Dans un contexte actuel préoccupé par la couche d’ozone et son recouvrement ainsi que par le réchauffement global, comprendre, représenter et modéliser la dynamique stratosphérique sont des enjeux majeurs. Une question récurrente est de pouvoir évaluer les biais des modèles à haute altitude dans la stratosphère. Or la stratosphère est une couche de l’atmosphère difficilement accessible à la mesure. Une solution est d’extraire des informations en analysant les trajectoires des ballons qui sont essentiellement conditionnées par les mouvements des masses d’air. Ces travaux de thèse ont donné naissance à une banque de données rassemblant des mesures de vent, de température et de pression issues des vols de ballons stratosphériques sur une période de plus de 20 ans à des régions variées (région polaire arctique, à moyenne latitude et en régions intertropicales) et à différentes saisons. Une méthode d’analyse des biais mesures/modèle a été mise en place et appliquée aux réanalyses ERA-Interim. Par comparaison avec une précédente étude faite dans des conditions de vortex polaire, les mesures ainsi que la méthode d’analyse des biais ont été validées. Enfin, une analyse systématique des biais sur le vent à haute altitude dans la stratosphère considérant plusieurs régions à différentes saisons a été implémentée. Les résultats montrent que les biais dans les réanalyses ERA-Interim varient et augmentent en fonction de l’altitude. Les régions associées à des régimes bien établis sont plutôt bien représentées alors que les zones de transition ou de changement de circulation présentent plus de difficultés. D’une manière générale, les résultats de biais rapportent que les réanalyses ERA-Interim sous-estiment la vitesse du vent dans la stratosphère. / In a current context focused on the ozone layer recovery and on the global warming, understand, represent and model the stratospheric dynamic became major issues. Realism of these models needs to be assessed. Unfortunately the stratosphere is an atmospheric layer hardly accessible to measurements. Balloons trajectories are mainly depending on the air masses displacements. Extract measurements from balloon trajectories is a solution. These works led to the creation of a database which contains winds, temperature and pressure measurements from stratospheric balloons flights data collected over a time period of more 20 years at several locations (arctic polar region, at mid-latitude and in tropics) and at different seasons. A methodology of winds biases analysis has been developed and has been applied to the ERA-Interim reanalysis data. By comparing our results with a previous study, we have been able to validate the database and the winds biases analyse methodology. A systematic analyse of winds biases at high altitude in the stratosphere considering several locations/seasons has been implemented. The results show the biases in ERA-Interim reanalyses vary and rise as function of the altitude. The locations which are associated to well established circulations are rather well represented while the locations where the variability is higher present more difficulty. Globally biases results report ERA-Interim reanalyses underestimate wind speed in the stratosphere.

abstract: The numerical climate models have provided scientists, policy makers and the general public, crucial information for climate projections since mid-20th century. An international effort to compare and validate the simulations of all major climate models is organized by the Coupled Model Intercomparison Project (CMIP), which has gone through several phases since 1995 with CMIP5 being the state of the art. In parallel, an organized effort to consolidate all observational data in the past century culminates in the creation of several "reanalysis" datasets that are considered the closest representation of the true observation. This study compared the climate variability and trend in the climate model simulations and observations on the timescales ranging from interannual to centennial. The analysis focused on the dynamic climate quantity of zonal-mean zonal wind and global atmospheric angular momentum (AAM), and incorporated multiple datasets from reanalysis and the most recent CMIP3 and CMIP5 archives. For the observation, the validation of AAM by the length-of-day (LOD) and the intercomparison of AAM revealed a good agreement among reanalyses on the interannual and the decadal-to-interdecadal timescales, respectively. But the most significant discrepancies among them are in the long-term mean and long-term trend. For the simulations, the CMIP5 models produced a significantly smaller bias and a narrower ensemble spread of the climatology and trend in the 20th century for AAM compared to CMIP3, while CMIP3 and CMIP5 simulations consistently produced a positive trend for the 20th and 21st century. Both CMIP3 and CMIP5 models produced a wide range of the magnitudes of decadal and interdecadal variability of wind component of AAM (MR) compared to observation. The ensemble means of CMIP3 and CMIP5 are not statistically distinguishable for either the 20th- or 21st-century runs. The in-house atmospheric general circulation model (AGCM) simulations forced by the sea surface temperature (SST) taken from the CMIP5 simulations as lower boundary conditions were carried out. The zonal wind and MR in the CMIP5 simulations are well simulated in the AGCM simulations. This confirmed SST as an important mediator in regulating the global atmospheric changes due to GHG effect. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2013

O objetivo principal deste trabalho é avaliar a capacidade do modelo RegCM3 (Regional Climate Model versão 3), para três diferentes simulações, em simular os padrões espaciais de tendência de alguns índices climáticos anuais e sazonais de temperatura e chuva na América do Sul para o clima presente (1961-1990). Para atingir o objetivo principal, desenvolveu-se um algoritmo baseado no software RClimDex para calcular os índices e investigou-se a habilidade dos dados de reanálise do NCEP/NCAR e do conjunto de dados de chuva produzido por Liebmann e Allured (2005) em estimar os índices anuais e sazonais de temperatura e chuva para a mesma região e período. A metodologia concentrou-se em análises dos coeficientes de correlação e de regressão linear. De maneira geral, os dois conjuntos de dados foram considerados válidos para representar os índices anuais e sazonais de temperatura e chuva observados na América do Sul durante o período de 1961-1990. Contudo, a reanálise do NCEP/NCAR mostrou sinais de tendência opostos às observações para os índices de temperatura e chuva sobre a Argentina. Dentre as três simulações com o modelo RegCM3, as simulações RegCM3(s2), com o esquema de convecção Emanuel, e RegCM3(s1), com o esquema de Grell, apresentaram um melhor desempenho em representar as tendências dos índices de temperatura e chuva, respectivamente, sobre a América do Sul. O modelo RegCM3(s1) simulou a predominante tendência de aumento das condições de umidade observadas na América do Sul através dos índices anuais r95p, prcptot e rx5day, particularmente na estação de inverno. As tendências opostas dos índices anuais cwd e cdd, que sugerem um aumento no número de dias com chuva, ao contrário, não foram bem detectadas pelo modelo RegCM3(s1). O aquecimento da temperatura mínima como conseqüência do aumento de noites quentes e diminuição de noites frias identificado em quase todo o continente foi simulado corretamente pelo modelo RegCM3(s2). Com relação à temperatura máxima, embora as observações não apresentem um padrão característico, a simulação do modelo RegCM3(s2) se mostra bastante semelhante ao apresentado pela reanálise do NCEP/NCAR. As distribuições espaciais de tendência dos índices anuais de temperatura e chuva resultantes das simulações do RegCM3(s2 e s1, respectivamente) e do HadAM3 sobre a América do Sul durante o período de 1961-1990 são bastante semelhantes entre si, embora o HadAM3 seja caracterizado por tendências mais suavizadas. Para os índices anuais e sazonais de temperatura, a simulação do HadAM3 se mostrou ligeiramente melhor à do RegCM3, enquanto que o inverso é encontrado para os índices anuais e sazonais de chuva. / The main goal of this study is to investigate if the RegCM3 model (Regional Climate Model version 3) is able to simulate the spatial patterns of some annual and seasonal climate indices of temperature and precipitation trend over South America for the present climate (1961-1990). The analysis is done for three different simulations where the model was running with different cumulus parametrization, i.e, Grell and Emanuel. An algorithm based on the software RClimDex was developed to calculate the indices. Different data sources such as the NCEP/NCAR reanalysis, individual observational stations and others were used to estimate the annual and seasonal indices of temperature and precipitation for the same region and period. Correlation and linear regression coefficients analysis were used in the results comparison. In general, the results suggest that the datasets can provide useful information about annual and seasonal indices of temperature and precipitation at individual grid cells in South America during the period 1961-1990. However, the NCEP/NCAR reanalysis showed reversal trend signals for some indices over Argentina. Within the three simulations with the RegCM3 model, the trends of the annual and seasonal indices of temperature and precipitation over South America are better reproduced by the Emanuel (s2) and Grell (s1) schemes, respectively. The RegCM3(s1) simulates the change to wetter conditions in South America through the r95p, prcptot and rx5day annual indices, particularly in the austral winter. On the other hand, the opposite signal of the trends in cdd and cwd indices, that indicates an increase in the number of days with precipitation, was not well represented by the model. The warming in minimum temperature as a consequence of the increase in the frequency of warm nights and the decrease of cold nights observed all over the South American continent was correctly simulated by the RegCM3(s2). Although the observed maximum temperature extremes did not show any special feature, the simulations with Grell scheme were able to represent the spatial patterns of the warm and cold days indices trend similar to the NCEP/NCAR reanalysis. The spatial distributions of the annual indices of temperature and precipitation trend obtained from the RegCM3(s2 and s1, respectively) and HadAM3 simulations over South America during the period 1961-1990 are very similar, though the HadAM3 shows a trend less intense. For the annual and seasonal indices of temperature, the HadAM3 simulation is slightly better than the RegCM3 running, while the opposite is found to the annual and seasonal indices of precipitation.

HURDAT is the main historical archive of all tropical storms and hurricanes in the North Atlantic Basin, which includes the Caribbean Sea and Gulf of Mexico, from 1851 to the present. HURDAT is maintained and updated annually by the National Hurricane Center at Miami, Florida. Today, HURDAT is widely used by research scientists, operational hurricane forecasters, insurance companies, emergency managers and others. HURDAT contains both systematic biases and random errors. Thus, the reanalysis of HURDAT is vital. For this thesis, HURDAT is reanalyzed for the period of 1954-1963. The track and intensity of each existing tropical cyclone in HURDAT is assessed in the light of 21st century understanding and previously unrecognized tropical cyclones are detected and analyzed. The resulting changes will be recommended to the National Hurricane Center Best Track Change Committee for inclusion in HURDAT.

Radio signals from Global Navigation Satellite Systems (GNSS) satellites suffer delay as they propagate through the atmosphere (neutral and non-neutral) and this delay is partially driven by the water vapour content in the atmosphere. The delay component due to the non-neutral atmosphere (ionosphere) is removed through the use of dual frequency GNSS receivers. The main tropospheric parameter is the zenith tropospheric (or total) delay (ZTD), which is a widely accepted parameter with which to express the total delay in the signal from all satellites due to the neutral atmosphere. The ZTD is a measure of the integrated tropospheric condition over a GNSS receiver station. Accordingly, the integrated water vapour or precipitable water vapour (PWV) can be obtained from a portion of the ZTD, if the atmospheric pressure and temperature at the station are known through a concept often referred to as GNSS meteorology. A number of GNSS receivers have been deployed for mapping and geodetic services in Nigeria under the African reference frame initiative, but unfortunately most of these receivers do not have co-located meteorological sensors for pressure and temperature measurements. The prospect of incorporating GNSS meteorology into weather monitoring and climate analysis in Nigeria was investigated and is reported in this thesis. During the first task of this research, the technical basis for ground-based GNSS meteorology was reviewed and the potentials and challenges of the approach to meteorological activities in Africa (including Nigeria) were identified. Thereafter an in-depth analysis of the spatial and temporal variability of ZTD over Nigeria for the period of 2010-2014 was conducted; results revealed weak spatial dependence among the stations. Tidal oscillations (of the diurnal and semidiurnal components) were observed at the GNSS stations of which the diurnal ZTD cycles exhibited significant seasonal dependence, affirming the prospective relevance of ground-based GNSS data to atmospheric studies. Also in this research, the accuracy and suitability of using reanalysis datasets (ERA-Interim and NCEP/NCAR) and a GPT2 neutral model in retrieving PWV from GNSS observations over Nigeria were investigated; results showed that PWV can be retrieved to within a precision of about 1 mm, provided GNSS-derived ZTD is of high precision. A fundamental issue for GNSS meteorology in the West African region was yet again addressed in this research; this is the development of a weighted tropospheric mean temperature model for use in current and future GNSS meteorology activities in the region. A multitechnique comparison of PWV estimates showed good agreement between GNSS estimates and other techniques (i.e. the atmospheric infrared sounder, and ERAInterim reanalysis). This result is suggestive of the potential of assimilating GNSS atmospheric products into reanalysis and climate models. Diurnal and seasonal variability of GNSS PWV estimates exhibits strong correlation with weather events that influence the region (i.e. solar activity and rainfall events); this further demonstrated the immense contribution of the approach to efficient weather forecasting and climate monitoring for Nigeria. / Thesis (PhD)--University of Pretoria, 2017. / Geography, Geoinformatics and Meteorology / PhD / Unrestricted

Several phenomena in the middle latitude stratosphere are studied in the thesis. The reanalyses are utilized as a source of data for our study. Different reanalyses are compared each other and with observations and problems of reanalyses are shown. We are interested in connection between ozone, dynamics and other phenomena (Sudden Stratospheric Warmings, solar cycle, NAO etc.) in the stratosphere, mainly from 1979 to present using ERA-Interim, NCEP/NCAR reanalyses or satellite observations. The linear connection between total ozone difference and 100 hPa eddy heat flux is found in winter middle latitudes. Ozone trends in Europe and China are analysed. The meridional and zonal wind is also studied using MERRA reanalysis and model output (CCM SOCOL v3.0). The comparison of geographical distribution of geopotential height and meridional wind is done. Differences between MERRA and CCM SOCOL v3.0 has been observed for geopotential hight and meridional wind for winter season.

We use ERA-Interim reanalysis data of 2 meter temperature to perform a pattern analysis of the Arctic temperatures exploiting an artificial neural network called Self Organizing-Map (SOM). The SOM method is used as a cluster analysis tool where the number of clusters has to be specified by the user. The different sized SOMs are analyzed in terms of how the size changes the representation of specific features. The results confirm that the larger the SOM is chosen the larger will be the root mean square error (RMSE) for the given SOM, which is followed by the fact that a larger number of patterns can reproduce more specific features for the temperature. / Wir benutzten das künstliche neuronale Netzwerk Self Organizing-Map (SOM), um eine Musteranalyse von ERA-Interim Reanalysedaten durchzuführen. Es wurden SOMs mit verschiedener Musteranzahl verglichen. Die Ergebnisse zeigen, dass SOMs mit einer größeren Musteranzahl deutlich spezifischere Muster produzieren im Vergleich zu SOMs mit geringen Musteranzahlen. Dies zeigt sich unter anderem in der Betrachtung der mittleren quadratischen Abweichung (RMSE) der Muster zu den zugeordneten ERA Daten.

No description available.

Climate change is expected to increase the frequency, intensity and duration of droughts in all parts of the United States (US). Snow packs are disappearing earlier in the spring and summer, with reduced stream-flow. Lower reservoir levels, higher temperatures, and greater precipitation variability have been observed. Drought events in the US have threatened drinking water supplies for communities in Maryland and Chesapeake Bay as observed in 2001 through September 2002; Lake Mead in Las Vegas in 2000 through 2004; Peace River and Lake Okeechobee in South Florida in 2006; and Lake Lanier in Atlanta, Georgia in 2007. ENSO influences the climate of Florida; where El Niño years tend to be cooler and wetter, while La Niña years tend to be warmer and drier than normal in the fall through the spring, with the strongest effect in the winter. Both prolonged heavy rainfall and drought potentially have impacts on land uses and many aspects of Florida's economy and quality of life. Drought indices could integrate various hydrological and meteorological parameters and quantify climate anomalies in terms of intensity, duration, and spatial extent, thus making it easier to communicate information to diverse users. Hence, understanding local ENSO patterns on regional scales and developing a new land use drought index in Florida are critical in agriculture and water resources planning and managements. Current drought indices have limitations and drawbacks such as calculation using climate data from meteorological stations, which are point measurements. In addition, weather stations are scarce in remote areas and are not uniformly distributed. Currently used drought indices like the iv PDSI and the Standardized Precipitation Index (SPI) could not fully demonstrate the land use effects. Other limitations include no single index that addresses universal drought impact. Hence, there is a renewed interest to develop a new “Regional Land Use Drought Index (RLDI) that could be applied for various land use areas and serve for short term water resources planning. In this study, the first and second research topics investigated water and energy budgets on the specific and important land use areas (urban, forest, agriculture and lake) in the State of Florida by using the North American Regional Reanalysis (NARR) reanalysis data. NARR data were used to understand how drought events, EI Niño, La Niña, and seasonal and inter-annual variations in climatic variables affect the hydrologic and energy cycle over different land use areas. The results showed that the NARR data could provide valuable, independent analysis of the water and energy budgets for various land uses in Florida. Finally, the high resolution land use (32km×32km) adapted drought indices were developed based on the NARR data from 1979 to 2002. The new regional land use drought indices were developed from normalized Bowen ratio and the results showed that they could reflect not only the level of severity in drought events resulting from land use effects, but also La Niña driven drought impacts.

Antarctic Wind Engineering deals with the effects of wind on the built environment. The assessment of wind induced forces, wind resource and wind driven snowdrifts are the main tasks for a wind engineer when participating on the design of an Antarctic building. While conventional Wind Engineering techniques are generally applicable to the Antarctic environment, there are some aspects that require further analysis due to the special characteristics of the Antarctic wind climate and its boundary layer meteorology. The first issue in remote places like Antarctica is the lack of site wind measurements and meteorological information in general. In order to complement this shortage of information various meteorological databases have been surveyed. Global Reanalyses, produced by the European Met Office ECMWF, and RACMO/ANT mesoscale model simulations, produced by the Institute for Marine and Atmospheric Research of Utrecht University (IMAU), have been validated versus independent observations from a network of 115 automatic weather stations. The resolution of these models, of some tens of kilometers, is sufficient to characterize the wind climate in areas of smooth topography like the interior plateaus or the coastal ice shelves. In contrast, in escarpment and coastal areas, where the terrain gets rugged and katabatic winds are further intensified in confluence zones, the models lack resolution and underestimate the wind velocity. The Antarctic atmospheric boundary layer (ABL) is characterized by the presence of strong katabatic winds that are generated by the presence of surface temperature inversions in sloping terrain. This inversion is persistent in Antarctica due to an almost continuous cooling by longwave radiation, especially during the winter night. As a result, the ABL is stably stratified most of the time and, only when the wind speed is high it becomes near neutrally stratified. This thesis also aims at making a critical review of the hypothesis underlying wind engineering models when extreme boundary layer situations are faced. It will be shown that the classical approach of assuming a neutral log-law in the surface layer can hold for studies of wind loading under strong winds but can be of limited use when detailed assessments are pursued. The Antarctic landscape, mostly composed of very long fetches of ice covered terrain, makes it an optimum natural laboratory for the development of homogeneous boundary layers, which are a basic need for the formulation of ABL theories. Flux-profile measurements, made at Halley Research Station in the Brunt Ice Shelf by the British Antarctic Survery (BAS), have been used to analyze boundary layer similarity in view of formulating a one-dimensional ABL model. A 1D model of the neutral and stable boundary layer with a transport model for blowing snow has been implemented and verified versus test cases of the literature. A validation of quasi-stationary homogeneous profiles at different levels of stability confirms that such 1D models can be used to classify wind profiles to be used as boundary conditions for detailed 3D computational wind engineering studies. A summary of the wind engineering activities carried out during the design of the Antarctic Research Station is provided as contextual reference and point of departure of this thesis. An elevated building on top of sloping terrain and connected to an under-snow garage constitutes a challenging environment for building design. Building aerodynamics and snowdrift management were tested in the von Karman Institute L1B wind tunnel for different building geometries and ridge integrations. Not only for safety and cost reduction but also for the integration of renewable energies, important benefits in the design of a building can be achieved if wind engineering is considered since the conceptual phase of the integrated building design process.