海浪破碎对海洋上混合层中湍能量收支的影响

海浪破碎产生一向下输入的湍动能通量,在近海表处形成一湍流生成明显增加的次层,加强了海洋上混合层中的湍流垂向混合。为了研究海浪破碎对混合层中湍能量收支的影响,文中分析了海浪破碎对海洋上混合层中湍流生成的影响机制,采用垂向一维湍封闭混合模式,通过改变湍动能方程的上边界条件,引入了海浪破碎产生的湍动能通量,并分别对不同风速下海浪破碎的影响进行了数值研究,分析了混合层中湍能量收支的变化。当考虑海浪破碎影响时,近海表次层中的垂直扩散项和耗散项都有显著的增加,该次层中被耗散的湍动能占整个混合层中耗散的总的湍能量的92.0%,比无海浪破碎影响的结果增加了近1倍;由于平均流场切变减小,混合层中的湍流剪切生成减小了3.5%,形成一种存在于湍动能的耗散和垂直扩散之间的局部平衡关系。在该次层以下,局部平衡关系与壁层定律的结论一致,即湍动能的剪切生成与耗散相平衡。研究结果表明,海浪破碎在海表产生的湍动能通量影响了海洋上混合层中的各项湍能量收支间的局部平衡关系。

EFFECT OF WAVE BREAKING ON TURBULENT ENERGY BUDGETS IN OCEAN SURFACE MIXED LAYER

The ocean surface mixed layer (OSML) plays an important role in exchanging heat, momentum and gas between the atmosphere and deeper part of ocean. Due to turbulence vertical mixing generated by various atmosphere and ocean physical processes on different temporal and special scales, the OSML has more complicated dynamical and thermal structures. As an important physical process on air-sea interface, the wave movement and its breaking have significant effect on the OSML. Breaking waves produce bubbles and turbulence, which will promote the exchanges of gas and momentum through air-sea interface. In this paper we considered the turbulence mixing due to breaking waves. When breaking waves exist at ocean surface, the turbulent kinetic energy (TKE) is input downwards, and a sub layer is formed near ocean surface, in which turbulence vertical mixing is intensively enhanced. Abundant observations in the past several years have confirmed this phenomenon, and the influence mechanism has been analyzed based on measurements and scale analysis. However, there have been few researches about the effect of breaking waves on the turbulent energy budgets in the OSML. A vertical one-dimensional oceanic mixed layer model was employed in this paper, in which the Mellor-Yamada level-2.5 turbulence closure equations were included. The influence of surface wave breaking could be introduced into ocean model by modifying existing surface boundary condition of TKE equation and specifying an input of it. The rate of energy loss due to wave breaking is estimated by parameterization of friction velocity, and incorporated into ocean model as a TKE input at ocean surface. Therefore, the mixing length originally set to be zero is not appropriate, and it is assumed in this paper that it has a linear relationship with the roughness length in the presence of wave breaking. Using modified vertical one-dimensional oceanic model, variations in turbulent energy budgets with in the OSML were investigated considering the effect of wave breaking at different wind speeds. The turbulent energy budgets were analyzed by vertically integrating different TKE production and dissipation terms in TKE equation of one-dimensional model. There is a local equilibrium among shear production, dissipation and buoyant product ion of TKE in cases without the effect of breaking waves. When enhanced turbulence mixing due to wave breaking was considered in modified ocean model, this local equilibrium was destroyed, and vertical diffusion and dissipation of TKE were effectively enhanced in the sub layer. The turbulent energy dissipated in this sublayer was about 92.0% of total depth-integrated dissipated TKE, increased by one time as compared with the results under zero wave breaking. The shear product ion of TKE decreased by 3.5% because the mean flow fields intended to be uniform under the action of enhanced turbulent mixing. Therefore, in the wave-enhanced layer, the downward flux of TKE balanced its dissipation. In the other words, a new local equilibrium between diffusion and dissipation of TKE was reached. Below the sublayer, the local equilibrium between the shear product ion and dissipation of TKE agreed with the conclusion drawn from the classical law-of-the-wall. These simulated results from modified model had a good agreement with some measurements