Observational force analysis and anisotropic characteristics of tropical cyclone sea surface wind fields over Chinese offshore areas

Abstract

In this paper, based on the observation data of sea surface buoys, 17 tropical cyclones (TCs) occurring in the East China Sea and South China Sea from 2017 to 2021 are synthesized to conduct a force analysis in a two-dimensional radial momentum equation. The analysis evaluates the magnitude, equilibrium, proportional relationships, and spatial distribution features of each force, leading to clarified universal laws. The findings reveal a pronounced radial variability feature in the mean TC sea surface structure. Notably, the regions where the changes are most dramatic are located approximately 80 km from the mean TC center and particularly at 20–40 km. Within an overall range of 0–400 km, the cumulative contributions of each force from high to low are the pressure gradient force, friction term, centrifugal force, Coriolis force and acceleration force. In addition, obvious anisotropic features are also observed. The four orientations can be arranged according to force strength as the right region (240–300°), left region (60–120°), rear region (150–210°) and front region (0–30° and 330–360°). The pronounced pressure gradient force is extremely unstable within the 250 km range of the right (240–300°) and 200 km range of the left (60–120°) regions of the mean TC. The sensitivity of the centrifugal force to radial changes surpasses that of azimuthal changes, and significant variations are concentrated within a radial distance of 50 km. For the Coriolis force, noticeable anisotropic features are evident within the 150 km radius of the 210–330° sector. Additionally, a comparison of the observed radial pressure profile with empirical pressure models was conducted. We found that B parameter values between 0.7 and 1.0 are optimal for our observational cases. Selecting an appropriate environmental pressure can bring the empirical model closer to the observed data. This study establishes a realistic foundation for enhancing existing sea surface parametric wind models of TCs and developing novel approaches, thereby ultimately improving the numerical calculation accuracy for storm surges and other air–sea interactions.