Mack Laura, Berntsen Terje Koren, Vercauteren Nikki, Pirk Norbert
Department of Geosciences, University of Oslo, Blindern, P.O. 1022, 0315 Oslo, Norway.
Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany.
Boundary Layer Meteorol. 2024;190(9):38. doi: 10.1007/s10546-024-00879-5. Epub 2024 Aug 28.
The exchange of momentum, heat and trace gases between atmosphere and surface is mainly controlled by turbulent fluxes. Turbulent mixing is usually parametrized using Monin-Obukhov similarity theory (MOST), which was derived for steady turbulence over homogeneous and flat surfaces, but is nevertheless routinely applied to unsteady turbulence over non-homogeneous surfaces. We study four years of eddy-covariance measurements at a highly heterogeneous alpine valley site in Finse, Norway, to gain insights into the validity of MOST, the turbulent transport mechanisms and the contributing coherent structures. The site exhibits a bimodal topography-following flux footprint, with the two dominant wind sectors characterized by organized and strongly negative momentum flux, but different anisotropy and contributions of submeso-scale motions, leading to a failure of eddy-diffusivity closures and different transfer efficiencies for different scalars. The quadrant analysis of the momentum flux reveals that under stable conditions sweeps transport more momentum than the more frequently occurring ejections, while the opposite is observed under unstable stratification. From quadrant analysis, we derive the ratio of the amount of disorganized to organized structures, that we refer to as organization ratio (OR). We find an invertible relation between transfer efficiency and corresponding organization ratio with an algebraic sigmoid function. The organization ratio further explains the scatter around scaling functions used in MOST and thus indicates that coherent structures modify MOST. Our results highlight the critical role of coherent structures in turbulent transport in heterogeneous tundra environments and may help to find new parametrizations for numerical weather prediction or climate models.
大气与地表之间的动量、热量和痕量气体交换主要由湍流通量控制。湍流混合通常采用莫宁-奥布霍夫相似理论（MOST）进行参数化，该理论是针对均匀平坦表面上的稳定湍流推导得出的，但仍经常应用于非均匀表面上的非稳定湍流。我们研究了挪威芬瑟一个高度异质的高山峡谷站点四年的涡度协方差测量数据，以深入了解MOST的有效性、湍流传输机制以及起作用的相干结构。该站点呈现出双峰地形跟随通量足迹，两个主要风区的特征是有组织且强烈的负动量通量，但亚中尺度运动的各向异性和贡献不同，导致涡扩散率闭合失效以及不同标量的传输效率不同。动量通量的象限分析表明，在稳定条件下，下扫输送的动量比更频繁出现的上抛输送的动量更多，而在不稳定分层条件下则观察到相反的情况。通过象限分析，我们得出了无组织结构与有组织结构数量的比率，我们将其称为组织比率（OR）。我们发现传输效率与相应的组织比率之间存在一种代数S形函数的可逆关系。组织比率进一步解释了MOST中使用的标度函数周围的离散情况，从而表明相干结构会修改MOST。我们的结果突出了相干结构在异质苔原环境中湍流传输中的关键作用，并可能有助于为数值天气预报或气候模型找到新的参数化方法。
