A Multiscale Analysis of the Most Extreme 24-Hour Rainfall Event in Brazil

The northern coast of São Paulo State, Brazil, experienced the most extreme 24-h rainfall event recorded in the country between the afternoon of 18 February and the morning of 19 February 2023, with accumulated precipitation exceeding 600 mm at several rain-gauge stations. This study investigates the atmospheric processes associated with the event, with emphasis on the potential contribution of a seeder–feeder mechanism, and evaluates the performance of convection-permitting (CP) WRF and RegCM5 models in simulating the event. A multiscale analysis was conducted using rain-gauge observations, ERA5 reanalysis, GOES-16 satellite imagery, polarimetric radar observations, and CP models. The event developed under a synoptic configuration commonly observed along the southeastern Brazilian coast, in which persistent southeasterly flow associated with an oceanic frontal system sustained moisture transport toward the coast. The exceptional rainfall intensity could not be explained by the large-scale environment alone, highlighting the importance of local topographic and cloud processes. Satellite and radar observations suggested that orographic precipitating clouds triggered by the Serra do Mar interacted with oceanic deeper convective bands that propagated inland. This interaction enhanced precipitation along the coastline and over the Serra do Mar through a seeder–feeder-like process. Radar observations captured signatures consistent with the main features of this mechanism: melted ice particles produced by the deeper convective band seeded the underlying orographic precipitation system, potentially enhancing collision–coalescence processes and, consequently, increasing precipitation. Both CP models reproduced the broad coastal precipitation band but displaced it inland. The WRF-3 km simulation exhibited the highest spatial correlation with radar-derived precipitation (r = 0.68), but also the largest positive bias (108.0 mm). Increasing the horizontal resolution to 1 km reduced the bias to 48.4 mm but did not improve the simulated location of the rainfall band. The RegCM5-CP experiments using different physical parameterizations highlighted the sensitivity of the simulated precipitation to model configuration. These findings indicate that the extreme rainfall resulted from the combined influence of sustained synoptic moisture supply, orographic lifting, and interactions between cloud systems, while also highlighting the challenges of accurately simulating the location and magnitude of extreme coastal precipitation.