Stationary and Transient Asymmetric Features in Tropical Cyclone Eye with Wavenumber-one Instability: Case Study for Typhoon Haishen (2020) with Atmospheric Motion Vectors from 30-second Imaging

Takeshi Horinouchi, Satoki Tsujino, Masahiro Hayashi, Udai Shimada, Wataru Yanase, Akiyoshi Wada, and Hiroyuki Yamada

2023
DOI

概要

Dynamics of low-level flows in the eye of Typhoon Haishen (2020) in its late phase of intensification are investigated with a special rapid-scan observation of the Himawari-8 geosynchronous satellite conducted every 30 seconds. This is accomplished by deriving storm-relative atmospheric motion vectors at an unprecedentedly high spatiotemporal resolution by tracking clouds across five consecutive visible-light reflectivity. The overall low-level circulation center was situated several kilometers away from the storm center defined in terms of the inner edge of the lower part of eyewall clouds. The shift direction is rearward the storm translation, consistently with a numerical study of tropical cyclone (TC) boundary layer. Over the analysis period of 10 hours, azimuthal-mean tangential wind around this center was increased at each radius within the eye, and the rotational angular velocity was nearly homogenized. The instantaneous low-level circulation center is found to orbit around the overall circulation center at distances around 5 km. Its orbital angular speed was close to the maximum angular speed of azimuthal-mean tangential winds. This rotating transient disturbance is found to transport angular momentum inward, which explains the tangential wind increase and the angular velocity homogenization in the eye. These features are consistent with an algebraically growing wavenumber-1 barotropic instability, whose impact on TC structures has not been explored. This instability enhances wavenuber-1 asymmetry in ring shaped vorticity, which can be induced by various processes such as translation, environmental shear, and exponential barotropic instability. Therefore, it may appear broadly in TCs to affect wind distribution in their eyes.

引用

Horinouchi T., S. Tsujino, M. Hayashi, U. Shimada, W. Yanase, A. Wada, and H. Yamada, 2023: Stationary and Transient Asymmetric Features in Tropical Cyclone Eye with Wavenumber-one Instability: Case Study for Typhoon Haishen (2020) with Atmospheric Motion Vectors from 30-second Imaging. Monthly Weather Review, 151(1), 253-273. https://doi.org/10.1175/MWR-D-22-0179.1

公開ソフトウェア

本研究で使用した VTTrac のソースコードは次のリポジトリにて公開しています： https://github.com/thorinouchi/VTTrac

ライセンス

上記ソフトウェアは2条項 BSD ライセンスに準拠しています。

プレスリリース

本研究についてプレスリリースを行いました

動画

  • Copy of the supplemental movie of the article (available in a ZIP file). This is a collection of the 30-sec visible images near the center of Haishen (120 km x 120 km), on which the 2-minute (5-step) trajectories of template subimages are indicated by "+" marks every 3 minutes.

  • Additional movie available on this website: 30-sec visible images near the center of Haishen (without cloud tracking results)