Identificação de jatos de baixos níveis na região do reservatório de Furnas/MG

Abstract

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.