Journal Article

How Did the Merger With a Tropical Depression Amplify the Rapid Weakening of Super Typhoon Hinnamnor (2022)?

Zhang, Weimin, Zhang, Ze, Zheng, Minglin

2024

Overview

Grasping the physical interactions when two tropical cyclones (TCs) (TC) are in proximity is essential for boosting the accuracy of TC forecasts. This study dissects an uncommon scenario wherein the merging with Tropical Depression 13 W significantly hastened the rapid weakening of Super Typhoon Hinnamnor (2022), utilizing comparative experiments with and without 13 W in simulation's initial field. The findings reveal strong correlations between the merger, amplified environmental vertical wind shear (VWS), and Hinnamnor's consecutive weakening, unfolding in two stages— “top‐down” (Stage 1) and “bottom‐up” weakening (Stage 2) stage. In Stage 1, 13 W led to downdrafts from upper level, hindering the eyewall updrafts and weakening the warm core. In Stage 2, 13 W merged into Hinnamnor's outer rainband, introduced low‐entropy air into the boundary layer and also vied with the eyewall for energy. This research emphasizes that even minor, less‐intense vortices can have profound impacts on the rapid intensity change in TCs. Plain Language Summary A tropical cyclone (TC)'s intensity and track can be affected by the atmospheric vortices through binary interaction (i.e., Fujiwhara effect). Yet little study has illustrated the merger of TCs with large differences in intensity and size, as well as how it impacts the TC structure and intensity. This study examines the rapid weakening of Super Typhoon Hinnamnor (2022) during its interaction with the tropical depression named 13 W, by two numerical experiments—with and without the existence of 13 W in the initial field. The results find that the merger‐induced increase in environmental vertical wind shear and outer rainband effect caused by 13 W contributed to Hinnamnor's weakening. The process unfolds in two stages. In the first stage, the 13 W in upshear quadrants induced downdrafts from the upper level, disrupting Hinnamnor's eyewall updrafts and diminishing the warm core from top to down. In the second stage, the 13 W in downshear quadrants merged into Hinnamnor's outer rainband, increasing the infusion of low‐entropy air into boundary layer, in turn weakening Hinnamnor from bottom to upper level. This research demonstrates the rainband effect from the other TC during binary interaction, and underlines the attention on impacts from weak and small vortices in TC operational forecasting. Key Points The merger with Tropical Depression 13 W amplified lower environmental wind and vertical wind shear, leading to Hinnamnor's further weakening In early “top‐down” weakening, the 13 W in upshear induced downdrafts, diminishing Hinnamnor's eyewall updrafts and warm core In later “bottom‐up” weakening, the 13 W in downshear merged into Hinnamnor's outer rainband, reducing the entropy of boundary layer inflow

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Boundary layers / Cyclones / Downdraft / Entropy / Hurricanes / Numerical experiments / Quadrants / Rainbands / Tropical cyclone intensities / Tropical cyclones / Tropical depressions / Typhoons / Updraft / Vertical air currents / Vertical wind shear / Vortices / Wind / Wind effects / Wind shear