Observed Characteristics of Weak Tropical Cyclones Evolving Toward Alignment

Although recent studies have emphasized the importance of achieving an aligned vortex for rapid intensification to occur, there has yet to be a robust, multi-storm observational examination of how TC precipitation is linked to changes in vortex tilt and how they co-evolve to initiate—or inhibit—TC intensification in weak TCs. This study uses an international, multi-site ground-based radar database to better understand the processes associated with TC precipitation that are responsible for a vertically misaligned vortex evolving towards an aligned state. Tropical Storm Fiona, a challenging forecast case from the 2022 Atlantic hurricane season, provides the first known observational documentation of core reformation from start to finish afforded by dual Doppler three-dimensional wind analyses obtained using ground radar with Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI). The high spatial and temporal resolution of the analyses enables not only feature tracking of the cooperative precipitation and vorticity dynamics but also trajectories to demonstrate changes in the flow of parcels near the reforming vortex. Since deep convection is shown to be a significant driver of the vorticity modulation in this case, the interplay of the thermodynamics and convection is also revealed using dropsondes and passive microwave retrievals of relative humidity and thermodynamic instability from the Advanced Technology Microwave Sounder (ATMS) and Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS). Finally, the phase space of precipitation and thermodynamic characteristics of Fiona are compared to other recently studied notable cases of (mis-) alignment including Dorian (2019), Isaias (2020), Sally (2020), and Ida (2021).