Quantification of Physical and Numerical Mixing in a Coastal Ocean Model Using Salinity Variance Budgets
Quantification of Physical and Numerical Mixing in a Coastal Ocean Model Using Salinity Variance Budgets
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DOI：
10.1029/2022ms003380
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发表时间：
2023-04
影响因子：
6.8
通讯作者：
Dylan Schlichting;Lixin Qu;D. Kobashi;R. Hetland
中科院分区：
文献类型：
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作者：
Dylan Schlichting;Lixin Qu;D. Kobashi;R. Hetland
数值混合，即主要由平流离散化产生的虚假混合，由于锋利、充满活力的锋面，在河口和沿海模型中通常很重要。我们比较了德克萨斯州-路易斯安那州大陆架海洋状态亚尺度解析现实模拟中数值混合的在线和离线估计。虽然数值混合的离线估计与在线估计不同，但离线方法可能是唯一可用的分析。我们使用两种方法根据盐度平方 s2 的残差和体积平均盐度方差 s′2 ${s}^{{\prime }^{2}}$ 预算来离线估计数值混合。 s′2 ${s}^{{\prime }^{2}}$ 预算高估了每小时输出的时间平均在线数值混合 60%。由于与趋势项和平流项相关的较大截断误差，s2 预算比较较差。当输出频率增加到 10 分钟（对于长期沿海海洋模拟来说是不切实际的频率）时，s2 预算的残差开始收敛到 s′2 ${s}^{{\prime }^{2}}$ 预算，但这两种方法都不能无条件收敛到在线方法，因此不推荐用于数值混合的通用分析。我们还使用双向嵌套网格和在线方法研究了水平分辨率对数值混合的影响。在粗模型中，体积积分数值混合占整体物理混合（由湍流闭合方案规定的混合）的 57%，并且可能超过物理混合半个数量级。我们发现嵌套模型中的数值混合平均减少了 35%，这可能是由于嵌套模拟中出现了新的动态过程。
Numerical mixing, the spurious mixing primarily generated by the discretization of advection, is often significant in estuarine and coastal models due to sharp, energetic fronts. We compare on‐ and offline estimates of numerical mixing in a submesoscale‐resolving realistic simulation of the ocean state over the Texas‐Louisiana continental shelf. While offline estimates of numerical mixing differ from online estimates, offline methods may be the only analysis available. We use two methods to estimate numerical mixing offline based on the residuals of the salinity squared s2 and volume‐mean salinity variance s′2 ${s}^{{\prime }^{2}}$ budgets. The s′2 ${s}^{{\prime }^{2}}$ budget overestimates the time‐averaged online numerical mixing by 60% at hourly output. The s2 budget compares poorly due to large truncation errors associated with the tendency and advection terms. The residual of the s2 budget starts to converge to the s′2 ${s}^{{\prime }^{2}}$ budget as output frequency increases to 10 min—an unrealistic frequency for long‐term coastal ocean simulations—but neither method unconditionally converges to the online method and therefore cannot be recommended for generic analysis of numerical mixing. We also investigate the effects of horizontal resolution on numerical mixing using a two‐way nested grid with the online method. The volume‐integrated numerical mixing constitutes 57% of the bulk physical mixing—the mixing prescribed by the turbulence closure scheme—in the coarse model and may exceed the physical mixing by half an order of magnitude. We find numerical mixing is reduced by 35% on average in the nested model, likely due to new dynamical processes that emerge in the nested simulation.