Multiscale analysis and simulations of an extreme rainfall event in Northeast Brazil with the MPAS model

Extreme rainfall events are becoming increasingly frequent in Northeast Brazil (NEB). The state of Alagoas, located on the eastern coast of the region, is one of the most affected areas in recent years, with records of high-magnitude events over the past four years. These events cause significant socioeconomic impacts, resulting in considerable human and material losses, underscoring the importance of a deeper understanding to mitigate short-term risks better. This study aims to investigate the synoptic and mesoscale conditions driving the extreme precipitation event occurred on May 6-7, 2024, which rainfall totals surpassed 270 mm/day across multiple areas of Alagoas, marking the highest 24-hour rainfall accumulation in the region this century. The study also evaluated the ability of the global Model for Prediction Across Scales (MPAS) to perform simulation for the extreme precipitation event, using a variable grid of 60-3 km and convection-permitting parameterization through two microphysics schemes: WSM6 and Thompson. Infrared channel (10.35 µm) images from the GOES-16 satellite were used to monitor the cloudiness development. ERA5 global reanalysis data were utilized to evaluate the synoptic conditions as a first analysis step. Observed precipitation data from MERGE/INPE, S-band meteorological radar, and rain gauges operated by CEMADEN were used to analyze accumulated precipitation. Synoptic analysis, through streamlines, revealed strong wind shear between 200 and 850 hPa, which was responsible for developing a Squall Line that propagated and reached Alagoas on May 6. The propagation of eastward-moving cloudiness towards the study area was observed on the same day, resulting from an Easterly Wave Disturbance (EWD) identified through the transport of kinetic energy originating near the African continent (1°E; 20°S). The displacement and intensification of this system towards NEB were confirmed by the intense vertical integrated moisture transport convergence (1000–200 hPa) over time, enhancing convection as it encountered the mesoscale system over the continent. As confirmed by anomalies, Sea Surface Temperature (SST) played an essential role in intensifying vertical motions, which were unusually high for this time of year. Overall, the EWD trough axis propagating along the trade winds, combined with intense moisture convergence, symbolized the intensification of upward movements in the region, where the dynamic conditions necessary for the development of the extreme precipitation event were established. The simulations showed that the MPAS underestimated the intensity of precipitation associated with the extreme event, although the simulations predicted values exceeding 50 mm/day in the most affected area. The results show similar performance in reproducing weather variables, with slightly better results for the WSM6 run. Preliminary results provide valuable insights into the performance of MPAS, emphasizing the need for further evaluation using additional physical parameterizations and alternative model configurations to enhance its predictive accuracy.