Turbulence in the stratified boundary layer under ice: observations from Lake Baikal and a new similarity model
Turbulence in the stratified boundary layer under ice: observations from Lake Baikal and a new similarity model
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DOI：
10.5194/hess-2019-608
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发表时间：
2019-12
影响因子：
6.3
通讯作者：
G. Kirillin;I. Aslamov;V. Kozlov;R. Zdorovennov;N. Granin
中科院分区：
文献类型：
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作者：
G. Kirillin;I. Aslamov;V. Kozlov;R. Zdorovennov;N. Granin
抽象的。湖泊和极地海洋上的季节性冰盖在冰底产生季节性发展的边界层，其特征是：固体边界处的固定温度和下面的稳定密度分层。边界层中的湍流输送决定了冰-水界面处的冰生长和融化条件，特别是在大型湖泊和边缘海中，大规模的水循环可以产生高度可变的混合条件。由于冰下边界混合难以测量，现有的冰盖动力学模型通常忽略或以非常简单的形式参数化它。我们提出了第一个详细的观察贝加尔湖冰下的混合，获得了先进的声学方法的帮助下。湍流动能的耗散率（TKE）来自于边界层内流速的相关性（结构函数）。耗散率变化的范围覆盖2个数量级，表现出强烈的湍流条件。混合的强度密切相关的大规模冰下电流的平均速度。混合是在稳定的密度（温度）层结的背景下发展起来的，它影响着边界层的垂直结构。考虑到分层效应，我们提出了一个模型的湍流能量预算的基础上，将耗散率和浮力频率的长度尺度（Dougherty-Ozmidov缩放）。该模型与观测结果吻合良好，并产生一个比例关系的冰水热通量的剪切速度平方的函数。该区域的冰水热通量是所有湖泊中报告的最大的（高达40 W m−2），并且与所提出的关系进行了很好的对比。最终的发现是，冰下的剪切速度的水冰热通量的强烈依赖。结果表明，当传统的“整体”方法应用于分层边界层时，热通量估计存在很大的误差。这也意味着冰下海流对冰融化的影响可能比传统模型估计的要大得多。
Abstract. Seasonal ice cover on lakes and polar seas creates seasonally developing boundary layer at the ice base with specific features: fixed temperature at the solid boundary and stable density stratification beneath. Turbulent transport in the boundary layer determines the ice growth and melting conditions at the ice–water interface, especially in large lakes and marginal seas, where large-scale water circulation can produce highly variable mixing conditions. Since the boundary mixing under ice is difficult to measure, existing models of ice cover dynamics usually neglect or parameterize it in a very simplistic form. We present the first detailed observations on mixing under ice of Lake Baikal, obtained with the help of advanced acoustic methods. The dissipation rate of the turbulent kinetic energy (TKE) was derived from correlations (structure functions) of current velocities within the boundary layer. The range of the dissipation rate variability covered 2 orders of magnitude, demonstrating strongly turbulent conditions. Intensity of mixing was closely connected to the mean speeds of the large-scale under-ice currents. Mixing developed on the background of stable density (temperature) stratification, which affected the vertical structure of the boundary layer. To account for stratification effects, we propose a model of the turbulent energy budget based on the length scale incorporating the dissipation rate and the buoyancy frequency (Dougherty–Ozmidov scaling). The model agrees well with the observations and yields a scaling relationship for the ice–water heat flux as a function of the shear velocity squared. The ice–water heat fluxes in the field were the largest among all reported in lakes (up to 40 W m−2) and scaled well against the proposed relationship. The ultimate finding is that of a strong dependence of the water–ice heat flux on the shear velocity under ice. The result suggests large errors in the heat flux estimations when the traditional “bulk” approach is applied to stratified boundary layers. It also implies that under-ice currents may have much stronger effect on the ice melt than estimated by traditional models.