Extreme Precipitation Events in Northeast Brazil Triggered by Different Synoptic Scale Systems

This study aims to analyze the synoptic patterns responsible for the different systems development that led to extreme precipitation events in Brazilian Northeast (BNE) on three different dates - May 27, 2017, February 20, 2020, and March 14, 2023. Despite having the driest region of Brazil (The Sertão) with rainfall values not exceeding 400 mm per year, BNE is characterized by the influence of several synoptic-scale systems throughout the year that contribute to extreme precipitation events over the region. ERA5 global reanalysis data have been used for synoptic and vertical structure evaluation as a first step of analysis. MERGE precipitation product and pluviometric stations were used to document accumulated precipitation produced by these events. The following step will consist of conducting simulations at a very high-resolution using the Model for Prediction Across Scales (MPAS) to assess its ability to represent the circulation patterns associated with the extreme precipitation events analyzed. The three cases were situated in different sectors of BNE and were caused by different synoptic-scale systems: Easterly Wave Disturbances (EWD), Upper Tropospheric Cyclonic Vortex (UTCV) and Intertropical Convergence Zone (ITCZ), respectively. The first analyzed event was caused by an EWD, where the axis of a trough at the 850 hPa level was observed along with intense moisture transport along the eastern coast of BNE, resulting in an intense rainfall of 173 mm/24h. The second event, considered more atypical, was triggered by an UTCV, also along the BNE eastern coast. The precipitation from this system resulted in 68 mm in just 4 hours, surpassing the historical February average in Maceió, Alagoas. Finally, the third event was caused by the ITCZ and a Mesoscale Convective System (MCS) convection, resulting in heavy rains (140 mm/24h) over the Maranhão state, especially in the city of São Luís. Despite the different circulation patterns of the analyzed synoptic systems, some similar characteristics were identified, such as intense moisture convergence and strong wind shear, in addition to negative surface relative vorticity. Topography played a significant role. The first case analyzed (on the east coast) recorded the highest rainfall among the three cases, despite not presenting cloudiness with great vertical development. This occurred due to the high convergence of moisture transport combined with the elevated relief to the west, which facilitated increased convective activity. Vertical profiles of specific humidity and vertical air movements proved to be very important fields for analyzing the circulation patterns of synoptic scale systems, which in turn have quite different characteristics. Furthermore, it demonstrated its significance as a valuable instrument for enhancing comprehension of the vertical thermodynamic structures of the extreme precipitation events.