Padrões de ecos de radar meteorológico no semiárido brasileiro e condições atmosféricas associadas.

Abstract

A unique set of C-band meteorological radar echoes is analyzed. The data were obtained in Petrolina-PE (9°24'S, 40°30'W), located in the semi-arid region of Northeast Brazil, during the first semester of 1985. The role played by thermodynamic and dynamic forcings during the period of study is investigated. A linear correlation analysis between the parcel convective energy and the number of echoes within different classes of horizontal area gave significant positive correlations for echoes with area < 400 km2 . Similar results were found for echo height. The parcel convective energy and rainfall measured by raingages were found to be correlated in April only. The echoes were classified following two criteria. The first one, based on the reflectivity gradients seen on the PPI and RHI images, allowed to identify convective, stratiform and stratoconvective echoes. The second classification, based on the apparent degree of organization seen on the PPI images, allowed to identify five main distinct patterns: scattered echoes, zone of echoes, line of echoes, strip of echoes and band of echoes. Results show convective echoes to account for 98,78% of the total number of echoes. They occurred throughout the observation period being more frequent in the southeast quadrant of the radar coverage. A relatively high frequency (0,8%) of stratiform echoes occurred in January only. Extensive and long-lasting predominantely stratiform precipitation fields covering about 35,000 km2 are found to be associated with upper tropospheric cyclonic vortices. Scattered echoes and zones of echoes were seen during the entire observation period. Lines and strips of echoes were more frequent in March and April. Bands of echoes occurred almost entirely in June. A system of frontal origin penetrating in the radar area provided the dynamic forcing to form the bands of echoes. All five distinct patterns were more frequent in the southeast quadrant of the radar coverage. It is hypothesized that this preferential location may be explained by, among other factors, the availability of moisture brought in by the southeast trade winds and the penetration and/or in situ formation of organized weather systems.