The influence of idealized surface heterogeneity on virtual turbulent flux measurements
The influence of idealized surface heterogeneity on virtual turbulent flux measurements
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DOI：
10.5194/acp-18-5059-2018
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发表时间：
2018-04-13
影响因子：
6.3
通讯作者：
Mauder, Matthias
中科院分区：
文献类型：
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作者：
De Roo, Frederik;Mauder, Matthias
涡动相关测量中的地表能量收支不平衡问题至今仍是一个未解决的问题。一个可能的原因是陆面不均匀性的存在，这影响了边界层湍流。为了研究对流条件下地表变量对近地层通量测量能量收支分区的影响，我们通过大涡模拟建立了一个系统的参数研究。对于这项研究，我们使用一个虚拟的控制体积的方法，它允许确定平流的平均流量，通量发散和存储项的能量预算在虚拟测量站点，除了标准的湍流通量。我们关注的是地表通量的不均匀性，并保持地形平坦。地表通量的强度在局部地区不同，这些斑块具有不同的长度尺度。强度和长度尺度可以在两个水平维度上变化，但遵循理想化的棋盘模式。我们主要关注的是千米尺度的地表异质性，以及小一个数量级。对于这两个长度尺度，我们调查的平均响应的通量在一些虚拟塔，当不同的长度尺度内的异质性长度和不同的补丁之间的对比度变化时。对于每个模拟，虚拟测量塔被定位在功能上不同的位置（例如，向下气流区域、向上气流区域、域之间的边界等）。由于储存项总是很小，非闭合由平均流的平流和通量散度之和给出。值得注意的是，缺失的通量可以分别用平均流的平流或通量-辐散来描述，因为后两者彼此具有高度相关性。对于千米尺度的异质性，我们注意到上升气流和下降气流对表面异质性的明显依赖，同样，我们也看到能量分配对塔位置的依赖。对于百米尺度，我们没有注意到这种明显的依赖性。最后，我们在模拟中寻找能量平衡比的修正因子。与摩擦速度的相关性不像以前发现的那么明显，但这可能是由于我们集中在有效的强对流条件下。
The imbalance of the surface energy budget in eddy-covariance measurements is still an unsolved problem. A possible cause is the presence of land surface heterogeneity, which affects the boundary-layer turbulence. To investigate the impact of surface variables on the partitioning of the energy budget of flux measurements in the surface layer under convective conditions, we set up a systematic parameter study by means of large-eddy simulation. For the study we use a virtual control volume approach, which allows the determination of advection by the mean flow, flux-divergence and storage terms of the energy budget at the virtual measurement site, in addition to the standard turbulent flux. We focus on the heterogeneity of the surface fluxes and keep the topography flat. The surface fluxes vary locally in intensity and these patches have different length scales. Intensity and length scales can vary for the two horizontal dimensions but follow an idealized chessboard pattern. Our main focus lies on surface heterogeneity of the kilometer scale, and one order of magnitude smaller. For these two length scales, we investigate the average response of the fluxes at a number of virtual towers, when varying the heterogeneity length within the length scale and when varying the contrast between the different patches. For each simulation, virtual measurement towers were positioned at functionally different positions (e.g., downdraft region, updraft region, at border between domains, etc.). As the storage term is always small, the non-closure is given by the sum of the advection by the mean flow and the flux-divergence. Remarkably, the missing flux can be described by either the advection by the mean flow or the flux-divergence separately, because the latter two have a high correlation with each other. For kilometer scale heterogeneity, we notice a clear dependence of the updrafts and downdrafts on the surface heterogeneity and likewise we also see a dependence of the energy partitioning on the tower location. For the hectometer scale, we do not notice such a clear dependence. Finally, we seek correlators for the energy balance ratio in the simulations. The correlation with the friction velocity is less pronounced than previously found, but this is likely due to our concentration on effectively strongly to freely convective conditions.