Large-Eddy Simulations of Surface Heterogeneity Effects on the Convective Boundary Layer During the LITFASS-2003 Experiment
Large-Eddy Simulations of Surface Heterogeneity Effects on the Convective Boundary Layer During the LITFASS-2003 Experiment
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DOI：
10.1007/s10546-012-9748-z
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发表时间：
2012
影响因子：
4.3
通讯作者：
B. Maronga;S. Raasch
中科院分区：
文献类型：
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作者：
B. Maronga;S. Raasch
我们调查的对流边界层（CBL）的LITFASS-2003实验期间观察到的表面非均匀性的影响。大涡模拟（LES），所观察到的近地面感热和潜热通量的驱动下，进行了日周期和比较与观测。在以前的研究理想化的一维和二维的非均匀性，二次环流的发展，叠加在湍流场，并部分接管垂直输送的热量和水分。二次环流型在局地和卷状结构之间变化，取决于背景风条件。对于更高的背景风速，流动感觉到一个有效的表面热通量模式，从原来的模式由流向平均。这种有效的模式产生了一个滚动轴沿着平均边界层风向的滚动状二次环流。主要是上游表面条件控制了二次环流模式，其中风场随着背景风速的增加而增加。不像夹带通量，似乎是略有减少相比，cabulously加热CBL，感热的垂直通量似乎没有被修改的混合层，而潜热的垂直通量显示不同的响应二次环流。研究表明，需要足够的时间平均和系综平均分离的非均匀性引起的信号从原始LES湍流数据。这可能是一个重要的原因，为什么实验在非均匀地形在过去没有给任何明确的证据，非均匀性引起的影响。
We investigate the impact of observed surface heterogeneities during the LITFASS-2003 experiment on the convective boundary layer (CBL). Large-eddy simulations (LES), driven by observed near-surface sensible and latent heat fluxes, were performed for the diurnal cycle and compare well with observations. As in former studies of idealized one- and two-dimensional heterogeneities, secondary circulations developed that are superimposed on the turbulent field and that partly take over the vertical transport of heat and moisture. The secondary circulation patterns vary between local and roll-like structures, depending on the background wind conditions. For higher background wind speeds, the flow feels an effective surface heat-flux pattern that derives from the original pattern by streamwise averaging. This effective pattern generates a roll-like secondary circulation with roll axes along the mean boundary-layer wind direction. Mainly the upstream surface conditions control the secondary circulation pattern, where the fetch increases with increasing background wind speed. Unlike the entrainment flux that appears to be slightly decreased compared to the homogeneously-heated CBL, the vertical flux of sensible heat appears not to be modified in the mixed layer, while the vertical flux of latent heat shows different responses to secondary circulations. The study illustrates that sufficient time averaging and ensemble averaging is required to separate the heterogeneity-induced signals from the raw LES turbulence data. This might be an important reason why experiments over heterogeneous terrain in the past did not give any clear evidence of heterogeneity-induced effects.