How does the Merge of the A Tropical Depression cause the Size Change of Typhoon HINNAMNOR (2022)?

How does the merge of the a tropical depression cause the size change of Typhoon HINNAMNOR (2022) ?

Mao-Cheng Li and Ming-Jen Yang

Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan

ABSTRACT

The size change of tropical cyclones has long been a scientific problem that remains unclear. In 2022, Typhoon Hinnamnor (TY 2211) in the northwest Pacific underwent a significant size change. According to ERA5 reanalysis and satellite observations (ASCAT/SAR), the radius of the 34-knot wind (R₃₄) was expanded approximately three times within two days, a significance size change for tropical cyclone. The reason for the increase in size is the scientific issue that we seeks to explore in this study. An additional tropical cyclone, TD 13W, appeared to be a possible key factor influencing the significant change in size associated with Typhoon Hinnamnor. TD 13W exhibited significant interactions with Typhoon Hinnamnor in terms of track, intensity, and structure. These interactions included the Fujiwhara effect and merging process, both contributing to the noticeable size increase of Typhoon Hinnamnor concurrently.

This study used the Weather Research and Forecasting (WRF) model to simulate the interaction between Typhoon Hinnamnor and TD 13W. Model results show that the WRF model could reasonably capture the interaction process between the two cyclones. Absolute angular momentum (AAM) budget were calculated on the simulation results to analyze three time periods which contributed to the size increase of Typhoon Hinnamnor:

1. Approach of TD 13W: The positive AAM contribution was primarily provided by the radial advection from the eddy.
2. Merger with TD 13W: The positive AAM contribution was mainly supplied by the radial advection from both the eddy and the axisymmetric mean flows.
3. Disappearance of TD 13W: The positive AAM contribution was predominantly provided by the axisymmetric mean vertical advection (associated with the active outer rainbands).

Future work will involve model sensitivity experiments to assess the impact of removing TD 13W, aiming to gain further insights into its role in the size change of Typhoon Hinnamnor.

Keywords: Typhoon Hinnamnor; size change; absolute angular momentum budget