Frontogenesis over south america and precursors for the cold surges in the state of São Paulo / Frontogênese na América do Sul e precursores de friagem no estado de São Paulo

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

Intense cold air incursions in São Paulo (freezes) occur on the average once a year during the months of April through September and 80% during the months of May through August. The central part of the South American continent is frontogenetic due to the horizontal deformation field in the lower troposphere on winter. Composites of the frontogenesis function, potential vorticity (PV), surface pressure, temperature, lower and upper troposphere winds, 500 hPa geopotential are constructed using the reanalyses data from the National Centers for Environmental Prediction and the European Center for Medium Range Weather Forecasts for 14 freeze events (1979-93). The precursors of the freezes are summarized here. (i) A cold frontal passage over northern Argentina and southern Brazil during day -7 through day -5 precedes the passage of a second front responsible for the freeze event. (ii) A surface high pressure anomaly off southern Chile coast favors anticyclogenesis on day -5 to day -1 over Argentina. (iii) Two surface cyclogeneses during day -3 to day -1 create a low pressure center in the Atlantic off the Uruguay coast. (iv) The lower tropospheric frontolysis region over the Andes expands eastward into northern Argentina on day -2, into Paraguay and Uruguay on day -1, and into southern Brazil and Bolivia on day 0. (v) An upper troposphere (500-200 hPa) trough deepens over western Argentina on day -3. (vi) A cyclonic anomaly in the PV on the 325 K isentrope intensifies gradually over Argentina north of 45°S from day -7 to form a well defined center of -2.0 PVU over southern Brazil on day 0. The cooling trends at 925 hPa start on day -4 in Patagonia and propagate north-northeastward to Paraná, São Paulo and Mato Grosso do Sul on day 0 and the -8°C/day isotherm occupies the region of the states of Paraná, São Paulo and Mato Grosso do Sul. The advection is mostly responsible for the cooling (-3°C/day), however, the diabatic affects (-2 to -3°C/day) are important in the western and central parts of the continent. The numerical simulations with the T62L28 model of the Centro de Previsão de Tempo e Estudos Climáticos show that when the strength of the precursor synoptic perturbation on day -4 is weakened the situation does not evolve into a freeze, indicating that the amplitude of the precursor has a strong bearing on the subsequent evolution. The skill of the model forecasts remains above 60% up to 7 days lead time. It is conjectured that the cyclogenesis on day -1 is triggered by the approach of cyclonic PV anomaly and the difluent 500 hPa trough. The strong zonal surface pressure gradient over eastern Argentina is responsible for intense cold advection into the central parts of Brazil. The radiative cooling over the surface high pressure center intensifies further the thermal contrast across the front. The two reanalysis data sets agree well in describing the evolution of the freezes and, therefore, the reliability of the results is high. These findings are helpful to improve the forecasts of the severe cold air incursions into Brazil. The PV field shows capability to extend the lead time up to 8 days.

ASSUNTO(S)

meteorologia heat budget freezing experimentos vorticity frontogenese brasil friagem meteorology frontogenesis balanço de calor experimentation vorticidade

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