Study on the characteristics of wind field and aerodynamic parameters based on multi-level observations on the Beibu Gulf island under typhoon conditions

Based on multi-level observation data from near-surface layer on the Weizhou Island, Beihai, Guangxi, the study selected the periods influenced by Typhoons Yagi, Sanba, and Talim during 2023–2024. First, quality control and correction were performed on the observation data for each typhoon event. Subsequently, the structural characteristics of typhoon boundary layer under the influence of complex underlying surface on the Beibu Gulf island were analyzed. This included the evolution of near-surface wind fields, wind profile characteristics, and the variations of aerodynamic parameters, i.e., friction velocity (u*), roughness length (z0), and drag coefficient (Cd), with near-surface wind speed and direction. The results indicate that the typhoon boundary layer between 10–32 m exhibits near-neutral stability, and the mean wind speed profile follows logarithmic distribution. Under the influence of complex underlying surface on the Beibu Gulf island, the variations of u*, z0, and Cd with near-surface wind speed resemble characteristics typical of land underlying surface, showing obvious non-monotonicity and cannot be simply categorized as homogeneous marine underlying surface condition. Specifically, u* ranges between 0.5–1.5 m s-1; when the 10 m wind speed exceeds approximately 15 m s-1, the magnitudes of z0 and Cd are on the order of 10-2 m and 10-3, respectively. Under easterly winds, the values of u*, z0, and Cd are generally higher than those under westerly winds. The findings contribute to deeper understanding of typhoon boundary layer structure over complex island underlying surface and hold significant importance for optimizing typhoon numerical simulations and parameterization schemes in the Beibu Gulf region.