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arXiv 2404.09545physics.flu-dynphysics.ao-ph
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充分混合状态下的湍流冰-洋边界层:来自直接数值模拟的洞见

Turbulent ice-ocean boundary layers in the well-mixed regime: insights from direct numerical simulations

Louis-Alexandre Couston

AI总结:

本研究通过直接数值模拟验证了冰-洋边界层MML模型中盐与温度有效扩散系数相等的假设,构建了基于热驱动的简化模型,其预测与完整模型吻合良好,可用于估算界面通量与融化速率。

摘要:

融水混合线(Meltwater Mixing Line,MML)模型可对冰附近水团特性作出理论预测,适用于与观测结果进行对比。若温盐图中呈现的海洋学测量数据与MML预测结果重合,通常可得出结论:局地动力过程由环境水团与附近融化冰的湍流混合主导。尽管MML模型与大量观测结果一致,但它建立在一个难以通过现场测量验证的假设之上,尤其是在冰边界附近,该假设即有效(湍流与分子层面的)盐扩散系数与温度扩散系数相等。\n本文通过对均匀环境中外强迫冰-洋边界层的典型模型开展直接数值模拟,对这一假设进行检验。我们通过选取接近冰点的环境温度聚焦于充分混合状态,并运行模拟直至达到统计稳态。结果验证了边界层大部分区域内有效扩散系数相等的假设。\n重要的是,MML模型的有效性意味着垂直于界面的平均盐度剖面与温度剖面之间存在线性相关性,这一关系可用于构建基于单一标量变量(称为热驱动,thermal driving)的简化冰-洋边界层模型。我们证明,简化热驱动模型预测的整体动力过程与完整温盐模型预测的整体动力过程吻合良好。随后,我们展示了如何利用热驱动模型的结果估算界面热通量、盐通量以及融化速率。

英文摘要:

The meltwater mixing line (MML) model provides a theoretical prediction of near-ice water mass properties that is useful to compare with observations. If oceanographic measurements reported in a temperature-salinity diagram overlap with the MML prediction, then it is usually concluded that the local dynamics are dominated by the turbulent mixing of an ambient water mass with nearby melting ice. While the MML model is consistent with numerous observations, it is built on an assumption that is difficult to test with field measurements, especially near the ice boundary, namely that the effective (turbulent and molecular) salt and temperature diffusivities are equal. In this paper, this assumption is tested via direct numerical simulations of a canonical model for externally-forced ice-ocean boundary layers in a uniform ambient. We focus on the well-mixed regime by considering an ambient temperature close to freezing and run the simulations until a statistical steady state is reached. The results validate the assumption of equal effective diffusivities across most of the boundary layer. Importantly, the validity of the MML model implies a linear correlation between the mean salinity and temperature profiles normal to the interface that can be leveraged to construct a reduced ice-ocean boundary layer model based on a single scalar variable called thermal driving. We demonstrate that the bulk dynamics predicted by the reduced thermal driving model are in good agreement with the bulk dynamics predicted by the full temperature-salinity model. Then, we show how the results from the thermal driving model can be used to estimate the interfacial heat and salt fluxes, and the melt rate.

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