The characterization of the onset of the rainy season in Paraíba do Sul Valley

As precipitation is the most important climatic element for agricultural productivity and urban planning, the study of its seasonal characteristics and its temporal-spatial variability are fundamental to the understanding and prevention of risks in human activities, whether in rural or urban areas. The region’s rainy season was analyzed in order to characterize its dynamics using rainfall observations and climatic simulation data. The work was divided into three steps: 1) a comparison of three different methods for determining the onset of the rainy season based on rainfall data from São José dos Campos-SP and Taubaté for the period from 1982 to 2010; 2) an investigation of whether the physical mechanism of generation and beginning of the rainy season is determined by the passage of an extratropical cyclone (or cold front) at the Paraiba´s valley; and 3) the use of data from a climatic simulation to investigate the behavior of the rainy season at the end of the XXI century. The onset of the rainy season, with little difference between the methods employed, occurs around October 23rd to November 6th in Taubaté and October 28th to November 11th in São José dos Campos. In 82.8% of the cases, the passage of a cold front contributed to the onset of the rainy season. In the climatic simulation results, it was found that there is a delay of 2 pentads (for scenario A2) or 3 (scenario B2) in the beginning of the rainy season. These results can contribute to planning in order to prevent natural disasters in the study region