Atmospheric characteristics that induce extreme precipitation in frontal systems over Southeastern Brazil during summer: Observations and atmospheric model simulation

Abstract

Cold fronts are the most frequent synoptic systems that affect Southeast Brazil during the whole year. The highest frequency of these systems at these latitudes occurs in the months of spring (SON) and winter (JJA), but the highest associated precipitation occurs in the spring (SON) and summer (DJF). Some of these fronts present light rains but others cause heavy precipitation, giving rise to flooding and landslides, mainly in the summer. Composites of the two cases (light and heavy rainfall) are analysed for the summer season. Large‐scale and regional features are shown to discuss the differences in precipitation. Regional differences are seen in the position and intensity of low pressure and postfrontal high. The differences are also associated with the humidity flux and convergence over the region. The main difference at high levels is the position of the frontal trough, which is close to the region in the wet cases and displaced to the ocean in the dry cases. Features of the Southern Annular Mode and Madden–Julian Oscillation show opposite patterns in the two cases. These modes of variability can modulate extra‐tropical wave trains over the Pacific, which have different behaviour downstream, over South America and South Atlantic Ocean. The eddy kinetic energy is stronger over Southeastern Pacific and Southern South America, and wave activity shows energy propagation towards the continent in the wet cases. Similar analyses with results of a global atmospheric model show that the general atmospheric characteristics of a frontal system are reproduced, as the temperature gradient, the wind confluence and the ridge‐trough pair associated with the frontal system. The model represents the differences with respect to moisture flux and the frontal trough position, but underestimates the moisture convergence, eddy kinetic energy and wave activity, in the wet cases.

Synoptic patterns of South Atlantic Convergence Zone episodes associated with heavy rainfall events in the city of Rio de Janeiro, Brazil

“…On D-1, this frontal system is located approximately to the southeast of the Santa Catarina state without reaching the coast of the Southeast region of Brazil on D0. A similar pattern was observed byAndrade and Cavalcanti (2018) during heavy rainfall summer fronts over Southeast Brazil. It is also noted the SASA located around 30°S, 20°W which moves slowly to the east over the entire analyzed period.…”

Aridity Indices to Assess Desertification Susceptibility: A Methodological Approach Using Gridded Climate Data

“…This area coincides with the climatological position of the SACZ (Carvalho et al 2004;Nielsen et al 2019). Coastal areas of São Paulo and Rio de Janeiro states presented also precipitation totals higher than 1400 mm, most likely associated with FS, and convective and orographic precipitation (Andrade and Cavalcanti 2018;Brito et al 2017;Seluchi et al 2011). Precipitation totals over mm were also observed in the coastal areas of SEB, likely associated with the combination of synoptic systems (FS and SACZ) with mesoscale systems, in addition to the influence of topography, sea and land breezes and mountain/valley circulations (Brito et al 2017;Lyra et al 2018).…”

“…Patterns of precipitation over Southeast Brazil (SEB) are associated with the passage of frontal systems and, during the summer, the South Atlantic Convergence Zone (SACZ), a band of clouds oriented NW-SE (Satyamurty et al 1998;Brito et al 2017;Andrade and Cavalcanti 2018;Lyra et al 2018). Frontal systems (FS) are more common in spring and winter, but total precipitation is highest for events during the months of spring and summer (Andrade and Cavalcanti 2018). The SACZ accounts for 25% of precipitation between October and April, on average, with peaks of 41% in January and 56% in March (Nielsen et al 2019).…”

Climate variability over South America‐regional and large scale features simulated by the Brazilian Atmospheric Model (BAM‐v0)

“…However, the relations of extremes in South America with convection in Indonesia region, identified in observational data were not reproduced. Characteristics of frontal systems that affect Southeastern South America were reasonably simulated by the model (Andrade and Cavalcanti, ).…”