With the global climate changing, the number of rapidly intensifying typhoons has shown an increasing trend. Investigating the spatiotemporal evolution characteristics of wave fields during typhoon rapid intensification by numerical simulation has a significant influence on coastal disaster prevention and mitigation. In this paper, a coupled typhoon-wave numerical model for the northeastern South China Sea based on the coupled ocean-atmosphere-wave-sediment transport (COAWST) model was developed in order to analyze the characteristics of wave fields under the rapidly intensifying Typhoon Mekkhala (2020). Results indicate that, with Mekkhala rapidly intensifying, there is a positive correlation between the significant wave height and the change of local wind speed on the right side of the track, forming the about 5 m difference of the significant wave height compared to the other side. Additionally, significant wave height seems more sensitive to typhoon intensity than to typhoon size. Within five times of the radius of maximum wind speed away from the typhoon center, wave age moderately decreases, while the mean wave direction is weak in response to the rapidly increasing wind field, maintaining a 60°counterclockwise deflection relative to background wave direction. Further analysis of the wave spectrum shows that, ignoring the background wave field, wave spectral energy in the right-front quadrant increases during typhoon rapid intensification, while a bimodal wave spectral dominated by wind waves emerges in the left-rear quadrant. Notably, wind wave components with faster group velocity transmit from this quadrant to the right-rear quadrant, generating low-frequency swells.

The rapid simulation of typhoon wave field is the core work of wave disaster warning. A fast typhoon wave field prediction model is constructed based on a two-dimensional wave-ray theory. This model is driven by the Holland wind field model, and uses the fourth-order Runge- Kutta method to solve differential equations in a polar coordinate grid, obtaining the spatiotemporal distribution of wave energy, peak wave direction, and peak frequency. Taking Typhoon LINFA (2009) as an example, the spatial structure and evolution process of the typhoon wave field are analyzed in the model. The simulation results show the wave field asymmetry caused by group velocity resonance; This reflects the development of wind waves under typhoons, as well as the propagation of swells. High energy and high group velocity wind waves generated in high wind speed areas of typhoons propagate outward; The formation of swells in the low wind speed area of a typhoon is the main component of the mixed wave field as the dominant wave. Moreover, the model was validated using Jason-1 altimeter data, and the results were in good agreement with the altimeter data, with the correlation coefficient R=0.79. It has been proven that the model can quickly and effectively simulate typhoon wave fields.