FE - DECA | Dissertações de Mestrado e Teses de Doutoramento
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Browsing FE - DECA | Dissertações de Mestrado e Teses de Doutoramento by advisor "Almeida, Pedro"
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- Análise do vapor de água precipitável em Moçambique usando observações GNSSPublication . Vicente, Armando Luchato; Fernandes, Rui Manuel da Silva; Almeida, Pedro; Bos, MachielAs observações GNSS (Global Navigation Satellite System) são considerados atualmente como umas tecnologias poderosas para a monitorização do vapor de água na atmosfera, a qual é uma variável de bastante interesse para as ciências meteorológicas. Os equipamentos GNSS proporcionam uma boa relação de custo-benefício quando comparados com os demais instrumentos meteorológicos tradicionais. O sistema GNSS tem vantagem assinaláveis que fez que o mesmo fosse estabelecido como uma técnica de observação standard para a estimativa de Vapor de Água Precipitável (PWV) na atmosfera, utilizando o Atraso Zenital Troposférico (ZTD) que é a variável diretamente estimada utilizando as observações GNSS. Uma melhor compreensão do vapor de água na atmosfera, pode mitigar os impactos dos riscos naturais através da prevenção que é mais eficaz quando possuímos maior capacidade de prever fenómenos atmosféricos extremos que contribuem no risco de rupturas económica em grandes escalas nacionais e internacionais. Esta dissertação tem como objectivo calcular e analisar a variabilidade do ZTD/PWV na implementação de um produto automatizado usando dados GNSS+Temperatura+Pressão para Moçambique. As estimativas ZTD são calculadas usando a estratégia PPP (Precise Point Positioning) que permite estimar os parâmetros de cada estação separadamente, e que se adequa com a situação em Moçambique por este ainda não possuir uma rede densa de estações GNSS.
- Analysis of Precipitable Water Vapour in Angola Using GNSS ObservationsPublication . Gomes, Isildo Ntemo; Fernandes, Rui Manuel da Silva; Almeida, Pedro; Bos, MachielFor accurate weather predictions and analysis of extreme events, a good estimate of the amount of water content in the atmosphere is essential. This information is provided by several techniques like radiosondes that measure this parameter at various heights. However, most of them are very limited spatially and temporarily or suffer from measurement specific constraints. To complement these techniques, Precipitable Water Vapor (PWV) can be measured with GNSS (Global Navigation Satellite System) at CORS (Continuously Operating Reference Stations) networks. when the temperature and pressure are also known at the station location. PWV can be derived from the delay in the GNSS signal when it passes through the troposphere. In the framework of SUGGEST-AFRICA, it is being implemented a system to use the national GNSS stations for the automatic computation of PWV in Angola. Thus, this dissertation intends to describe the necessary steps to develop a system to be used for supporting meteorological and climate applications in Angola. SUGGEST-AFRICA also funded the installation of 5 weather stations, collocated with GNSS stations in Angola namely: Benguela, Cabinda, Cuito, Luanda and Namibe, in order to obtain pressure and temperature which is necessary to obtain the PWV estimates. When there are no nearby meteorological stations, the potential alternative is to use values from global/regional models. Methodologies have been optimized to passive and actively access the GNSS data; the PWV estimations are computed using PPP (Precise Point Positioning), which permits the estimation of each station separately; solutions have been validated using internal values. In addition, analyses are presented to evaluate the reliability of the network. This work presents preliminary results for the variation of the ZTD data available all around the territory in Angola and how they relate to the seasonal variations in water vapour. Also, presents preliminary results for the time-series variation of PWV in the Luanda station (collocated by the SEGAL group). This study is supported by SUGGEST-AFRICA, funded by Fundação Aga Khan and FCT. It uses computational resources provided by C4G – Collaboratory for Geosciences (PINFRA/22151/2016). It is also supported by project FCT/UIDB/50019/2020 – IDL funded by FCT.
- Analysis of Precipitable Water Vapour in Nigeria using GNSS ObservationsPublication . Bala, Ipalibo Ateriye; Fernandes, Rui Manuel da Silva; Almeida, Pedro; Bos, MachielWater Vapour estimation using ground-based Global Navigation Satellite System (GNSS) observations is a well-established technology that contributes to weather forecast, research, and climate monitoring. Water vapour in the atmosphere is directly related with precipitation that may lead to extreme event (e.g., floods). The application of GNSS to sense the total amount of water vapour integrated along the signal path in the troposphere is what is referred to as GNSS meteorology. GNSS has the advantage of all-weather condition, low cost with high temporal and spatial resolution when compared to other classical methods of water vapour measuring that are expensive and/or with low spatial and temporal coverage. When GNSS signals are transmitted from GNSS satellites in space to ground-based GNSS receivers, they experience a tropospheric delay (an error source in GNSS positioning) often represented in GNSS meteorology as the Zenith Total Delay (ZTD). The ZTD is the sum of the Zenith Hydrostatic Delay and the Zenith Wet Delay and it is one of the products of GNSS data processing. The ZTD can be converted to Precipitable Water Vapour (PWV) when surface temperature and pressure values are known at the GNSS site using a conversion factor (?) that is dependent on the weighted mean temperature (Tm) and pressure. This dissertation focuses on the estimation and analysis of water vapour in Nigeria using GNSS observations. The Nigerian Permanent GNSS Network (NIGNET) stations observations and products were retrieved from the infrastructure implemented by Office of the Surveyor General of the Federation (OSGoF). Processing of the data was carried out using online software (GipsyX) for the estimation of ZTD. Fifteen GNSS stations were used in this research and the period 2009 to 2021 was considered. The characteristics of the ZTD over the territory of Nigeria was investigated. The range of ZTD variation in Nigeria for the period used in this research was found to be approximately between 1900mm to 2700mm in the NIGNET stations. The two main seasons in Nigeria were significantly noticed as low peaks were found to be occurring during the dry (winter) season while high peaks were remarkably seen during the rainy (summer) season. The amplitude of the seasonal variation within the period under investigation is between a minimum of 36mm to a maximum of 124mm with the Northern region having higher values than the Southern part. It was discovered ultimately by the results obtained from the analyses, that ZTD variation in both the Northern and Southern regions are influenced by the 4 distinct climates and other local weather conditions including temperature and the trade wind from Sahara Desert and the Atlantic Ocean.