地形和风速影响下的海气相互作用大涡模拟研究

LARGE EDDY SIMULATION OF AIR-SEA INTERACTION UNDER THEINFLUENCE OF TOPOGRAPHY AND WIND SPEED

当水流经过海洋地形时,水流的不稳定性会引起垂向混合并伴随大量湍流过程。针对传统海气耦合模式缺少在湍流尺度上讨论海洋地形与风速对海气相互作用影响的问题,使用并行大涡模拟海气耦合模式(the parallelized large eddy simulation model,PALM)在5 m/s的背景风场下,引入理想立方体地形,对比有无地形的影响;设置地形边长为L,高为3L(其中大气部分高L),L与水深H之比为L/H=1/2;然后保持地形条件不变。设置5、10和15 m/s三种风速,讨论风速对小尺度海气相互作用的影响。研究表明:地形在大气部分减弱顺风向速度,增强侧风向速度,影响0~5L的高度区域,而对垂向作用较小;无地形条件下湍流垂向涡黏系数K_(m)在-0.3L时,水深达到最大值0.024 m^(2)/s,有地形条件下K_(m)在-0.8L时,达到最大值为0.16 m^(2)/s,地形的存在使得上层海洋混合加强,K_(m)最大值增加1个数量级。随风速增大海洋和大气中的净热通量、淡水通量和浮力通量都相应增大,在近海面处,5 m/s和10 m/s风速下三个通量的数值接近,当风速为15 m/s时净热通量和淡水通量相较于前者数值大小增加2倍,浮力通量增加近3倍,说明大风加剧了各通量在海表的交换;海洋混合层中湍动能收支各项也响应风速的变化,其中剪切项、Stokes剪切、耗散项随风速增大而增加,且在区域-0.2L~0变化明显,在近海表面处剪切项、传输项、压力项和耗散项的值达到最大,同时耗散项由传输项和剪切项平衡;随风速增大,K_(m)达到最大值的深度基本一致为-0.8L。

When water flows through ocean terrain,vertical mixing and a large amount of turbulent may occur due to water flow instability.Aiming at the issue that the traditional air-sea coupling model lacks of discussing the impact of ocean topography and wind speed on air-sea interaction on turbulent scale,the Parallelized Large Eddy Simulation Model(PALM)was used to introduce an ideal cube topography under background wind field of 5 m/s.The impact of topography was simulated under three wind speeds of 5,10,and 15 m/s.In the simulation,the side length of the cube was 1L and the height was 3L of which the height above water surface was 1L,and the ratio of L and water depth H was 1/2.The impact of wind speed on small-scale air-sea interactions was discussed.Results show that topography could weaken the downwind velocity in the atmosphere,enhances the crosswind velocity,and affect the altitude range of 0~5L,while the impact on the vertical direction is small.Under no-topography conditions,the turbulent vertical eddy viscosity coefficient K_(m)reached the maximum value of 0.024 m^(2)/s at-0.3L water depth without topography,while the maximum value of 0.16 m^(2)/s at-0.8L with topography.The presence of topography enhanced the mixing of the upper ocean and increased the maximum value of K_(m)by one order of magnitude.As the wind speed increases,all the net heat flux,freshwater flux,and buoyancy flux in the ocean and atmosphere increased accordingly.At near the sea surface level,the values of the three fluxes under 5 m/s and 10 m/s wind speeds are similar,while at 15 m/s,the net heat flux and freshwater flux increased by two times compared to the former,and the buoyancy flux increased by nearly three times,indicating that strong winds could intensify the exchange of various fluxes at the sea surface.In addition,the turbulent kinetic energy budget in the mixed layer of the ocean responded to the changes of wind speed.The shear term,the Stokes shear term,and the dissipation term increased with the wind speed increase.The variation was significant in the region of-0.2L~0,and the values of shear term,transmission term,pressure term and dissipation term reached the maximum near the sea surface.Meanwhile,the dissipation term was balanced by the transfer term and shear term.As the wind speed increased,the depth at which K_(m)reached its maximum value was-0.8L in overall.