en dia, la CLE és un règim que encara no està prou ben caracteritzat. La turbulència, que no és homogènia ni isòtropa, i la gran importància dels efectes locals com l'orografia, entre d'altres factors, dificulten l'estudi d'aquest règim. Per aquest motiu, la CLE és objecte d'especial atenció, sobretot a l'hora de millorar la seva representació en models tant de temps com de clima.Aquest treball es centra en l'estudi de la CLE mitjançant 3 eines diferents: 1) simulacions explícites de grans remolins (més conegudes com a simulacions LES), per determinar el comportament dels moviments turbulents, on les resolucions són de l'ordre de metres; 2) simulacions mesoscalars, per caracteritzar els efectes locals, on les resolucions són de l'ordre de kilòmetres; 3) anàlisi de les observacions sota aquestes condicions per tal de caracteritzar i entendre millor els fenòmens observats.En primer lloc s'estudia el rang d'estabilitats a on el model LES, que considera la teoria de Kolmogorov per la dissipació de l'energia, funciona correctament. Els resultats del model són realistes tal com mostra la seva comparació amb les mesures de dues campanyes experimentals (SABLES-98 i CASES-99). Per explorar més a fons els resultats LES i per comparar-los amb les mesures s'han utilitzat les Funcions de Distribució de Probabilitat (PDF). Aquests resultats LES són també comparables als obtinguts amb altres models LES, tal com mostra la intercomparació de models LES, més coneguda com a GABLS.Un cop desenvolupades totes les eines necessàries es fa un LES d'un cas més realista, basat en les observacions d'un màxim de vent de capes baixes (més conegut com a Low-Level Jet, LLJ). L'anàlisi combinat dels resultats LES i les mesures permet entendre millor els processos de barreja que tenen lloc a través de la inversió. Finalment, la contribució dels efectes locals s'estudia mitjançant les simulacions mesoscalars, en aquest cas centrades a l'illa de Mallorca. Durant el vespre es veu com les circulacions locals es desenvolupen a les conques (de longitud al voltant de 25km), formant-se, per exemple, vents catabàtics o LLJ com l'estudiat anteriorment. En aquest cas les simulacions es verifiquen amb imatges de satèl·lit NOAA i observacions de les estacions automàtiques de mesures, donant resultats semblants. / The atmospheric boundary layer is the area directly influenced by the presence of the Earth's surface and its height is from hundreds of meters to few kilometres. During the night, the radiative cooling stratifies the layer close to the surface and it forms the Stably-stratified Atmospheric Boundary Layer (SBL). Nowadays, the SBL is a regime not well enough characterized, yet. Turbulence, which is not homogeneous either isotropic, and the great importance of the local effects, like the orography, among other factors, make the SBL be a difficult regime to study. Even so, the SBL is an object of special attention, especially when improving its representation in numerical prediction models or climate models.This work focuses on the study of the SBL through 3 different tools: 1) Large-Eddy Simulations (LES), to determine the turbulent motions, where the resolutions are about 1m; 2) Mesoscale simulations, to characterize the local effects, where resolutions are about 1km; 3) Analysis of the observations under these conditions in order to better characterize and understand the observed phenomena.In first place, it is studied the range of stabilities where the LES model, that considers the Kolmogorov theory for the dissipation of the energy, works correctly. The results are realistic as the comparison with measures from two experimental campaigns (SABLES-98 and CASES-99) shows. To explore the results more thoroughly, and to compare the LES results to the measurements, the Probability Density Functions (PDF) have been used. The LES results are also comparable to the ones obtained with other LES models, as the intercomparison of different LES models show, better known as GABLS.Then, a more realistic case is performed using the LES model, based on observations of a Low-Level Jet (LLJ). The combined inspection of the LES results and the observations allow to better understand the mixing processes that take place through the inversion layer. Finally, the contribution of the local effects is studied through a mesoscale simulation. Here the attention is focused on the Mallorca Island. During the night, the model is able to reproduce the local circulations is a basin of a characteristic size of 25km. The main features obtained previously from the LES of the LLJ are also reproduced by the mesoscale model. These runs are verified with NOAA satellite images and observations from the automatic surface weather stations, giving that the model is able to reproduce realistic results.