Atividade convectiva no sul da América do Sul

Abstract

In this work synoptic and mesoscale aspects are analyzed in association to the formation of Mesoscale Convective Systems (MCS) at South of South America (SA) that reached the State of Rio Grande do Sul (RS) in some moment during its life time. The selected period involves the month of February of the years 2002 to 2005. Among the identified MCS, two cases had been chosen for individual study because one entered on the RS and another one not, although they presented similar total duration and resulted from the interaction between several MCS. Also it were analyzed six MCS, three observed in March/2002 and three in February/2003, to identify predictors patterns of its occurrence some hours before its formation. The experimental base to the accomplishment of this work included geostationary satellite imagery, Sea Surface Temperature (SST) anomalies values on the Atlantic and Pacific Oceans, observed data at the surface meteorological stations located on RS and simulations obtained with the MM5 model. The comparison between the precipitation anomalies with the climatologic field showed that February 2003 was extremely rainy while in February 2002, 2004 and 2005 it had precipitation deficit. The amount of MCS that reached the RS in February of 2003 was approximately six times greater than the observed in the other months. The values of the anomalies of SST in the Atlantic Ocean had influenced decisively on the intensity of the observed convective activity at the south of SA. The schedules and geographic localizations of beginning and ending of the MCS, as well as the preferential direction of displacement of them, had been different for each one of the analyzed months. The individual analysis of two cases showed differences in the thermodynamic and kinematic fields. Particularly, the field of wind at the surface acted decisively, favoring or hindering the penetration of the MCS in the State. The best results to the prediction of MCS had been found for the interval of six hours before its formation. In the region where the MCS had formed the values of mixing ratio were next to 10 g/kg and the air was sufficiently humid in the layer superface/850hPa. The instability index K and Total-Totals had indicated that values from 15ºC and 40ºC, respectively, are good pointers of the MCS occurrence. Values of horizontal velocity greater than 15 m/s in the level 850 hPa were found to the north/northeast of MCS more intenses. A methodology of forecast for the period of six hours before the formation of the MCS observed in similar synoptic conditions to the analyzed ones in this work was proposal.