Estrutura e evolução de frentes frias intensas na região amazônica brasileira

Abstract

Some observed characteristics of the cold frontal incursions into the Amazon Basin during the ten year period 2001-2010 are described. The precipitation and the subsequent fall of temperature at Manaus (3oS, 60oW) associated with the cold frontal passages occur two days later than at São Paulo. The mean precipitation of 34 mm/day on the day of the passage of the front over Manaus in the austral summer and 38 mm/day in austral winter are substantially larger than the precipitation on the preceding and the succeeding days, showing the importance of frontal passages for the rainfall in the basin. The fall of minimum temperature associated with the cold air incursion is much smaller than the fall of maximum temperature, of the order of 8oC in 48 hours observed at Manaus. The low-level wind anomalies to the south of 10°S over Bolivia, Paraguay and southern Brazil in the austral summer frontal event composites show cold southerlies. The development of an inverted trough in the lower troposphere over the continent in the subtropics makes the southerly winds to strengthen and reach the Amazon Basin. The extratropical synoptic scale trough at 850 hPa level to the south of 30°S in the South Atlantic moves from 40°W to 30°W in two days. The upper troposphere wind anomalies show a closed cyclonic circulation with its center off the coast of southern Brazil. The moisture flux convergence over the Amazon basin during frontal passages is more intense and accounts for the rainfall. The rainfall and the fall of temperature associated with fronts in the Amazon Basin have small correlation, which complicates the study of these synoptic phenomena in the Amazon region. The simulations using the Eta regional model, showed that both the convective parameterization Betts-Miller (BM) and Kain-Fritsch with momentum flux (KFm) showed good consistency with the pattern observed, satisfactorily simulating the diurnal variation of the meteorological variables. However, none of them proved to be better over the other, alternating between underestimation and overestimation. The frontogenetic function is a good diagnostic of the case studied, indicating the areas of frontogenesis and frontolysis satisfactorily.