A numerical study on the effects of a midlatitude upper-level trough on the track and intensity of Typhoon Bavi (2020)

Abstract

The complex interactions between a tropical cyclone (TC) and the midlatitude upper-level trough or cutoff low (sometimes called a cold-core vortex) can lead to an unusual track, such as sudden turning motion, and intensity change of the tropical cyclone, often leading to large forecast errors. In this study, the track and intensity changes of Typhoon Bavi (2020) under the influence of the upper-level trough and the detached cutoff low from its base are investigated based on numerical experiments using the advanced Weather Research and Forecasting (WRF) model. Bavi formed over the western North Pacific between Taiwan and Okinawa Islands on 23 August 2020. Under the influence of an upper-level trough to the north, Bavi initially moved northeastward. As the trough propagated eastward, a cutoff low was detached from the base of the trough on the next day. The downward penetration of the upper-level cutoff low led to a sudden northwestward turning motion of Bavi when it intensified to a severe typhoon and then moved almost due north and made landfall over North Korea. These changes and processes are well captured in a control numerical experiment. In a sensitivity experiment with the mid-latitude upper-level trough removed in the initial conditions, the tropical cyclone moved primarily north-northeastward after 1-day simulation without the sudden northwestward turning. This track change led to an earlier landfall of the storm over South Korea and, thus, an earlier weakening than in the control experiment, which is termed as the indirect effect on the storm intensity due to the change in the tropical cyclone track. The results also show that eddy angular momentum flux convergence associated with the upper-level trough contributed little to the simulated tropical cyclone intensification, while the environmental vertical wind shear is key to the intensity change of Bavi.