Resumo:
Commonly, at the top of the Planetary Boundary Layer (PBL), in its nocturnal/stable phase,
the formation of a maximum in the vertical wind profile known as Low Level Jet (LLJ)
occurs. These events are observed in various parts of the world and have a great influence on
activities that depend on weather and climate, such as air transport. Several studies have been
carried out to evaluate the mechanisms capable of inducing the formation of LLJs. Among
these mechanisms are inertial oscillation, baroclinicity induced by sloping terrain and the
presence of synoptic systems. In this study, the occurrence of LLJs in the Furnas reservoir
region, located in Minas Gerais, MG, is investigated. In addition, an evaluation of the
synoptic conditions on the days of occurrence of the LLJs is made, as well as the response of
the surface flow. For that, 10 years of data from the ERA5 reanalysis were used along with
data measured in situ during a field campaign and one year of data from an anemometric
tower installed on the banks of the Furnas reservoir. The LLJs detection criteria were based on
a case study that had already been analyzed in previous studies. LLJs occur mostly in spring,
mainly in the months of September and October, between late evening (21 LT) and early
morning (8 LT). As for the synoptic characteristics, the LLJs occur predominantly in clear or
partially cloudy sky scenarios where there is a ridge penetration (high pressure) over the
Southeast or the western edge of the South Atlantic Subtropical High (SASH). The ERA5 data
showed a good ability to represent the wind speed, making it possible to identify profiles of
LLJs in the period under study. Two numerical simulations were performed with the WRF
model, the first with the original topography (control simulation) and the other with the
modified topography, removing the sloping relief around the reservoir. With the simulations it
was possible to infer about the influence of the sloping relief on the occurrence of LLJs. After
analyzing the observational and model data, it is considered the possibility that the LLJs in the
Furnas reservoir region are modulated mainly by baroclinicity induced by synoptic systems
with potential influence of sloping topography. The decoupling of the top of the PBL with the
levels closer to the surface in the day/night transition cannot be neglected, as it provides
support for the flow acceleration due to the temperature gradient. The presence of the sloping
topography causes a thermal gradient due to differential cooling/heating between
valley/mountain, in this case, the available potential energy can contribute to complement the
formation of the JBN due to baroclinicity in synoptic scale.