Observations of the turbulent exchange between a river surface and the atmosphere in a mountainous area in southern Brazil are presented and discussed. A micrometeorological tower was installed directly above the surface of a 60-m-wide river. This paper describes the observed turbulent fluxes over 12 days of observations at this site. Eddy correlation sensible and latent heat fluxes are directed toward the river during daytime and from the river at night, and they are controlled by differences between water and air temperatures. The magnitude of the vertical fluxes between the river and the atmosphere increases during daytime with increasing temperature gradient up to a threshold, beyond which the increasing stability starts to dampen the fluxes. Water and air temperatures show very little variations across the width of the river, indicating that the measurements taken at one margin may be representative of the mean river exchange. Local scalar budgets show that daytime warming and moistening rates above the river are controlled by local transport from the riverbanks. The main vertical fluxes have a very small magnitude: 0.8 W m−2 for sensible heat and 1.1 W m−2 for latent heat. Events of very large sensible heat fluxes from the river to the atmosphere and very large latent heat fluxes from the atmosphere to the river happened on 3 days, following nights with a very deep fog layer in the valley. These events represented the passage of a warm and dry air mass down the river. A process to explain the occurrence of these large fluxes is suggested that is associated with differential fog dissipation over the valley.
Neste trabalho analisam-se as características espectrais da turbulência atmosférica na região do Centro de Lançamento de Alcântara (CLA), que sofre a influência de uma falésia e da brisa marítima provinda do oceano, visando obter informações sobre a estrutura turbulenta do local. Determinaram-se os espectros das 3 componentes do vento (u, v e w) na camada limite superficial sobre esse terreno homogêneo, visando aplicações no Projeto e Desenvolvimento dos foguetes de sondagem e do Veículo Lançador de Satélite (VLS) realizado pelo IAE. Os dados foram coletados no mês de agosto do ano de 1999 através de um anemômetro sônico tridimensional. Também se obteve resultados envolvendo as relações σu/u*, σv/u* e σw/u*, todos condizentes com os previstos na literatura.
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