强潮狭长海湾中垂直涡黏性系数与底拖曳系数的估计

Estimation of Vertical Eddy Viscosity and Bottom Drag Coefficients in Tidally Energetic Narrow Bay

垂直动量交换与底拖曳应力的准确刻画是目前区域海洋环流模拟中的主要困难,而基于现场观测的垂直涡黏性系数与底拖曳系数的估计是解决这一问题的最有效途径。本文通过3个海床基观测平台在1个强潮狭长海湾中的成功应用,获得了观测站位处水位、平均流速和高频湍流脉动的2周日连续观测资料,并由此计算分析了垂直涡黏性系数和底拖曳系数。结果表明:垂直涡黏性系数(AZ)具有显著的潮内变化,在M2分潮流占优的观测海区,AZ大致呈M4分潮变化,但同时存在明显的涨-落潮不对称性,涨急时较大,落急时较小。在垂向上,AZ大致呈抛物线分布,其最大值出现在底上4m左右,约为5.4×10-3 m2.s-1。基于方差方法和动量平衡法所估计的底拖曳系数的平均值分别为0.17×10-3和0.81×10-3,表明在岸线和底形复杂的狭长海湾内,形状阻力对底摩擦的贡献可显著大于界面阻力,数值模式中必须充分考虑形状阻力的作用。

The accurate quantification of vertical momentum exchange and bottom drag poses the major difficulty in regional ocean modeling, to which in-situ observations may provide the most effective refer- ence. In this paper, the vertical eddy viscosity and bottom drag coefficient were estimated with two-day- long measurements of tidal elevation, currents and turbulence data, which were conducted by using three bottom-mounted quardrapods located along the major axis of a tidally energetic narrow bay, the Xiangshan Bay, East China Sea. The flow in the bay was predominantly driven by the M2 constituent. It was found that the vertical eddy viscosity coefficient （Az） experienced evident tidal variations, approximately at a fre- quency of M4. There was also pronounced flood-ebb asymmetry in Az, being larger in flood and smaller in ebb. The vertical profile of Az roughly followed a parabolic form, with a maximum value of about 5.4 ×10^-3 m2 · s-1 at a height of 4 meters above the bottom. The mean bottom drag coefficient estimated with the variance method and the dynamical balance method were 0. 17×10^-3 and 0.81 ×10^-3 , respectively. This indicated that the form drag due to irregular topography was much larger than the skin friction. The suggest was given that the effects of form drag should be fully taken into consideration in hydrodynamic simulations.