Rapid Outer‐Size Expansion of Typhoon Hinnamnor (2022): Role of Merger With a Tropical Depression

Abstract Rapid expansion (RE) of tropical cyclones (TCs), which quickly increases their outer size and destructive potential, has not been thoroughly investigated in the merger of binary TCs. This study examines a rare case where typhoon Hinnamnor (2022) experienced RE during its merger with tropical depression (TD) 13W, growing from 125 to 275 km within 24 hr. Two experiments (with/without 13W in the initial field of simulation) were conducted to examine the roles of merger on RE. Only the experiment involving TD 13W successfully replicated the observed changes in size and intensity. While the favorable large‐scale flow caused relatively steady growth and contributed approximately 70% to the total expansion, the binary TC interaction abruptly induced the remaining 30% contribution by substantial asymmetric positive radial flux of vorticity into the outer core on the mesoscale. The influx of vorticity resulted from the in‐phase superposition of the positive vorticity anomaly of 13W and enhanced boundary layer inflow. On the basis of large‐scale low‐level southwesterlies in downshear, the binary interaction with 13W introduced inflow in the downwind region, while the stratiform‐induced mesoscale descending inflow driven by latent cooling also assisted in the enhancement of inflow. By pinpointing the role of binary TC interaction in size expansion, this study shows that accurately describing such interactions is essential for improving TC size forecasting.