• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 44
  • 6
  • 5
  • 3
  • 2
  • 2
  • 1
  • 1
  • 1
  • Tagged with
  • 79
  • 79
  • 25
  • 21
  • 14
  • 13
  • 8
  • 8
  • 8
  • 7
  • 7
  • 7
  • 6
  • 6
  • 6
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Inversion of lunar FeO and numerical simulation of the detached dust layers on Mars / Etude de la Lune et de Mars par télédétection infrarouge

Wang, Chao 24 November 2016 (has links)
Les travaux menés dans cette thèse se partagent entre la Lune et Mars, cibles privilégiées pour les missions d'exploration spatiales. La première partie porte sur l'instrument Interference Imaging Spectrometer (IIM) qui était à bord du satellite lunaire chinois Chang’e-1. Une méthode inédite utilisant l'angle spectrale et le concept de distance Euclidienne, et visant à supprimer les mauvais pixels de IIM, est proposée. Une nouvelle procédure de calibration est utilisée, et l'inter-étalonnage des données IIM avec des données télescopiques est amélioré. Ce nouveau jeu de données permet, après inversion, d'estimer l'abondance de FeO dans le sol lunaire. Les valeurs trouvées sont comparables aux observations du satellite américain Clementine et fournissent une nouvelle référence pour les études lunaires à venir. La seconde partie est consacrée à la modélisation du phénomène de tempêtes de poussière-fusée ("rocket dust storms") générées par des mouvements convectifs meso-echelle liés au chauffage solaire des poussières. L'objectif est de reproduire numériquement les couches de poussières détachées découvertes par l'instrument Mars Climate Sounder (NASA) dans le Modèle de Climat Global (GCM) du LMD. Les simulations montrent que, durant la saison des tempêtes de poussières (printemps et été austral), ce phénomène permet d'expliquer les couches détachées, et de reproduire les observations. Cependant, durant l'autre partie de l'année où il y a très peu de tempêtes de poussières, il semble nécessaire d'inclure dans le GCM un autre processus impliquant les vents de pente, capable de réinjecter les poussières en altitude pour maintenir les couches détachées. / Moon and Mars have been the important targets for deep space missions. The studies in this thesis include two parts. The first part is concerning Chang’e-1 Interference Imaging Spectrometer (IIM) data preprocessing and global lunar FeO inversion. In order to better preprocess the IIM data, a new method using spectral angle and Euclidean distance for removing bad pixels has been proposed. A new in-flight calibration has been conducted. And cross calibration of IIM data by using the telescopic data is improved. The processed IIM data have also been used to inverse lunar FeO abundance. The IIM-derived FeO is comparable to Clementine FeO results, and can be an alternative dataset for Moon studies. The second part is concerning parameterizing rocket dust storms and daytime slope winds in LMD (Laboratoire de Météorologie Dynamique) Mars GCM (Global Climate Model) to reproduce the detached dust layers (DDLs) observed by Mars Climate Sounder (MCS) on Mars. The simulations by the GCM including rocket dust storm parameterization show that, during the Martian dusty seasons, the rocket dust storms are the key factors to explain the observed DDLs. The formation and evolution of GCM simulated DDLs are in agreement with those of MCS observation. Meanwhile, the simulation also suggests that the large variation of the DDLs’ altitudes in dusty season are contributed by the deep convection induced by rocket dust storms. The simulations by the GCM including daytime slope winds parameterization show that with the help of daytime slope winds, the GCM can reproduce the detached dust layers in Martian clear seasons, which cannot be simulated by the rocket dust storm process.
12

Klimavariabilität der Tropo- und Stratosphäre in einem globalen gekoppelten Atmosphäre-Ozean-Modell mit vereinfachter stratosphärischer Chemie / Tropo- and stratospheric climate variability in a global coupled atmosphere-ocean-model with simplified stratospheric chemistry

Brand, Sascha January 2007 (has links)
In dieser Arbeit wurde die Variabilität der Atmosphäre in einem neuen gekoppelten Klimamodell (ECHO-GiSP) untersucht, welches eine vereinfachte Stratosphärenchemie (bis 80 km Höhe) enthält. Es wurden 2 Simulationen über 150 Jahre durchgeführt. In einer der Simulationen wurde die atmosphärische Chemie modelliert, hatte aber keinen Einfluß auf die Dynamik des Klimamodelles. In der zweiten Simulation wurde hingegen die Wirkung der Chemie auf die Klimadynamik explizit berücksichtigt, die über die Strahlungsbilanz des Modelles erfolgt. Dies ist die erste Langzeitsimulation mit einem voll gekoppelten globalen Klimamodell mit interaktiver Chemie. Die Simulation mit rückgekoppelter Chemie zeigt eine Abschwächung des atmosphärischen Variabilitätsmusters der Arktischen Oszillation (AO). Zudem kommt es in der Troposphäre zu einer Reduzierung der mittleren Windgeschwindigkeiten der gemäßigten Breiten aufgrund verringerter Temperaturgegensätze zwischen den Tropen und den Polargebieten. Auch in der Stratosphäre ergibt sich eine Abschwächung und Erwärmung des Polarwirbels. Diese Auswirkungen der Kopplung zwischen der atmosphärischen Chemie und der Dynamik des Klimamodelles sind eine wichtige Erkenntnis, da in früheren Klimasimulationen die Variabilität der AO oft zu stark ausgeprägt war. In der Stratosphäre reduziert sich infolge des abgeschwächten Polarwirbels auch die großräumige Zirkulation zwischen den beiden Hemisphären der Erde. In der Troposphäre werden hingegen die allgemeine Zirkulation, und damit auch die subtropischen Strahlströme des Windes verstärkt. Zudem kommt es in den Tropen zu Temperaturänderungen durch stratosphärische Ozonschwankungen in Abhängigkeit von der AO. Allgemein verändert sich die Kopplung zwischen Troposphäre und Stratosphäre, einschließlich des durch die Anregung von langen atmosphärischen Wellen erfolgenden vertikalen Energieübertrages aus der Troposphäre in die Stratosphäre. / In this work the atmospheric variability in a new coupled climate model (ECHO-GiSP) was analyzed, which includes a simplified stratospheric chemistry (up to 80 km height). Two simulations of 150 years were performed. In one of those simulations the atmospheric chemistry was modeled without having any influence back on the model dynamics. In the second simulation the impact of the chemistry on climate dynamics, taking place via the models radiation balance, was explicitly recognized. This is the first long term simulation using a fully coupled global climate model with interactive chemistry. The simulation with interactive chemistry shows a weakening of the Arctic Oscillation (AO) pattern of atmospheric variability. At the same time there is a reduction of the mean wind speeds in middle latitudes in the troposphere, which is caused by weaker temperature gradients between the tropics and the polar regions. Also, in the stratosphere a weakening and warming of the polar vortex is obvious. These effects of the coupling between atmospheric chemistry and the dynamics of the climate model are an important result, since in earlier climate simulations the variability of the AO often was overestimated. Due to the weakened polar vortex in the stratosphere also the large scale interhemispheric mean circulation is reduced. On the other hand, the tropospheric meridional mean circulation, and thus also the subtropical jetstreams of the zonal wind are enhanced. Furthermore there are tropical temperature variations in the troposphere and lower stratosphere, which are induced by stratospheric ozone variations associated to the phase of the AO. Generally, the coupling between tropo- and stratosphere is changed, which includes the vertical energy and momentum transfer by ascending planetary waves from the troposphere to the stratosphere.
13

Simulation der indischen Monsunzirkulation mit dem Regionalen Klimamodell HIRHAM / Simulation of the Indian Monsoon Circulation with the regional climate model HIRHAM

Polanski, Stefan January 2011 (has links)
In dieser Arbeit wird das regionale Klimamodell HIRHAM mit einer horizontalen Auflösung von 50 km und 19 vertikalen Schichten erstmals auf den asiatischen Kontinent angewendet, um die indische Monsunzirkulation unter rezenten und paläoklimatischen Bedingungen zu simulieren. Das Integrationsgebiet des Modells erstreckt sich von etwa 0ºN - 50ºN und 42ºE - 110ºE und bedeckt dabei sowohl die hohe Topographie des Himalajas und Tibet Plateaus als auch den nördlichen Indischen Ozean. Das Ziel besteht in der Beschreibung der regionalen Kopplung zwischen der Monsunzirkulation und den orographischen sowie diabatischen Antriebsmechanismen. Eine 44-jährige Modellsimulation von 1958-2001, die am seitlichen und unteren Rand von ECMWF Reanalysen (ERA40) angetrieben wird, bildet die Grundlage für die Validierung der Modellergebnisse mit Beobachtungen auf der Basis von Stations- und Gitterdatensätzen. Der Fokus liegt dabei auf der atmosphärischen Zirkulation, der Temperatur und dem Niederschlag im Sommer- und Wintermonsun, wobei die Qualität des Modells sowohl in Bezug zur langfristigen und dekadischen Klimatologie als auch zur interannuellen Variabilität evaluiert wird. Im Zusammenhang mit einer realistischen Reproduktion der Modelltopographie kann für die Muster der Zirkulation und Temperatur eine gute Übereinstimmung zwischen Modell und Daten nachgewiesen werden. Der simulierte Niederschlag zeigt eine bessere Übereinstimmung mit einem hoch aufgelösten Gitterdatensatz über der Landoberfläche Zentralindiens und in den Hochgebirgsregionen, der den Vorteil des Regionalmodells gegenüber der antreibenden Reanalyse hervorhebt. In verschiedenen Fall- und Sensitivitätsstudien werden die wesentlichen Antriebsfaktoren des indischen Monsuns (Meeresoberflächentemperaturen, Stärke des winterlichen Sibirischen Hochs und Anomalien der Bodenfeuchte) untersucht. Die Ergebnisse machen deutlich, dass die Simulation dieser Mechanismen auch mit einem Regionalmodell sehr schwierig ist, da die Komplexität des Monsunsystems hochgradig nichtlinear ist und die vor allem subgridskalig wirkenden Prozesse im Modell noch nicht ausreichend parametrisiert und verstanden sind. Ein paläoklimatisches Experiment für eine 44-jährige Zeitscheibe im mittleren Holozän (etwa 6000 Jahre vor heute), die am Rand von einer globalen ECHAM5 Simulation angetrieben wird, zeigt markante Veränderungen in der Intensität des Monsuns durch die unterschiedliche solare Einstrahlung, die wiederum Einflüsse auf die SST, die Zirkulation und damit auf die Niederschlagsmuster hat. / In this study the regional climate model HIRHAM with a horizontal resolution of 50 km and 19 vertical levels is applied over the Asian continent to simulate the Indian monsoon circulation under present-day and past conditions. The integration domain extends from 0ºN - 50ºN and 42ºE - 110ºE and covers the high topography of Himalayas and Tibetan Plateau as well as the northern Indian Ocean. The main objective is the description of the regional coupling between monsoon circulation and orographic as well as thermal driving mechanisms of monsoon. A 44-years long simulation from 1958-2001, driven at the lateral and lower boundaries by European reanalysis (ERA40), is the basis for the validation of model results with observations based on station and gridded data sets. The focus is on the the long-term and decadal summer and winter monsoon climatology and its variability concerning atmospheric circulation, temperature and precipitation. The results successfully reproduce the observations due to a realistic simulation of topographic features. The simulated precipitation shows a better agreement with a high-resolution gridded data set over the central land areas of India and in the higher elevated Tibetan and Himalayan regions than ERA40. In different case and sensitivity studies the main driving mechanisms of the Indian monsoon (Sea Surface Temperatures, strength of the Siberian High in winter and soil moisture anomalies) are investigated. The results show, that the simulation of these mechanisms with a regional climate model is also difficult related to the complex non linear monsoon system and the small-scale processes, which are not just sufficiently parameterized and understood in the model. A paleoclimatic experiment for a 44-years long time slice in mid-holocene (6000 years before present), which is driven by a global ECHAM5 simulation, shows significant changes in the monsoon intensity due to the different solar forcing, which influences the SST, the circulation and the precipitation.
14

Datormodellering av en värmelagrande betongväggs inverkan på det termiska klimatet i ett växthus

Agebro, Andreas January 2010 (has links)
This report describes the building of a computer model that makes it possible to simulate the thermal climate in a greenhouse. The computer model is built on the physical theory of heat exchange that occur in a greenhouse, such as radiation and convective heat exchange. The model also includes the heat storage that is active in a greenhouse. The computer model is used to simulate the thermal climate in a greenhouse under three periods, winter, spring and summer. It also investigates which effect a concrete wall has on the thermal climate in a greenhouse. The purpose of putting a concrete wall in the greenhouse model is to investigate the possibility to store heat during the day and then use this heat when the temperature drops during the night. The result from the simulations shows that a concrete wall levels the big difference in temperature that normally occurs under a day in a greenhouse. It also shows that heat is stored in the concrete wall and during the night the wall temperature is higher than both the outdoor temperature and the greenhouse temperature. This makes the wall a source of heat during this time. / Växthus är pga. sin utformning väldigt känsligt för klimatets påverkan. Detta resulterar i att klimatet i växthuset under soliga dagar kan uppnå väldigt höga temperaturer medans temperaturen under kalla dagar och nätter kan bli lika låg som den rådande utomhustemperaturen. Växthusets klimat blir därför väldigt extremt och temperatursvängningarna stora. Temperatursvängningarna beror till stor del på växthusets låga värmetröghet och genom att öka trögheten i en byggnad kan temperatur svängningar minskas och ett jämnare termisk klimat uppnås. En ökning av trögheten kan också bidra till att värme lagras under varmare perioder och på så sätt minska ett eventuellt uppvärmningsbehov under de kalla perioderna. För att undersöka tröghetens inverkan och möjligheterna till värmelagring hos ett växthus har detta examensarbete inriktats på uppbyggnaden av en datormodell som kan simulera ett växthus termiska förhållanden. Modellen har sedan använts för att undersöka hur en betongvägg påverkar det termiska klimatet i växthuset samt betongväggens förmåga att lagra värme. Datormodellen har byggts upp i MATLAB vilket gör det möjligt att med klimatdata från olika perioder simulera växthuset inre klimat. Datormodellen bygger på matematiska beräkningar som grundar sig på fysikaliska och termiska samband. Växthuset som undersöks i datormodellen bygger på ett växthus som är planerat att uppföras på trädgårdsanläggningen Wij trädgårdar i Ockelbo. Växthuset går under namnet Eldtemplet och ingår i projektet ”Ny energi i gamla landskap” som ska utforska möjligheterna till nya energikällor inom trädgårds- och odlingsverksamhet. Simuleringar har genomförts med klimatdata från ett dygn under tre olika årstider, vinter, vår och sommar. Resultatet från simuleringarna visar att temperaturerna i växthuset påverkas väldigt mycket av den infallande solstrålningen. Införandet av en betongvägg ökar växthusets tröghet och jämnar ut temperatursvängningarna i växthuset. Simuleringarna visar också att betongväggen får en värmelagrande förmåga och under vissa perioder kan tillföra växthuset värme under natten då temperaturen i växthuset sjunker. Genom resultatet kan slutsatsen dras att det finns åtgärder att ta till som kan förbättra växthusets termiska egenskaper väsentligt och göra växthus mer energieffektiva.
15

Modeling Aerosol - Water Interactions in Subsaturated and Supersaturated Environments

Fountoukis, Christos 05 June 2007 (has links)
The current dissertation is motivated by the need for an improved understanding of aerosol water interactions both in subsaturated and supersaturated atmospheric conditions with a strong emphasis on air pollution and climate change modeling. A cloud droplet formation parameterization was developed to i) predict droplet formation from a lognormal representation of aerosol size distribution and composition, and, ii) include a size-dependant mass transfer coefficient for the growth of water droplets which explicitly accounts for the impact of organics on droplet growth kinetics. The parameterization unravels most of the physics of droplet formation and is in remarkable agreement with detailed numerical parcel model simulations, even for low values of the accommodation coefficient. The parameterization offers a much needed rigorous and computationally inexpensive framework for directly linking complex chemical effects on aerosol activation in global climate models. The new aerosol activation parameterization was also tested against observations from highly polluted clouds (within the vicinity of power plant plumes). Remarkable closure was achieved (much less than the 20% measurement uncertainty). The error in predicted cloud droplet concentration was mostly sensitive to updraft velocity. Optimal closure is obtained if the water vapor uptake coefficient is equal to 0.06. These findings can serve as much needed constraints in modeling of aerosol-cloud interactions in the North America. Aerosol water interactions in ambient relative humidities less than 100% were studied using a thermodynamic equilibrium model for inorganic aerosol and a three dimensional air quality model. We developed a new thermodynamic equilibrium model, ISORROPIA-II, which predicts the partitioning of semi-volatiles and the phase state of K+/Ca2+/Mg2+/NH4+/Na+/SO42-/NO3-/Cl-/H2O aerosols. A comprehensive evaluation of its performance was conducted against the thermodynamic module SCAPE2 over a wide range of atmospherically relevant conditions. Based on its computational rigor and performance, ISORROPIA-II appears to be a highly attractive alternative for use in large scale air quality and atmospheric transport models. The new equilibrium model was also used to thermodynamically characterize aerosols measured at a highly polluted area. In the ammonia-rich environment of Mexico City, nitrate and chloride primarily partition in the aerosol phase with a 20-min equilibrium timescale; PM2.5 is insensitive to changes in ammonia but is to acidic semivolatile species. When RH is below 50%, predictions improve substantially if the aerosol follows a deliquescent behavior. The impact of including crustal species (Ca2+, K+, M2+) in equilibrium calculations within a three dimensional air quality model was also studied. A significant change in aerosol water (-19.8%) and ammonium (-27.5%) concentrations was predicted when crustals are explicitly included in the calculations even though they contributed, on average, only a few percent of the total PM2.5 mass, highlighting the need for comprehensive thermodynamic calculations in the presence of dust.
16

Toward understanding predictability of climate: a linear stochastic modeling approach

Wang, Faming 15 November 2004 (has links)
This dissertation discusses the predictability of the atmosphere-ocean climate system on interannual and decadal timescales. We investigate the extent to which the atmospheric internal variability (weather noise) can cause climate prediction to lose skill; and we also look for the oceanic processes that contribute to the climate predictability via interaction with the atmosphere. First, we develop a framework for assessing the predictability of a linear stochastic system. Based on the information of deterministic dynamics and noise forcing, various predictability measures are defined and new predictability-analysis tools are introduced. For the sake of computational efficiency, we also discuss the formulation of a low-order model within the context of four reduction methods: modal, EOF, most predictable pattern, and balanced truncation. Subsequently, predictabilities of two specific physical systems are investigated within such framework. The first is a mixed layer model of SST with focus on the effect of oceanic advection.Analytical solution of a one-dimensional model shows that even though advection can give rise to a pair of low-frequency normal modes, no enhancement in the predictability is found in terms of domain averaged error variance. However, a Predictable Component Analysis (PrCA) shows that advection can play a role in redistributing the predictable variance. This analytical result is further tested in a more realistic two-dimensional North Atlantic model with observed mean currents. The second is a linear coupled model of tropical Atlantic atmosphere-ocean system. Eigen-analysis reveals that the system has two types of coupled modes: a decadal meridional mode and an interannual equatorial mode. The meridional mode, which manifests itself as a dipole pattern in SST, is controlled by thermodynamic feedback between wind, latent heat flux, and SST, and modified by ocean heat transport. The equatorial mode, which manifests itself as an SST anomaly in the eastern equatorial basin, is dominated by dynamic feedback between wind, thermocline, upwelling, and SST. The relative strength of thermodynamic vs dynamic feedbacks determines the behavior of the coupled system, and enables the tropical Atlantic variability to be more predictable than the passive-ocean scenario.
17

Development of an integrated building load-ground source heat pump model as a test bed to assess short- and long-term heat pump and ground loop performance

Gaspredes, Jonathan Louis 08 February 2012 (has links)
Ground source heat pumps (GSHP) have the ability to significantly reduce the energy required to heat and cool buildings. Historically, deployment of GSHP's in the cooling-dominated Texas and Southwest region has been significantly less than in other regions of the United States. The long term technical and economic viability of GSHPs in arid regions such as Texas has been questioned due to failures of ground loop heat pump systems by early adopters. A proposed solution is to include a supplemental heat rejection (SHR) device to help offset the unbalanced ground loads. An integrated building load-ground source heat pump model is developed in this thesis and is designed to be a test bed for potential SHR devices. The model consists of discrete component models that can be mixed and matched to represent various types of buildings and ground source heat pumps. One of the unique features of the integrated model is the use of the Simulink/Matlab environment. This environment allows the user to develop component models that take advantage of the built-in functionality of Matlab and Simulink. Another unique feature is the full coupling of the building load, heat pump, and ground loop at every time step. The building load, heat pump, and ground loop models were chosen to allow for short time step simulations, which allows for a range of dynamic response times to be modeled and for different heat pump/SHR control methods to be explored. The integrated model can be used on any computer that has the Matlab and Simulink software. The building load model used, called HAMBASE, can model both residential and commercial buildings. HAMBASE was validated using the ASHRAE 140-2007 standard. The heat pump model uses readily available data provided by GSHP manufacturers to accurately model operation across a wide range of input conditions. The vertical borehole ground loop model, developed at Oklahoma State University, is based on Eskillson's g-function model, but included a one-dimensional numerical model to calculate the short term thermal response of the borehole and ground. The ground loop model utilizes GLHEPRO, a ground loop sizing and simulation tool, to create the required parameter files. Using the integrated building load-ground source heat pump model, a model of a single family house with a ground source heat pump was developed. The house model was validated by the results from eQuest and GELHPRO. A series of sensitivity studies were completed to determine dominant factors affecting the use of GSHPs in Texas and the Southwest regions of the United States. The results show that the life of a vertical borehole can be significantly extended/cut short if the ground parameters are properly/not properly designed prior to ground loop sizing. / text
18

Investigation of Changes in Hydrological Processes using a Regional Climate Model

Bhuiyan, AKM Hassanuzzaman 23 August 2013 (has links)
This thesis evaluates regional hydrology using output from the Canadian Regional Climate Model (CRCM 4.1) and examines changes in the hydrological processes over the Churchill River Basin (CRB) by employing the Variable Infiltration Capacity (VIC) hydrology model. The CRCM evaluation has been performed by combining the atmospheric and the terrestrial water budget components of the hydrological cycle. The North American Regional Reanalysis (NARR) data are used where direct observations are not available. The outcome of the evaluation reveals the potential of the CRCM for use in long-term hydrological studies. The CRCM atmospheric moisture fluxes and storage tendencies show reasonable agreement with the NARR. The long-term moisture flux over the CRB was found to be generally divergent during summer. A systematic bias is observed in the CRCM precipitation and temperature. A quantile-based mapping of the cumulative distribution function is applied for precipitation adjustments. The temperature correction only involves shifting and scaling to adjust mean and variance. The results indicate that the techniques employed for correction are useful for hydrological studies. Bias-correction is also applied to the CRCM future climate. The CRCM bias-corrected data is then used for hydrological modeling of the CRB. The VIC-simulated streamflow exhibits acceptable agreement with observations. The VIC model's internal variables such as snow and soil moisture indicate that the model is capable of simulating internal process variables adequately. The VIC-simulated snow and soil moisture shows the potential of use as an alternative dataset for hydrological studies. Streamflow along with precipitation and temperature are analyzed for trends. No statistically significant trend is observed in the daily precipitation series. Results suggest that an increase in temperature may reduce accumulation of snow during fall and winter. The flow regime may be in transition from a snowmelt dominated regime to a rainfall dominated regime. Results from future climate simulations of the A2 emission scenario indicate a projected increase of streamflow, while the snow depth and duration exhibit a decrease. Soil moisture response to future climate warming shows an overall increase with a greater likelihood of occurrences of higher soil moisture.
19

On the Arctic Seasonal Cycle

Mortin, Jonas January 2014 (has links)
The seasonal cycle of snow and sea ice is a fundamental feature of the Arctic climate system. In the Northern Hemisphere, about 55 million km2 of sea ice and snow undergo complete melt and freeze processes every year. Because snow and sea ice are much brighter (higher albedo) than the underlying surface, their presence reduces absorption of incoming solar energy at high latitudes. Therefore, changes of the sea-ice and snow cover have a large impact on the Arctic climate and possibly at lower latitudes. One of the most important determining factors of the seasonal snow and sea-ice cover is the timing of the seasonal melt-freeze transitions. Hence, in order to better understand Arctic climate variability, it is key to continuously monitor these transitions. This thesis presents an algorithm for obtaining melt-freeze transitions using scatterometers over both the land and sea-ice domains. These satellite-borne instruments emit radiation at microwave wavelengths and measure the returned signal. Several scatterometers are employed: QuikSCAT (1999–2009), ASCAT (2009–present), and OSCAT (2009–present). QuikSCAT and OSCAT operate at Ku-band (λ=2.2 cm) and ASCAT at C-band (λ=5.7 cm), resulting in slightly different surface interactions. This thesis discusses these dissimilarities over the Arctic sea-ice domain, and juxtaposes the time series of seasonal melt-freeze transitions from the three scatterometers and compares them with other, independent datasets. The interactions of snow and sea ice with other components of the Arctic climate system are complex. Models are commonly employed to disentangle these interactions. But this hinges upon robust and well-formulated models, reached by perpetual testing against observations. This thesis also presents an evaluation of how well eleven state-of-the-art global climate models reproduce the Arctic sea-ice cover and the summer length—given by the melt-freeze transitions—using surface observations of air temperature. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: In press. Paper 4: Submitted.</p>
20

Investigation of Changes in Hydrological Processes using a Regional Climate Model

Bhuiyan, AKM Hassanuzzaman 23 August 2013 (has links)
This thesis evaluates regional hydrology using output from the Canadian Regional Climate Model (CRCM 4.1) and examines changes in the hydrological processes over the Churchill River Basin (CRB) by employing the Variable Infiltration Capacity (VIC) hydrology model. The CRCM evaluation has been performed by combining the atmospheric and the terrestrial water budget components of the hydrological cycle. The North American Regional Reanalysis (NARR) data are used where direct observations are not available. The outcome of the evaluation reveals the potential of the CRCM for use in long-term hydrological studies. The CRCM atmospheric moisture fluxes and storage tendencies show reasonable agreement with the NARR. The long-term moisture flux over the CRB was found to be generally divergent during summer. A systematic bias is observed in the CRCM precipitation and temperature. A quantile-based mapping of the cumulative distribution function is applied for precipitation adjustments. The temperature correction only involves shifting and scaling to adjust mean and variance. The results indicate that the techniques employed for correction are useful for hydrological studies. Bias-correction is also applied to the CRCM future climate. The CRCM bias-corrected data is then used for hydrological modeling of the CRB. The VIC-simulated streamflow exhibits acceptable agreement with observations. The VIC model's internal variables such as snow and soil moisture indicate that the model is capable of simulating internal process variables adequately. The VIC-simulated snow and soil moisture shows the potential of use as an alternative dataset for hydrological studies. Streamflow along with precipitation and temperature are analyzed for trends. No statistically significant trend is observed in the daily precipitation series. Results suggest that an increase in temperature may reduce accumulation of snow during fall and winter. The flow regime may be in transition from a snowmelt dominated regime to a rainfall dominated regime. Results from future climate simulations of the A2 emission scenario indicate a projected increase of streamflow, while the snow depth and duration exhibit a decrease. Soil moisture response to future climate warming shows an overall increase with a greater likelihood of occurrences of higher soil moisture.

Page generated in 0.0254 seconds