Tesi sul tema "Air sea interaction"

This thesis is a study of the air-sea fluxes of momentum, sensible heat and latent heat. Fluxes are estimated using the covariance, COARE2.6b bulk flux algorithm, and inertial dissipation methods. The bulk algorithm is validated against the covariance fluxes for the first time in a light-wind, shallow tropical sea, with strong atmospheric instability and low sea state conditions. The removal of ship motion contamination is investigated. This is the first study to quantify the errors associated with corrections for ship motion contamination, and the effects of motion contamination on the covariance calculated heat fluxes. Flow distortion is investigated. Bulk transfer coefficients and roughness lengths are computed and related to the sea state. Ship motion contamination is successfully removed in 86% of the runs. Error analysis of the motion removal algorithm indicates maximum uncertainties of 15% in the wind fluctuations, and 0.002 N/m/m for the wind stress. Motion correction changes the stress by more than 15% in half of the runs analysed. The ship is found to accelerate the mean air flow and deflect it above the horizontal. A correction is developed for the air flow acceleration. The scalar fluxes show good agreement on average for all the methods. As wind speed approaches zero, covariance wind stress is significantly larger than the bulk and inertial dissipation derived wind stress. The non-zero covariance wind stress is reflected in the drag coefficient, CdN10, and momentum roughness length, z0, which are much larger than the parameterisations used in the bulk algorithm. The MCTEX CdN10, wind speed (u10N) relation is 1000 x Cd10N = 1.03 + 7.88/(u10N)^2 0.8 < u10N < 7.5 m/s z0 is primarily a function of wind speed rather than sea state, with largest roughness lengths occurring as wind speed approaches zero. This relation is used in the bulk algorithm, yielding good agreement between covariance and bulk derived wind stress. A new parameterisation for the effects of gustiness, based on wind variance is developed. This brings the bulk wind stress into agreement with the covariance derived wind stress.

Thesis (M.S. in Meteorology and M.S. in Physical Oceanography) Naval Postgraduate School, December 1993.
Thesis advisor(s): James Thomas Murphree ; Peter Chu. "December 1993." Bibliography: p. 88-89. Also available online.

Cette thèse concerne l'étude de l'interaction air-mer, due aux échanges de mouvements, avec un modèle idéalisé mais consistant. Les études sont réalisées à partir d'un modèle shallow-water bicouches (une pour l'océan et une pour l'atmosphère), avec une fine résolution spatiale et temporelle. L'interaction est uniquement due à la friction de surface entre les deux couches.Elle est implémentée par une loi de friction quadratique. La force appliquée à l'océan est calculée en utilisant la différence de vitesse entre les vents et les courants. Pour la force appliquée à l'atmosphère on distingue deux cas l'interaction ``1way'' et ``2way''. Pour la première, la friction appliquée à l'atmosphère néglige la dynamique de l'océan; elle est calculée en utilisant uniquement les vents. Pour l'interaction ``2W'', la friction appliquée à l'atmosphère est l'opposée de celle appliquée à l'océan.Trois configurations idéalisées sont explorées ici.La première configuration explique la génération d'une instabilité barotrope dans l'océan due à la force de friction quadratique et la dissipation visqueuse horizontale de l'atmosphère. Dans le cas 1W le cisaillement entraîne une instabilité barotrope dans l'océan. Dans le cas 2W, l'instabilité est amplifiée en amplitude et en dimension et est transférée à l'atmosphère. L'échelle principale de cette instabilité correspond à celle du rayon de Rossby dans l'océan. Elle est uniquement visible dans les modèles numériques, lorsque la dynamique est résolue à cette échelle à la fois dans l'océan mais aussi dans l'atmosphère.Dans la deuxième configuration, des expériences pour différentes valeurs du coefficient de traînée de surface sont réalisées. Le forçage diffère de la première configuration, et permet d'avoir une dynamique turbulente dans l'océan et l'atmosphère. L'énergie perdue par l'atmosphère et gagnée par l'océan par cisaillement à l'interface sont déterminées et comparées aux estimations basées sur les vitesses moyennes. La corrélations entre la vorticité océanique et atmosphérique est déterminée à l'échelle synoptique et méso-échelle de l'atmosphère. L'océan a un rôle passif, et absorbe l'énergie cinétique à quasiment tout les instants et tous les lieux. Les résultats différent des études réalisées à l'échelle du bassin. De par les faibles vitesses de l'océan, le transfert d'énergie dépend que faiblement des courants. La dynamique de l'océan laisse cependant son empreinte dans la dynamique de l'atmosphère conduisant à un état `quenched disorder' du système océan-atmosphère, pour le plus fort coefficient de friction utilisé.La dernière configuration, considère l'échange de mouvements entre l'océan et l'atmosphère autour d'une île circulaire. Dans les simulations actuelles de la dynamique océanique, le champs du forçage atmosphérique est généralement trop grossier pour inclure la présence de petites îles (<100km). Dans les calculs présentés ici, l'île est représenté dans la couche atmosphérique par un coefficient de traînée cent fois plus fort au dessus de l'île que l'océan. Cela engendre de la vorticité dans l'atmosphère , autour et près du sillage de l'île. L'influence de la vorticité atmosphérique sur la vorticité de l'océan, l'upwelling, la turbulence et le transfert d'énergieest considéré en utilisant des simulations couplées océan-atmosphère.Les résultats sont comparés avec des simulations ayant un forçage atmosphérique constant dans le temps et l'espace (pas de sillage) et des simulations "1W" (pour lesquelles les courants n'ont pas d'influence sur l'atmosphère).Les résultats des simulations sont en accords avec les travaux et les observations précédemment réalisés, et confirment que le sillage atmosphérique est le principal processus générant des tourbillons océanique dans le lit de l'île. Il est aussi montré que la vorticité est injectée directement par le rotationel du vent, mais aussi par la force du vent perpendiculaireau gradient d'épaisseur de la couche de surface océanique
This thesis considers air-sea interaction, due to momentum exchange, in an idealized but consistent model. Two superposed one-layer fine-resolution shallow-water models are numerically integrated. The upper layer represents the atmosphere and the lower layer the ocean. The interaction is only due to the shear between the two layers. The shear applied to the ocean is calculated using the velocity difference between the ocean and the atmosphere.The frictional force between the two-layers is implemented using the quadratic drag law. Three idealized configurations are explored.First, a new mechanism that induces barotropic instability in the ocean is discussed. It is due to air-sea interaction with a quadratic drag law and horizontal viscous dissipation in the atmosphere. I show that the instability spreads to the atmosphere. The preferred spatial scale of the instability is that of the oceanic baroclinic Rossby radius of deformation.It can only be represented in numerical models, when the dynamics at this scale is resolved in the atmosphere and the ocean.In one-way interaction the shear applied to the atmosphere neglectsthe ocean dynamics, it is calculated using the atmospheric wind, only. In two-way interaction it is opposite to the shear applied to the ocean.In the one-way interaction the atmospheric shear leads to a barotropic instability in the ocean. The instability in the ocean is amplified, in amplitude and scale, in two-way interaction and also triggers an instability in the atmosphere.Second, the air-sea interaction at the atmospheric synoptic and mesoscale due to momentum transfer, only, is considered. Experiments with different values of the surface friction drag coefficient are performed, with a different atmospheric forcing from the first configuration, that leads to a turbulent dynamics in the atmosphere and the ocean. The actual energy loss of the atmosphere and the energy gain by the ocean, due to the inter-facial shear,is determined and compared to the estimates based on average speeds.The correlation between the vorticity in the atmosphere and the ocean is determined. Results differ from previous investigations where the exchange of momentum was considered at basin scale. It is shown that the ocean has a passive role, absorbing kinetic energy at nearly all times and locations.Due to the feeble velocities in the ocean, the energy transfer depends only weakly on the ocean velocity. The ocean dynamics leaves nevertheless its imprint in the atmospheric dynamics leading to a quenched disordered state of the atmosphere-ocean system, for the highest value of the friction coefficient considered. This finding questions the ergodic hypothesis, which is at the basis of a large number of experimental, observational and numericalresults in ocean, atmosphere and climate dynamics.The last configuration considers the air-sea interaction, due to momentum exchange, around a circular island. In todays simulations of the ocean dynamics, the atmospheric forcing fields are usually too coarse to include the presence of smaller islands (typically $<$ 100km).In the calculations presented here, the island is represented in the atmospheric layer by a hundred fold increased drag coefficient above the island as compared to the ocean. It leads to an increased atmospheric vorticity in the vicinity and in the wake of the island. The influence of the atmospheric vorticity on the ocean vorticity, upwelling, turbulence and energy transfer is considered by performing fully coupled simulations of the atmosphere-oceandynamics. The results are compared to simulations with a constant, in space and time, atmospheric forcing (no wake) and simulations with one-waycoupling only (where the ocean velocity has no influence on the atmosphere).Results of our simulations agree with previous published work and observations, and confirm that the wind-wake is the main process leading to mesoscale oceanic eddies in the lee of an island

The world ocean is rich in mesoscale structures that locally affect the overlying atmosphere. The interaction of these mesoscale oceanic features with the overlying atmosphere is an interesting topic because of the ubiquity of mesoscale structures in the ocean and the lacking of a definitive representation of the interaction mechanisms. This thesis presents two case studies of air-sea interaction using data collected during the EUREC4A campaign in the tropical north-western Atlantic. The objective is to learn about mesoscale air-sea interaction by case study analysis, particularly for what concerns the effect of sea surface temperature structures. SST measurements from Merian’s thermosalinograph identify the case studies. Then, ship-borne observations from the two weather stations, wind lidar, cloud radar on the R/V Merian, characterize each case study. Radiosondes launched from the R/Vs Merian and Atalante complete the ground-based dataset. Satellite SST and cloud cover observations complement the dataset and allow for comparison. Also, wind data from the ICON-LEM model output are exploited to provide field information to the ship-based point measurements. For the first case study, no effects of the SST on the overlaying atmosphere can be detected. The absence of a detectable SST effect in the first case study is attributed to the limited temporal and spatial extent of the SST anomaly. In the second case study a connection between an SST cold front, vertical velocity, and cloud cover is found. It is proposed that the colder SST dampen the vertical motion on the overlaying atmosphere reducing cloud formation.

Dissertation (Ph.D. in Meteorology)--Naval Postgraduate School, June 2010.
Dissertation supervisor: Montgomery, Michael. "June 2010." Description based on title screen as viewed on July 14, 2010. Author(s) subject terms: Air-sea interaction, tropical cyclones, surface fluxes, drag coefficient, CBLAST. Includes bibliographical references (p. 125-131). Also available in print.

Air-sea interactions are a key process in the forcing of the ocean circulation and the climate. Water Mass Formation is a phenomenon related to extreme air-sea exchanges and heavy heat losses by the water column, being capable to transfer water properties from the surface to great depth and constituting a fundamental component of the thermohaline circulation of the ocean. Wind-driven Coastal Upwelling, on the other hand, is capable to induce intense heat gain in the water column, making this phenomenon important for climate change; further, it can have a noticeable influence on many biological pelagic ecosystems mechanisms. To study some of the fundamental characteristics of Water Mass Formation and Coastal Upwelling phenomena in the Mediterranean Sea, physical reanalysis obtained from the Mediterranean Forecating System model have been used for the period ranging from 1987 to 2012. The first chapter of this dissertation gives the basic description of the Mediterranean Sea circulation, the MFS model implementation, and the air-sea interaction physics. In the second chapter, the problem of Water Mass Formation in the Mediterranean Sea is approached, also performing ad-hoc numerical simulations to study heat balance components. The third chapter considers the study of Mediterranean Coastal Upwelling in some particular areas (Sicily, Gulf of Lion, Aegean Sea) of the Mediterranean Basin, together with the introduction of a new Upwelling Index to characterize and predict upwelling features using only surface estimates of air-sea fluxes. Our conclusions are that latent heat flux is the driving air-sea heat balance component in the Water Mass Formation phenomenon, while sensible heat exchanges are fundamental in Coastal Upwelling process. It is shown that our upwelling index is capable to reproduce the vertical velocity patterns in Coastal Upwelling areas. Nondimensional Marshall numbers evaluations for the open-ocean convection process in the Gulf of Lion show that it is a fully turbulent, three-dimensional phenomenon.

This dissertation investigates wind-wave-current interaction, wave breaking detection and the analysis of breaking characteristics at the air-sea interface. In-situ data measured during the Shoaling Waves Experiment (SHOWEX) and Baltic Sea Swell Experiment (BASE) are applied in the studies and analysis. Wind, wind stress and wave data were obtained from several Air Sea Interaction Spar (ASIS) buoys. Surface currents were measured by a High-Frequency Ocean Surface Current Radar. Two distinct types of wave-current-wind interaction were observed in the presence of a strong along-coast current. First, the horizontal current shear resulted in wind-sea waves shifting away from the wind direction. This motion resulted in a steering of the stress away from the mean wind direction. Second, short wind waves on a uniform current are shifted to the current direction, and the wind stress is steered toward the current direction by the short waves. The wind stress veering has been confirmed by data from the SeaWind scatterometer on board the QuikSCAT satellite. This finding is in agreement with the results from some recent studies. The present study also describes an experimental investigation of breaking wave detection by ASIS buoys. A method, developed from the laboratory, and using local wave parameters to provide a detailed description of breaking, is applied to wave data from ASIS buoys. One the basis of these data, the relation between breaking probability and wind speed shows characteristics similar to those from several field experiments with different conditions. Furthermore, additional parameters, wave age and wave steepness, are also shown to affect the breaking probability during our in-situ measurements. Upper ocean shear, which can modulate wave breaking as predicted by both theory and laboratory work, are also observed to change the breaking properties. This characteristic is rarely reported by in-situ experiment.

The main focus of this study is the influence of waves on the heat transfer over sea. In particular, the bulk transfer coefficient CH (the Stanton number), has been investigated for possible wave influence. Measurements from the site Östergarnsholm in the Baltic Sea have been used. The site has a large sector with undisturbed over water fetch. Data during the period 1995-1999 have been used. It is shown that CH behaves differently as it approach z/L=0 from the unstable side depending on the wave state. During growing sea, CH makes a rapid drop as it passes over neutrality, strikingly different from swell conditions where CH makes a much ’smoother’ transition. This difference is also shown to exist for the kinematic heat flux. Based on the definition of CH, it is suggested that one of the reasons of CH’s different behaviour for different stratification and wave state, is ought to be sought in the kinematic heat flux itself. A comparison of the w,θ cospectra during growing sea and swell conditions, showed differences. For growing sea, the larger size eddies dominates the heat flux during unstable conditions. There is no significant difference in peak frequency for different grade of instability. The swell cases showed a more inconsistent behaviour as it approached neutrality, with the peak frequency shifting for different stability ranges. The correlation coefficient between u, the longitudinal wind component, and w, the vertical wind component, Ru,w is also investigated in this study. It is shown that Ru,w is exposed to some wave influence. A comparison of Ru,w as a function of wave age, for neutral and non-neutral stratification is made. For swell cases and non-neutral stratification Ru,w makes a rapid drop and assumes values close to zero. This is not seen for the neutral cases although there is a slight decrease. It is concluded that a certain amount of positive heat flux and inactive turbulence is needed to see this drop in the correlation coefficient.
Sammanfattning av ”Våginflytandet på värmeutbytet över hav” Syftet med studien är att undersöka om det existerar ett våginflytande för värmeutbytet över hav och speciellt eventuellt våginflytande på utbyteskoefficienten CH (Stantons tal). Mätdata från Östergarnsholm utanför Gotland har använts. Denna mätstation har en stor sektor i vilken vindens anloppssträcka ostört är påverkad av hav. Data från perioden 1995–1999 har använts. Stantons tal CH beter sig annorlunda vid övergången från instabil till stabil skiktning beroende på havsytans tillstånd. Vid uppbyggande sjö gör CH ett ’hopp’ då det passerar neutral skiktning. För dyning finns inte detta hopp utan övergången är mycket mjukare. Denna skillnad observeras också hos det turbulenta värmeflödet. Baserat på definitionen av CH föreslås det att dess olika beteende för olika skiktning och vågtillstånd finns att söka i beteendet hos det turbulenta värmeflödet. En jämförelse av cospektrat för vertikal vind, w, och potentiell temperatur, θ, visar att där finns olikheter mellan uppbyggande sjö och dyning. Under instabila förhållanden och uppbyggande sjö domineras värmeflödet av storskaliga virvlar. Det existerar ingen signifikant skillnad i maximal värdets frekvens för olika grad av instabilitet. Dyningsfallen visar ett mer varierat beteende med en maximalvärdes frekvens som skiftar för olika stabilitetsområden. I studien undersöks också korrelationskoefficienten mellan longitudinal vind u, och vertikal vind w, Ru,w. Det visas att Ru,w är utsatt för ett visst våginflytande. Ru,w som en funktion av vågålder jämförs för neutral och icke-neutral skiktning. För dyning och icke-neutral skiktning så faller Ru,w snabbt till små värden nära noll. Detta resultat skiljer sig för neutral skiktning där Ru,w bara gör en svag minskning. Slutsatsen är att det krävs en viss mängd positivt värmeflöde och inaktiv turbulens för att se det kraftiga avtagandet hos korrelationskoefficienten.

Les embruns sont des aérosols en phase aqueuse générés à la surface de l’eau. Au large, ils sont générés par des mécanismes tel que le déferlement et l’écrêtage. Aujourd’hui, la connaissance portant sur les embruns excédant 20 µm de rayon reste limitée. Cette thèse vise à améliorer la compréhension des processus de génération, de transport, et les impacts sur les propriétés de la couche limite atmosphérique marine (CLAM). La campagne MATE2019 est ainsi menée à l’installation air-mer de Luminy (Marseille, France) afin d’étudier le rôle des interactions vague-vent sur la génération. Une analyse d’échelle révèle que la génération d’embruns corrèle le mieux avec la variance de pente de vagues pour les plus grosses gouttelettes ‘spume’ générées par écrêtage. Pour les plus petites gouttelettes ‘jet’ générées par éclatement de bulles, la meilleure corrélation est obtenue avec un nombre adimensionnel combinant la variance de pentes de vagues, l’age de vague, et un nombre de Reynolds adapté aux mers de vent. Il en résulte la formulation de deux fonctions de génération d’embruns dépendantes sur l’état de mer, valides pour des vents de 12–20 m s-1 et des rayons de 3–35 µm. Extrapolées aux conditions in situ, les fonctions de génération issues du laboratoire sont paramétrées dans les modèles numériques MACMod et MESO-NH, à leurs tours validés à l’aide de mesures terrain, dont une nouvelle campagne de mesure effectuée pendant la thèse dans le Golfe de Gascogne. Les meilleures performances de modélisation sont obtenues avec les fonctions de génération issues du laboratoire. Ces résultats permettent de mieux appréhender l’impact des embruns sur la CLAM
Sea spray droplets are aqueous phase aerosols generated from the water surface. In the open ocean, they are generated as a result of wind-forced wave breaking and surface-tearing mechanisms. To this day, knowledge of sea spray particles larger than 20 µm radius is sparse. The present thesis aims to improve knowledge of the sea spray generation flux, as well as transport and impacts on the properties of the marine atmospheric boundary layer (MABL). To this end, the effects of wind–wave interactions on the surface sea spray generation flux are investigated during the MATE2019 experiment, conducted at the large wave–wind facility in Luminy (Marseille, France). Scaling analysis shows that the sea spray generation is best correlated with the wave-slope variance for thelarger spume droplets generated by surface tearing. For the smaller jet droplets generated by bubble bursting, the highest correlation is found with a nondimensional number combining the wave-slope variance, the wave age, and a windsea Reynolds number. This resulted in the formulation of two wave-state-dependent sea spray generation functions, each valid for wind speeds 12–20 m s-1 and radii 3–35 µm. Upscaled to the field, the laboratory-derived generation functions are parameterized in the MACMod and MESO-NH numerical models, and validated using field data collected during the thesis in the Bay of Biscay for this purpose. Best model performance is found with the laboratory generation functions. Such results are encouraging for the study of sea spray impacts on the properties of the MABL

Les surcotes de tempête sont souvent sous-estimées dans les modèles hydrodynamiques, ainsi que les grandes vagues dans les modèles de vagues. Les causes possibles sont une sous-estimation des vents dans les modèles atmosphériques et/ou une formulation incorrecte de la tension de vent. Les objectifs de cette thèse sont (1) d’estimer les biais par vents forts dans les modèles atmosphériques (2) de développer une nouvelle paramétrisation du coefficient de traı̂née permettant de réduire ce biais (3) d’étudier l’impact des vagues sur la tension de vent. La méthode consiste à étudier la réponse de l’atmosphère et de l’océan à la tension de vent. Dans une première partie, nous utilisons le modèle couplé vagues-atmosphère d’ECMWF. Nous montrons que les vents forts sont sous-estimés, avec un biais de l’ordre de -7 m/s à 30 m/s. Des écarts significatifs existent aussi entre les observations, les bouées et les vents issus de ASCAT-KNMI étant généralement inférieurs à ceux des plateformes et des autres données satellites utilisées dans cette étude (AMSR2, ASCAT-RSS, WindSat, SMOS et JASON-2). La nouvelle paramétrisation développée permet d’obtenir des vents plus forts qu’avec celle d’ECMWF par défaut. Dans une deuxième partie (réponse de l’océan), nous utilisons le modèle global océanique TUGO du LEGOS forcé par le modèle couplé vagues-atmosphère d’ECMWF. Nous montrons qu’une paramétrisation de la tension de vent dépendant des vagues plutôt que du vent est plus appropriée quand l’état de mer est jeune. Elle conduit à des surcotes plus proches des observations (marégraphes et traces altimétriques de JASON-2). L’impact des vagues sur la surcote est significatif, et peut atteindre 20 cm
Storm surges may be underestimated in hydrodynamic models, as well as large wave heights in wave models. This could come from an underestimation of strong winds in atmospheric models and/or an inappropriate wind stress formulation. The objectives of the present work are (1) to estimate how strong are the biases for high winds in atmospheric models (2) to develop a new drag parameterization that could reduce this bias (3) to investigate the impact of the waves on the wind stress. The method consists of studying the response of the atmosphere and the ocean to the wind stress.In a first part, we use the coupled wave-atmosphere model from ECMWF. We show that strong winds may be underestimated, as much as -7 m/s at 30 m/s.Significant differences also exist between observations, with buoys and ASCAT-KNMI generally showing lower wind speeds than the platforms and other remote-sensing data used in this study(AMSR2, ASCAT-RSS, WindSat, SMOS and JASON-2).The newly empirically adjusted Charnock parameterization leads to higher winds compared to the default ECMWF parameterization. In a second part, we use the global ocean model TUGO fromLEGOS forced with ECMWF coupled wave-atmopshere model. We show that a wave-dependent rather than wind-dependent stress formulation is more appropriate, when the sea state is young and the sea rougher. It yields to simulated surges closer to observations (i.e. tide gauges and JASON-2 altimeter tracks). The wave impact on the surges is significant, and may reach 20 cm

La Southern Annular Mode (SAM) è il modo di variabilità dominante dell'Emisfero Australe extratropicale, definito come la prima funzione ortogonale empirica del geopotenziale a 500 hPa e caratterizzato da un pattern di anomalie del campo di pressione al livello del mare di polarità opposta sull'Antartide e alle medie latitudini. I meccanismi dietro a questo modo di variabilità sono un argomento di crescente interesse per via del trend positivo che la SAM ha mostrato negli ultimi decenni, ricondotto all'aumento di concentrazioni di gas serra e alla riduzione di ozono stratosferico. Comprendere in che misura le variazioni nella SAM, predetta sempre più verso la sua fase positiva, influenzeranno il clima nel futuro richiede una solida comprensione degli impatti che la SAM ha sul clima presente. A questo scopo, è essenziale identificare i modelli climatici capaci di rappresentare lo stato corrente della SAM con grande accuratezza, condizione necessaria affinchè essi siano affidabili per le proiezioni future. Partendo da questo contesto, la tesi ha l'obiettivo di indagare la relazione tra la SAM e la temperatura superficiale del mare (SST) basandosi sulla rianalisi ERA5 e di valutare l'accuratezza della sua rappresentazione da parte dei modelli che partecipano alla sesta fase del Coupled Model Intercomparison Project (CMIP6). Nella prima parte del lavoro, è analizzata la rianalisi ERA5 per identificare la fenomenologia della SAM ed i processi fisici che conducono al pattern delle anomalie di SST in risposta alla SAM. La seconda parte del lavoro consiste nell'identificare le caratteristiche che i modelli climatici allo stato dell'arte riescono a riprodurre con realismo e quelle che sono invece più difficili da rappresentare adeguatamente. Risultato della tesi è una diagnosi dell'affidabilità dei modelli CMIP6 nella caratterizzazione della relazione tra SAM e SST allo stato presente.

The investigation of climate variability in different timescales such as daily, monthly, seasonal and inter-annual has utmost importance for managing the socio-economic processes on regional to global scale. Indeed, the variation in the climate has a crucial impact on agriculture, water, health, tourism, economy and transportation. Therefore the development of climate forecasting tools is necessary which helps to manage these sectors more efficiently. However, there are limitations on producing accurate climate forecast for more than two weeks in advance due to the chaotic nature of the climate system, especially for the region like the Mediterranean, which is characterized by high interannual variability. Due to its importance and challenging nature, a collective effort is being done to improve the skill of models and climate forecasting in the Mediterranean. The contribution of this thesis is a overall effort, which consists of developing a high resolution model application in the Mediterranean, which can provide reliable estimate of Sea Surface Temperature (SST) and Mixed Layer Depth (MLD). This approach is based on the fact that the atmospheric predictability in seasonal to interannual time scale is significantly dependent on slowly varying lower boundary conditions (e.g. Charney and Shukla 1981) i.e. Mediterranean SSTs. The spatial resolution of model is increased for taking into account the mesoscale processes in the Mediterranean. Since the first internal Rossby radius of deformation in the Mediterranean sea is of the order of 10-15 kms, the spatial resolution of an eddy resolving model should have at least a resolution one half of the Rossby radius. Based on this assumption, the spatial resolution is explored to the order of -5 km (1/16°). The regional ocean modeling system (ROMS) adopted from the Rutgers University is used in the current study. The objective is to validate certain fields (such as SST and MLD) obtained from model simulations and study air-sea interactions. The validation is done by performing two experiments namely, climatological and interannual simulations. The model simulated results are validated with observations as well as intercompared to evaluate the skill of model. The monthly mean SST climatology is obtained from ten years of model run forced with climatological air-sea fluxes is well captured by the model configuration and follows the annual cycle. Model simulated summer SST climatology shows biases of the order of 0.8-1.0 °C with observation (MedAtlas) and 1.0-1.2 °C with other datasets (intercomparison). The vertical structure of temperature climatology is found to be well simulated by model in which upper layer shows a difference of 1.0 °C and it further decreased at intermediate layers. The simulated sea surface height and surface currents is validated with Aviso altimetry data. On the large scales the surface currents generated by model captures general structures of surface circulation. The monthly mean mixed layer depth (MLD) climatology computed from model is validated with observed monthly MLD climatology and found that the winter MLD is overestimated by model. In second experiment, model is forced with six hourly air-sea interaction fluxes from ERA-Interim and interannual simulations are obtained for the period 1998-2007. The monthly mean SST climatology obtained from above interannual simulation follows climatological annual cycle with cold biases in summer season. The weak SSTs (bias of the order of 1.0 to 1.5 °C) are observed in the summer for the period 2002-2007 in the model simulations. The monthly mean SST anomalies are well simulated by model except for the year 2006. The time evolution of monthly mean SST anomalies area averaged over different sub-basins are exhibits interannual variability. The comparison with satellite derived SSTs reveals that our model is able to capture both, the seasonal and inter-annual variability, although it still has a bias of the order of 1 to 1.2 °C. The model is able to reproduce the temperature at subsurface layer having the signatures of existence of intermediate water masses. The monthly mean mixed layer climatology derived from interannual simulations is quite well reproduced by model. In the Gulf of Lions, MLD values are reached upto 1500 meter deep in winter whereas it shows 50 meter in summer season. The time evolution of monthly mean mixed layer climatology derived from the model is able to reproduce annual variability. The interannual variability of monthly mean mixed layer depth is simulated quite well by model for the year 2004-2007. The timeseries of climatological, monthly and daily mixed layer depth which is area averaged over various sub-basins follows seasonal cycle. The high resolution regional model application developed in the current study is thus able to reproduce certain fields. The surface currents and eddy kinetic energy in the model shows small scale structures and strong variability. The model is also capable to generate mesoscale eddies in the western Mediterranean although model overestimated surface fields.
La investigación de la variabilidad climática en diferentes escalas de tiempo, como diario, mensual, estacional e interanual tiene suma importancia para la gestión de los procesos socio-económica en la región a escala global. De hecho, la variación en el clima tiene un impacto crucial en la agricultura, el agua, la salud, el turismo, la economía y el transporte. Por lo tanto el desarrollo de herramientas de predicción del clima es necesario que ayuda a gestionar estos sectores de manera más eficiente. Sin embargo, existen limitaciones en la producción de pronóstico climático precisa durante más de dos semanas de antelación debido a la naturaleza caótica del sistema climático, especialmente para la región como el Mediterráneo, que se caracteriza por una alta variabilidad interanual. Debido a su importancia y naturaleza desafiante, se está haciendo un esfuerzo colectivo para mejorar la habilidad de los modelos y la predicción del clima en el Mediterráneo. La contribución de esta tesis es un esfuerzo global, que consiste en el desarrollo de una aplicación de modelo de alta resolución en el Mediterráneo, que puede proporcionar una estimación fiable de la temperatura superficial del mar (TSM) y la profundidad de la capa mixta (MLD). Este enfoque se basa en el hecho de que la previsibilidad atmosférica en estacional a interanual escala de tiempo es significativamente dependiente de variación lenta condiciones límite inferior (por ejemplo, Charney y Shukla 1.981), es decir TSM mediterráneos. La resolución espacial de modelo se incrementa por tomar en cuenta los procesos de mesoescala en el Mediterráneo. Desde la primera radio interno de deformación de Rossby en el mar Mediterráneo es del orden de 10-15 kms, la resolución espacial de un remolino resolución de modelo debe tener al menos una resolución de la mitad del radio de Rossby. Basándose en esta suposición, la resolución espacial se explora a la orden de — 5 km (1/16°). El sistema de modelado regional de los océanos (ROMS), aprobada por la Universidad de Rutgers se utiliza en el estudio actual. El objetivo es validar ciertos campos (como SST y MLD) obtenidos a partir de simulaciones de modelos y estudiar las interacciones aire-mar. La validación se realiza mediante la realización de simulaciones de dos experimentos saber, climatológicos e interanuales. Los resultados del modelo simulado se validan con las observaciones, así como intercomparados para evaluar la habilidad del modelo. La media mensual climatología SST se obtiene a partir de diez años de ejecución del modelo forzada con climatológicas flujos aire-mar es bien capturados por la configuración del modelo y sigue el ciclo anual. Simulado Modelo SST climatología verano muestra sesgos del orden de 0,8 a 1,0 °C con observación (MEDATLAS) y 1,0-1,2 °C con otros conjuntos de datos (de intercomparación). La estructura vertical de la climatología de temperatura se encuentra para ser bien simulado por modelo en el que la capa superior muestra una diferencia de 1,0 °C y disminuyó aún más en las capas intermedias. Las corrientes simulados altura y la superficie de la superficie del mar se valida con los datos de altimetría aviso. En las grandes escalas de las corrientes superficiales generadas por el modelo capta las estructuras generales de la circulación superficial. La climatología media mensual profundidad de la capa mixta (MLD) calculada a partir del modelo se valida con la observada climatología mensual MLD y encontró que el invierno MLD se sobreestima el modelo. En segundo experimento, el modelo se ve obligado a seis por hora aire-mar flujos de interacción a partir de simulaciones ERA-Interim e interanuales se obtienen para el período 1998-2007. La climatología TSM mensual media obtenida desde arriba simulación interanual sigue el ciclo anual climatológica con sesgos fríos en la temporada de verano. Los TSM débiles (sesgo del orden de 1,0 a 1,5 °C) se observan en el verano para el período 2002-2007 en las simulaciones del modelo. Las anomalías medias mensuales de la TSM son bien simuladas por el modelo, excepto para el año 2006. La evolución en el tiempo de media área de anomalías de TSM mensual promedio durante diferentes subcuencas son exposiciones variabilidad interanual. La comparación con TSM satélite derivada revela que nuestro modelo es capaz de capturar tanto, la variabilidad estacional e interanual, a pesar de que todavía tiene un sesgo del orden de 1 a 1,2 °C. El modelo es capaz de reproducir la temperatura a la capa subsuperficial tener las firmas de la existencia de masas de agua intermedias. La media climatología capa de mezcla mensual derivado de simulaciones interanuales está bastante bien reproducido por modelo. En el Golfo de León, los valores de MLD se alcanzan hasta 1.500 metros de profundidad en invierno mientras que muestra 50 metros en temporada de verano. La evolución en el tiempo de la media climatología capa de mezcla mensual derivada del modelo es capaz de reproducir la variabilidad anual. La variabilidad interanual de media profundidad de la capa mixta mensual se simula bastante bien por el modelo para el año 2004-2007. Las series de tiempo de profundidad de la capa mixta climatológica, mensual y diaria, que es el área de media sobre varias subcuencas del siguiente ciclo estacional. La aplicación modelo regional de alta resolución desarrollado en el presente estudio es, pues, capaz de reproducir ciertos campos. Las corrientes superficiales y la energía cinética de Foucault en el modelo muestra las estructuras de pequeña escala y fuerte variabilidad. El modelo también es capaz de generar remolinos de mesoescala en el Mediterráneo occidental, aunque el modelo sobreestima campos superficiales.

During an upwelling event the cold bottom-water is brought to the sea surface. This cools the atmosphere from below and the stratification becomes more stable. When the atmosphere is more stable the turbulence is reduced and, as a consequence, so are the turbulent fluxes. This study is investigating four periods of upwelling from the Östergarnsholm-site, in the Baltic Sea east of Gotland, during the summer of 2005. The air measurements are taken at a tower at the southernmost tip of Östergarnsholm while the measurements in the water are from a buoy moored 1 km south-southeast of the tower. During all the upwelling events the wind is south-westerly, along the coast of Gotland. This means that the buoy is not within the flux footprint area and is perhaps not always representative of what happens there. All the periods show a stabilization of the atmosphere as the SST (Sea Surface Temperature) decreases. The heat fluxes, especially the latent heat flux, decreases as the SST decreases. The amount of CO2 in the atmosphere, in the summer, is usually higher than the amount in the surface water of the seas because the oceans are a net sink of CO2. The air-sea flux of CO2 is to a large extent controlled by this difference. Therefore the flux of CO2 is usually directed to the sea. The deep-water contains more CO2 than the surface water because the phytoplankton near the surface removes CO2 through photosynthesis. The deep-water is also colder and can solve more CO2. During an upwelling event this CO2-rich water is brought to the surface. As an upwelling event progresses the difference in CO2-concentration between the air and the sea is reduced, sometimes reversed, and the flux decreases. This is what happens in three of the investigated periods in this study. During the fourth period a counter gradient flux is observed.
När en uppvällning inträffar förs kallt djupvatten upp till havsytan. Det kalla vattnet kyler atmosfären nedifrån, något som leder till mer stabil skiktning. När atmosfären blir mer stabilt skiktad dämpas turbulensen och det medför att de turbulenta flödena också avtar. I den här studien analyseras fyra perioder med uppvällning. Mätningarna kommer från Östergarnsholm, öster om Gotland, under sommaren 2005. Mätningarna i luften är tagna från en mast vid Östergarnsholms södra udde. Mätningarna i vattnet kommer från en boj som är förankrad 1 km sydsydöst om masten. Vid samtliga uppvällnings-perioder i den här studien är vinden sydvästlig (längs Gotlandskusten). Det betyder att bojen inte befinner sig inom flödenas footprint-area och dess mätningar är kanske inte hela tiden representativa för vad som händer i footprint-arean. Samtliga undersökta perioder visar på en stabilisering av atmosfären då havsytans temperatur avtar. Värmeflödena, i synnerhet det latenta värmeflödet, avtar i samband med att temperaturen i havsytan sjunker. Halten av CO2 i atmosfären är vanligtvis högre än halten i havens ytvatten (under sommaren) eftersom de är en nettosänka för CO2 globalt sett. CO2-flödet mellan havsytan och atmosfären styr till en stor del av denna skillnaden i CO2-halt. Det innebär att CO2-flödet är riktat neråt, mot havet. Havens djupvatten innehåller mer CO2 därför att växtplankton nära ytan reducerar CO2-halten genom fotosyntesen. Djupvattnet är också kallare och kan därför lösa mer CO2. Under en uppvällning förs detta CO2-rika vatten upp till ytan. När en uppvällning fortskrider minskar skillnaden i CO2-halt mellan hav och atmosfär (ibland kan CO2-halten i ytvattnet även komma att överstiga atmosfärens halt) och flödet avtar. Tre av perioderna i den här studien visar på ett avtagande flöde. Den fjärde perioden uppvisar ett flöde motriktat CO2-gradienten.

The uncertainty in the determination of the momentum and scalar fluxes remains one of the main obstacles to accurate numerical forecasts in low to moderate wind conditions. For example, latent heat fluxes computed from data using direct covariance and bulk aerodynamic methods show that there is good agreement in unstable conditions when the latent heat flux values are generally positive. However, the agreement is relatively poor in stable conditions, particularly when the moisture flux is directed downward. If the direct covariance measurements are indeed accurate, then they clearly indicate that the bulk aerodynamic formula overestimate the downward moisture flux in stable conditions. As a result, comparisons of the Dalton number for unstable and stable conditions indicate a marked difference in value between the two stability regimes. Investigations done for this thesis used data taken primarily at the Air-Sea Interaction Tower (ASIT) during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) Experiment 2003 from the 20-27 August 2003. Other data from the shore based Martha's Vineyard Coastal Observatory (MVCO) and moored buoys in the vicinity of the ASIT were also incorporated. During this eight day period, the boundary layer was often characterized by light winds, a stably stratified surface layer and a swell dominated wave field. Additionally, the advection of warm moist air over cooler water resulted in fog formation and a downward flux of moisture on at least three occasions. Therefore, a primary objective of this thesis is to present a case study to investigate the cause of this shortcoming in the bulk formula under these conditions by examining the physical processes that are unique to these boundary layers. Particular attention will be paid to the behavior of the Dalton number in a stable marine atmospheric boundary layer under foggy conditions using insights derived from the study of fog formation and current flux parameterization methods.

A quantitative description of wind-wave and wind-current momentum transfer in high wind conditions is currently unresolved, mainly due to the severe character of the problem. It is, however, necessary for accurate wave models, storm and hurricane forecasting, and atmosphere-ocean model coupling. In this research, strongly forced wind-wave conditions were simulated in a laboratory tank. On the air side, a static pressure probe mounted on a vertical wave follower measured wave-induced airflow pressure fluctuations in close proximity to the surface. Vertical profiles of wave-induced pressure fluctuations were resolved and wave phase dependent features, such as airflow separation, identified. Based on the pressure measurements, wind-wave momentum fluxes were obtained. The dependence of the spectral wave growth function on wind forcing, wave steepness, and wave crest sharpness was also investigated. The bulk air-sea momentum fluxes were estimated using the "total budget" experimental technique. It provided information on the contribution of a wind-wave flux induced by a single wave to the total air-sea momentum flux. The percentile contribution of wind-wave momentum flux into one wave was found to be dependent on the wave's steepness. An arbitrary change in steepness, however, was found to modify the wave field in such a way that it had little effect on the total wind stress. To complement wind stress measurements velocity profiles in the water were measured using Particle Image Velocimetry technique. Mean current, turbulent stress, turbulent kinetic energy and turbulent dissipation rate vertical profiles were studied as a function of wind speed. Together with wave spectrum evolution measurements they form a complete empirical description of momentum fluxes in the laboratory tank. The results provide a detailed empirical view on airflow pressure fluctuations over a wavy surface, on total wind stress, and on the velocity response in the water. A new wave growth parameterization with wind forcing range extended into storm conditions is the most significant stand alone result of this work. Combined with the near surface vertical profiles, these empirical data also serve as a test bed for coupled air-sea numerical models.

The enthalpy (sensible and latent heat) exchange processes within the surface layers at an air-water interface have been examined in 15-m wind-wave tunnel at the University of Miami. Measurements yielded 72 mean values of fluxes and bulk variables in the wind speed (referred to 10 m) range form 0.6 to 39 m/s, covering a full range of aerodynamic conditions from smooth to fully rough. Meteorological variables and bulk enthalpy transfer coefficients, measured at 0.2-m height, were adjusted to neutral stratification and 10-m height following the Monin-Obukhov similarity approach. The ratio of the bulk coefficients of enthalpy and momentum was estimated to evaluate Emanuel's (1995) hypothesis. Indirect "Calorimetric" measurements gave reliable estimates of enthalpy flux from the air-water interface, but the moisture gained in the lower air from evaporation of spray over the rough water remained uncertain, stressing the need for flux measurements along with simultaneous spray data to quantify spray's contribution to the turbulent air-water enthalpy fluxes.

O papel dos fluxos de superfície de calor sensível e latente (FCSL) em dois ciclones extratropicais com desenvolvimento distinto na costa do sul do Brasil foi avaliado a partir de simulações numéricas utilizando o modelo de área limitada WRF versão 2.2. Em um dos ciclones, a circulação se originou em baixos níveis e propagou-se para a média troposfera (ciclone 1). No outro ciclone (ciclone 2) a circulação originou-se em níveis médios e propagou-se até a superfície. Foram realizadas simulações numéricas com e sem FCSL para cada um dos ciclones. A trajetória do ciclone 1 foi fortemente alterada na ausência de FCSL, exibindo deslocamento incorreto (para nordeste) e menor tempo de vida. Este comportamento esteve associado às mudanças no padrão de advecção de temperatura em baixos níveis e à diminuição da convergência de massa induzida pelo calor sensível, na ausência de FCSL. No experimento sem FCSL, ocorre também desacoplamento entre o ciclone em superfície e a onda em níveis médios e altos, com consequente enfraquecimento do sistema. O aumento da estabilidade estática e o mecanismo de convergência de Ekman são responsáveis por menor convergência nas regiões frontais na ausência de FCSL. A relação de fase entre os campos de altura geopotencial e temperatura em baixos níveis e o perfil vertical de aquecimento diabático também mostram condições mais favoráveis ao desenvolvimento do ciclone na presença dos FCSL. O ciclone 2 não teve a trajetória alterada entre as duas simulações. A advecção de temperatura e a convergência em baixos níveis devido ao calor sensível foram semelhantes, explicando a similaridade na trajetória nos experimentos com e sem FCSL. A convergência de Ekman diferenciou-se entre as duas simulações, especialmente no final do ciclo de vida do ciclone, mostrando que este processo também altera a intensidade de ciclones fracos. A influência dos FCSL mostrou-se dependente do mecanismo dominante de formação dos ciclones. O ciclone 1, com forçante dinâmica menos intensa, sofreu grandes variações em trajetória e tempo de vida na ausência de FCSL. Já o ciclone 2, sob forçante dinâmica mais definida e intensa, mostrou-se menos dependente dos processos de superfície para o seu deslocamento. Os mecanismos de aprofundamento foram mais intensos no ciclone 1.
The role of latent and sensible heat fluxes (LSHF) between ocean and atmosphere during the development of two extratropical cyclones over the southwestern Atlantic Ocean is analyzed using the WRF (Weather Range and Forecast) Mesoscale Model, version 2.2. In cyclone 1, the circulation has originated in low levels and propagated to the middle troposphere; the cyclone 2s circulation has originated in middle levels propagating towards the surface during its life cycle. The trajectory of cyclone 1 was strongly influenced by the surface heat fluxes, showing an incorrect displacement and a shorter lifetime in the absence of these fluxes. This behavior is associated with changes in low level temperature advection and the reduction of low level mass convergence is induced by sensible heat fluxes from surface. In the absence of LSHF there is also a decoupling of the surface low and the upper level wave, causing the weakening of the system. Without surface fluxes, the higher static stability and the weaker Ekman convergence mechanism are responsible for less convergence in the frontal regions of the cyclone. The lagging of the geopotential wave and the temperature wave in low levels, and the diabatic heating profile in the troposphere also show more favorable conditions to the cyclone deepening in the presence of surface fluxes. The trajectory of cyclone 2 showed no significant modification in the absence of LSHF. The temperature advection field is similar and the low level convergence related to sensible heat fluxes didn´t has an impact on the displacement of this system. The Ekman convergence had smaller magnitude in the no-LSHF simulation, especially in the final stages of the cyclone life cycle, indicating that this mechanism can be important also for the deepening of weak systems and not only for explosive systems, as considered in previous studies. This work shows that the role of the LSHF seems to be dependent on the cyclone development main mechanisms. In cyclone 1, where the dynamic forcings are less intense, the absence of surface fluxes had a great impact on the trajectory, intensity and duration of the system. In cyclone 2, with more intense dynamic forcings, the displacement was less influenced by surface processes. The deepening mechanisms had greater impact on the cyclone 1.

To understand and to predict the weather and climate, numerical models are important tools and it is crucial that the controlling processes are described correctly. Since 70% of the global surface is covered with water the description how the ocean and atmosphere communicates has a considerable impact. The ocean–atmosphere exchange occurs through transport of momentum (friction) and heat, governed by turbulent eddies. The sea surface is also an important source of turbulence in both directions. The scales of the turbulent eddies cannot be resolved in ocean and climate models. Therefore, the turbulent exchanges have to be related to mean variables, such as wind speed and temperature differences. By using measurements, new methods to describe the air–sea exchange during two specific processes were developed. These processes are the so-called UVCN-regime (Unstable Very Close to Neutral stratification) and swell, i.e. waves which are not produced by the local wind. These processes were included in an ocean model and in a regional atmospheric climate model and the impact was investigated. The UVCN-regime enhances the heat transport significantly during the autumn and winter months in the ocean model. This results in a shallower well-mixed surface layer in the ocean. Wind-following swell reduces the surface friction, which is very important for the atmosphere. Some secondary effects in the climate model are reduced low-level cloud cover and reduced precipitation by more than 10% over sea areas. Locally and for short periods the impact is large. It is important to include the UVCN-regime and the swell impact in models, to make simulations more reliable.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2007.
Includes bibliographical references (p. 153-166).
The role of ocean dynamics in driving air-sea interaction is examined at two contrasting sites on 125°W in the eastern tropical Pacific Ocean using data from the Pan American Climate Study (PACS) field program. Analysis based on the PACS data set and satellite observations of sea surface temperature (SST) reveals marked differences in the role of ocean dynamics in modulating SST. At a near-equatorial site (3°S), the 1997-1998 El Nifio event dominated the evolution of SST and surface heat fluxes, and it is found that wind-driven southward Ekman transport was important in the local transition from El Nifio to La Nifia conditions. At a 10'N site near the summertime position of the Inter-tropical Convergence Zone, oceanic niesoscale motions played an important role in modulating SST at intraseasonal (50- to 100-day) timescales, and the buoy observations suggest that there are variations in surface solar radiation coupled to these mesoscale SST variations. This suggests that the mesoscale oceanic variability may influence the occurrence of clouds. The intraseasonal variability in currents, sea surface height, and SST at the northern site is examined within the broader spatial and temporal context afforded by satellite data.
(cont.) The oscillations have zonal wavelengths of 550-1650 km and propagate westward in a manner consistent with the dispersion relation for first baroclinic mode, free Rossby waves in the presenice of a, mean westward flow. The hypothesis that the intraseasonal variability and its annual cycle are associated with baroclinic instability of the North Equatorial Current is supported by a spatio-temporal correlation between the amplitude of intraseasonal variability and the occurrence of westward zonal flows meeting an approximate necessary condition for baroclinic instability. Focusing on 100N in the eastern tropical Pacific, the hypothesis that mesoscale oceanic SST variability can systematically influence cloud properties is investigated using several satellite data products. A statistically significant relationship between SST and columnar cloud liquid water (CLW), cloud reflectivity, and surface solar radiation is identified within the wavenumber-frequency band corresponding to oceanic Rossby waves. Analysis of seven years of CLW data and 20 years surface solar radiation data indicates that 10-20% of the variance of these cloud-related properties at intraseasonal periods and wavelengths on the order of 100 longitude can be ascribed to SST signals driven by oceanic Rossby waves.
by J. Thomas Farrar.
Ph.D.

L'obiettivo di questo lavoro è quello di sviluppare un metodo efficiente di decomposizione dei domini per la simulazione di vari flussi di fluido, di interesse per la meccanica dei fluidi, geofisica e ambientale, utilizzando griglie non sovrapposte in un solutore LES. Viene mostrato un metodo di decomposizione del dominio di sub-strutturazione esplicita che applica le condizioni di compatibilità presenti tra flussi di fluido non miscibili. L'attuale metodologia è utilizzata per lo studio dei flussi archètipo accoppiati in direzione co-corrente e controcorrente, per numeri di Reynolds bassi. Inoltre, viene mostrato lo studio sui ostacoli immersi nei flussi di Poiseuille-Couette accoppiati.
The objective of this work is to develop an efficient domain decomposition method for the simulation of multiple fluid flows, of interest in geophysical and environmental fluid mechanics, using non-overlapping grids. An explicit sub-structuring domain decomposition method that enforces the compatibility conditions present between non-miscible fluid flows is shown. The current methodology is used for the study of coupled canonical co-current, and counter-current, flows for low Reynolds Numbers. Additionally, the study around immersed obstacles in coupled Poiseuille-Couette flows is made.

I present the development of instrumentation and methods for the measurement of coastal processes, ocean surface phenomena, and air-sea interaction in two parts. In the first, I discuss the development of a portable scanning lidar (light detection and ranging) system for manned aircraft and demonstrate its functionality for oceanographic and coastal measurements. Measurements of the Southern California coastline and nearshore surface wave fields from seventeen research flights between August 2007 and December 2008 are analyzed and discussed. The October 2007 landslide on Mt. Soledad in La Jolla, California was documented by two of the flights. The topography, lagoon, reef, and surrounding wave field of Lady Elliot Island in Australia's Great Barrier Reef were measured with the airborne scanning lidar system on eight research flights in April 2008. Applications of the system, including coastal topographic surveys, wave measurements, ship wake studies, and coral reef research, are presented and discussed.

In the second part, I detail the development of instrumentation packages for small (18 – 28 kg) unmanned aerial vehicles (UAVs) to measure momentum fluxes and latent, sensible, and radiative heat fluxes in the atmospheric boundary layer (ABL), and the surface topography. Fast-response turbulence, hygrometer, and temperature probes permit turbulent momentum and heat flux measurements, and short- and long-wave radiometers allow the determination of net radiation, surface temperature, and albedo. Careful design and testing of an accurate turbulence probe, as demonstrated in this thesis, are essential for the ability to measure momentum and scalar fluxes. The low altitude required for accurate flux measurements (typically assumed to be 30 m) is below the typical safety limit of manned research aircraft; however, it is now within the capability of small UAV platforms. Flight tests of two instrumented BAE Manta UAVs over land were conducted in January 2011 at McMillan Airfield (Camp Roberts, CA), and flight tests of similarly instrumented Boeing-Insitu ScanEagle UAVs were conducted in April 2012 at the Naval Surface Warfare Center, Dahlgren Division (Dahlgren, VA), where the first known direct flux measurements were made from low-altitude (down to 30 m) UAV flights over water (Potomac River). During the October 2012 Equatorial Mixing Experiment in the central Pacific aboard the R/V Roger Revelle, ship-launched and recovered ScanEagles were deployed in an effort to characterize the marine atmospheric boundary layer structure and dynamics. I present a description of the instrumentation, summarize results from flight tests, present preliminary analysis from UAV flights off of the Revelle, and discuss potential applications of these UAVs for marine atmospheric boundary layer studies.

Radar observations of the ocean surface are becoming increasingly important. Common applications are wind retrieval and global weather forecasting and characterization. Because of the common use of ocean radar measurements, it is important to understand the sensitivity of the backscatter to both radar parameters and surface parameters. At near-normal incidence angles, it has been assumed that the radar backscatter exhibits little or no azimuth dependence (Colton, 1989). However, recent data taken by the BYU YSCAT radar system suggests that this is not the case. At an incidence angle of 10°, the YSCAT radar data shows from a fraction of a decibel to up to 10 decibels of azimuth modulation, depending on the surface conditions. In this thesis, a physical optics approach is used with a two-dimensional surface model to derive the electromagnetic backscatter from the ocean surface. If the waves on the ocean surface are directed, azimuth modulation is predicted at near-normal incidence angles. The effects of surface and radar parameters on the azimuth modulation are studied, and the results are compared to data taken by the YSCAT radar system. It is shown that the theory correctly predicts of the shape of the curve when the normalized radar cross-section is plotted as a function of azimuth angle. The theory also predicts the correct trend of the modulation magnitude as function the surface roughness. However, the simplifications in the model limit its prediction of the frequency dependence of the modulation. Relaxing some of the assumptions of the model is likely to correct this problem.

The ocean mixed layer response to a tropical cyclone within, and immediately adjacent to, the Gulf of Mexico Loop Current is examined using a combination of ocean profiles and a numerical model. A comprehensive set of temperature, salinity, and current profiles acquired from aircraft-deployed expendable probes is utilized to analyze the three-dimensional oceanic energy and circulation evolution in response to Hurricane Lili's (2002) passage. Mixed-layer temperature analyses show that the Loop Current cooled <1 degree C in response to the storm, in contrast to typically observed larger decreases of 3-5 degrees C. Correspondingly, vertical current shears, which are partly responsible for entrainment mixing, were found to be up to 50% weaker, on average, than observed in previous studies within the directly-forced region. The Loop Current, which separates the warmer, lighter Caribbean Subtropical water from the cooler, heavier Gulf Common water, was found to decrease in intensity by -0.18 plus/minus 0.25 m/s over an approximately 10-day period within the mixed layer. Contrary to previous tropical cyclone ocean response studies which have assumed approximately horizontally homogeneous ocean strucutre prior to storm passage, a kinetic energy loss of 5.8 plus/minus 6.3 kJ/m^2, or approximately -1 wind stress-scaled energy unit, was observed. Using near-surface currents derived from satellite alimetery data, the Loop Current is found to vary similarly in magnitude, suggesting storm-generated energy is rapidly removed by the pre-exiting Loop Current. Further examination of the energy response using an idealized numerical model reveal that due to: 1) favorable coupling between the wind stress and pre-existing current vectors; and 2) wind-driven currents flowing across the large horizontal pressure gradient; wind energy transfer to mixed-layer kinetic energy can be more efficient in these regimes as compared to the case of an initially horizontally homogeneous ocean. However, nearly all of this energy is removed by advection by 2 local inertial periods after storm passage, and little evidence of the storm's impact remains. Mixed-layer vorticity within the idealized current also shows a strong direct response, but little evidence of an near-inertial wave wake results.

The turbulence structure over the sea was studied with the emphasis on humidity. The data sets used came from the island of Östergarnsholm outside Gotland in the Baltic Sea. The study included spectral and quadrant analyses of the wind, temperature and humidity parameters from one measuring level. The wave state of the sea was deduced from data from a wave rider buoy anchored 4 km from the site.Two turbulence instruments for humidity were compared, the MIUU instrument (hot wire) and an open pass infrared gas analyser from LI-COR. The comparison showed that the LI-COR instrument resolved the high frequency fluctuations of the humidity better. The unstable cospectra of the sensible and latent heat fluxes were studied and categorised. It was found that many cospectra have two or more maxima. The higher frequency maxima gained influence when the stratification became near neutral.The quadrant analyses showed that the structures of humidity flux were similar to those of the heat flux. The sources of the flux were studied using different ratios. The ratio between events of moist updrafts and dry downdrafts were extensively studied. It was shown that the events of moist updrafts were more dominating during swell than during growing sea. When the results of the spectral and quadrant analyses were combined, it was shown that the smaller sized eddies of heat dominate the events of warm updrafts and that the large eddies dominate the cold downdrafts. The bulk transfer number for moisture, the Dalton number (CE), was found to be almost constant with stratification for unstable runs. The mean value was calculated to (1.0±0.3)·10-3.
Målet för denna studie var turbulensstrukturer över hav med särskild tonvikt på fuktigheten. I denna studie har använts observationer från en mast på ön Östergarns-holm, strax öster om Gotland. Arbetet innefattar spektral- och kvadrantanalys av vind, temperatur och fuktighet från en mätnivå. Havets aktuella tillstånd mättes med en vågboj förtöjd 4 km från masten. Två turbulensinstrument för fuktighet jämfördes, MIUU-instrumentet (varmtråds-instrument) och ett instrument från företaget LI-COR som mäter infraröd absorption. Jämförelsen visade att LI-COR-instrumentet löser upp de högfrekventa fuktighets-fluktuationerna bättre. Instabila cospektra för sensibelt och latent värmeflöde studerades och kategoriserades. Det visade sig att många cospektra hade två eller flera maxima. Det högfrekventa maximumet fick ökad betydelse när skiktningen blev nära neutral. Kvadrantanalyserna visade att strukturerna för värme- och fuktighetsflödet är liknande. Källan för flödena studerades med hjälp av olika kvoter. Av särskilt intresse var kvoten mellan tillfällen med fuktiga uppvindar och torra nedvindar. Det visade sig att tillfällen med fuktiga uppvindar var mer dominerande vid dyning än vid upp-byggande vågor. När resultaten från spektral- och kvadrantanalysen kombinerades, visade det sig att de små virvlarna med värme dominerar vid tillfällen med varma uppvindar och att de stora virvlarna dominerar vid kalla nedvindar. Utbyteskoefficienten CE för fuktighet, även kallad Dalton-talet, är nästan konstant för de instabila fallen. Medelvärdet av CE beräknades till (1,0±0,3)·10-3.

Intraseasonal oscillations (ISOs) are important elements of the tropical climate with time-scales of 20-80 day. The ISO is poorly simulated and predicted by numerical models. This work presents a joint diagnostic and modeling study of the ISO that examines the hypothesis that local coupling between the ocean and the atmosphere is essential to the existence and evolution of the ISO in the Indo-Pacific warm pool region. Low-level moistening during the transition phase preconditions the atmosphere for deep convection. The vertical structure of ISO from the ECMWF coupled model during different phases of the oscillation as well as the skill of the model in simulating the processes that occur during the transition phase were studied. The forecast skill of the vertical structure associated with the ISO is greater for winter than for summer events. Predictability of the convective period is poor when initialized before the transitional phase. When initialized within the transition period including lower tropospheric moistening, predictability increases substantially, although the model parameterizations appears to trigger convection quickly without allowing an adequate buildup of CAPE during the transition. The model tends to simulate a more stable atmosphere compared to data, limiting the production of deep convective events. Two different one-dimensional coupled models are used to analyze the role of local ocean-atmosphere coupling in generating ISO. The ocean component is a one-dimensional mixed layer model. In the first model the atmospheric component corresponds to the SCCM. Results suggest that convection in the model tends to be "overactive," inhibiting development of lower frequency oscillations in the atmosphere. In the second case, the atmospheric component is a semi-empirical model that allows reproducing the coupled ISO over long integration periods including only local mechanisms. In the semi-empirical scheme the rate of change of atmospheric variables is statistically related to changes in SST. The stable state of this model is a quasi-periodic oscillation with a time scale between 25 and 80 days that matches well the observed ISO. Results suggest that the period of the oscillation depends on the characteristics of the ocean mixed layer, with a higher frequency oscillation for a shallow mixed layer.

Au cours des dernières décennies, les communautés de recherches océanographiques et atmosphérique ont démontrées que pour améliorer notre compréhension du couplage entre l'atmosphère et l'océan, et le paramétrage du flux de masse entre l'océan et l'atmosphère (gaz, aérosols, par exemple) , de moment (pour la génération de vagues et de courants marins) et d'énergie (flux de chaleur et énergie cinétique pour les courants et le processus de mélange près de la surface ) dans les modèles couplés océan-atmosphère, les vagues doivent être prises en compte. La physique du couplage dépend de la cinématique et de la dynamique du champ de vagues, y compris les processus de génération de vagues liées au vent, les interactions non-linéaires, ondes-ondes et la dissipation des vagues, cette dernière étant normalement considérée comme dominée par le déferlement. Nous présentons ici une série d'études expérimentales et numériques, démontrant l'importance du champ de vagues sur les interactions océan - atmosphère
Over the last several decades there has been growing recognition from both the traditional oceanographic and atmospheric science communities that to better understand the coupling between the atmosphere and the ocean, and reflect that understanding in improved air-sea fluxes of mass (e.g. gases, aerosols), momentum (e.g. generation of waves and currents) and energy (e.g. heat and kinetic energy for currents and mixing) in coupled ocean-atmosphere models, surface-wave processes must be taken into account. The underlying physics of the coupling depends on the kinematics and dynamics of the wave field, including processes of wind-wave growth, nonlinear wave-wave interactions, wave-current interactions and wave dissipation, with the last normally considered dominated by wave breaking. Here we present a series of experiments, both numerical and field observations, focusing on surface wave effects on air-sea interaction processes

L'Asie du Sud-Est (SEA pour Southeast Asia) regroupe 10% de la population mondiale et est soumise à un large éventail de facteurs et aléas climatiques : typhons, mousson, Oscillation Australe El Niño (ENSO), changement climatique... A l'interface entre l'Océan Indien, l'Océan Pacifique et l'atmosphère, cette région, qui comprend le continent maritime, est de plus un élément clé du fonctionnement de la circulation océanique et atmosphérique globale. L'objectif général de cette thèse est de mieux comprendre le fonctionnement et l'impact des interactions air-mer dans le climat d'Asie du Sud-Est. Ceci est d'une importance primordiale pour une connaissance approfondie et une meilleure capacité de prédiction de la variabilité climatique à toutes les échelles dans la région, depuis les événements extrêmes et la variabilité interannuelle jusqu'aux projections futures, mais aussi pour mieux comprendre, modéliser et prévoir le climat global. Nous nous sommes concentrés sur deux processus qui jouent un rôle important dans le climat du d'Asie du Sud-Est : ENSO et les échanges de chaleur à l'interface air-mer. Tout d'abord, l'impact d'ENSO et de sa variante, ENSO Modoki, sur la variabilité des précipitations dans la région SEA a été étudié pour la période 1979-2019. La diminution (augmentation) observée des précipitations sur la SEA pendant les événements Modoki par rapport aux événements ENSO canoniques a été expliquée par une réduction (augmentation) du transport d'humidité dans la région et un affaiblissement (renforcement) de la branche ascendante de la circulation de Walker. Deuxièmement, nous avons analysé des jeux de données observationnelles et numériques disponibles et effectué des simulations de sensibilité afin d'explorer et d'évaluer la gamme d'estimations des flux de chaleur air-mer dans la région SEA. Cela a révélé une énorme incertitude dans les estimations provenant de divers ensembles de données, avec des valeurs de flux de chaleur net variant d'environ -30 à +40 W.m-2. Le modèle numérique SYMPHONIE a été utilisé avec deux méthodes de forçage des flux de chaleur de surface (formules bulk vs. flux prescrits à partir de jeux de données atmosphériques) pour étudier la sensibilité de la température de surface de la mer simulée à ces flux. Les résultats ont fourni une estimation de +12.5 W.m-2 du gain net de chaleur pour l'océan sur la période 2009-2018, et ont suggéré qu'ERA5, la cinquième génération de réanalyse de l'ECMWF ("European Centre for Medium-Range Weather Forecasts"), peut être utilisée comme référence bien qu'elle surestime légèrement le flux net de chaleur. Enfin, les flux air-mer produits par 30 modèles issus des simulations de la phase 6 du "Coupled Model Intercomparison Project" (CMIP6) ont été évalués par rapport à ERA5. Sur la période historique, la moyenne d'ensemble CMIP6 reproduit bien la variabilité spatiale des flux de chaleur, mais sous-estime de deux tiers le gain net de chaleur pour l'océan par rapport à ERA5. Les principaux facteurs contribuant à ce biais net sont le rayonnement en ondes courtes (SW) et le flux de chaleur latente (LH). Le gain de chaleur net devrait augmenter au cours du XXème siècle, en raison d'une augmentation du gain de SW et de la perte de LH et d'une diminution du rayonnement de grande longueur d'onde (LW) et des pertes de chaleur sensible (SH). Les modèles qui prévoient un réchauffement plus intense de la surface de la mer présentent des changements plus importants dans les flux de chaleur. Ceux-ci devraient changer le plus dans le cadre du scénario SSP5-8.5 (+3,7, +1,0, -8,4, +9,2, et +1,9 W.m-2, respectivement pour Qnet, SW, LH, LW, et SH), suivi par le scénario SSP2-4.5, et enfin le scénario SSP1-2.6
Southeast Asia (SEA) gathers 10% of the world population and is subject to a wide range of climate factors and hazards : typhoons, monsoon, El Niño Southern Oscillation (ENSO), climate change... At the interface between the Indian Ocean, the Pacific Ocean and the atmosphere, the SEA region, which includes the maritime continent, is moreover key to the functioning of global oceanic and atmospheric circulation. The general objective of this thesis is to better understand the functioning and impact of air-sea interactions in the SEA climate. This is of primary importance for an in-depth knowledge and a better prediction capacity of climate variability at all scales in the region, from extreme events and interannual variability to future projections, but also to better understand, model and forecast global climate. We focused on two processes that play an important role in SEA climate : El Niño Southern Oscillation (ENSO) and air-sea heat exchanges. First, the impact on SEA rainfall variability of ENSO and its variant, ENSO Modoki, were investigated for the period 1979-2019. The observed decrease (increase) in rainfall over SEA during Modoki events compared to the canonical ENSO events was explained by a reduced (enhanced) moisture transport into the region and a weakening (strengthening) of the ascending branch of the Walker circulation. Second, we analyzed available observational and numerical datasets and conducted sensitivity simulations to explore and assess the range of estimates of air-sea heat fluxes in the SEA region. This revealed a huge uncertainty in estimates from various datasets, with values of net heat flux varying from approximately -30 to +40 W.m-2. The SYMPHONIE numerical model was used with two methods of surface heat flux forcing (bulk formulae vs. prescribed fluxes from atmospheric datasets) to investigate the sensitivity of the model's sea surface temperature to those fluxes. Results provided a +12.5 W.m-2 estimate of net heat gain for the ocean over 2009-2018, and suggested that ERA5, the fifth generation of the European Centre for Medium-Range Weather Forecasts reanalysis, can be used as a reference though a slight overestimation of net heat flux. Last, air-sea fluxes produced by 30 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations were evaluated against ERA5. Over the historical period, the CMIP6 ensemble average reproduces well the spatial variability of heat fluxes, but underestimates the net heat gain for the ocean by two-third compared to ERA5. The primary contributors to the net bias are shortwave radiation (SW) and latent heat flux (LH). The net heat gain is projected to increase during the XXIst century, resulting from an increase of SW gain and LH loss and a decrease of longwave radiation (LW) and sensible heat (SH) losses. Models with higher projected sea surface warming exhibit larger changes in heat fluxes. Heat fluxes are predicted to change the most under the SSP5-8.5 scenario (+3.7, +1.0, -8.4, +9.2, and +1.9 W.m-2, respectively for Qnet, SW, LH, LW, and SH), followed by the SSP2-4.5 scenario, and finally the SSP1-2.6 scenario

Thesis (S.M.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Ocean Engineering; and the Woods Hole Oceanographic Institution), 2005.
Includes bibliographical references (leaves 70-72).
The uncertainty in the determination of the momentum and scalar fluxes remains one of the main obstacles to accurate numerical forecasts in low to moderate wind conditions. For example, latent heat fluxes computed from data using direct covariance and bulk aerodynamic methods show that there is good agreement in unstable conditions when the latent heat flux values are generally positive. However, the agreement is relatively poor in stable conditions, particularly when the moisture flux is directed downward. If the direct covariance measurements are indeed accurate, then they clearly indicate that the bulk aerodynamic formula overestimate the downward moisture flux in stable conditions. As a result, comparisons of the Dalton number for unstable and stable conditions indicate a marked difference in value between the two stability regimes. Investigations done for this thesis used data taken primarily at the Air-Sea Interaction Tower (ASIT) during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) Experiment 2003 from the 20-27 August 2003. Other data from the shore based Martha's Vineyard Coastal Observatory (MVCO) and moored buoys in the vicinity of the ASIT were also incorporated.
(cont.) During this eight day period, the boundary layer was often characterized by light winds, a stably stratified surface layer and a swell dominated wave field. Additionally, the advection of warm moist air over cooler water resulted in fog formation and a downward flux of moisture on at least three occasions. Therefore, a primary objective of this thesis is to present a case study to investigate the cause of this shortcoming in the bulk formula under these conditions by examining the physical processes that are unique to these boundary layers. Particular attention will be paid to the behavior of the Dalton number in a stable marine atmospheric boundary layer under foggy conditions using insights derived from the study of fog formation and current flux parameterization methods.
by Robert Farrington Crofoot.
S.M.

Thesis (Ph. D.)--Joint Program in Chemical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2007.
Includes bibliographical references (p. 215-225).
The five noble gases (helium, neon, argon, krypton, and xenon) are biologically and chemically inert, making them ideal oceanographic tracers. Additionally, the noble gases have a wide range of solubilities and molecular diffusivities, and thus respond differently to physical forcing. Tritium, an isotope of hydrogen, is useful in tandem with its daughter helium-3 as a tracer for water mass ages. In this thesis, a fourteen month time-series of the five noble gases, helium-3 and tritium was measured at the Bermuda Atlantic Time-series Study (BATS) site. The time-series of five noble gases was used to develop a parameterization of air-sea gas exchange for oligotrophic waters and wind speeds between 0 and 13 m s-1 that explicitly includes bubble processes and that constrains diffusive gas exchange to ± 6% and complete and partial air injection processes to ± 15%. Additionally, the parameterization is based on weeks to seasonal time scales, matching the time scales of many relevant biogeochemical cycles. The time-series of helium isotopes, tritium, argon, and oxygen was used to constrain upper ocean biological production. Specifically, the helium flux gauge technique was used to estimate new production, apparent oxygen utilization rates were used to quantify export production, and euphotic zone seasonal cycles of oxygen and argon were used to determine net community production. The concurrent use of these three methods allows examination of the relationship between the types of production and begins to address a number of apparent inconsistencies in the elemental budgets of carbon, oxygen, and nitrogen.
by Rachel H.R. Stanley.
Ph.D.

As part of the ONR sponsored Coupled Boundary Layer/Air-Sea Transfer (CBLAST) experiment, data from the NOAA WP-3D research aircraft measurements into major Hurricanes in the 2002-2004 seasons are analyzed to investigate the structure of the boundary layer. The turbulent fluxes of momentum and enthalpy are derived using the eddy correlation method. For the first time, the momentum and enthalpy fluxes were directly measured in the boundary layer of a hurricane with wind speeds up to 30 m/s. A new bulk parameterization of the momentum and enthalpy flux is developed. The vertical structure of turbulence and fluxes are presented for the entire boundary layer in the rain free region between the outer rainbands. The turbulent kinetic energy budget was estimated for the hurricane boundary layer between the outer rainbands. The universal spectra and cospectra of the wind velocity, temperature and humidity are also derived. A case study on the effects of roll vortices on the turbulent fluxes is conducted, which confirmed the existence of the boundary layer rolls and gave the first estimate of their modulation of the momentum and sensible heat flux. The CBLAST data provided an invaluable perspective on the evaluation and development of the boundary layer parameterization suited for the hurricane models. Studies on entrainment processes above of the mixed layer and turbulent transport processes induced by the inflow are recommended in the future.

This study reports on a new formulation of the spectral dissipation source term Sds for wind-wave modelling applications. This new form of Sds features a nonlinear dependence on the local wave spectrum, expressed in terms of the azimuthally integrated saturation parameter B(k)=k^4 F(k). The basic form of this saturation-dependent Sds is based on a new framework for the onset of deep-water wave breaking due to the nonlinear modulation of wave groups. The new form of Sds is succesfully validated through numerical experiments that include exact nonlinear computations of fetch-limited wind-wave evolution and hindcasts of two-dimensional wave fields made with an operational wind-wave model. The newly-proposed form of Sds generates integral spectral parameters that agree more closely with observations when compared to other dissipation source terms used in state-of-the-art wind-wave models. It also provides more flexibility in controlling properties of the wave spectrum within the high wavenumber range. Tests using a variety of wind speeds, three commonly-used wind input source functions and two alternative full-development evolution limits further demonstrate the robustness and flexibility of the new saturation-dependent dissipation source term. Finally, improved wave hindcasts obtained with an implementation of the new form of Sds in a version of the WAM model demonstrate its potential usefulness in operational wind-wave forecasting applications.

Chaque automne, des événements de précipitations intenses (HPEs) ont lieu en Méditerranée nord-occidentale. Cette thèse adopte une approche par la modélisation climatique régionale couplée atmosphère-océan pour traiter de la sensibilité de ces événements à des changements de température de surface de la mer (SST) résultant soit de biais dans le modèle couplé, soit de la réponse de la couche de mélange océanique à des forçages atmosphériques. Deux cas d’études mettent en évidence la sensibilité particulière des zones de convergence d’humidité aux changements de SST. L’élaboration d’indices synthétiques de changements dans les précipitations et de changements de SST en amont des zones précipitantes met en lumière dans plusieurs régions (Cévennes, région de Valence, Calabre) une relation linéaire entre ces deux quantités dans deux plateformes de modélisation différentes : MORCE et CNRM-RCSM4. Dans la région de Valence, en Espagne, nous montrons en outre que les événements de précipitations intenses sont souvent précédés d’un épisode de Mistral qui refroidit la zone amont des précipitations dans les jours précédant celles-ci, refroidissement qui tend ensuite à réduire l’intensité de l’événement précipitant
Every year in autumn, heavy precipitation events (HPEs) occur in the northwestern Mediterrranean. This thesis uses coupled atmosphere-ocean regional climate modeling to tackle the sensitivity of these events to sea surface temperature (SST) changes coming either from model biases or from the oceanic mixed layer response to atmospheric forcing. Two case studies show the particular sensitivity of moisture convergence zones to SST changes. The use of synthetic indexes of precipitation changes and SST changes in the upstream zones shows a linear relationship between the two indexes in several regions (Cévennes, the region of Valencia, Calabria) in the modeling platforms MORCE and CNRM-RCSM4. Furthermore, we show that the HPEs in the region of Valencia are often preceded by a Mistral event which cools the upstream zone whithin 5 days before the HPEs. In turn, this cooling tends to reduce the intensity of the HPE

Ocean wind waves represent the atmosphere-ocean boundary, playing a central role in the air-sea exchanging processes. Heat, mass and momentum are transferred across this boundary, with waves mediating the exchange of principally the momentum between the winds and the ocean surface. During the generation process waves are called wind sea. When they leave their generation area or outrun their generating wind they are called swell. The wave field can be said to be dominated either by wind sea or swell. Depending on the wave regime the momentum and energy exchanging processes and the degree of coupling between the waves and the wind is different. During the growing process, waves act as a drag on the surface wind and the momentum flux is directed downward. When swell dominates the wave field a reverse momentum flux mechanism occurs triggered by swell waves traveling considerably faster than the surface winds. The momentum transfer is now directed from the waves to the atmosphere, and takes place because swell waves perform work on the atmosphere as part of their attenuation process. This upward momentum transfer has an impact on the lower atmosphere dynamics, and on the overall turbulence structure of the boundary layer. A detailed qualitative climatology of the global wind sea and swell fields from wave reanalysis data, is presented, revealing a very strong swell dominance of the World Ocean. The areas of larger potential impact of swell on the atmosphere, from a climatological point of view, are also studied. A model that reproduces the swell impact on the lower atmosphere dynamics, conceptually based on the energy transfer from the waves to the atmosphere, is presented – a  new parameterization for the wave-induced stress is also proposed. The model results are compared with field observations. A modeling simulation, using a coupled wave-atmosphere model system, is used to study the impact of swell in a regional climate model, by using different formulations on how to introduce the wave state effect in the modeling system.
Gränsen mellan hav och atmosfär beskrivs av vågor, dessa spelar en central roll i utbytesprocesser mellan hav och atmosfär. Värme, massa och rörelsemängd överförs vid ytan och utbytet av rörelsemängd mellan vind och havsyta styrs i stor utsträckning av vågorna. Då vågor skapas kallas de för vinddrivna vågor. När vågorna sedan lämnar området där de genererats eller rör sig fortare än den vind som genererat dem kallas de dyning. Ett vågfält kan sägas vara dominerat av antingen vinddrivna vågor eller dyningsvågor. Beroende på vilken vågregim som råder så är kopplingen mellan vågor och vind olika och därmed också utbytesprocesserna för rörelsemängd och energi. Då vågorna genereras fungerar de som en bromsande kraft för vinden och impulsutbytet är nedåtriktat. När dyning dominerar vågfältet inträffar en mekanism för omvänt impulsutbyte som sätts igång av dyningsvågor som färdas avsevärt snabbare än vinden. Rörelsemängd överförs då från vågorna till atmosfären, eftersom dyningsvågorna utför arbete på atmosfären då de dämpas. Den uppåtriktade transporten av rörelsemängd har en stor effekt på dynamiken och turbulensstrukturen i lägre delen av atmosfären. En detaljerad kvalitativ klimatologi av globala vågfält (vinddrivna och dyning) från återanalysdata presenteras och visar att dyning dominerar vågfältet på världshaven. Områden där man kan förvänta sig störst effekt av dyning på atmosfären har identifierats. En konceptuellt baserad modell som reproducerar effekten av dyning på dynamiken i lägre delen av atmosfären presenteras. Modellen styrs av överföring av energi från vågor till atmosfären. I modellen föreslås även en ny parameterisering för våginducerad kraft på havsytan. Modellresultaten är utvärderade mot fältmätningar. En regional klimatmodell, med ett kopplat våg-atmosfärssystem, har använts för att studera den långtida effekten av dyning vid klimatsimulering. Olika formuleringar för beskrivningen av vågornas effekt på atmosfären har använts, beroende på om vinddrivna vågor eller dyning dominerar vågfältet.

The understanding and prediction of the monsoon variability over South Asia region is one of the biggest challenges for climatologist and meteorologist today. The Indian Summer Monsoon (ISM) has different temporal and spatial scales of variability and it is mainly driven by strong air sea interactions. In this thesis we evaluate the performance of the new regional Earth System Model (ESM) RegCM-ES in reproducing the main characteristics of the ISM rainfall (ISMR). We performed two sets of simulations, one with the new RegCM-ES and another with the stand-alone version of the atmospheric component i.e. the regional climate model (RCM) RegCM4. RegCM-ES is composed mainly by three components, the RegCM4, the ocean model MITgcm and the hydrological model HD. Another experiment, performed using RegCM-ES with a more high-resolution hydrological model implemented ad hoc for this study, has been added to this two set of experiments. The climatological mean state of the monsoon is well represented by mostly all the experiments although not with the same skills. The most interesting results are observed in simulating the variability of the monsoon and here we highlight some of them. The intraseasonal northward(eastward) propagation of the convection has been analysed using lag/lead map of the regressed anomalies as a function of latitudes(longitudes). The two propagations are better reproduced by the set of ESM simulations thanks to the role of the air-sea coupling. For what concerns the interannual variability (IAV) of the ISMR the air-sea coupling plays an important role. The time series of simulated anomalies by RCM exhibit no correlation with the observed anomalies obtained from the dataset of the Indian Meteorological Department (IMD). On the other hand, the corresponding ESM simulations exhibits good skills in reproducing the IAV of ISMR and good correlation coefficients are observed with IMD. One new finding of this study is a new source of predictability with one-year lag for the ISMR. It is well known that El Niño Southern Oscillation (ENSO) plays a quite important role in modulating the precipitation over most of the intertropical belt and over the South Asia region. As seen in previous findings, the response of the ISMR to ENSO can be delayed of two seasons through the contribution of different air-sea coupled mechanisms as the decrease of Western Arabian Sea Upwellingthe Indo-western Pacific ocean capacitor and the Indo-Tropical northwest Pacific ocean-atmosphere interaction. Our findings extend and confirm the possibility that this response may have a longer feedback time, a year or so. The coupled simulations are used to explain the mechanism and investigate the models response. They appear quite similar to those proposed in the previous studies but further investigations are needed to understand more in deep the phenomena involved. The role of the new hydrological model (CHyM) implemented inside RegCM-ES on the ISMR is also investigated. Due to the main role that the freshwater discharge plays on the formation of a shallow mixed layer depth on the Bay of Bengal that influences the air-sea coupling and the formation of deep convection over this area, the correct estimation of the freshwater discharge is quite important. Although the new hydrological model produces a more realistic annual cycle of the discharge (verified only for a limited set of data due to the lack of observations over this region), this doesn't seem to have a relevant effect on both the mean climatological state of the monsoon and in its variability. A possible explanation for this comes from a missing representation of the barrier layer (BL) in the Bay of Bengal (BoB) due to many different reasons. Two of them are a not enough resoluted ocean model as well as a too strong wind stress forcing that doesn’t allow the formation of the BL.

Field-wise wind estimation (also known as model-based wind estimation) is a sophisticated technique to derive wind estimates from radar backscatter measurements. In contrast to the more traditional method known as point-wise wind retrieval, field-wise techniques estimate wind field model parameters. In this way, neighboring wind vectors are jointly estimated, ensuring consistency. This work presents and implementation for field-wise wind retrieval for the SeaWinds scatterometer on the QuikSCAT satellite. Due to its sophistication, field-wise wind retrieval adds computational complexity and intensity. The tradeoffs necessary for practical implementations are examined and quantified. The Levenberg-Marquardt algorithm for minimizing the field-wise objective function is presented. As the objective function has several near-global local minima, several wind fields represent ambiguous wind field estimates. A deterministic method is proposed to ensure sufficient ambiguities are obtained. An improved method for selecting between ambiguous wind field estimates is also proposed. With a large set of Sea-Winds measurements and estimates available, the σ° measurement statistics are examined. The traditional noise model is evaluated for accuracy. A data-driven parameterization is proposed and shown to effectively estimate measurement bias and variance. The parameterized measurement model is used to generate Cramer-Rao bounds on estimator performance. Using the Cramer-Rao bound, field-wise and point-wise performances are compared.

Chaque automne, des événements de précipitations intenses (HPEs) ont lieu en Méditerranée nord-occidentale. Cette thèse adopte une approche par la modélisation climatique régionale couplée atmosphère-océan pour traiter de la sensibilité de ces événements à des changements de température de surface de la mer (SST) résultant soit de biais dans le modèle couplé, soit de la réponse de la couche de mélange océanique à des forçages atmosphériques. Deux cas d’études mettent en évidence la sensibilité particulière des zones de convergence d’humidité aux changements de SST. L’élaboration d’indices synthétiques de changements dans les précipitations et de changements de SST en amont des zones précipitantes met en lumière dans plusieurs régions (Cévennes, région de Valence, Calabre) une relation linéaire entre ces deux quantités dans deux plateformes de modélisation différentes : MORCE et CNRM-RCSM4. Dans la région de Valence, en Espagne, nous montrons en outre que les événements de précipitations intenses sont souvent précédés d’un épisode de Mistral qui refroidit la zone amont des précipitations dans les jours précédant celles-ci, refroidissement qui tend ensuite à réduire l’intensité de l’événement précipitant
Every year in autumn, heavy precipitation events (HPEs) occur in the northwestern Mediterrranean. This thesis uses coupled atmosphere-ocean regional climate modeling to tackle the sensitivity of these events to sea surface temperature (SST) changes coming either from model biases or from the oceanic mixed layer response to atmospheric forcing. Two case studies show the particular sensitivity of moisture convergence zones to SST changes. The use of synthetic indexes of precipitation changes and SST changes in the upstream zones shows a linear relationship between the two indexes in several regions (Cévennes, the region of Valencia, Calabria) in the modeling platforms MORCE and CNRM-RCSM4. Furthermore, we show that the HPEs in the region of Valencia are often preceded by a Mistral event which cools the upstream zone whithin 5 days before the HPEs. In turn, this cooling tends to reduce the intensity of the HPE

In May of 2000 Jason-1, a joint project between NASA and the French space agency CNES, will be launched. Its mission is to continue the highly successful gathering of data which TOPEX/Poseidon has collected since August of 1992. The main goal of Jason-1 is to achieve higher accuracy in measuring the mean sea level (MSL). In order to do so, the electromagnetic (EM) bias must be estimated more accurately because it is the largest contributing error. This thesis presents two different studies which add to the knowledge and improve estimation of the EM bias, and thus assists Jason-1 in achieving its primary goal. Oceanographic data collected from two different experiments are analyzed; on in the Gulf of Mexico (GME) and the other in Bass Strait, Australia (BSE). The first study is a spatial analysis of the backscattered power versus the phase of the wave. Its purpose is to determine why the normalized EM bias stops increasing and levels out at high wind speeds (about 11 m/s) and then decreases at higher wind speeds. Two possible causes are investigated. First, it could be due to a shift in the backscatter power modulation to the forward or rear face of the wave crests. Second, it may be due to the backscatter power becoming more homogeneous throughout the wave profile. This study is novel because it uses the knowledge of the spatial distribution of both the backscatter and wave displacement for the study of the EM bias. Both contribute to the EM bias decrease, but the latter cause seems to be the dominant effect. This study is performed on GME data. The second study uses two different nonparametric regression (NPR) techniques to estimate the EM bias. A recent study of satellite data from the TOPEX/Poseidon altimeter supports that the bias is modeled better using NPR regression. A traditional parametric fit is compared to two NPR techniques with GME data. The parametric fit is a variation of NASA's equation used to estimate EM bias for their Geophysical Data Records (GDRs). The two NPR techniques used are the Nadaraya-Watson Regression (NWR) and Local Linear Regression (LLR) estimators. Two smoothing kernel functions are used with each NPR technique, namely the Gaussian and the Epanechnikov kernels. NPR methods essentially consist of statistically smoothing the measured EM bias estimates are compared in the wind and significant wave height plane. Another recent study has shown that wave slope is strongly correlated to EM bias. With this knowledge, EM bias is estimated over several two-dimensional planes which include wave slope in attempt to reduce the residual bias. This portion of the study is performed on GME and BSE data. It is shown that a combination of slope, significant wave height, and wind speed used in conjunction with these NPR methods produces the best EM bias estimate for tower data.

YSCAT is a unique ultrawideband microwave scatterometer developed to investigate the sea surface under a variety of environmental and radar parameters. The YSCAT94 experiment consisted of a six month deployment on the WAVES research tower operated by the Canada Center for inland Waters (CCIW). Over 3500 hours of data were collected at 2Γ 3.05Γ 5.3Γ 10.02Γ and 14 GHz and at a variety of wind speeds, relative azimuth angles, and incidence angle. A low wind speed "rolloff" of the normalized radar cross section (σ°) in YSCAT94 data is found and quantified. The rolloff wind speedΓ γΓ is estimated through regression estimation analysis using an Epanechnikov kernel. For YSCAT94 data, the rolloff is most noticeable at mid-range incidence angles with γ values ranging from 3 to 6 m/s. In order to characterized YSCAT94 backscatter distributions, a second order polynomial in log space is developed as a model for the probability of the radar cross sectionΓρ(σ°). Following Gotwols and ThompsonΓρ(σ°) is found to adhere to a log-normal distribution for horizontal polarization and a generalized log-normal distribution for vertical polarization. If ρ(α|σ°) is assumed to be Rayleigh distributed, the instantaneous amplitude distribution ρ(α) is found to be the integral of a Rayleigh/generalized log-normal distribution. A robust algorithm is developed to fit this probability density function to YSCAT94 backscatter distributions. The mean and variance of the generalized log-normal distribution are derived to facilitate this algorithm. Over 2700 distinct data cases sorted according to five different frequencies, horizontal and vertical polarizations, upwind and downwind, eight different incidence angles Γ1-10 m/s wind speeds, and 0.1-0.38 mean wave slope are considered. Definite trends are recognizable in the fitted parameters a1Γ a2Γ and C of the Rayleigh/generalized log-normal distribution when sorted according to wind speed and mean wave slope. At mid-range incidence angles, the Rayleigh/generalized log-normal distribution is found to adequately characterize both low and high amplitude portions of YSCAT94 backscatter distributions. However, at higher incidence angels (50°and 60°) the more general Weibull/generalized log-normal distributions is found to better characterized the low amplitude portion of the backscatter distributions.

La mer Méditerranée est souvent considérée comme un océan laboratoire pour comprendre les changements globaux liés à l’augmentation de CO2 atmosphérique. Ce travail, basé sur l’étude de données recueilles dans trois régions méditerranéennes, étudie les variations du CO2 océanique dans ce bassin. À l’échelle de la saison, outre les changements de température, le contenu en alcalinité influe sur le contenu en CO2 en Méditerranée orientale, tandis que les changements en carbone total sont responsables des variations dans le bassin occidental. En zone côtière urbanisée, l’émission de CO2 anthropique conditionne les échanges air-mer de CO2. Cette étude montre que l’augmentation de carbone et l’acidification à l’échelle de plusieurs années ne sont pas seulement dues à l’augmentation du CO2 atmosphérique : le contenu en alcalinité module ces tendances dans le bassin oriental, tandis que, dans le bassin occidental, ces tendances sont vraisemblablement influencées par la dynamique des courants
The Mediterranean Sea is often considered as a laboratory ocean for understanding global changes related to the atmospheric CO2 increase. This work, based on the study of data collected in three Mediterranean regions, investigates the variations of oceanic CO2 in this basin. On a seasonal timescale, in addition to temperature changes, alkalinity content influences the CO2 content in the Eastern Mediterranean, while total carbon changes are responsible for variations in the Western Basin. In urbanised coastal areas, anthropogenic CO2 emission’ influences air-sea CO2 exchanges. This study shows that the carbon increase and the acidification on a multi-year timescale is not only due to the increase in atmospheric CO2: the alkalinity content modulates these trends in the eastern basin, while, in the western basin, these trends are likely influenced by current dynamics

Os padrões de circulação do Atlântico Sudoeste são caracterizados por uma diversidade de massas de água. A presença das correntes associadas ao giro subtropical e a incursão para norte da Corrente Circumpolar Antártica (CCA) determinam extensas regiões de largo gradiente de Temperatura da Superfície do Mar (TSM). Ao mesmo tempo, circulações de mesoescala geram intensos gradientes setorizados, ou reforçam o contraste de larga-escala. Consequentemente, frentes oceânicas de diferentes escalas são formadas ao longo desses gradientes. Quando o vento sopra sobre essas frentes, os fluidos trocam calor e momentum alterando suas propriedades dinâmicas e termodinâmicas. Nesse trabalho visamos caracterizar as alterações no campo de vento superficial que podem ser atribuídas a essas trocas. Para isso, foi aplicado um algoritmo de detecção de frentes em campos de Temperatura da Superfície do Mar (TSM) derivados do conjunto OSTIA. Em situações de escoamento atmosférico sinótico homogêneo, foram calculados o divergente e rotacional do vento medido pelo escaterômetro QuickSCAT ao longo das zonas frontais, bem como suas componentes perpendicular e paralela às frentes. Ao longo de 8 anos mais 96.000 frentes oceânicas foram detectadas, co-localizadas com a disponibilidade de dados de vento dando origem a 40.000 composições. O sistema de coordenadas dessas composições foi rotacionado para que as frentes oceânicas tivessem a mesma orientação. Nós empilhamos as composições em um arranjo 3D e foram obtidas médias das circulações atmosféricas induzidas. As perturbações médias obtidas indicaram que há convergência do vento quando este sopra do lado quente para o lado frio da frente com a frenagem do escoamento. De forma oposta, há divergência e aceleração do vento quando este sopra no sentido oposto. Nós identificamos alterações locais no rotacional do campo de vento capazes de induzir o bombeamento de Ekman no oceano. Esse processo pode gerar mecanismos de retroalimentação no sistema. Nossos resultados corroboram o de diversos estudos sobre o tema presentes literatura.
The southwest Atlantic circulation patterns are characterized by a diversity of water masses. The presence of currents associated with the subtropical gyre and the northward incursion of Antarctic Circumpolar Current (ACC) determine extensive regions of strong sea surface temperature gradient (SST). At the same time, mesoscale circulations generate intense local gradients, or reinforce the large-scale contrast. Consequently, oceanic fronts are formed along these gradients on different scales. When the wind blows over these fronts, the fluids exchange heat and momentum, and that changes their dynamic and thermodynamic properties. In this work we aim to characterize the changes in the surface wind field that might be attributed to these exchanges. To do it, a frontal detection algorithm was applied to the SST field derived from OSTIA set. We selected situations of synoptic homogeneous atmospheric flow and calculated the divergent and rotational wind, in addition to its perpendicular and parallel components to the oceanic fronts. We used wind measurements recorded by the QuickSCAT scatterometer along the frontal zones. More than 96,000 oceanic fronts were detected along 8 years of data. We matched them to available wind data and formed more than 40,000 SST-WIND compositions. The coordinate system of these compositions was rotated so that all oceanic fronts have the same horizontal orientation. We piled up the compositions in a 3D array and the temporal mean of the induced circulations was calculated. The average disturbance obtained indicated that there is wind convergence when it blows from the warm side of the front to the cold side because the flow is slowed down. Conversely, there is wind divergence when it blows in the opposite direction due to speeding up flow. We identified local changes in wind field curl capable of inducing Ekman pumping over the ocean. This process can generate feedback mechanisms in the system. Our results were consistent with the literature.