Mesoscale and Synoptic Environment in Three Orographically Enhanced Rain Events on the Coast of Santa Catarina (Brazil)

Abstract

Abstract In this study three cases of extreme rainfall events are analyzed in Florianopolis, Santa Catarina (SC), on Brazil’s coast, lying between the mountains and the South Atlantic Ocean in southeastern South America. The largest rainfall totals, 200–300 mm in 12–24 h, resulted in flash floods. ECMWF 6-hourly 1.5° × 1.5° data were used to determine the synoptic fields and quasigeostrophic forcing, along with observational data from local networks, for subsynoptic analyses. In case C1, during May 2010, there was a frontal passage. In C2 and C3 (January 2008 and February 1979, respectively) the coastline separated a ridge over the continent from a trough over the sea at the surface, with an atmosphere of low baroclinicity predominating at 500 hPa. All three events were characterized by postfrontal conditions; the presence of an anticyclone over the sea, centered at 35°–40°S; and slow-moving or quasi-stationary midlevel cyclones over the continent, at 27 °S. The circulation generated in the cyclone resulted in strong (10 m s−1) and persistent low-level east-northeast winds along the coast of SC, favoring the influx of heat and humidity on days with heavy rain. The upward motion at 850 hPa was concentrated over the affected areas and stronger than that at 500 hPa. Even so, the moist air that reached the mountain in a conditionally unstable environment rose only to middle levels. The features presented were similar to those observed for orographic rain events in Europe and the United States.