Extreme Rainfall-Enhancing Mechanisms in the Tail Rainband of Hurricane Fiona (2022)

Despite making landfall as a category 1 hurricane, total precipitation from Hurricane Fiona (2022) exceeded 32 inches locally over the island of Puerto Rico. Satellite imagery suggests that Hurricane Fiona’s tail-rainband strengthened and persisted after landfall, contributing to the heavy rainfall production. We hypothesize that environmental modulation during the early stages of the storm led to enhancing mechanisms such as isentropic uplift and boundary layer convergence in the downshear tail rainband. Additional enhancements of the rainfall occurred over Puerto Rico’s high terrain by orographic effects. To examine our hypotheses, we investigate three stages in the life cycle of Hurricane Fiona, each focusing on different processes. The first stage includes rainfall associated with the primary eyewall while the center of the storm made landfall, the second stage focuses on the external rainbands affecting the island overnight, and the third and final stage analyzes the enhancement of the tail rainband both over open water and the southern portion of the island as Hurricane Fiona kept strengthening over the Atlantic Ocean. To analyze the rainfall mechanisms we use output from the Hurricane Analysis and Forecast System (HAFS) configuration “B” modeling system in concert with observations from the Puerto Rico Next Generation Weather Radar (NEXRAD) Level 2 Doppler radar and rain gauges around the island. The NEXRAD data was processed by the Lidar Radar Open Software Environment (LROSE) to estimate the surface rainfall and conduct a HAFS-B rainfall accumulation verification. Quantitative precipitation estimates from radar and rainfall measurements suggest that HAFS-B simulated reasonably well the precipitation amounts and location. HAFS-B track differences from the real trajectory contributed to discrepancies between the simulated and observed rainfall. Our findings support the hypothesis that evaporative cooling within inner core rainfall sets up a favorable environment for isentropic uplift to enhance rainfall production. Additionally, we found that heavy rainfall was also associated with interactions between enhanced potential vorticity and strong vertical motions. This study assesses in detail the rainfall enhancing mechanisms that contributed to Hurricane Fiona’s heavy rainfall over Puerto Rico and the nearby ocean.