Abstract Changes in the physical and biogeochemical properties of water columns are frequently associated with cold fronts and mesoscale convective systems due to increased cloud cover. The effects of low-level jet (LLJ) events on thermal stratification and water quality, however, remain undescribed, particularly for tropical reservoirs. Here, water temperature time series are combined with meteorological data, LIDAR observations, ERA5 reanalysis data, and hydrodynamical modeling to investigate the impact of an event of LLJ over the Furnas hydropower reservoir in Brazil. The LLJ event was characterized by dry, intense, and persistent winds (~10 m s-1) blowing for more than 12 hours over the main fetch of the reservoir. In the downwind side of the lake, the surface mixed layer depth increased by 50% during the LLJ event. The changes to the water column were produced by a combination of wind-induced upwelling, shear-driven mixing, and nocturnal convective overturning, different from the heat balance expected during passing cold fronts and mesoscale convective systems. The results suggest that both momentum and heat fluxes during LLJ events need to be accounted for in lake modelings to reproduce the vertical mixing process.

Resumo Este artigo é uma contribuição do Projeto “uso de VANT para prospecção eólica em sistemas aquáticos”, que consistiu num esforço multi-instrumental e multi-institucional para estudar a camada limite atmosférica influenciada por extensos corpos d’água e pelas feições do entorno como colinas, rampas e mudanças abruptas de rugosidade. O trabalho apresenta o resultado de um experimento específico, realizado em novembro de 2016, que visou estudar a camada limite atmosférica (CLA) influenciada por colinas. Durante os experimentos foram levantados perfis verticais do vento a partir de um VANT, utilizado pela primeira vez no Brasil, para esta finalidade. O escoamento do vento, no topo da colina, apresentou forte intensificação a 20 m de altura. Os resultados foram comparados com um modelo analítico baseado na teoria linear aplicado a um terreno real.

Abstract This paper represents a contribution of the Project “unmanned aerial vehicles to wind assessment in aquatic systems”, a multi-institutional and multi-instrumented experiment carried out to measure, in detail, the characteristics of winds over a large tropical aquatic system and how the flow is modulated by hill, ramp and abrupt roughness changes. It presents results from experiments conducted during november 2016 over a typical hill site that is adequate for installation of wind turbines. During the experiments, both hilltop and upstream wind profile data were obtained using an unmanned rotorcraft, commonly known as a hexacopter. The flow speed at 20 m over the top of hill was found to be increased by a factor of 1.8. The results were compared with an analytical model which makes a careful application of linear theory to real terrain.