High-Resolution Simulation of the Rapid Intensification of Super Typhoon Rammasun (2014)

Super Typhoon Rammasun (2014) was the most intense tropical cyclone (TC) ever recorded in the South China Sea (SCS), with a minimum sea surface pressure of 888 hPa and a maximum surface wind of 72 m s-1 at landfall in Wenchang, Hainan Province of China. Rammasun experienced rapid intensification (RI) from 17 July 2014 when it moved over the north of the SCS, with the deepening rate of about 6.3 m s-1 (6 h)-1 during its 30-h RI phase, which was really rare in the SCS and was a great challenge to TC intensity forecasting.

In this study, an 120-h high-resolution numerical simulation of Rammasun was conducted to investigate the storm structural change and the associated effects of environmental factors during the explosive intensifying stage using the operational real-time typhoon prediction system of the State Key Laboratory of Severe Weather in Chinese Academy of Meteorological Sciences. The typhoon prediction system is constituted by three major parts: the dynamic-balanced vortex initialization, the nonhydrostatic version of the Advanced Research core of the WRF (ARW), and the post-processing program. Super Typhoon Rammasun is explicitly predicted using a two-way interactive, movable, triply nested (18/6/2 km) grid, with mesh sizes of 310 × 250, 270 × 270, and 210 × 210, respectively. There are 35 layers in the vertical with the model top at 50 hPa. The NCEP's Final Operational Global Analysis data is adopted to provide the initial and boundary conditions. The grid nudging technique is applied to retain the large-scale information from global analysis fields.

Model verification indicates that the track and intensity of Rammasun are successfully reproduced, with the averaged 72h track error less than 44.1 km and 72h intensity error less than 4.9 m s-1. In particular, the RI process is captured reasonably well, with an intensification rate of 25.5 m s-1 per day. Moreover, the simulated TC structure agrees well with satellite and radar observations, including the cloud-free eye, the outward-sloping eyewall, the inner-core convective asymmetry, and the outer rainband activity.

As Rammasun enters the SCS, the low-level monsoon trough located in the Indo-China Peninsula weakens and gradually merges into the TC circulation. With the influence of the monsoon circulation, strong northeasterly 200-850-hPa vertical shear over 11 m s-1 is found, unfavorable for RI and contributing to the occurrence of the eyewall tilt as well as asymmetric convection in the inner core. As indicated in previous studies, the most active convection and rainfall tend to arise downshear left. Afterward, the cross-equatorial flow strengthens and the horizontal water vapor flux significantly increases at the onset of RI. In addition, high sea surface temperature (SST) exceeding 30 ℃ is found over the SCS in our model, although it seems to be overestimated compared with the satellite observations. An in-depth investigation of the RI of Rammasun will be conducted based on the high-resolution simulation results.