Estudo de mecanismos dinâmicos responsáveis pela ocorrência de chuvas extremas na bacia hidrográfica do Alto São Francisco.

Abstract

In the period from December 13th to 20th, 2011, an episode of SACZ over the southeastern region of Brazil caused several records of intense and persistent rains in the Alto São Francisco watershed, with very high accumulations in several meteorological stations. The fundamental objective of this work was to detect the mechanisms, both dynamic and thermodynamic, responsible for precipitation, using the WRF numerical model. The synoptic conditions prevailing in the event period were analyzed using reanalysis data from ERA5, however data provided by the GFS model were used to initialize the model and evaluate the sensitivity of three cloud microphysics parameterizations. With the use of the spectral relaxation technique, it was possible to correct the simulated circulation on a large scale without harming the smaller scales, resulting in a better behavior of the model in the local scale simulations. The presence of a trough in the South-Southeast region at all levels of the troposphere, together with other dynamic factors and the effects of the warming of the atmosphere by the release of latent heat and the high moisture content, led to the SACZ episode. The event was the result of the coupling influence between the convective scale and the large scale. The main mechanism responsible for the occurrence of extreme rainfall was CISK (Conditional Instability of Second Order), due to a deep convection with strong release of latent heat, which together with topographical effects contributed to the maintenance of cyclonic circulations at low levels. Other mechanisms were identified, resulting in more accentuated instability conditions, such as the presence at some times of a small Mesoscale Cyclonic Vortex, embedded in the stratiform region of the SACZ, together with a strong convergence of the wind at medium and low levels. and the transport of moisture through the Low Level Jet.