Numerical simulation of the interaction between the subtropical jet and low level fronts in the south and southeast regions of Brazil

The society has realized the importance of knowing the meteorological systems which act over a region. Specifically in the south and southeast regions of Brazil, there is great interest in those ones that give rise to large amount of precipitation and strongly affect the day by day of people and the economy in general. Observational studies have suggested that an interaction between the subtropical jet and low level fronts can be responsible for intense precipitation over these regions. However, it is not clear how this interaction occurs and through which mechanisms.

Using the Regional Atmospheric Modeling System (RAMS), this work aims the identification of the interaction between the subtropical jet and low level fronts in the south and southeast of Brazil, focusing on the ageostrophic circulations in the transverse and vertical plane.

A case study occured on June 25th, 26th and 27th 1996 is presented. A low level front displaced from Argentina/Uruguay to the southeast of Brazil while upper level flow patterns show the subtropical jet over the south of Brazil. From the numerical simulations, it can be seen that the presence of the jet does develop and intensify the precipitation associated to the low level front when the latter moves to the region below the upper level divergence due to the jet. This overlap results in strong updrafts and intense precipitation.

The agreement of the results from the numerical simulations themselves and the observational data analysis allow us to rely upon them.

Outstanding simulations considered the water vapour as a passive tracer and a surface with flat topography. In this case study, the flat topography does not interfere in the dynamics associated to these two meteorological systems, leading to almost any impact on the precipitation development. On the other hand, the latent heat release was found to be crucial to accelerate the updraft.