Small-amplitude trochoidal oscillations in Typhoons Rammasun (2014) and Lekima (2019)

Abstract

Tropical cyclones (TCs)tend to oscillate around a mean path and the trochoidal oscillation has been documented based on radar, satellite and aircraft observational data and in numerical simulations. Currently,no consensus has been reached on the mechanisms responsible for the trochoidal oscillation in TC motion although a few theories have been proposed since 1950. In this study, the small-amplitude track oscillations of Super Typhoons Rammasun (2014) and Lekima (2019) are examined based on the 6-minute radar refelctivity and the detected oscillation in Rammasun is verified through numerical simulation. The trochoidal oscillation in the tracks of Super Typhoons Rammasun and Lekima can be retrieved with the 6-minute radar data. The detected track oscillation always exhibits cyclonic rotations around a mean track, with the amplitude of a few kilometers and the period of a few hours. Thenumerical simulation of Typhoon Rammasun indicates that the small-amplitude track oscillation can be simulated in terms of the period and amplitude. Based on two different center-detecting methods, it is shown that the simulated track oscillation represents the wobble of the whole inner-core circulation. Since the small-scale oscillations are closely associated with the dynamics of the TC inner-core circulation, this study suggests thatthe trochoidal oscillation in TC tracks may provide important information for understanding the intensity and structure changes of TCs.

Key points

• The tracks of Typhoons Rammasun and Lekima exhibited small-amplitude oscillations

• The oscillation can be numerically simulated in terms of the period and amplitude

• The trochoidal oscillation is the wobble of the whole TC inner-core circulation