Rain Type and Flood Events in Southern of Brazil

In Brazil there is more than 12,000 flood and flash flood events already registered (1991-2012) (CEPED, 2013), with a total number of impacted people by such events exceeding 20 million. The Southern and Southeastern regions of Brazil together account 69% and 56% of floods and flash floods occurrences, respectively, with events throughout the year, but more frequently during the summer period of the Southern Hemisphere. The main phenomena related to flood events usually occur associated with intense and prolonged rainfall events in the rainy periods, which correspond to the summer in the South and Southeast regions and the winter in the Northeast Region. In Brazil, a large part of the scientific community associates natural disasters to meteorological systems, but there is still a gap in the knowledge of the relationship between rainfall type and flood events. In addition, results regarding these relationships may bring improvements in the representation of rainfall in meteorological models, especially for short periods of time, thus producing subsidy for more effective warnings for natural disasters related to precipitation. In this context, the objective of this work is to identify patterns of rainfall and main atmospheric characteristics, directly related to the occurrence of floods in some states of southern of Brazil, where initially were cataloged 444 flood events. In order to determine the type of rainfall for each occurrence of cataloged flood, the product 2A25/PR-V7 obtained by the radar on the Tropical Rainfall Measuring Mission (TRMM) satellite was used, and for identify the meteorological systems and atmospheric patterns associated with flood events analyzed, we used data from the Modern Era Retrospective Analysis for Research and Applications (MERRA). Considering the proposed methodology, 192 events were recorded for the study. Seasonally, 61% of the events occurred during the summer, 12% during autumn, 11% during winter, and 16% in the spring. The results allowed to observe that of all the analyzed events there is predominance of stratiform rainfall, followed by shallow convective and deep convective rainfall. It is noted that the frequency of occurrence of rainfall types for the events analyzed in the states is shown according to the climatology. When analyzed the rainfall rate for each type of rainfall, it is observed that for the events occurred in the north of the study area, the highest rates are related to deep convective rainfall, while for the south part of the study area the stratiform rainfall events are those that present higher rates for the studied events. When analyzing the spatial distribution of the rain type, it is observed that a great part of the events of stratiform rain are concentrated in the east part of continent, and nucleus on the coast and another one in the extreme west. The events of deep convective rainfall are more frequent on the north part, especially on the northwest, whereas the events of shallow convective rainfall are well distributed over the study area, with low frequency in the south part. The precipitation rate characteristics for each type of rain show to be directly related to the type of meteorological system that is active in the study area, where for the south a large part of the events (86%) are related to the frontal systems and, for the north, a large part of the events (63%) are related to the South American Convergence Zone (SACZ). The geopotential and flow anomalies show that on the surface there is an intensification of a low pressure system and a displacement of an anomalous cyclonic flow towards the south of Brazil during the flood events, supporting the condition of frontal and SACZ. The predominant occurrence of stratiform rainfall during flood events in the south part of the study area may also have a direct relation of the interaction between transient systems, observed in flow and geopotential anomalies, and orography. On the north part of the study area, the anomalous conditions of circulation and geopotential support the performance of the SACZ and the convective activity associated to this system, having a direct relationship with the found results. Finally, the results obtained will provide subsidy for the monitoring of flood events from remote sensing, and contributing significantly to the improvement in precipitation parameterization schemes in meteorological models for weather forecasting and nowcasting.