Large-eddy simulation is used to examine the afternoon and early evening transition of the atmospheric boundary layer (ABL) over heterogeneous surfaces with respect to the decay of the turbulence kinetic energy (TKE). The heterogeneous surface sensible heat flux is prescribed based on the inverse transform of the Fourier spectrum whose slope is controlled to behave as $$ \kappa^{0} $$ κ 0 , $$ \kappa^{ - 1} $$ κ - 1 , $$ \kappa^{ - 2} $$ κ - 2 , and $$ \kappa^{ - 3} $$ κ - 3 (where $$ \kappa $$ κ is the wavenumber) in the wavelength range from 18 to 0.2 km. The $$ \kappa^{0} $$ κ 0 slope spectrum corresponds to a randomly homogeneous field of surface flux, which is implicitly assumed in the grid cells of most large-scale models. The spectral slopes of negative exponents represent heterogeneous surface fields with a larger negative exponent describing stronger mesoscale heterogeneity. The results for a homogeneous surface flux are consistent with that of previous studies in which, during the transition period, the decay of the volume-averaged TKE normalized with the convective velocity scale follows the power law $$ \tau^{ - n} $$ τ - n (where $$ \tau $$ τ is time normalized by the eddy turnover time scale) for exponents n = 1, 2, and 6. While the $$ \tau^{ - 6} $$ τ - 6 portion still remains over the surface with weak mesoscale heterogeneity, the $$ \tau^{ - 6} $$ τ - 6 portion of the spectrum is replaced with a $$ \tau^{ - 2/3} $$ τ - 2 / 3 portion over moderate heterogeneity, and the $$ \tau^{ - 2} $$ τ - 2 and $$ \tau^{ - 6} $$ τ - 6 portions are replaced with $$ \tau^{ - 2/3} $$ τ - 2 / 3 and $$ \tau^{ - 1} $$ τ - 1 portions of the spectrum over strong heterogeneity, resulting from the surface-heterogeneity-induced horizontal flow persisting after sunset. In fact, mesoscale features across the ABL become more apparent with the reduced number of turbulent eddies in the evening.

中文翻译：

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中尺度表面非均质性对大气边界层下午和傍晚过渡的影响

大涡模拟用于检查大气边界层 (ABL) 在非均质表面上的下午和傍晚过渡与湍流动能 (TKE) 的衰减有关。异质表面感热通量是基于傅立叶谱的逆变换规定的，其斜率被控制为 $$ \kappa^{0} $$ κ 0 , $$ \kappa^{ - 1} $$ κ - 1 、 $$ \kappa^{ - 2} $$ κ - 2 和 $$ \kappa^{ - 3} $$ κ - 3（其中 $$ \kappa $$ κ 是波数）在以下波长范围内18 至 0.2 公里。$$ \kappa^{0} $$ κ 0 斜率谱对应于表面通量的随机均匀场，这是在大多数大型模型的网格单元中隐式假设的。负指数的光谱斜率表示具有更大负指数的异质表面场，描述更强的中尺度异质性。均匀表面通量的结果与先前研究的结果一致，其中在过渡期间，用对流速度标度归一化的体积平均 TKE 的衰减遵循幂律 $$ \tau^{ - n} $ $ τ - n（其中 $$ \tau $$ τ 是由涡旋周转时间标度归一化的时间）对于指数 n = 1、2 和 6。而 $$ \tau^{ - 6} $$ τ - 6部分仍然保留在表面具有弱中尺度异质性，$$ \tau^{ - 6} $$ τ - 6 部分的频谱被$$ \tau^{ - 2/3} $$ τ - 2 / 3 部分超过中等异质性，而 $$ \tau^{ - 2} $$ τ - 2 和 $$ \tau^{ - 6} $$ τ - 6 部分被替换为 $$ \tau^{ - 2/3} $$ τ - 2 / 3 和 $$ \tau^{ - 1} $$ τ - 1 强异质性的光谱部分，这是由日落后持续存在的表面异质性引起的水平流造成的。事实上，随着晚上湍流涡流数量的减少，整个 ABL 的中尺度特征变得更加明显。