Extreme Rainfall and Intensification Mechanisms in Hurricane Fiona (2022)

Hurricane Fiona (2022) caused devastating flooding over Puerto Rico as an intensifying category 1 hurricane and in some southern locations left more than 823 mm of precipitation. The event ranks fourth in tropical cyclone (TC) rainfall total records over a 24 hour period in Puerto Rico. The primary cause of the rainfall totals can be attributed to a persistent rainband visible in radar and satellite data. This study will encompass an overview of the mechanisms affecting Hurricane Fiona's intensification and precipitation enhancement as it interacted with vertical wind shear. We hypothesize that enhanced potential vorticity displaced from the inner core by vertical wind shear was a primary contributor to the heavy rainfall production, resulting in both isentropic uplift and boundary layer convergence in the downshear rainband. Additional enhancements of the rainfall occurred over Puerto Rico’s high terrain by orographic effects. To analyze the rainfall mechanisms we use the Hurricane Analysis and Forecast System (HAFS-B) model in concert with observations from the Puerto Rico NEXRAD radar data and aircraft data collected as part of the joint Office of Naval Research Tropical Cyclone Rapid Intensification (TCRI) Departmental Research Initiative and the NOAA Advancing the Prediction of Hurricanes Experiment (APHEX). A rainfall accumulation verification of the HAFS-B forecasts is first conducted, indicating that the rainfall rates over Puerto Rico were reasonably well-simulated but were displaced over the ocean due in part to track differences from the real storm. Preliminary results support the hypothesis that heavy rainfall is associated with interactions between enhanced potential vorticity and moist southerly flow in the downshear rainbands. Strong vertical motions and rainfall continued even after the eyewall was located well to the northwest of the island 24 hours after landfall in Puerto Rico. The physical mechanisms related to the extreme rainfall and its relationship to the concurrent intensification of Fiona in the presence of moderate vertical wind shear will be discussed.