Estudo observacional de jatos de baixos níveis na região metropolitana de Porto Alegre durante o período de 1989 a 2003

Abstract

To analyze the occurrences of the Low-Level Jets (LLJ) at the metropolitan region of Porto Alegre, fifteen years of collected data of sounding between 00 and 12UTC in the period 1989 to 2003. For this, the criteria of classification proposed by Bonner (1968) and modified by Whiteman et al. (1997) have been applied. After being selected these LLJ were divided in function of the season of the year (cold tempered season, cold season, hot tempered season and hot season). Afterwards a classification of these LLJ was made according to predominant height, direction of the wind and levels where the same ones had occurred. After that, a case study was selected, the most intense LLJ that occurred on january 27th in 2002 at 12UTC, observed during the hot season (the most commom season for occurrencies of LLJ at 12UTC). To study it addicional tools were utilized: imagery of geostationary satellite GOES-8, fields of wind, specific humidity and temperature supplied by Global and ETA models and sounding before, during and after the occurrence of the JBN. As the Global and ETA models didn’t present a space and temporal resolution refined enough to analyze the LLJ, the numerical model of mesoescale BRAMS was used to simulate the study case. The observationals results showed that the LLJ had been more frequent in the hot tempered season (12UTC) and in hot season (00UTC), they present speeds between 10 and 16 m s-1, and occurred in distinct heights depending on the time (at 00UTC in 600 m and at 12UTC in 900 m). The predominant direction of the LLJ showed that at 00UTC, the LLJ had been more frequent in the first quadrant, while at 12UTC, this predominance was verified in the fourth quadrant. With the numerical analysis it was possible to verify that the three models, in the time of the occurrence of the LLJ, had overestimated the values of specific humidity and potential temperature. The speed of the wind was underestimated by the models, being the BRAMS, the model that underestimated most the observed value. It was observed in the study region, in the time of LLJ (12UTC), a stream of north, with hot and humid air, that favored the formation and intensification of the observed Mesoescale Convective System (MCS).