Defense
Student: Lucas Tártaro Pereira
Program: Meteorology
Title: "Ocean–Atmosphere Interaction and the Modulation of Coastal Extreme Precipitation During the February 19, 2023 Event on the Northern Coast of São Paulo"
Advisor: Prof. Dr. Ricardo Hallak
Judging Committee:
- Prof. Dr. Ricardo Hallak - Presidente e Orientador - IAG
- Prof. Dr. Renato Ramos da Silva - UFSC
- Dr. Danilo Couto de Souza - Pós-doc IAG
Other Members:
- Prof. Dr. Ricardo de Camargo - IAG
- Prof. Dr. Reinaldo Haas - UFSC
- Prof. Dr. Jorge Alberto Martins - UTFPR
Abstract:
During the early hours of February 19, 2023, the northern coast of São Paulo State was impacted by the largest accumulated precipitation event ever recorded in the region, with rainfall totals exceeding 648 mm within 24 hours concentrated between the municipalities of São Sebastião and Bertioga, causing dozens of fatalities, hundreds of displaced residents, and widespread destruction. This study investigated the dynamical and thermodynamical mechanisms that led to the occurrence of this catastrophic event, with emphasis on the role of Sea Surface Temperature (SST) through numerical simulations using the Weather Research and Forecasting (WRF) model. The synoptic analysis revealed the presence of an extratropical cyclone exhibiting a T-bone structure, consistent with the Shapiro–Keyser conceptual model, associated with a cold front and an Upper-Level Cyclonic Vortex (ULCV) over northeastern Brazil. This synoptic configuration established persistent south-southeasterly winds perpendicular to the coastline, favoring orographic uplift along the Serra do Mar mountain range. The analysis of observational datasets demonstrated that the ERA5 and GFS global reanalyses, as well as the IMERG satellite-based precipitation product, were unable to adequately capture the magnitude of the event. In particular, IMERG systematically smoothed extreme precipitation values, even when pentad accumulations were considered for the 2001–2023 period. SST observations showed positive pre-event anomalies (+1.3°C above the climatological February mean of the last 40 years), followed by an abrupt cooling exceeding 2°C during the event. This cooling was attributed to the deepening of the ocean mixed layer mechanically induced by wind stress. Five WRF simulations were conducted following an incremental refinement strategy of the model configuration by modifying initial conditions (GFS and ERA5), vertical resolution (56 and 76 levels), and microphysics schemes (Purdue-Lin and Thompson), followed by a sensitivity analysis focused on SST representation (fixed and evolving SST). The results demonstrated that transient SST variability played a modulating role in the spatial distribution of precipitation, whereby SST cooling stabilized the planetary boundary layer, reduced precipitation over the ocean, and allowed convective systems to reach the Serra do Mar, where they produced extreme orographic precipitation. In addition to promoting orographic uplift, the wind was responsible for transferring momentum to the ocean through wind stress, deepening the mixed layer and reducing SST.
Keywords: Numerical modeling, tipping point, boundary layer, ocean-atmosphere interaction, orographic precipitation, extreme events