Wind stress variability at the meso–gamma scale: Comparative observations from two adjacent offshore towers

The spatial variability of wind stress was investigated through eddy covariance measurements conducted at two offshore towers separated by 8.6 km. Observations showed that while wind speed spatial variability remains minimal (< 3%) for both the NW (parallel to the inter-site transect) and NE (perpendicular) directions, wind stress at the two sites exhibits significant directional dependence, with a greater than 10% difference under NW winds and less than 1% difference under NE winds. Further comparative analysis indicated that under similar wave conditions, the bulk wind stress generally follows wind speed patterns in both directions. However, vertical differences emerge at different heights under NW winds, with wind stress at 20 m height being less than that at 12 m height, unlike bulk stress. This discrepancy can be statistically attributed to the horizontal pressure gradient in the motion equation. The combination of observations and sea-state-dependent bulk results demonstrates that meso–gamma scale wind stress variability can reach 10% or more. 摘要 基于两座相距8.6 km的海上观测塔获取的观测资料表明: 虽然风速在西北方向 (平行于两塔连线) 和东北方向 (垂直于两塔连线) 的空间变异性均较小 (<3%), 但两站点的风应力表现出显著的方向依赖性—在西北风向下差异超过10%, 而在东北风向下差异不足1%. 对比分析表明, 在相似海浪条件下, 块体风应力在两方向上均基本遵循风速的变化规律. 然而在西北风向下, 不同高度层出现垂直差异: 20 m 高度处的风应力低于12 m 高度处, 这与块体风应力的分布特征不同. 该差异可通过运动方程中的水平压力梯度项进行解释.