垂直分辨率对长江口海域M_2分潮模拟的影响

The effect of vertical resolution on the simulation of M_2 tide in Changjiang Estuary

基于EFDC(Environmental Fluid Dynamics Code)模式建立了长江口及其邻近海域的三维水动力学模型,研究模型的垂直分辨率对该海域M2分潮模拟的影响。结果表明:垂直分辨率的变化对M2分潮传播方向的模拟结果影响较小,但其可通过底摩擦和湍流耗散两个计算过程来影响潮能通量的模拟结果,最终对长江口和杭州湾内的M2分潮振幅产生显著的影响。最底层厚度较大时,上层自由水体的高流速特征在最底层过于明显,进而导致计算的底摩擦应力偏高,此时提高底层的垂直分辨率会降低底摩擦对能量的耗散。另一方面,垂直湍流混合作用会随垂直分辨率的增加而增强,所以垂直分辨率增加到一定程度后,上层自由水体的高流速会经由增强的湍流混合而更多的传入底层,使计算的底摩擦应力随垂直分辨率的提高而有重新增加的趋势,进而又增强底摩擦对潮能的耗散。

A three-dimensional hydrodynamic model for Changjiang Estuary and the adjacent sea was developed within the framework of Environmental Fluid Dynamics Code （EFDC）. The effect of vertical resolution on the simulation of Me tide was analyzed. The direction of M2 tide propagation was simulated correctly. The M2 tide propagated from southeast to northwest in open sea,then it propagated inland along coastline in Changjiang River and Hangzhou Bay. The bottom stress and turbulence varying with vertical resolution determined the energy prop- agated into Changjiang River and Hangzhou Bay. This made the amplitude of Me tide decrease with increasing ver- tical resolution in Changjiang River. But in Hangzhou Bay, the amplitude of Me tide increased with increasing ver- tical resolution at first, then decreased with increasing vertical resolution. The computation of bottom stress was based on the mixing length theory and the boundary layer was assumed to be a constant fluxes layer. The bottom layer should be thin enough so that the higher velocity of the upper water, which resulted in higher bottom stress, was of little importance in bottom layer. The intensity of vertical turbulence increased with increasing vertical reso- lution. This transmitted more high velocity of upper water into bottom layer and resulted in higher bottom stress again. Finally, the more consumption of energy caused by higher bottom stress made the amplitude of M2 tide be- come smaller.