How Do Weak, Misaligned Tropical Cyclones Evolve Toward Alignment? A Multi‐Case Study Using the Hurricane Analysis and Forecast System

Abstract

Abstract This study simulates five initially weak, moderately sheared tropical cyclones (TCs) from the 2020–2021 North Atlantic basin hurricane seasons using the Hurricane Analysis and Forecast System (HAFS). Four of the five simulations rapidly evolve from misaligned vortices with asymmetric precipitation and thermodynamic distributions toward more aligned and symmetric configurations. The displaced low‐level (LLC) and mid‐level circulations (MLC) non‐monotonically progress toward alignment with periods of reformation, precession, and advection. Beginning 12–18 hr pre‐alignment, TCs have increasingly greater mid‐tropospheric humidity and areal coverages of precipitation downshear left than the simulation that fails to align. Alignment precedes the most sustained symmetrization of favorable thermodynamics and precipitation, but deep convection (not necessarily symmetric) plays a critical role in alignment. Ida (2021), a high impact US storm, undergoes a vortex‐scale evolution where an increase in areal coverage and intensity of deep convection promotes a reformation of the vortex into a smaller compact core with a closed MLC (a closed LLC does not immediately form). This convective behavior downtilt helps to reshape the irrotational velocity field in the lower troposphere toward the reformed vortex. The increasingly convergent flow of favorable boundary layer thermodynamics within the inflow region thereby increases the instability, which maintains the persistent intense convection. The confluent flow ultimately promotes an advection of the pre‐existing LLC toward the reformed vortex resulting in alignment. Tilt reductions are also shown to be temporally linked with the diurnal cycle, wherein convection preferentially increases near the center during the early morning hours (local time).

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