Low-Level Jet seasonal climatology on Porto Alegre-RS-Brazil metropolitan region, using 1989-2003 upper-air observations

Abstract

The aim of this work was to develop a climatology observational study of the Low-Level Jet (LLJ), observed at Porto Alegre-RS-Brazil metropolitan region, using 1989-2003 at 00:00 and 12:00 UTC upper-air observations, from Porto Alegre International Airpot station. The LLJ classification criteria proposed by Bonner (1968) and modified by Whiteman et al. (1997) were applied to determine the LLJ occurrence. Afterwards the LLJ selected were separated by season (autumn, winter, spring and summer) and then were classified according to: i) LLJ intensity, ii) more frequent height of LLJ occurrence and iii) LLJ predominant direction. The results showed that the LLJ have been more frequent in spring (12:00 UTC) and in summer (00:00 UTC). Mostly of them presented velocity between 10 and 16 m s-1, and seems to occur in distinct heights depending on the time (at 00:00UTC in 600 m and at 12:00 UTC in 900 m). The LLJ predominant direction showed that at 00:00 UTC, the LLJ were more frequent in the east quadrant, while at 12:00 UTC, this predominance was verified in the west and north quadrants.