Extreme rainfall mechanisms in Hurricane Fiona (2022)

Abstract

Hurricane Fiona's (2022) historical heavy precipitation devastated the Caribbean Island of Puerto Rico after it made landfall as a category 1 hurricane. Rainfall accumulation totals in southern interior region areas surpassed 900 mm during 18 – 19 September 2022. To analyze the rainfall mechanisms, we use output from the Hurricane Analysis and Forecast System (HAFS) configuration "B" modeling system and observations from the Puerto Rico Next Generation Weather Radar (NEXRAD) Level 2 Doppler radar and rain gauges around the island. Quantitative precipitation estimates from radar and rainfall measurements suggest that HAFSB simulated reasonably well the precipitation amounts and location. HAFSB track differences from the real trajectory contributed to discrepancies between the simulated and observed rainfall. We investigate three stages of the Hurricane Fiona rain event, each focusing on different processes. The first stage is associated with the primary eyewall and rainfall produced through boundary layer convergence. The second stage focuses on the principal rainband affecting the island and is associated with rainfall enhancement from vertical wind shear interactions with Fiona's potential vorticity. The third and final stage analyzes the enhancement of a "tail rainband" both over open water and the southern portion of the island as Hurricane Fiona kept strengthening west of Puerto Rico. Our findings support the hypothesis that evaporative cooling within inner core rainfall from stage one and two sets up a favorable environment for isentropic uplift to enhance rainfall production in stage three. Additional enhancements of the rainfall occurred over Puerto Rico's high terrain by orographic effects.