Supporting data for PhD thesis "Turbulence in Thermally-Stratified Boundary Layers over Idealized Urban Morphology"

Thesis AbstractLarge-eddy simulations (LESs) are used to study stratified atmospheric boundary layers over idealized urban surfaces under various conditions—unstable, neutral, and stable—ranging from free-convective to very stable regimes. A large computational domain captures large-scale coherent motions, such as horizontal convective rolls and thermal plumes in convective boundary layers (CBLs), and wave-like motions in stable boundary layers (SBLs). These interactions with urban roughness lead to complex, multiscale processes.In urban CBLs, both the momentum correlation and the flux correlation exhibit non-monotonic trends with increasing instability due to the formation of large-scale convective rolls. Roughness length and zero-plane displacement decrease with stronger stratification. Buoyancy affects momentum transport within the roughness sublayer (RSL), reducing the impact of urban roughness and shear. This enhances upward momentum fluxes while reducing downward momentum fluxes by over 20%, even in weakly unstable conditions. Urban-type roughness promotes the formation of convective rolls under less unstable conditions compared to canonical settings.The multiscale dynamics within urban CBLs are explored using spatial and amplitude modulation (AM) methods. Large-scale coherent structures play a key role in spatially modulating momentum fluxes in urban canopy layers (UCLs). Large-scale accelerating flows in the UCL enhance small-scale turbulence, showing a monotonic decrease in AM with increasing instability. The AM of small scales by large-scale vertical velocities is more pronounced in the UCL, with downdrafts enhancing turbulence under shear-dominant conditions and updrafts under buoyancy-dominant conditions. Building wakes dominate the AM of adjacent small-scale turbulence, with building-scale turbulence being highly influenced by AM from CBL structures. Unstable conditions significantly alter the phase relationship between large- and small-scale turbulence within UCLs.A height-dependent scalar quantifies the nonlocal contributions within urban CBLs. With increasing instability, velocity variances and vertical heat fluxes shift from downdraft- to updraft-dominant. Despite these shifts, downdrafts primarily influence vertical momentum flux within the UCL. Wavelet analysis reveals turbulence and momentum-transporting eddies characterized by smaller length scales in downdrafts and larger scales in updrafts, which is reversed within UCLs due to urban buildings. Scale variations explain parameterization variability, with differences exceeding 100% between updrafts and downdrafts. Nonlocal processes contribute significantly to UCL turbulence and fluxes, accounting for up to 40.5% of vertical velocity variance and 56.0% of vertical heat flux.In urban SBLs, increasing stratification confines turbulence to roughness scales and induces wave-like motions that dominate within the RSL under very stable conditions. Similarity theory with Businger-Dyer relationships, which apply under weak stability, overestimates the vertical gradients of mean velocity in more stable conditions. Dispersive flux initially increases from neutral to moderately stable conditions due to the buoyancy suppression of turbulence but declines as stability limits building-induced recirculations, forming a quiescent layer that restricts momentum and heat exchange within the UCL. Non-fully-turbulent fluctuations dominate up to 71.5% of streamwise velocity variance, 51.1% of vertical momentum flux, and 59.0% of heat flux, with wave-like motions alone contributing 46.8%, 27.2%, and 35.1%, respectively. A recovery of -5/3 scaling occurs at scales smaller than buildings, decoupled from wave-like motions, as evidenced by a distinct spectral gap.

论文摘要
本研究采用大涡模拟（Large-eddy simulations, LESs），针对从自由对流状态到极强稳定状态的多种工况（不稳定、中性、稳定分层），探究理想化城市下垫面上方的分层大气边界层流动。通过设置大计算域，本研究可捕捉大尺度相干运动：对流边界层（Convective Boundary Layers, CBLs）内的水平对流涡与热羽流，以及稳定边界层（Stable Boundary Layers, SBLs）内的类波动运动。上述流动与城市下垫面粗糙元的相互作用，会催生复杂的多尺度物理过程。

在城市对流边界层中，由于大尺度对流涡的形成，动量关联与通量关联均会随不稳定性增强呈现非单调变化趋势。粗糙长度与零平面位移高度均会随层结增强而减小。浮力会对粗糙次层（Roughness Sublayer, RSL）内的动量输运产生影响，削弱城市下垫面粗糙元与剪切力的作用效果。即便在弱不稳定工况下，这一效应也会使向上动量通量增强，同时使向下动量通量降低超过20%。相较于理想均匀下垫面工况，城市型粗糙元会在不稳定性较弱的条件下更易促进对流涡的形成。

本研究采用空间调制与振幅调制（Amplitude Modulation, AM）方法，探究城市对流边界层内的多尺度动力学特性。大尺度相干结构对城市冠层（Urban Canopy Layers, UCLs）内的动量通量空间调制起到关键作用。城市冠层内的大尺度加速流会增强小尺度湍流，且振幅调制程度随不稳定性增强呈现单调递减趋势。大尺度垂直速度对小尺度运动的振幅调制在城市冠层中更为显著：在剪切主导工况下，下沉气流会增强湍流；而在浮力主导工况下，上升气流则会增强湍流。建筑尾流主导了邻近小尺度湍流的振幅调制，而建筑尺度的湍流则会受到对流边界层结构振幅调制的显著影响。不稳定工况会显著改变城市冠层内大、小尺度湍流之间的相位关系。

引入高度相关的标量指标，可量化城市对流边界层内的非局域贡献。随着不稳定性增强，速度方差与垂直热通量的主导来源会从下沉气流转向上升气流。尽管存在上述转变，下沉气流仍是城市冠层内垂直动量通量的主要影响因素。小波分析结果显示：下沉气流中的湍流与输运动量的涡旋特征尺度更小，上升气流中的则更大；但由于城市建筑的影响，这一规律在城市冠层内发生反转。尺度差异可解释参数化方案的不确定性，上升气流与下沉气流之间的参数化差异甚至超过100%。非局域过程对城市冠层的湍流与通量贡献显著，其在垂直速度方差中的占比最高可达40.5%，在垂直热通量中的占比最高可达56.0%。

在城市稳定边界层中，层结增强会将湍流限制在粗糙元尺度范围内；在极强稳定工况下，类波动运动会主导粗糙次层内的流动。适用于弱层结工况的布辛格-戴尔（Businger-Dyer）相似理论，在更强层结条件下会高估平均速度的垂直梯度。弥散通量会随层结从中性到中等稳定状态先升高：这是由于湍流受到浮力抑制所致；但随着层结增强，建筑诱导的再循环流受到限制，弥散通量会随之下降，并形成静稳层以阻碍城市冠层内的动量与热交换。非完全湍流脉动在顺流速度方差、垂直动量通量与热通量中的占比分别可达71.5%、51.1%与59.0%，其中仅类波动运动的贡献就分别达到46.8%、27.2%与35.1%。在小于建筑尺度的范围内，湍流会恢复-5/3次方标度律，且该过程与类波动运动解耦，这一点可通过显著的频谱间隙得到验证。