湍流交换系数的水平非均匀对Ekman抽吸的影响研究

利用地转动量近似理论，讨论了湍流交换系数（K）存在水平非均匀的条件下，边界层顶Ekman抽吸，研究表明：K的水平变化对Ekman抽吸有明显的影响，这种影响决定于边界层内的气压场、边界层顶的风场和K的梯度之间的关系，在气旋性涡旋中，当K由涡旋中心向外增大时，它增加边界层顶的Ekman抽吸，反之，减少Ekman抽吸。Ekman抽吸增加的幅度与K的梯度值有关，与边界层内水平气压梯度有关。

Variable Eddy Diffusivities and Ekman Pumping

Ekman pumping is the vertical flow at the interface between the boundary layer and the interior flow which is created by the cross-isobar convergence and divergence within the boundary layer. This vertical flow is important for the exchange of momentum, mass and energy between the boundary layer and the free atmosphere. A lot of studies point out that Ekman pumping is proportional to the eddy diffusivities, the thickness of the boundary layer and the geostrophic vorticity. As we know, the eddy diffusivities is a function of the flow and it varies as the turbulence varies. Thus one must parameterize it as a function or other variables such as z/L, Richardson number of the stability. Because these variables are inhomogeneous, the eddy diffusivities are also inhomogeneous. Using the Geostrophic Momentum Approximation, the effect of horizontal variation of eddy diffusivities on Ekman Pumping is investigated in this paper. Such an effect is determined by the horizontal distribution of eddy diffusivities, the wind field at the top of the boundary layer and the pressure field. If the direction of eddy diffusivities gradients is parallel to the pressure gradients, the convergence within the boundary layer will be stronger than that of the uniform eddy diffusivities and the Ekman Pumping will increase in a cyclonic vortex. Whereas, If the direction of eddy diffusivities gradients is opposite to the pressure gradients, the convergence within the boundary layer will be weaker than that of the uniform eddy diffusivities and the Ekman Pumping will decrease in a cyclonic vortex. It is also demonstrated that the pressure distribution is a more important factor. The physical interpretations of results are provided.