Abstract
Seasonal rainfall forecasts generated with regional Eta model for the rainy season of the Sao Francisco River Basin, upstream Hydroeletric Power Plant (HPP) Tres Marias in Minas Gerais are evaluated in this work. The use of such forecasts as an input in energy planning models, represent a better management in the generation, transmission and distribution of electricity. However, it is well known that most of the globe, seasonal climate predictability is much reduced, since the effects of boundary conditions to determine the evolution of the average state of the atmosphere compete with internal variability associated with the chaotic instabilities and nonlinear interactions of the atmospheric flow. The forecasts of the Eta-Seasonal-15km model are initiated on days 13 to 17 October, extending until February 28, during the years 2001-2010. The evaluation results indicate that the noise (inter members variability) is higher than the signal (interannual variability), suggesting low reliability of the forecasts for the region during the rainy season. Forecasts of precipitation are underestimated and the Root Mean Square Error of 77.03 mm / month is high, almost twice its standard deviation. Note A high spatial variability of rainfall due to very steep topography, which further reduces the performance of numerical models. A hit rating category of precipitation (IACP), based on the distribution of tertiles, was applied to account for the number of times that the forecasts point to the same category of observed rainfall: rainfall below, within or above the normal range. The IACP for the whole area and during the ten years was low (mean 29%), however in the southern Basin IACP is a bit higher, around 50% to 70% in the Southeast region. The annual review of the forecasts for the entire area indicated that the best model performance occurred in 2005, when conditions ATMS negative in Pacific, near the coast of Peru were persisted in October 2005 and in fact such anomalies occurred through the month of February 2006.
Highlights
Neste trabalho previsões climáticas sazonais (4,5 meses), obtidas por modelagem numérica regional de alta resolução, são utilizadas para a área do reservatório da Usina Hidrelétrica (UHE) de Três Marias, localizada na sub-bacia do Alto São Francisco, entre os paralelos 18o e 21o S e os meridianos 43o30 ́ e 46o40 ́ W no Estado de Minas Gerais (Região Sudeste do Brasil)
Iniciada em maio de 1957 e concluída em janeiro de 1961, teve como principais objetivos a regularização das vazões do Rio São Francisco em parte de seu trecho Médio, o controle das enchentes nesse trecho durante as cheias periódicas, melhoria da navegabilidade, utilização do potencial hidrelétrico e fomento da indústria e irrigação (ONS, 2009)
A seguir, anomalias de precipitação previstas são confrontadas com os valores observados no mesmo período em 58 estações meteorológicas localizadas na área em estudo
Summary
Instituto de Geociências, Departamento de Meteorologia, Avenida Athos da Silveira Ramos, 274 – Bloco G. As previsões do modelo Eta-Sazonal-15km são iniciadas nos dias 13 a 17 de outubro, estendendo-se até 28 de fevereiro, durante os anos de 2001 a 2010. O IACP para toda a área e durante os dez anos foi baixo (média de 29%), contudo no sul da Bacia o IACP é um pouco mais elevado, em torno de 50%, chegando a 70% no sudeste da região. A avaliação anual das previsões para toda a área indicou que a melhor performance do modelo ocorreu no ano de 2005, quando condições de ATMS negativas no Pacífico, próximo a costa do Peru foram persistidas no mês de outubro de 2005 e de fato tais anomalias se verificaram até o mês de fevereiro de 2006.
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