Dynamical and synoptic characteristics of heavy rainfall episodes in southern Brazil

摘要

The dynamical and synoptic characteristics that distinguish heavy rainfall episodes from non-heavy rainfall episodes in southern Brazil are discussed. A heavy rainfall episode is defined here as one in which the 50 mm day isohyet encloses an area of not less than 10 000 km in the domain of southern Brazil. 170 such events are identified in the 11-year period 1991-2001. The mean flow patterns in the period of one to three days preceding the episodes show some striking synoptic-scale features which may be considered as forerunners of these episodes: (a) a deepening mid-tropospheric trough in the eastern South Pacific approaches the continent 3 days before the episode; (b) a surface low-pressure center forms in northern Argentina one day before; (c) a northerly low-level jet develops over Paraguay two days before; and (d) a strong moisture flux convergence over southern Brazil becomes prominent one day before the episode. A parameter called Rainfall qunatity (RQ), defined as the product of the area enclosed by the 50 mm day isohyet and the average rainfall intensity, is correlated with fields of atmospheric variables such as 500-hPa geopotencial and 850-hPa meridional wind. Significant lag-correlations show that the anomalies of some atmospheric variables could be viewed as precursors of heavy rainfall in southern Brazil that can be explored for use in improving the forecasts.