Análise dos fluxos de calor em um ciclone extratropical explosivo: experimentos numéricos de sensibilidade

Abstract

Extratropical cyclones are one of the atmospheric systems that most cause weather changes in the regions where they operate. In some situations, these systems have a deepening rate of ~24 hPa/24 h, which gives them the name of explosive cyclones. An important source of energy influencing the development of extratropical cyclones is surface sensible and latent heat fluxes (FCSL), especially over the oceans. Between late June and early July 2020, an explosive cyclone occurred off the southern coast of Brazil. As little is known about the physical development processes of explosive extratropical cyclones in the South Atlantic, the aim of this work is to verify whether turbulent heat flows from the ocean to the atmosphere contributed to deepen the system (pressure drop >= 24 hPa /24 h) and describe the drivers of cyclogenesis; to investigate the contribution of the sea-air interaction, through numerical simulations with the Weather Research and Forecast Model (WRF), and to present the physical properties of the clouds associated with the squall line, which caused a lot of environmental and material damages and, even, the death of at least 12 people in the State of Santa Catarina (SC). Cyclogenesis started at 1200 UTC on June 30, 2020 at the southern border of Brazil and Uruguay, with a valley at mid-upper levels as forcing, which is a common parameter of cyclogenesis in the studied region. ERA5 reanalysis data were used to describe the configuration of the cyclone and associated systems. The description of the impacts in the State of SC was carried out through the review of news published by the media and data provided by the civil defense of the State. To verify the role of turbulent heat fluxes, a comparison between numerical experiments with turbulent fluxes of sensible and latent heat on and off in the WRF model was used. It was observed through the experiments that the sea-air interaction (turbulent heat flows) contributed to the deepening of the cyclone, leading it to an explosive state, because when the flows were turned off, the cyclone lost its status as a bomb. The cyclone directly influenced the formation of a cold front, which in turn led to the development of a prefrontal squall line, responsible for the adverse conditions in the state of Santa Catarina. While the satellite images did not show the squall line located ahead of the cold front in the cyclone wave due to its resolution, the radar reflectivity data clearly represented the propagation of the squall line over southern Brazil.