浪花和飞沫水滴在海面大风边界层中的垂直传输

Vertical Transport of Sea Spray and Spume Droplets in High-Wind Marine Atmospheric Boundary Layer

对我们在南海海域建立的大气边界层观测站的资料进行分析表明,在冷涌和热带气旋(包括强台风)过境的大风期间,在边界层底层10 min平均的水平风速ū基本不随高度而变,甚至大都伴随有明显的上升气流。而且风场脉动中含有强相干性的阵风扰动(vg,频率位于1/60~1/600 Hz频段),以及近于随机性的高频湍流脉动(v_t,频率大于1/60 Hz),它们的特性以及都可以很好地用水平风速ū来参数化表示。取实测的(ū,)和脉动v′=v_g+v_t,或取实测的ū与参数化的v_g、v_t和,应用拉格朗日随机模式作数值模拟,结果表明:由破头浪发射出来的浪花和飞沫水滴(半径r_p为10~500μm)有相当大的一部分可以飞离大气底层而进入100 m高以上的大气中,继而对进入大气中的海盐气溶胶通量有重要贡献,不可以被忽略。在水滴的垂直传输过程中,阵风扰动起了极重要的作用,而在>0且较显著时更起重要作用。我们对上扬率(可上升至100 m以上高度的水滴数与由海面发射出的水滴数之比)作出了初步的参数化公式,有很高的精度,主要的参量是无量纲量ū~2/(r_gp),其中r_p和g分别是水滴半径和重力加速度。

Through analyzing the observational data obtained during high-wind conditions over the South China Sea, our results have shown that：（a） The 10-min average horizontal wind speed ū is almost independent of height;（b） the 10-min average vertical velocity w^- is almost always positive;（c） there exists strong coherent gusty wind with low-frequency（vg, between 1/600 Hz and 1/60 Hz）, and nearly random and isotropic turbulent fluctuations with high-frequency（vt, larger than 1/60 Hz）. The vertical velocity w^- and the characteristics of the gusty wind and turbulence can be well parameterized through the horizontal wind speed u^-. Using the observations of（u^-,w^-） and the parameterized gusty wind and turbulence, the Lagrangian stochastic model is implemented to study and simulate vertical transport of spume droplets. The results show that a quite high proportion of spray and spume droplets（the radius is between 10 and 500 μm） teared from the breaking waves can reach the 100 m height in ABL（Atmospheric Boundary Layer） and contribute to the sea salt aerosol flux. The coherent gusty wind during high winds plays an important role in the vertical transport of droplets. Besides, we also parameterize the uplifting fraction of spume droplets that can reach the 100 m height in the ABL by using a nondimensional factor u^-^2/（rpg）, where rp is the radius of droplet and g is the gravitational acceleration.