Estudo da atuação de um sistema de latitudes médias de semiárido do Nordeste do Brasil: aspectos de meso e grande escalas.

Abstract

This work investigates the consequences of a mid-latitude system invasion of semi-arid Northeast Brazil in the beginning of June 1985. Large and mesoscalc aspects of the episode are diagnosed using; (a) NCEP gridpoint data, (b) METEOSAT images, (c) precipitation fields obtained by a O-band meteorological radar installed in Pelrolina-PU (9°24'S, 40°29'W), (d) data from daily radiosonde ascents at Pclrolina. (c) rain rales and daily rainfall totals obtained by raingauges located within the radar coverage area. Diagnostics of the large scale scenario shows that a cyclogenesis starting on June 3 within a pre-existing frontal band off the southeastern coast of Brazil causes a sequence of events which includes; (a) the advance, up to 7°S approximately, of a midlatitude trough whose axis is meridionally oriented in the middle troposphere, near 45°W, from the Northeast Brazil up to the South Atlantic; (b) the weakening of the trade winds regime starting on June 4, (c) the presence of low level moisture convergence in most of the Northeast, on June 5 and 6, (d) an extensive cloud coverage on the Northeast on June 5 and 6, in contrast with the previous days. Ambient conditions favorable to development of precipitating convective systems are analyzed by considering as stability index the maximum parcel convective energy ( E M A X ) . Results are compared with the degree of organization and intensification of the echoes detected by the Pelrolina radar. It is found that EMAX values indicate a degree of instability which is insufficient to explain the convective systems' development within the radar coverage area, despite the gradual increase of E M A X during the period of study. Deep cells with tops as high as 15 km and organization on the mesoscalc are responsible by high rainfall rates and large areas covered by stratiform rain which last longer than 6 hours. Low level moisture convergence associated with the mid-latitude system is the mechanism responsible by the development and intensification of the precipitating systems.