Tesi sul tema "Atmospheric cycle"

Thesis (Ph. D.)--University of California, San Diego, 2007.
Title from first page of PDF file (viewed July 10, 2007). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 146-154).

Dans un contexte de changement climatique, la connaissance des climats passés permet de mieux cerner l'évolution future du climat. Les isotopes stables de l'eau constituent un excellent proxy paléo-climatique. Les propriétés physiques des isotopes lourds de l'eau (H182 O; HDO) induisent des fractionnements isotopiques, qui dépendent de la température et du taux de distillation. Sous réserve d'une inversion bien conditionnée du signal isotopique, on peut reconstruire les variations passées du climat à partir d'archives isotopiques. Les carottes de glace andines constituent un enregistrement unique de la variabilité du climat tropical. En revanche, la complexité de la circulation atmosphérique rend plus ardue l'interprétation de leur signal isotopique. En conséquence, nous avons développé au cours de cette thèse un module traitant du fractionnement des isotopes stables de l'eau au sein du modèle de circulation régionale REMO pour application au cas de l'Amérique du Sud. Le manuscrit retrace les principales étapes de la thèse. Il s'agit de la mise en perspective du travail de thèse dans la problématique du changement climatique ; la description du modèle de circulation régionale REMOiso et de son module traitant des isotopes de l'eau ; la validation initiale de REMOiso sur l'Europe ; l'étude des variations saisonnières des précipitations, de la circulation atmosphérique régionale et du signal isotopique en Amérique du Sud ; de l'enregistrement par les isotopes stables de l'eau de la mousson sud-américaine
Climate change has recently become a major concerning among scientists and the general public. A better knowledge of past climates helps forecasting the future evolution of climate. Stable water isotopes stand as an outstanding paleo-climate proxy. Physical properties of heavy stable water isotopes (H182 O; HDO) cause fractionation processes related to temperature and degree of distillation. If the isotopic signal is correctly inverted, past climate change can be inferred from isotopic archives. Andean ice-cores offer a unique records of tropical climate and its variability through time. However, the interpretation of the isotopic signal is difficult because of complex atmospheric dynamic over South America. For this purpose, we developed a module handling the stable water isotope fractionation processes within the regional circulation model REMO and applied it to South America. The manuscript outlines the major milestones of the present PhD. We first introduce the research topic in the wider scope of climate change; the description of the stable water isotope enabled regional circulation model REMOiso; an initial validation of REMOiso over Europe; an investigation of the seasonal variations of precipitation, atmospheric circulation and isotopic signal over South America; and at last the recording of the south American monsoon system (SAMS) by stable water isotope diagnostics

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1992.
Title as it appears in the M.I.T. Graduate List, Feb. 1992: Diurnal cycle of deep cloud cover in tropics.
Includes bibliographical references (leaf 53).
by Sewon Park.
M.S.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1999.
Includes bibliographical references (p. [84]-[87]).
In this thesis, I designed and implemented a simple atmosphere-ocean coupled carbon cycle model which can be used as a tool to uncover the mechanisms of the interaction between the dynamics of the atmosphere-ocean system and the oceanic reservoir of CO 2 on the 101 to 103 years time scale. The atmosphere-ocean coupled model is originally developed by Marotzke (20,21), and the biogeochemical model is developed by Follows(personal communication). The atmosphere-ocean-carbon model makes the atmosphere-ocean dynamics and the carbon cycle fully interactive, and results in two stationary states characterized by two distinct patterns of the thermohaline circulation. The temperature driven, high latitudes sinking mode showed significantly lower atmospheric pCO2 than the salinity-driven, low latitudes sinking mode. The atmosphere-ocean dynamics dominates the system behavior of the model. The carbon cycle weakly feedbacks on the atmosphere-ocean system through the radiation balance. The model reveals two feedback mechanisms, the global warming feedback and the thermohaline pCO 2 feedback. The thermohaline pCO2 feedback has three sub-components, which are the biological pump feedback, the outgassing feedback and the DIC exporting feedback. The numerical experiments estimate the relative importance among them. The system becomes less stable when all the feedback mechanism is introduced. The model could be used to understand some basic mechanism of the situations similar to the anthropogenic global warming. The stability analysis is applied to evaluate the model runs. The current rate of 7 GTC yr - 1 can induce the spontaneous shutdown of thermohaline circulation after 550 years of constant emission. The stability of the thermohaline circulation rapidly decreases even before the system stops the thermohaline circulation. The model parameterized surface alkalinity as a simple function of sea surface salinity or as a constant, rather than solving the alkalinity cycle explicitly. The system is sensitive to the parameterization, in which different assumptions on alkalinity lead to different results both analytically and numerically.
by Takamitsu Ito.
S.M.

ENGLISH:

The natural nitrogen cycle has been and is significantly perturbed by anthropogenic emissions of reactive nitrogen (Nr) compounds into the atmosphere, resulting from our production of energy and food. In the last century global ammonia (NH3) emissions have doubled and represent nowadays more than half of total the Nr emissions. NH3 is also the principal atmospheric base in the atmosphere and rapidly forms aerosols by reaction with acids. It is therefore a species of high relevance for the Earth's environment, climate and human health (Chapter 1). As a short-lived species, NH3 is highly variable in time and space, and while ground based measurements are possible, they are sparse and their spatial coverage is largely heterogeneous. Consequently, global spatial and temporal patterns of NH3 emissions are poorly understood and account for the largest uncertainties in the nitrogen cycle. The aim of this work is to assess distributions and saptiotemporal variability of NH3 using satellite measurements to improve our understanding of its contribution to the global nitrogen cycle and its related effects.

Recently, satellite instruments have demonstrated their abilities to measure NH3 and to supplement the sparse surface measuring network by providing global total columns daily. The Infrared Atmospheric Sounding Interferometer (IASI), on board MetOp platforms, is measuring NH3 at a high spatiotemporal resolution. IASI circles the Earth in a polar Sun-synchronous orbit, covering the globe twice a day with a circular pixel size of 12km diameter at nadir and with overpass times at 9:30 and 21:30 (local solar time when crossing the equator). An improved retrieval scheme based on the calculation of Hyperspectral Range Index (HRI) is detailed in Chapter 2 and compared with previous retrieval methods. This approach fully exploits the hyperspectral nature of IASI by using a broader spectral range (800-1200 cm-1) where NH3 is optically active. It allows retrieving total columns from IASI spectra globally and twice a day without large computational resources and with an improved detection limit. More specifically the retrieval procedure involves two steps: the calculation of a dimensionless spectral index (HRI) and the conversion of this index into NH3 total columns using look-up tables (LUTs) built from forward radiative transfer simulations under various atmospheric conditions. The retrieval also includes an error characterization of the retrieved column, which is of utmost importance for further analysis and comparisons. Global distributions using five years of data (1 November 2007 to 31 October 2012) from IASI/MetOp-A are presented and analyzed separately for the morning and evening overpasses. The advantage of the HRI-based retrieval scheme over other methods, in particular to identify smaller emission sources and transport patterns over the oceans is shown. The benefit of the high spatial sampling and resolution of IASI is highlighted with the regional distribution over China and the first four-year time series are briefly discussed.

We evaluate four years (1 January 2008 to 31 December 2011) of IASI-NH3 columns from the morning observations and of LOTOS-EUROS model simulations over Europe and Western Russia. We describe the methodology applied to account for the variable retrieval sensitivity of IASI measurements in Chapter 3. The four year mean distributions highlight three main agricultural hotspots in Europe: The Po Valley, the continental part of Northwestern Europe, and the Ebro Valley. A general good agreement between IASI and LOTOS-EUROS is shown, not only over source regions but also over remote areas and over seas when transport is observed. The yearly analyses reveal that, on average, the measured NH3 columns are higher than the modeled ones. Large discrepancies are observed over industrial areas in Eastern Europe and Russia pointing to underestimated if not missing emissions in the underlying inventories. For the three hotspots areas, we show that the seasonality between IASI and LOTOS-EUROS matches when the sensitivity of the satellite measurements is taken into account. The best agreement is found in the Netherlands, both in magnitude and timing, most likely as the fixed emission timing pattern was determined from experimental data sets from this country. Moreover, comparisons of the daily time series indicate that although the dynamic of the model is in reasonable agreement with the measurements, the model may suffer from a possible misrepresentation of emission timing and magnitude. Overall, the distinct temporal patterns observed for the three sites underline the need for improved timing of emissions. Finally, the study of the Russian fires event of 2010 shows that NH3 modeled plumes are not enough dispersed, which is confirmed with a comparison using in situ measurements.

Chapter 4 describes the comparisons of IASI-NH3 measurements with several independent ground-based and airborne data sets. Even though the in situ data are sparse, we show that the yearly distributions are broadly consistent. For the monthly analyzes we use ground-based measurements in Europe, China and Africa. Overall, IASI-derived concentrations are in fair agreement but are also characterized by less variability. Statistically significant correlations are found for several sites, but low slopes and high intercepts are calculated in all cases. At least three reasons can explain this: (1) the lack of representativity of the point surface measurement for the large IASI pixel, (2) the use of a single profile shape in the retrieval scheme over land, which does therefore not account for a varying boundary layer height, (3) the impact of the averaging procedure applied to satellite measurements to obtain a consistent quantity to compare with the in situ monthly data. The use of hourly surface measurements and of airborne data sets allows assessing IASI individual observations. Much higher correlation coefficients are found in particular when comparing IASI-derived volume mixing ratio with vertically resolved measurements performed from the NOAA WP-3D airplane during CalNex campaign in 2010. The results demonstrate the need, for validation of the satellite columns, of measurements performed at various altitudes and covering a large part of the satellite footprint.

The six-year of IASI observations available at the end of this thesis are used to analyze regional time series for the first time (Chapter 5). More precisely, we use the IASI measurements over that period (1 January 2008 to 31 December 2013) to identify seasonal patterns and inter-annual variability at subcontinental scale. This is achieved by looking at global composite seasonal means and monthly time series over 12 regions around the world (Europe, Eastern Russia and Northern Asia, Australia, Mexico, South America, 2 sub-regions for Northern America and South Asia, 3 sub-regions for Africa), considering separately but simultaneously measurements from IASI morning and evening overpasses. The seasonal cycle is inferred for the majority of these regions. The relations between the NH3 atmospheric abundance and emission processes is emphasized at smaller regional scale by extracting at high spatial resolution the global climatology of the month of maxima columns. In some region, the predominance of a single source appears clearly (e.g. agriculture in Europe and North America, fires in central South Africa and South America), while in others a composite of source processes on small scale is demonstrated (e.g. Northern Central Africa and Southwestern Asia).

Chapter 6 presents the achievements of this thesis, as well as ongoing activities and future perspectives.

FRANCAIS:

Le cycle naturel de l'azote est fortement perturbé suite aux émissions atmosphériques de composés azotés réactifs (Nr) résultant de nos besoins accrus en énergie et en nourriture. Les émissions d'ammoniac (NH3) ont doublé au cours du siècle dernier, représentant aujourd'hui plus de la moitié des émissions totales de Nr. De plus, le NH3 étant le principal composé basique de notre atmosphère, il réagit rapidement avec les composés acides pour former des aérosols. C'est dès lors un constituant prépondérant pour l'environnement, le climat et la santé publique. Les problématiques environnementales y étant liées sont décrites au Chapitre 1. En tant que gaz en trace le NH3 se caractérise par une importante variabilité spatiale et temporelle. Bien que des mesures in situ soient possibles, elles sont souvent rares et couvrent le globe de façon hétérogène. Il en résulte un manque de connaissance sur l'évolution temporelle et la variabilité spatiale des émissions, ainsi que de leurs amplitudes, qui représentent les plus grandes incertitudes pour le cycle de l'azote (également décrites au Chapitre 1).

Récemment, les sondeurs spatiaux opérant dans l'infrarouge ont démontré leurs capacités à mesurer le NH3 et par là à compléter le réseau d'observations de surface. Particulièrement, l'Interféromètre Atmosphérique de Sondage Infrarouge (IASI), à bord de la plateforme MetOp, mesure le NH3 à une relativement haute résolution spatiotemporelle. Il couvre le globe deux fois par jour, grâce à son orbite polaire et son balayage autour du nadir, avec un temps de passage à 9h30 et à 21h30 (temps solaire local quand il croise l'équateur). Une nouvelle méthode de restitution des concentrations basée sur le calcul d'un index hyperspectral sans dimension (HRI) est détaillée et comparée aux méthodes précédentes au Chapitre 2. Cette méthode permet d'exploiter de manière plus approfondie le caractère hyperspectral de IASI en se basant sur une bande spectrale plus étendue (800-1200 cm-1) au sein de laquelle le NH3 est optiquement actif. Nous décrivons comment restituer ces concentrations deux fois par jour sans nécessiter de grandes ressources informatiques et avec un meilleur seuil de détection. Plus spécifiquement, la procédure de restitution des concentrations consiste en deux étapes: le HRI est calculé dans un premier temps pour chaque spectre puis est ensuite converti en une colonne totale de NH3 à l'aide de tables de conversions. Ces tables ont été construites sur base de simulations de transfert radiatif effectuées pour différentes conditions atmosphériques. Le processus de restitution des concentrations comprend également le calcul d'une erreur sur la colonne mesurée. Des distributions globales moyennées sur cinq ans (du 1 novembre 2007 au 31 Octobre 2012) sont présentées et analysées séparément pour le passage diurne et nocturne de IASI. L'avantage de ce nouvel algorithme par rapport aux autres méthodes, permettant l'identification de sources plus faibles de NH3 ainsi que du transport depuis les sources terrestres au-dessus des océans, est démontré. Le bénéfice de la haute couverture spatiale et temporelle de IASI est mis en exergue par une description régionale au-dessus de la Chine ainsi que par l'analyse de premières séries temporelles hémisphériques sur quatre ans.

Au Chapitre 3, nous évaluons quatre ans (du 1 janvier 2008 au 31 décembre 2011) de mesures matinales de IASI ainsi que de simulations du modèle LOTOS-EUROS, effectuées au-dessus de l'Europe et de l'ouest de la Russie. Nous décrivons une méthodologie pour prendre en compte, dans la comparaison avec le modèle, la sensibilité variable de l'instrument IASI pour le NH3. Les comparaisons montrent alors une bonne concordance générale entre les mesures et les simulations. Les distributions pointent trois régions sources: la vallée du Pô, le nord-ouest de l'Europe continentale et la vallée de l'Ebre. L'analyse des distributions annuelles montre qu'en moyenne, les colonnes de NH3 mesurées sont plus élevées que celles simulées, à part pour quelques cas spécifiques. Des différences importantes ont été identifiées au-dessus de zones industrielles en Europe de l'est et en Russie, ce qui tend à incriminer une sub-estimation voire une absence de ces sources dans les inventaires d'émissions utilisés en entrée du modèle. Nous avons également montré que la saisonnalité est bien reproduite une fois la sensibilité des mesures satellites prise en compte. La meilleure concordance entre le modèle et IASI est observée pour les Pays-Bas, ce qui est certainement dû au fait que le profil temporel des émissions utilisé pour les simulations LOTOS-EUROS est basé sur des études expérimentales réalisées dans ce pays. L'étude des séries temporelles journalières indique que la dynamique du modèle est raisonnablement en accord avec les mesures mais pointe néanmoins une possible mauvaise représentation du profil temporel ainsi que de l'ampleur des émissions. Finalement, l'étude des importants feux ayant eu cours en Russie à l'été 2010 a montré que les panaches modélisés sont moins étendus que ceux observés, ce qui a été confirmé grâce à une comparaison avec des mesures sols.

Le chapitre 4 est dédié à la confrontation des mesures IASI avec différents jeux de données indépendants acquis depuis le sol et par avion. Les distributions globales annuelles sont concordantes, bien que la couverture spatiale des mesures sols soit limitée. Des mesures effectuées à la surface en Europe, en Chine et en Afrique sont utilisées pour les comparaisons mensuelles. Ces dernières révèlent une bonne concordance générale, bien que les mesures satellites montrent une plus faible amplitude de variations de concentrations. Des corrélations statistiquement significatives ont été calculées pour de nombreux sites, mais les régressions linéaires sont caractérisées par des pentes faibles et des ordonnées à l'origine élevées dans tous les cas. Au minimum, trois raisons contribuent à expliquer cela: (1) le manque de représentativité des mesures ponctuelles pour l'étendue des pixels IASI, (2) l'utilisation d'une seule forme de profil vertical pour la restitution des concentrations, qui ne prend dès lors pas en compte la hauteur de la couche limite, (3) l'impact de la procédure utilisée pour moyenner les observations satellites afin d'obtenir des quantités comparables aux mesures sols mensuelles. La prise en compte de mesures en surface effectuées à plus haute résolution temporelle ainsi que de mesures faites depuis un avion permet d'évaluer les observations IASI individuelles. Les coefficients de corrélation calculés sont bien plus élevés, en particulier pour la comparaison avec les mesures effectuées depuis l'avion NOAA WP-3D pendant la campagne CalNex en 2010. Ces résultats démontrent la nécessité de ce type d'observations, effectuées à différentes altitudes et couvrant une plus grande surface du pixel, pour valider les colonnes IASI-NH3.

Les six ans de données IASI disponibles à la fin de cette thèse sont utilisées pour tracer les premières séries temporelles sub-continentales (Chapitre 5). Plus spécifiquement, nous explorons les mesures IASI durant cette période (du 1 janvier 2008 jusqu'au 31 décembre 2013) pour identifier des structures saisonnières ainsi que la variabilité inter-annuelle à l'échelle sous-continentale. Pour arriver à cela, des moyennes saisonnières composites ont été produites ainsi que des séries temporelles mensuelles au-dessus de 12 régions du globe (Europe, est de la Russie et nord de l'Asie, Australie, Mexique, Amérique du Sud, 2 sous-régions en Amérique du nord et en Asie du sud et 3 sous-régions en Afrique), considérant séparément mais simultanément les mesures matinales et nocturnes de IASI. Le cycle saisonnier est raisonnablement bien décrit pour la plupart des régions. La relation entre la quantité de NH3 atmosphérique et ses sources d'émission est mise en exergue à l'échelle plus régionale par l'extraction à haute résolution spatiale d'une climatologie des mois de colonnes maximales. Dans certaines régions, la prédominance d'un processus source apparait clairement (par exemple l'agriculture en Europe et en Amérique du nord, les feux en Afrique du Sud et en Amérique du Sud), alors que, pour d'autres, la diversité des sources d'émissions est démontrée (par exemple pour le nord de l'Afrique centrale et l'Asie du sud-ouest).

Le Chapitre 6 reprend brièvement les principaux aboutissements de cette thèse et présente les différentes recherches en cours et les perspectives associées.

Doctorat en Sciences agronomiques et ingénierie biologique
info:eu-repo/semantics/nonPublished

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics; and, (S.B.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 59-60).
A recent article by Kidston et al. [8] demonstrates that the length of atmospheric eddies increases in simulations of future global warming. This thesis expands on Kidston et al.'s work with additional studies of eddy length in the NCEP2 reanalysis (a model-data synthesis that reconstructs past atmospheric circulation) and general circulation models (GCMs) from the Coupled Model Intercomparison Project phase 3. Eddy lengths are compared to computed values of the Rossby radius and the Rhines scale, which have been hypothesized to set the eddy length. The GCMs reproduce the seasonal variation in the eddy lengths seen in the reanalysis. To explore the effect of latent heating on the eddies, a modification to the static stability is used to calculate an effective Rossby radius. The effective Rossby radius is an improvement over the traditional dry Rossby radius in predicting the seasonal cycle of northern hemisphere eddy length, if the height scale used for calculation of the Rossby radius is the depth of the free troposphere. There is no improvement if the scale height is used instead of the free troposphere depth. However, both Rossby radii and the Rhines scale fail to explain the weaker seasonal cycle in southern hemisphere eddy length. In agreement with Kidson et al., the GCMs robustly project an increase in eddy length as the climate warms. The Rossby radii and Rhines scale are also generally projected to increase. Although it is not possible to state with confidence what process ultimately controls atmospheric eddy lengths, taken as a whole the results of this study increase confidence in the projection of future increases in eddy length.
by Todd A. Mooring.
S.B.

This work studies the geographical and the scale variability of the summer diurnal cycle of precipitation over Continental United States through the analysis of 12 years of radar mosaics during the period 1996-2007. Wavelet analysis was performed to understand the importance of different spatial scales ranging from 8 km to 512 km. The objective of this thesis is to study the dependence of the variability of the rainfall field on location and time of day and how spatial scale explains this variability.The results of this analysis show that the maximum activity for the different spatial scales occurs at different times of day: the larger the scale, the later their maximum influence occurs. The initiation of precipitation is scale-dependent: the onset is associated with the small scales, which later on they grow to form larger rainfall patterns. Over the Great Plains the average precipitation field presents two temporal maxima: one occurring during the afternoon and related to small scales (associated with the radiative forcing) and another one occurring during night hours related to larger scales (organized rainfall systems propagating from the west). In the eastern U.S., the precipitation pattern is also scale-dependent, it presents a similar behavior to that over the eastern lee of the Rocky Mountains. In particular, the maximum rainfall rates occur in the afternoon in association with small-scale processes. Monthly precipitation, and its scale decomposition, is also analyzed to present the differences in the diurnal cycle of precipitation for each summer month.
Cette étude porte sur la variabilité géographique et d'échelle du cycle diurne des précipitations estivales, au-dessus des États-Unis continentaux, par l'analyse de mosaïques radars de 12 ans pour la période de 1996-2007. Les transformées en ondelettes sont utilisées pour saisir l'importance des différentes échelles spatiales de 8 à 512 km dans le cycle diurne. L'objectif de cette mémoire est d'étudier la dépendance de la variabilité du champ de précipitation sur l'endroit géographique et la période du jour, et comment cette variabilité est expliquée par les différentes échelles.Les résultats de cette analyse montre que l'activité maximale de précipitation survient à un temps différent pour différentes échelles spatiales : plus large est l'échelle, plus tard se produit l'importance maximal de l'échelle. L'initiation de précipitation dépend de l'échelle : le début est associé aux petites échelles, qui s'organisent plus tard pour former des chutes de pluie à plus grande échelle. Sur les Grandes Prairies, la moyenne du champ de précipitation montre deux maximums : un en après-midi et relié aux petites échelles (associé au forçage radiative); un second durant la nuit, relié aux grandes échelles (systèmes de chute de pluie se propageant de l'ouest). Au dessus de la région Est, la distribution de précipitation est également dépendante de l'échelle; elle montre un comportement similaire au-dessus du côté Est des Rocheuses. En particulier, un taux maximal de chute de pluie advient en après-midi en association avec des processus de petites échelles. La précipitation mensuelle, et sa décomposition par échelles, est aussi analysée afin d'identifier les différences dans le cycle diurne de précipitation de chaque mois estival.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2006.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (p. 213-230).
This dissertation describes the most comprehensive study to date of the diurnal cycle of air pollution in the Kathmandu Valley, Nepal -- a bowl-shaped mountain valley of two million people with a growing air pollution problem but little past research. Field measurements and computer simulations were used to study the interplay of emissions and ventilation. From September 2004 through June 2005, CO (carbon monoxide), ozone, PM10 (particles smaller than 10 micrometers), wind speed and direction, solar radiation, temperature, and humidity were continuously measured east of Kathmandu. Sensors towers and mountains measured the diurnal cycle of the vertical temperature structure and stability. A sodar measured the mixed layer height and upper-level winds. Bag sampling provided the diurnal cycle of CO on mountains, passes and around the valley. Winds were measured on a mountain pass and ozone on a mountaintop. Patterns of air pollution and meteorology in the valley showed remarkable day-to-day similarity, with daily twin peaks of CO and PM10, a noon ozone maximum, afternoon westerly winds, and a stagnant cold pool at night. On mountaintops at night, ozone remained high, while CO dropped to regional background levels.
(cont.) The meso-scale meteorological model MM5 was adapted to the Kathmandu Valley for days in February and May 2005. It was able to capture the essential features of the valley's meteorology and was used to address three specific questions: The break-up of the valley's temperature inversion was found to be dominated in February by up-slope winds on the valley rim, plus subsidence over the valley center; in May surface heating of the valley bottom also played a major role. The pathways of pollutant transport out of the valley were found to be up the valley rim slopes in the morning, but out the eastern and southern passes in the afternoons. At night pollutants remained within the valley except near the river outlet. They were lifted off the ground at night and re-circulated in the morning. The eulerian chemistry transport model CAMx, was used in tracer mode, with MM5 meteorology to simulate the emission, transport and removal of CO from the Kathmandu Valley. The simulations were limited by the accuracy of Kathmandu's emissions inventory, especially the spatial distribution of emissions.
by Arnico K. Panday.
Sc.D.

Context. The Martian methane and water cycle are subject of ongoing research through simulation. Exchange with the subsurface has a potentially strong impact, but is often neglected. Aims. For methane, I determine if adsorption with an increased enthalpy can explain the observed seasonal variations and conflicting observations by the Trace Gas Orbiter and the Curiosity rover. For water, the impact of adsorption and ice formation in the subsurface on the global cycle is studied. A new way of initializing the soil, by running a decoupled subsurface model, is tested. Depths of stable subsurface ice and subsurface water distributions are studied. Methods. A General Circulation Model (GCM) is used with a purely diffusive subsurface model. For methane, different initial states, source scenarios, and decay times are tested. For water, a model without an active atmosphere is implemented to provide an initial state. The effect of the subsurface with this initial state on the full atmospheric water cycle is tested. Results. For methane, a strong influence on the global methane cycle is observed. Seasonal variations measured at Gale Crater are reproduced, but the conflicting observations cannot be explained by adsorption. For water, the new initialization can be used without completely disrupting the water cycle. It leads to a generally wetter atmosphere, in conflict with observations. Found ice table depths do not match well with observations, but ice profiles reproduce previous findings. Conclusion. Methane adsorption is able to partly explain observed variations, but cannot be the only process to influence methane abundances. The new initialization method for water works well in principle, but a more refined model is needed for more realistic results.

Reanalysis products are widely used to study the land-atmosphere exchanges of energy, water, and carbon fluxes, and have been evaluated using in situ data above or below ground. Here measurements for several years at five flux tower sites in the U.S. (with a total of 315,576 hours of data) are used for the coupled evaluation of both below- and above-ground processes from three global reanalysis products and six global land data assimilation products. All products show systematic errors in precipitation, snow depth, and the timing of the melting and onset of snow. Despite the biases in soil moisture, all products show significant correlations with observed daily soil moisture for the periods with unfrozen soil. While errors in 2 meter air temperature are highly correlated with errors in skin temperature for all sites, the correlations between skin and soil temperature errors are weaker, particularly over the sites with seasonal snow. While net shortwave and longwave radiation flux errors have opposite signs across all products, the net radiation and ground heat flux errors are usually smaller in magnitude than turbulent flux errors. On the other hand, the all-product averages usually agree well with the observations on the evaporative fraction, defined as the ratio of latent heat over the sum of latent and sensible heat fluxes. This study identifies the strengths and weaknesses of these widely-used products, and helps understand the connection of their errors in above- versus below-ground quantities.

The last glacial cycle (c. 115-12 kyr BP) was the most recent in a series of recurring glaciations of the subpolar continents. Massive ice sheets evolved in Eurasia and North America, which, at their maximum, were of continental scale and together lowered the global sea-level by approximately 100 m. The paleo-modelling community has focused on the last glacial maximum (LGM, ~ 20 kyr BP), leaving the longer period when the ice sheets evolved to their LGM configurations largely unexplored. In this thesis we study the mutual interaction between the time-mean atmospheric circulation and the evolution of the Northern Hemisphere ice sheets over the build-up phase of the last glacial cycle. Experiments are conducted with coupled atmosphere-ice-sheet models and a circulation model forced by geologically consistent reconstructions of the ice-sheet topography at key stages of the glacial cycle. The main findings from these studies are that the ice evolution in North America may have been controlled by circulation anomalies induced by the background topography in conjunction with the ice sheets themselves. A geologically consistent pre-LGM ice sheet could only be obtained when including the North American Cordillera. However, the ice sheets' influence on the local climate conditions is also found to be paramount for this configuration. We further suggest that the incipient ice sheets may have had a limited influence on the large-scale winter circulation as a result of their location relative the westerly mean flow. The LGM Laurentide Ice Sheet (LIS) was, however, different because of its continent-wide extent, and it may therefore have had a large influence on the planetary-scale circulation, especially in the Atlantic sector. We find that the planetary waves forced by the LIS were considerably larger than at earlier times, and, as a result of a more frequent planetary wave reflection over the Atlantic Ocean basin, an altered stationary wave field and a zonalised winter jet.

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 4: Manuscript.

Most undisturbed peatlands sequester carbon, and rising levels of atmospheric nitrogen deposition may have the potential to destabilize this function, possibly resulting in an increased release of carbon dioxide into the atmosphere.  It is therefore of vital importance to investigate further the link between atmospheric nitrogen deposition and carbon dynamics in exposed ecosystems such as peatlands. The work described in this thesis aimed to elucidate the impact of increasing nitrogen on aspects of carbon turnover in peatlands.  Using a long-term field-based experiment, I tested the effects of 4 years of ammonium and nitrate addition (8, 24 and 56 kg N ha^-1 y^-1) on the fate of newly photosynthesised carbon by plants and the turnover of labile and recalcitrant carbon.  A second set of experiments undertaken in the laboratory assessed the use of plant wax analysis as potential biomarkers of past changes in vegetation and carbon status in peat. Overall, this work has shown that the form of nitrogen (ammonium versus nitrate) is a crucial component of atmospheric pollution and must be taken into consideration when investigating or predicting effects of reactive nitrogen on peatlands.  In addition, nitrogen addition affected the fate of newly synthesised carbon differently in Eriophorum vaginatum and Calluna vulgaris, revealing the importance of considering plant traits when investigating environmental changes in terrestrial ecosystems.  Furthermore, it has led to the development of an investigative tool for further exploration of the historical effects of atmospheric nitrogen deposition on vegetation an carbon content in peatlands.

In this thesis, an indirect effect of sulfuric acid particles on Arctic climate during the cold season, from November to May, is investigated. Specifically, the research is focused on the alteration of the air mass dehydration rate due to sulfuric acid aerosols. These anthropogenic aerosols differ from natural aerosols by their poor ability to nucleate ice, their strong solubility and their reduced homogeneous freezing temperature when internally mixed with other compounds.
Simulations performed with three column models and analysis of observed data at Alert (1991--94) are used to investigate an indirect effect of these aerosols on climate: the dehydration - greenhouse feedback. Each model covers different levels of physical basis and realism of their simulation. Two scenarios have been compared in the simulations: an acid aerosols scenario and a natural aerosols scenario.
Results show that aerosol acidification leads to a depletion of the ice crystal number concentration and an increase of their mean size. As a result, clear sky precipitation (CSP) occurs more frequently than ice fogs during Arctic haze episodes. This result is in agreement with observations that indicate an increase by more than 50% of the weekly mean CSP frequency, when the proportion of sulfuric acid is greater than the mean observed value of 20%. Consequently, the sedimentation flux of ice crystals and the dehydration rate of the lower troposphere are accelerated. The radiative effect is a weaker atmospheric emissivity in the boundary layer, up to the height corresponding approximately to the top of the ice crystal layer. As a result, the infrared flux reaching the surface and the greenhouse effect are decreased. Simulations performed for the period 1991 to 1994 at Alert show a negative radiative forcing of about --3 W/m2 at the surface between November and May. The net result is a strengthening of the surface-based temperature inversion of 1.3°C, with a surface cooling of 0.4°C and a warming of 0.9°C at 800 hpa.
The indirect effect of the dehydration - greenhouse feedback due to anthropogenic acid aerosols can explain in part the observed strengthening of the surface-based temperature inversion in the Arctic (Kahl et al., 1993).

A numerical model of the global carbon cycle is presented which includes the effects of anthropogenic &CO\sb2& emissions &(CO\sb2& produced from fossil fuel combustion, biomass burning, and deforestation) on the global carbon cycle. The model is validated against measured atmospheric &CO\sb2& concentrations. Future levels of atmospheric &CO\sb2& are then predicted for the following scenarios: (1) Business as Usual (BaU) for the period 1990-2000; (2) Same as (1), but with no biomass burning; (3) Same as (1), but with no fossil fuel combustion; (4) Same as (1), but with a doubled atmospheric &CO\sb2& concentration and a 2 K warmer surface temperature associated with the doubled atmospheric &CO\sb2& concentration. The global model presented here consists of four different modules which are fully coupled with respect to &CO\sb2.& These modules represent carbon cycling by the terrestrial biosphere and the ocean, anthropogenic &CO\sb2& emissions, and atmospheric transport of &CO\sb2.&. The prognostic variable of interest is the atmospheric &CO\sb2& concentration field. The &CO\sb2& concentration field depends on both the sources and sinks of &CO\sb2& as well as the atmospheric circulation. In addition, the sources and sinks vary significantly as a function of both time and geographic location. The model output agrees well with measured data at the equatorial and mid latitudes, but this agreement weakens at higher latitudes. This is due to the less adequate representation of the terrestrial ecosystem models at these latitudes. In the first scenario, the predicted concentration of atmospheric &CO\sb2& is 362 parts per million by volume (ppmv) at the end of the 10 year model run. This establishes a baseline for the next three scenarios, which predict that biomass burning will contribute 3 ppmv of &CO\sb2& to the atmosphere by the year 2000, while fossil fuel combustion will contribute 5 ppmv. The net effect of a 2 K average global warming was to increase the atmospheric &CO\sb2& concentration by approximately 1 ppmv, due to enhanced respiration by the terrestrial biosphere.

This research has investigated the ability to model precipitation over East Africa using the RegCM regional climate model, and the ability of a stochastic model to predict Lake Victoria lake level one season in advance. The stochastic model was built using precipitation, sea surface temperatures and temperature, and provides detail about the steps used to develop the model. Precipitation modeling was carried out using RegCM and several convective schemes were compared to determine which performed best for East Africa. Additionally the microphysical scheme SUBEX was investigated thoroughly and several tuning parameter changes were made. Finally the precipitation from RegCM was split into 9 rainfall classifications which were then studied to determine how the regional climate model performed for representing rainfall events in the model, in terms of duration, intensity, and overall structure between all the event types.

This thesis investigates some important aspects of the atmospheric branch of the hydrological cycle in the modern day climate from an observational perspective. Data quality is evaluated, focusing on two state-of-the-art reanalysis products, ERA-I and JRA-55. Regional-scale discrepancies among reanalyses and observations, especially in their annual cycles, are found in the warm pool, Amazon, Gulf stream and Indian subcontinent regions. In the tropics, oceanic evaporation and its temporal variability are notably greater in JRA-55 than in ERA-I and satellite-based estimates, while both reanalyses overestimate precipitation. Higher tropical precipitation and evaporation, accompanied by a slightly lower level of total column water (TCW), might suggest a more intense hydrological cycle, but this can be an ill-defined concept especially when analysis increments mask “spin-down” errors in reanalysis models. Analysis increments arise to remove unphysical residuals in the atmospheric water budget, and these are explored via a cluster analysis to identify regimes with common behavior. Consistent for ERA-I and JRA-55, the regime with the largest negative residuals (greater moisture outputs than inputs) exceeding 50% of mean precipitation occurs during the dry season of some low latitude regions that feature strong seasonality, high evapotranspiration and high moisture divergence. Errors in the moisture divergence are likely responsible because they correlate strongly with the budget residual. Empirical Orthogonal Function (EOF) and Self Organizing Map (SOM) analyses are applied to identify the dominant inter-annual patterns of vertically-integrated moisture divergence variability. They reveal that the transition from strong La Niña through to extreme El Niño events is not a linear one and that the EOF orthogonality constraint results in the patterns being split between leading EOFs that are non-linearly related. The SOM analysis captures the range of responses to the El Niño Southern Oscillation (ENSO), indicating that the distinction between the moderate and extreme El Niños can be as great as the difference between La Niña and moderate El Niños, from a moisture divergence point of view. On diurnal time scales, horizontal moisture fluxes vary in response to thermodynamic and dynamic effects. TCW shows a global scale diurnal cycle that peaks around 1800 - 2100 local time with a peak-to-trough magnitude of 0.4mm. Semi-diurnal variations in surface winds and pressure, consistent with atmospheric tidal theory, create a westward propagating moisture convergence/divergence wave along the equator. Finally, the importance of Tropical Cyclones (TCs) as a source of freshwater for the North American continent is estimated using an ensemble of schemes designed to attribute onshore moisture fluxes to TCs. Averaged over the 2004–2012 hurricane seasons and integrated over the western, southern and eastern coasts of North America, the seven schemes attribute 7 to 18% (mean 14 %) of total net onshore flux to Atlantic TCs. A reduced contribution of 10% (range 9 to 11 %) was found for the 1980–2003 period, though only two schemes could be applied to this earlier period. Over the whole 1980–2012 period, a further 8% (range 6 to 9% from two schemes) was attributed to East Pacific TCs, resulting in a total TC contribution of 19% (range 17 to 22 %) to the ocean-to-land moisture transport onto the North American continent between May and November. The inter-annual variability does not appear to be strongly related to ENSO.

A land-surface module to couple a meteorological and a hydrologic model is described. lt was implemented and tested in the Leipzig\'s version of GESIMA. Preliminary results of a coupling with NASMO are presented, although this article mainly focuses on the description of the module and its effect on the atmospheric water cycle. One positive impact of the module is that it allows to produce subgrid-scale evapotranspiration in more details and to heterogenize precipitation. This strongly affects soil wetness, cloudiness and the thermal regime of the atmospheric boundary layer
Ein Bodenmodul zur Kopplung eines meteorologischen mit einem hydrologischen Modell wird vorgestellt. Er wurde implementiert und getestet in der Leipziger Version von GESIMA. Obgleich der Schwerpunkt des Artikels auf der Beschreibung des Moduls und seiner Auswirkung auf den atmosphärischen Wasserkreislauf liegt, werden auch vorläufige Ergebnisse einer Kopplung mit NASMO präsentiert. Ein positiver Effekt des Moduls ist, daß er ermöglicht, detaillierter die subskalige Evapotranspiration zu beschreiben und den Niederschlag zu heterogenisieren. Dies wirkt sich stark auf die Bodenfeuchte, die Bewölkung und das thermische Regime der atmosphärischen Grenzschicht aus

Atmospheric oxygen (O2) measurements represent an important tool for investigating carbon cycle processes that determine the magnitude of the fluxes of carbon dioxide (CO2) to and from the atmosphere. By combining atmospheric O2 and CO2 measurements, one can derive the tracer Atmospheric Potential Oxygen, (APO = O2 +1.1CO2) which is a conservative tracer with respect to terrestrial O2 and CO2 exchange processes and therefore primarily represents ocean exchange processes. The primary aim of this research was to assess the spatial and temporal variability of atmospheric O2, CO2 and APO at two contrasting locations: The Halley Research Station, Antarctica and the Weybourne Atmospheric Observatory, U.K. The measurements collected at Halley were made possible by establishing a high precision, continuous, in situ, atmospheric O2 and CO2 measurement system at the station, which I built, tested and installed as part of this research. The aim of the new measurement system was to fill in the observational O2 gap in the South Atlantic sector of the Southern Ocean; a key region with respect to the global oceanic sink for anthropogenic CO2 emissions. At the Weybourne Atmospheric Observatory, I have extended and re-evaluated an existing atmospheric O2 and CO2 measurement record (2008-2015). The inter-annual variability of the seasonal cycles and growth rates of atmospheric O2, CO2 and APO were examined to assess the temporal variability of the carbon cycle processes that control them. The data were also compared to other O2 monitoring stations in the northern hemisphere to understand the spatial variability of the processes. Throughout this thesis, I have used a range of analysis techniques, including model-observation comparisons, to assess what drives the variability of atmospheric O2, CO2 and APO observed at these two locations.

The University of Victoria Earth System Climate Model (UVic ESCM) v. 2.9 is used in this thesis to investigate two important topics in paleoclimate research: the glacial-to-interglacial rise in CO2 and the Holocene carbon cycle. The UVic ESCM belongs to a class of models known as Earth system Models of Intermediate Complexity (EMICs) (Claussen et al. 2002) and provides a simplified yet comprehensive representation of the climate system and carbon cycle dynamics, including a three-dimensional ocean model, a dynamic-thermodynamic sea ice model, a dynamic global vegetation model, ocean sediments, and a fully-interactive inorganic and organic carbon cycle in the ocean. First, a suite of transient simulations were conducted to cover the period from the Last Glacial Maximum (LGM) to the present (2000 A.D). Simulations including only prescribed orbital forcing and continental ice sheet changes failed to produce an increase in atmospheric CO2 for the simulation period, although they demonstrated significant long-term sensitivity (10-15 ppm) to small (1.9 Tmol yr-1) variations in the weathering rate. Modelling experiments incorporating the full CO2 radiative forcing effect since the Last Glacial Maximum, however, resulted in much higher CO2 concentrations (a 20 ppm increase over those without CO2 radiative forcing) due to a greater ventilation of deep-ocean DIC and decreased oceanic CO2 uptake, related in part to a larger decrease in southern hemisphere sea ice extent. The more thorough ventilation of the deep ocean in simulations with CO2 radiative forcing also caused a larger net alkalinity decrease during the late deglacial and interglacial, allowing atmospheric CO2 to increase by an additional 10 ppm in the simulations presented here. The inclusion of a high latitude terrestrial carbon reservoir provided a net release of carbon to the atmosphere, mostly during the early deglacial, increasing atmospheric CO2 levels to 240-250 ppm. This terrestrial release also provided better agreement with observed changes in carbonate concentrations in the deep ocean since the LGM (Yu et al. 2010). The addition of freshwater fluxes from ice sheet melting in North America added emphasis on the importance of a lower weathering rate during the LGM and early deglacial and indicated that deep water in the North Pacific may become more positively buoyant during freshwater fluxes in the Atlantic due to greater diffusion of heat to the deep ocean by enhanced Pacific intermediate water formation.Second, our results for the Holocene carbon cycle indicate that atmospheric CO2 should decrease between 6000 B.C. and 2000 A.D. without some kind of external forcing not represented in the model. However, the amount of the decrease (8-15 ppm) varied for different ocean circulation states. Furthermore, our simulations demonstrated significant sensitivity to Antarctic marine ice shelves, and these results indicate that more extensive marine ice shelves during the Holocene (relative to previous interglacials) may increase atmospheric CO2 levels by ~5 ppm (from purely physical mechanisms) and as much as 10 ppm when different ocean circulation states or alkalinity changes are included. Adding various anthropogenic land use scenarios to the Holocene carbon cycle were unable to explain the CO2 trend, accounting for only a third of the ice core CO2 increase by 1 A.D. in our most extreme scenario. However, the results imply that external mechanisms leading to a decrease in alkalinity during the Holocene (such as declining weathering rates, more extensive marine ice shelves, terrestrial uptake, more calcifiers, coral reef expansion, etc.) may prevent the ocean from absorbing more of the anthropogenic terrestrial release, allowing the deforestation flux to balance a greater fraction of the Holocene peatland uptake (not modelled) and permitting CO2 to increase from oceanic processes that are normally overwhelmed by northern peatlands.
Cette thèse détaille l'application du modèle du système climatique terrestre de l'Université de Victoria (version 2.9) dans le cadre de deux importants champs de recherche en modélisation paléoclimatique : l'augmentation du niveau de dioxyde de carbone (CO2) dans l'atmosphère durant la plus récente transition glaciaire-interglaciaire, ainsi que l'évolution du cycle du carbone durant l'Holocène. Le modèle utilisé dans cette étude est répertorié comme modèle de complexité intermédiaire (Claussen et al. 2002), offrant un traitement à la fois simplifié et exhaustif de la dynamique du système climatique terrestre et du cycle du carbone. Celui-ci comprend un modèle océanique tridimensionnel, un modèle de glace marine dynamique/thermodynamique, un modèle dynamique et global de la végétation, les sédiments océaniques ainsi qu'un traitement interactif du cycle du carbone organique et inorganique.Premièrement, une série de simulations transitoires sont effectuées afin de couvrir la période s'étendant du plus récent maximum glaciaire (LGM) jusqu'à aujourd'hui (2000 apr. J.-C.). Les simulations fondées uniquement sur une prescription des paramètres orbitaux et des calottes glaciaires ne reproduisent pas l'augmentation du CO2 dans l'atmosphère durant la période transitoire tel que mentionné ci-haut, mais exposent toutefois une certaine sensibilité (10-15 ppm) à de faibles (1.9 Tmol/an) variations dans le taux d'érosion. Dans le cas de simulations prenant en compte la gamme complète des effets radiatifs associés au CO2, par contre, la concentration du CO2 dans l'atmosphère s'avère beaucoup plus élevée (une augmentation de 20 ppm par rapport à celles sans effets radiatifs). Cette différence est causée par une plus importante ventilation de carbone inorganique dissous en eaux profondes ainsi qu'une diminution du taux d'absorption de CO2 par l'océan, qui s'explique en partie par une fonte accélérée de la glace marine dans l'hémisphère Sud. Le changement du régime de ventilation en profondeur a également pour effet de diminuer l'alcalinité marine à partir de la fin de la période de déglaciation, augmentant de 10ppm la concentration de CO2 dans l'atmosphère. La présence d'un réservoir de carbone terrestre an hautes latitudes fournit une source additionnelle de carbone, principalement durant les stages initiaux de la période de déglaciation, permettant ainsi aux niveaux de CO2 dans l'atmosphère d'atteindre les 240-250 ppm. En outre, ceci facilite la validation de nos résultats par rapport aux changements dans la concentration de carbonate observées depuis le dernier maximum glaciaire dans les profondeurs marines (Yu et al. 2010). Le faible taux d'érosion terrestre durant le maximum glaciaire et la période de déglaciation qui a suivi est d'autant plus significatif en raison d'un apport accru d'eau douce de fonte en provenance des calottes glaciaires Nord-Américaines. Deuxièmement, nos résultats quant au cycle du carbone durant l'Holocène pointent vers une certaine diminution du niveau de CO2 dans l'atmosphère se manifestant vers 6000 av. J.-C. et qui, en l'absence de forçage externe au modèle, devrait se maintenir jusqu'à aujourd'hui ; celle-ci semble toutefois varier (8-15 ppm) en fonction du mode de circulation océanique. De plus, la concentration atmosphérique de CO2 dans nos simulations démontre une importante sensibilité à l'étendue des barrières de glace en Antarctique, d'où notre conclusion qu'une présence accrue de glace marine durant l'Holocène (par rapport aux autres périodes interglaciaires) pourrait augmenter le niveau de CO2 atmosphérique de près de 5 ppm (effets physiques directs), et de pas moins de 10 ppm en considérant la gamme de modes de circulation océanique ainsi que les changements dans l'alcalinité marine.

Many studies have recently reported estimates of greenhouse gas (GHG) emissions and associated potential climate impacts of biofuel and natural gas (NG) use. U.S. corn ethanol production keeps increasing under federal mandates, and NG production soars due to successful tapping of unconventional resources in North America, particularly shale gas. Numerous life cycle assessment (LCA) studies document technology specific corn ethanol and NG GHG estimates. The estimates often include all life cycle stages from fuel supply to combustion, and point out potential for emissions reductions. Several studies suggest that using GHG emissions as an evaluation metric underestimates corn ethanol’s radiative forcing (RF) impact – a precursor and indicator for global temperature change – by 10-90% over the next few decades. This emissions timing effect may overestimate (i) ethanol’s climate benefits over gasoline and (ii) the effectiveness of U.S. policies mandating and subsidizing ethanol. This work revisits the above studies, and builds upon existing models to quantify RF impacts across the corn ethanol life cycle. The emissions timing factor (ETF) is significantly smaller than previous estimates (2-13% depending on the chosen impact time frame), and the effect is dwarfed by uncertainty in the GHG emissions estimates. Nevertheless, ETF reduces ethanol’s probability of meeting the federal target of 20% GHG reduction relative to gasoline from 53% (according to EPA GHG estimates) to 7-29%. However, the small potential climate impacts from U.S. ethanol use may not actually be observable based on estimated initial increases in global average surface temperature of < 0.001 °C. About 25% of global primary energy production comes from NG, whose life cycle GHG emissions and potential future climate impacts from substituting coal are highly uncertain due to fugitive methane (CH4) emissions from the NG industry. Accurately quantifying the NG fugitive emissions (FE) rate – the percentage of produced NG, mainly CH4 and ethane (C2H6) – released to the atmosphere is challenging due to the size and complexity of the NG industry. Recent LCA estimates suggest that the current NG FE rate could be as high as 8% and 6%, from shale and conventional NG, respectively, and other bottom-up studies indicate even higher rates several decades ago. This work analyzes possible ranges of the global average NG FE rate based on atmospheric CH4, C2H6, and carbon isotope (δ13C-CH4) measurements recorded since 1984, and top-down modeling of their sources and sinks. Box-model, δ13C-CH4mass balance, and 3D-modeling results agree on best estimate NG FE rates of 3-5% (of dry NG production and dry NG composition) globally over the past decade, and 5-8% around 1990. Upper bound FE rates are 5% and 7% in 2010 and 2000, respectively. Best estimate and upper bound values may be overestimated because both assume lower bound emissions from oil and coal production as well as complete absence of natural hydrocarbon seepage. While LCA studies are useful for identifying processes with the greatest NG FE reduction potential, the recent high bottom-up estimates do not appear representative of the U.S. national average based on top-down modeling results. Given the steadily declining NG FE rates one may expect that further emissions abatement is possible if industry practices are further improved.

Observations of atmospheric gas concentrations are very useful in the study of globally important ecosystems. Past observational efforts, however, have been focused on atmospheric measurements of ‘background air’, leaving the continental interiors under-represented. I present results from pilot, multi-species, atmospheric measurement campaigns in the Hainich Forest, Germany in 2005, and I describe the development, deployment, and results from high-precision continuous atmospheric measurements of CO2, O2, CH4, CO and N2O at the Zotino Tall Tower Observatory (ZOTTO) in the boreal forest of central Siberia from November 2005 to June 2007. Atmospheric variations were studied on seasonal, synoptic and diurnal time scales. Among the interesting features of the ZOTTO record are: 1) CO2 and O2 seasonal amplitudes of 26.6 ppm and 190 per meg (equivalent to 39.8 ppm in CO2); 2) a west-east gradient of –7 ppm of CO2 (in July 2006) between Shetland Islands (Scotland) and ZOTTO that reflects summertime continental CO2 uptake; 3) attenuation of the oceanic component of the O2 seasonal amplitude (Atmospheric Potential Oxygen; APO) at ZOTTO resulting in an amplitude of 45 per meg compared to 56 per meg observed at Shetlands; 4) high fire emissions of CH4 and CO in summertime with the minima of their monthly averages similar to seasonal cycles of these gases in the marine boundary layer; 5) large vertical gradients in CO2, CH4 and CO during ‘cold events’ (air temperatures below -30°C), suggesting separated layers of air and local sources possibly combined with other effects; 6) lower CO/CO2 ratios (1-4 ppb/ppm) from fossil fuel burning compared to those measured in Europe, with large CH4 contributions; 7) diurnal vertical CO2 gradients in spring 2007 giving estimates of night-time respiration fluxes of 0.04±0.02 mol C m-2 d-1. A comparison with REgional MOdel (REMO) simulations showed discrepancies in daily averages of CO2 attributed to errors in the model’s vertical mixing and prescribed terrestrial fluxes. Nevertheless, REMO exhibited good agreement in meteorological variables (compared to weather stations close to ZOTTO) and seasonal cycles of CO2, APO and CO. Studies of fire events showed high emission ratios of CO/CO2 and CH4/CO2, comparable with those reported for similar ecosystems, and in relatively good agreement with the model estimates.

There is much confidence in the global temperature change and its attribution to human activities. Global climate models have attained unprecedented complexity in representing the climate system and its response to external forcings. However, climate prediction remains a serious challenge and carries large uncertainty, particularly when the scale of interest becomes small. With the increasing interest in regional impact studies for decision-making, one of the urgent tasks is to make a systematic, quantitative evaluation of the expected skill of climate models over a range of spatiotemporal scales. The first part of this dissertation was devoted to this task, with focus on the predictive skill in the linear trend of surface air temperature. By evaluating the hindcasts for the last 120 year period in the form of deterministic and probabilistic predictions, it was found that the hindcasts can reproduce broad-scale changes in the surface air temperature, showing reliable skill at spatial scales larger than or equal to a few thousand kilometers (30° x 30°) and at temporal scales of 30 years or longer. However, their skill remains limited at smaller spatiotemporal scales, where we saw no significant improvement over climatology or a random guess. Over longer temporal scales, the feedbacks from the carbon cycle to atmospheric CO₂ concentration become important. Therefore the rest of the dissertation attempts to find key processes in the climate-carbon cycle feedback using one of the leading land-climate models, the National Center for Atmospheric Research Community Land Model. Evaluation of site-level simulations using field observations from the Amazon forest revealed that the current formulation for drought-related mortality, which lacks the ecophysiological link between short- and long-term drought stress, prevent the model from simulating realistic forest response. Global simulations showed that such dynamics of vegetation strongly influences the control of the nitrogen cycle on vegetation productivity, which then alters the sensitivity of the terrestrial biosphere to surface air temperature. This implies that if the state of the terrestrial biosphere is inconsistent with the simulated climate, either biased or prescribed, then its feedback to anthropogenic forcing could be also inconsistent.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998.
Includes bibliographical references (leaves 90-95).
Key uncertainties in the global carbon cycle are reviewed and a simple model for the oceanic carbon sink is developed and described. This model for the solubility sink of excess atmospheric CO2 has many enhancements over the more simple 0-D and 1-D box-diffusion models upon which it is based, including latitudinal extension of mixed-layer inorganic carbon chemistry, climate-dependent air-sea exchange rates, and mixing of dissolved inorganic carbon into the deep ocean that is parameterized by 2-D eddy diffusion. By calibrating the key parameters of this ocean carbon sink model to various "best guess" reference values, it produces an average oceanic carbon sink during the 1980s of 1.7 Pg yr-1, consistent with the range estimated by the IPCC of 2.0 Pg yr~1 ± 0.8 Pg (1992; 1994; 1995). The range cited in the IPCC study and widely reported elsewhere is principally the product of the structural uncertainty implied by an amalgamation of the results of several ocean carbon sink models of varying degrees of complexity. This range does not take into account the parametric uncertainty in these models and does not address how this uncertainty will impact on future atmospheric CO 2 concentrations. A sensitivity analysis of the parameter values used as inputs to the 2-D ocean carbon sink model developed for this study, however, shows that the oceanic carbon sink range of 1.2-2.8 Pg/yr for the 1980s is consistent with a broad range of parameter values. By applying the Probabilistic Collocation Method (Tatang, et al. 1997) to this simple ocean carbon sink model, the uncertainty of the magnitude of the oceanic sink for carbon and hence atmospheric CO2 concentrations is quantitatively examined. This uncertainty is found to be larger than that implied by the structural differences examined in the IPCC study alone with an average 1980s oceanic carbon sink estimated at 1.8 ± 1.3 Pg/yr (with 95% Confidence). It is observed that the range of parameter values needed to balance the contemporary carbon cycle yield correspondingly large differences in future atmospheric CO2 concentrations when driven by a prescribed anthropogenic CO2 emissions scenario over the next century. For anthropogenic CO 2 emissions equivalent to the IS92a scenario of the IPCC (1992), the uncertainty is found to be 705 ppm ± 47 ppm (one standard deviation) in 2100. This range is solely due to uncertainty in the "solubility pump" sink mechanism in the ocean and is only one of the many large uncertainties left to explore in the global carbon cycle. Such uncertainties have implications for the predictability of atmospheric CO2 levels, a necessity for gauging the impact of different rates of anthropogenic CO2 emissions on climate for policy-making purposes. Since atmospheric CO 2 levels are one of the primary drivers of changes in radiative forcing this result impacts on the uncertainty in the degree of climate change that might be expected in the next century.
by Gary Louis Holian.
S.M.

The land surface skin temperature diurnal cycle (LSTD) is very important for the understanding of surface climate and for evaluating climate models. This variable, however, cannot be obtained globally from polar-orbiting satellites because the satellites usually pass a given area twice per day and because their infrared channels cannot observe the surface when the sky is cloudy. In order to more optimally use the satellite data, this research is designed, for the first time, to solve the above two problems by advance use of remote sensing techniques and climate modeling. Specifically, this work is divided into two parts. Part one deals with obtaining the skin temperature diurnal cycle for cloud-free cases. We have developed a "cloud-free algorithm" to combine model results with satellite and surface-based observations, thus interpolating satellite twice-daily observations to the diurnal cycle. Part two studies the cloudy cases. The "cloudy-pixel treatment" presented here is a hybrid technique of "neighboring-pixel" and "surface air temperature" approaches. The whole algorithm has been tested against field experiments and climate model CCM3/BATS in global and single column mode simulations. It shows that this proposed algorithm can obtain skin temperature diurnal cycles with an accuracy of 1-2 K at the monthly pixel level.

Le Crétacé a été une période d'instabilité climatique et du cycle du carbone, dont le CO2 atmosphérique a été désigné comme le driver majeur. Cependant, les reconstitutions du CO2 atmosphérique ne reflètent ni les dynamiques climatiques ni les grands évènements de perturbation du cycle du carbone décrits pour cette période. J'ai utilisé la composition isotopique de carbone de la plante fossile Frenelopsis (d13Cleaf) comme un nouvel proxy pour reconstituer le CO2 atmosphérique du Crétacé en termes de composition isotopique de carbone (d13CCO2) et de concentration (pCO2). La première courbe de d13CCO2 pour toute la durée du Crétacé a été construite à partir du d13C des carbonates marins. Sa comparaison avec des estimations de d13CCO2 à partir du d13Cleaf a révélé que les modèles développés jusqu'à maintenant ont une tendance à exagérer les valeurs de d13CCO2. Des estimations du fractionnement isotopique du carbone issu par des plantes (13Cleaf) obtenues à partir des nouvelles données d e d13Cleaf et d13CCO2 ont permis de reconstituer l'évolution à grande échelle de la pCO2. Ces résultats indiquent que le CO2 a probablement été une conséquence à long terme du changement climatique durant le Crétacé. Des cycles de d13CCO2 de ~1.2, ~2.1, ~5.4 et ~10.2 Ma ont été détectés, synchrones à ceux du niveau de la mer et à la cyclicité des paramètres de l'orbite terrestre décrits pour le Mésozoïque. Mes résultats fournissent une nouvelle perspective du système climatique et du cycle du carbone du Crétacé, dominés principalement par les paramètres orbitaux de la Terre et secondairement par des évènements catastrophiques de libération de CO2 d'origine volcanique dans l'atmosphère
The Cretaceous was a period characterized by strongly marked climate change and major carbon cycle instability. Atmospheric CO2 has repeatedly been pointed out as a major agent involved in these changing conditions during the period. However, long-term trends in CO2 described for the Cretaceous are not consistent with those of temperature and the large disturbance events of the carbon cycle described for the period. This raises a double question of whether descriptions of the long-term evolution of atmospheric CO2 made so far are accurate or, if so, atmospheric CO2 was actually a major driver of carbon cycle and climate dynamics as usually stated. In this thesis the close relationship between the carbon isotope composition of plants and atmospheric CO2 is used to address this question. Based on its ecological significance, distribution, morphological features and its excellent preservation, the fossil conifer genus Frenelopsis is proposed as a new plant proxy for climate reconstructions during the Cretaceous. The capacity of carbon isotope compositions of Frenelopsis leaves (d13Cleaf) to reconstruct past atmospheric CO2, with regards to both carbon isotope composition (d13CCO2) and concentration (pCO2), is tested based on materials coming from twelve Cretaceous episodes. To provide a framework to test the capacity of d13Cleaf to reconstruct d13CCO2 and allowing for climate estimates from carbon isotope discrimination by plants (?13Cleaf), a new d13CCO2 curve for the Cretaceous based on carbon isotope compositions of marine carbonates has been constructed. Comparison with d13Cleaf-based d13CCO2 estimates reveals that although d13CCO2 and d13Cleaf values follow consistent trends, models developed so far to estimate d13CCO2 from d13Cleaf tend to exaggerate d13CCO2 trends because of assuming a linear relationship between both values. However, given the hyperbolic relationship between ?13Cleaf and pCO2, by considering an independently-estimated correction factor for pCO2 for a given episode, d13Cleaf values may be a valuable proxy for d13CCO2 reconstructions. ?13Cleaf estimates obtained from d13CCO2 and d13Cleaf values were used to reconstruct the long-term evolution of pCO2. The magnitude of estimated pCO2 values is in accordance with that of the most recent and relevant model- and proxy-based pCO2 reconstructions. However, these new results evidence long-term drawdowns of pCO2 for Cretaceous time intervals in which temperature maxima have been described

Eyewall replacement cycle (ERC) is frequently observed during the evolution of intensifying Tropical Cyclones (TCs). Although intensely studied in recent years, the underlying mechanisms of ERC are still poorly understood, and the forecast of ERC remains a great challenge. To advance our understanding of ERC and provide insights in improvement of numerical forecast of ERC, a series of numerical simulations is performed to investigate ERCs in TC-like vortices on a f-plane. The simulated ERCs possess key features similar to those observed in real TCs including the formation of a secondary tangential wind maximum associated with the outer eyewall. The Sawyer-Eliassen equation and tangential momentum budget analyses are performed to diagnose the mechanisms underlying the secondary eyewall formation (SEF) and ERC. Our diagnoses reveal crucial roles of outer rainband heating in governing the formation and development of the secondary tangential wind maximum and demonstrate that the outer rainband convection must reach a critical strength relative to the eyewall before SEF and the subsequent ERC can occur. A positive feedback among low-level convection, acceleration of tangential winds in the boundary layer, and surface evaporation that leads to the development of ERC and a mechanism for the demise of inner eyewall that involves interaction between the transverse circulations induced by eyewall and outer rainband convection are proposed. The tangential momentum budget indicates that the net tendency of tangential wind is a small residual resultant from a large cancellation between tendencies induced by the resolved and sub-grid scale (SGS) processes. The large SGS contribution to the tangential wind budget explains different characteristics of ERC shown in previous numerical studies and poses a great challenge for a timely correct forecast of ERC. The sensitivity experiments show that ERCs are strongly subjected to model physics, vortex radial structure and background wind. The impact of model physics on ERC can be well understood with the interaction among eyewall/outer rainband heating, radilal inflow in the boundary layer, surface layer turbulent processes, and shallow convection in the moat. However, further investigations are needed to fully understand the exhibited sensitivities of ERC to vortex radial structure and background wind.

Thesis: Ph. D., Joint Program in Chemical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Particulate organic carbon (POC) in the ocean and mobilized by rivers on land transfers -0. 1% of global primary productivity to the deep ocean sediments. This small fraction regulates the long-term carbon cycle by removing carbon dioxide from the atmosphere for centuries to millennia. This thesis investigates mechanisms of POC transfer to the deep ocean by analyzing particles collected in transit through two globally significant carbon reservoirs: the Southern Ocean and the Amazon River Basin. These endeavors test the hypothesis that organic matter composition controls the recycling and transfer efficiency of POC to the deep ocean, and illustrate new applications for ramped pyrolysis/oxidation (RPO), a growing method of POC characterization by thermal stability. By coupling RPO to stable and radiocarbon isotope analyses of riverine POC, I quantify three thermally distinct soil organic carbon pools mobilized by the Amazon River, and evaluate the degradability and fate of these different pools during transport to the coastal Atlantic Ocean. More directly, RPO analyses of marine samples suggest that POC transfer in the water column is in fact selective. Observations of consistent biomolecular changes that accompany transport of phytoplankton-derived organic matter to depth across the Southern Ocean support the argument for preferential degradation of specific POC pools in the water column. Combining discussions of POC recycling and transfer across both marine and terrestrial systems offer new perspectives of thermal stability as a proxy for diagenetic stability and POC degradation state. The challenges of interpreting RPO data in these two environments set the stage for applying the technique to more controlled experiments that trace POC from source to long-term sink.
by Sarah Zhou Rosengard.
Ph. D.

The Earth’s atmosphere constitutes a complex system subject to a large number of forcings of both natural and anthropogenic origin; these influence its evolution on a range of timescales. This thesis makes use of the UMUKCA global chemistry-climate model to explore several aspects relating to the atmospheric response to the 11-year solar cycle forcing and future stratospheric ozone recovery. Firstly, following recent improvements in the model, the atmospheric response to the solar cycle forcing simulated in UMUKCA is discussed. It is shown that while some features show a broad resemblance to observations/reanalysis, there are clear differences with regard to other features; the latter could result from model deficiencies and/or uncertainties in the observed response. The role of analysis method and of interannual variability is also addressed. Secondly, the solar cycle response is separated into the individual contributions from direct radiative heating and from ozone production using a set of sensitivity experiments. It is shown that while the tropical yearly mean responses to the two components are generally linearly additive, this is not necessarily the case in the high latitudes. It is suggested that solar-induced ozone changes could be important for modulating the Southern Hemisphere dynamical response. Thirdly, the role of the representation of the solar ozone response is studied. It is shown that the choice of the solar ozone response prescribed in the radiation scheme in non-interactive ozone experiments has a substantial impact on the simulated temperature response to the solar cycle forcing. The Northern Hemisphere dynamical responses are found to be generally similar within the uncertainty. A comparison with an interactive ozone case is also discussed. Lastly, future ozone recovery is investigated using a seven-member ensemble of 1960- 2099/1980-2080 integrations. The long-term evolution of ozone in different regions is found to be generally consistent with previous modelling studies. The long-term trends and variability in springtime Arctic ozone and its chemical, radiative and dynamical drivers are assessed. It is shown that Arctic ozone increases in the future, consistent with future reduction in stratospheric chlorine, stratospheric cooling and strengthening large-scale circulation. Yet, the large interannual variability is found to continue and to facilitate episodic ozone reductions, with halogen chemistry becoming a smaller but non-negligible driver of future springtime Arctic ozone variability for many decades.

Uncertainty about the causes of glacial-interglacial CO2 variations demonstrates our incomplete grasp of fundamental processes that govern our climate and thus one of the foremost problems in palaeoceanography and Earth System Science regards the mechanism(s) responsible for natural changes in atmospheric CO2 concentration. It is becoming clear that the Southern Ocean overturning circulation plays an important role in the global carbon cycle because altered communication between the atmosphere and abyss in the Southern Ocean is relatively well documented and often implicated in explanations of past and future climate changes, but the ambiguity of the paleoceanographic record defies interpretation of the mechanisms involved. Using a coarse resolution ocean general circulation model and coupled biogeochemistry code, an ensemble of idealised perturbations to external forcing and internal physics of the Southern Ocean is examined to explain the processes that link ocean circulation, nutrient distributions and biological productivity, and determine the extent to which the Southern Ocean governs the partitioning of CO2. Strengthened or northward-shifted winds result in oceanic outgassing and increased atmospheric carbon dioxide levels, while weakened or southward-shifted winds cause oceanic carbon uptake and reduced atmospheric carbon dioxide concentration. Driven by the work done on the ocean by the winds, changes in the rate or spatial pattern of the Southern Ocean residual overturning circulation lead to alteration of upper ocean stratification and the rate and depth from which carbon and nutrient-rich deep waters are upwelled to the surface. These surface waters, imprinted with the pattern of air-sea gas exchange, are subducted to intermediate depths in the ocean interior, not the abyss as previous suggested. These results are robust to significant alterations to surface heat and freshwater boundary conditions, mesoscale eddy activity and rates of air-sea gas exchange and represent a significant proportion of the change in glacial-interglacial CO2 that can be currently generated by altered ocean circulation in a variety of models, revealing that the upper limb of the Southern Ocean overturning circulation is important in determining atmospheric CO2 levels.

A land-surface module to couple a meteorological and a hydrologic model is described. lt was implemented and tested in the Leipzig\''s version of GESIMA. Preliminary results of a coupling with NASMO are presented, although this article mainly focuses on the description of the module and its effect on the atmospheric water cycle. One positive impact of the module is that it allows to produce subgrid-scale evapotranspiration in more details and to heterogenize precipitation. This strongly affects soil wetness, cloudiness and the thermal regime of the atmospheric boundary layer.
Ein Bodenmodul zur Kopplung eines meteorologischen mit einem hydrologischen Modell wird vorgestellt. Er wurde implementiert und getestet in der Leipziger Version von GESIMA. Obgleich der Schwerpunkt des Artikels auf der Beschreibung des Moduls und seiner Auswirkung auf den atmosphärischen Wasserkreislauf liegt, werden auch vorläufige Ergebnisse einer Kopplung mit NASMO präsentiert. Ein positiver Effekt des Moduls ist, daß er ermöglicht, detaillierter die subskalige Evapotranspiration zu beschreiben und den Niederschlag zu heterogenisieren. Dies wirkt sich stark auf die Bodenfeuchte, die Bewölkung und das thermische Regime der atmosphärischen Grenzschicht aus.

En raison de leur accumulation possible dans la glande thyroïde, les radio-isotopes de l'iode, 131I (t1/2 = 8,07 jours) et 129I (t1/2 = 15,7 106 ans), sont préoccupants pour la santé humaine. En raison de sa longue demi-vie, 129I est également d’une importance radio-écologique majeure du fait de son intégration potentielle au cycle biogéochimique naturel de son isotope stable (127I). Les forêts dont la surface couvre près du tiers du territoire en France et en Europe, se caractérisent par leur longévité, un haut niveau de recyclage de la biomasse et une forte influence sur les cycles hydrologiques et des nutriments. La végétation forestière peut intercepter puis recycler et accumuler les polluants et radionucléides émis dans l’environnement. Dans ce contexte, des axes de recherche ont été développés dans cette thèse afin de décrire qualitativement et quantitativement le cycle de l’iode et améliorer les prédictions à long-terme du comportement de 129I dans les écosystèmes forestiers.Les pluies constituent une source majeure d’iode pour les sols et la végétation forestière, les quantités apportées dépendant de la distance côtière et de la pluviométrie. La végétation modifie les quantités et la spéciation de l’iode apporté par les pluies. L'humus représente un compartiment d'accumulation provisoire d'iode associé à la matière organique. Ce compartiment constitue également une zone de réémission possible de l’iode par lessivage lors d’évènements pluvieux et/ou par volatilisation. Cette thèse a montré que les teneurs d'iode dans les sols forestiers dépendaient à la fois des apports atmosphériques et des chutes de biomasse, mais également de la capacité du sol à fixer l'iode. Ainsi, les conditions environnementales conduisant à une accumulation de matière organique et la présence d’(hydr)oxydes métalliques dans les sols favorisent la rétention de l’iode. A l’échelle de la placette forestière, le sol constitue le principal réservoir en iode (~99,9%). Du fait de la proportion d'iode au sein des arbres (< 0,03% du stock total en iode), les quantités d’iode recyclées par la végétation sont faibles, l’iode n’étant pas un oligo-élément pour les végétaux. Ainsi ces travaux de thèse ont permis de combler le manque de données relatives d’une part à la répartition de l’iode au sein des écosystèmes forestiers et d’autre part, aux processus de réémissions (lessivage et volatilisation) liés à la dégradation de la matière organique. Les données générées par cette thèse permettent de mieux appréhender la modélisation du cycle de l’iode dans cet écosystème forestier
As radioisotopes of iodine may concentrate in thyroid gland, 131I (t1/2 = 8.07 days) and 129I (t1/2 = 15.7 106 years) are of health concern. 129I is of major radioecological importance because it can potentially integrate natural biogeochemical cycle of its stable isotope (127I) due to its long half-life. Forests, characterized by their longevity, a high biomass turnover and a strong influence on hydrological and nutrients cycles, can intercept, recycle and accumulate a significant amount of pollutants and radionuclides released into environment. In this context, research developed in this thesis had the purpose of correct long-term prediction of iodine behaviour in forest ecosystems.Rainfall appears to be a significant input flux of iodine in forest soils and vegetation, depending on coastal distance and precipitation amount. Forest canopy modifies quantities and speciation of iodine initially present in rainfall. Humus would act as temporary iodine accumulation layer through its association with organic matter. However, humus layer also behaves as potential iodine mobilisation source by leaching and/or volatilization. This thesis has demonstrated that iodine levels in soils depend both on its atmospheric and litterfall inputs, but also on the soil's ability to fix iodine. Thus, environmental conditions characterized by organic matter accumulation and presence of metal (hydr)oxides in soils promote iodine retention. Thereafter, soil is the main iodine reservoir at the forest plot scale (~99.9%). Given small amounts of iodine in the trees (<0.03% of the total iodine stock), recycling of iodine by vegetation is low, iodine being not an essential element for vegetables.This thesis contributes to fill some gaps in the knowledge of iodine distribution within forests and on the processes of iodine reemission related to organic matter degradation. Data generated by this thesis help to better understand the modeling of iodine cycle in this forest ecosystem