On the Effect of Surface Heat-Flux Heterogeneities on the Mixed-Layer-Top Entrainment
On the Effect of Surface Heat-Flux Heterogeneities on the Mixed-Layer-Top Entrainment
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DOI：
10.1007/s10546-014-9913-7
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发表时间：
2012-07
影响因子：
4.3
通讯作者：
M. Sühring;B. Maronga;F. Herbort;S. Raasch
中科院分区：
文献类型：
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作者：
M. Sühring;B. Maronga;F. Herbort;S. Raasch
我们使用一组大涡模拟来研究一维条纹状表面热通量非均匀性对混合层顶部夹带的影响。感热通量和边界层深度的时间演化曲线显示，与均匀加热的混合层相比，小热流幅值的夹带减小，大热流幅值的夹带增大。对于较大的热通量幅值，观测到的最大夹带是按边界层深度顺序排列的斑块尺寸，而对于明显较小或较大的斑块尺寸，夹带与均匀情况相似。为了了解这种撞击的基本物理性质，研究人员开发了一种新的方法，通过局部通量散度来推断有关夹带的局部信息。我们发现在较强加热表面斑块的中心处有一个夹带最大值，在那里热能是由表面非均匀性引起的二次环流（SC）积累的。此外，我们还观察到在温度较低的区域也有一个夹带最大值，我们认为这与对流边界层（CBL）顶部sc诱导的水平流辐合有关。对于小的热通量幅值，抵消作用占主导地位，减少夹带，我们认为这是低层冷空气和上层暖空气的水平平流，由SC稳定CBL，从而减弱热对流。此外，我们发现平均风可以减少非均质性对夹带的影响。如果气流垂直于差热块之间的边界，即使在中等风速下，由于水平混合的增加，SC及其对夹带的影响也会消失。然而，如果气流方向平行于差热斑块之间的边界，则SC及其对夹带的影响将持续存在。
We used a set of large-eddy simulations to investigate the effect of one-dimensional stripe-like surface heat-flux heterogeneities on mixed-layer top entrainment. The profiles of sensible heat flux and the temporal evolution of the boundary-layer depth revealed decreased entrainment for small heat-flux amplitudes and increased entrainment for large heat-flux amplitudes, compared to the homogeneously-heated mixed layer. For large heat-flux amplitudes the largest entrainment was observed for patch sizes in the order of the boundary-layer depth, while for significantly smaller or larger patch sizes entrainment was similar as in the homogeneous case. In order to understand the underlying physics of this impact, a new approach was developed to infer local information on entrainment by means of the local flux divergence. We found an entrainment maximum over the centre of the stronger heated surface patch, where thermal energy is accumulated by the secondary circulation (SC) that was induced by the surface heterogeneity. Furthermore, we observed an entrainment maximum over the less heated patch as well, which we suppose is to be linked to the SC-induced horizontal flow convergence at the top of the convective boundary layer (CBL). For small heat-flux amplitudes a counteracting effect dominates that decreases entrainment, which we suppose is the horizontal advection of cold air in the lower, and warm air in the upper, CBL by the SC, stabilizing the CBL and thus weakening thermal convection. Moreover, we found that a mean wind can reduce the heterogeneity-induced impact on entrainment. If the flow is aligned perpendicular to the border between the differentially-heated patches, the SC and thus its impact on entrainment vanishes due to increased horizontal mixing, even for moderate wind speeds. However, if the flow is directed parallel to the border between the differentially-heated patches, the SC and thus its impact on entrainment persists.