Um estudo de caso sobre a influência de sistemas extratropicais no Nordeste do Brasil utilizando a vorticidade potencial.

Abstract

The period from 13 to 18 June 1975 was studied with the objective of determining if the use of the potential vorticity and its horizontal advection, in the middle and upper troposphere, is efficient in providing a qualitative prognosis of the evolution of extratropical systems which influence Northeast Brazil. Most of the analyzed days had mesoscale precipitation areas on the northern and eastern coasts of Northeast Brazil. The analysis of sequences of visible and infrared satellite imagery indicated that areas of cloudiness moving westward to the south of the Intertropical Convergence Zone (ITCZ) were responsible for rainfall areas located on the eastern coast of Rio Grande do Norte state and on Northeast Brazil's northern coast. Rainfall on the remaining of the eastern coastal area was caused by dissipating frontal bands that moved northeastward over the coast. On the middle (500 hPa) and high (300 hPa) troposphere, an extratropical trough moved northeastward over Argentina, Uruguay, the South and Southeast regions of Brazil and the southern area of Northeast Brazil, in the first half of the period of study. On the three remaining days, a west-east moving trough crossed South America within the latitudinal band of 15°S-25°S. Intense potential vorticity nuclei were associated with these troughs at the 500 hPa level. At 300 hPa an area of strong horizontal potential vorticity gradient indicated the location of the subtropical jet. This jet had a northwest-southeast band-like structure that passed over Caravelas-BA (17°S), about 15° to the north of its mean position, in five of the analyzed days. Two secondary maxima were seen over the Northeast Brazil on some days; one on the eastern coast and another on its interior. The use of the 300 hPa field of potential vorticity and its horizontal advection gave good results in the qualitative prognosis of the development and displacement of the extratropical systems within the analyzed area, during the entire period of study.