日间对流边界层中的非局地动量混合

日间对流边界层最显著的结构特征是在热力作用下所形成的组织化对流。与小尺度湍涡不同的是，组织化对流具有边界层尺度的垂直相干性，可实现垂直贯穿边界层的非局地物质和能量传输。本文针对对流边界层中的动量混合，探究组织化对流对动量输送的贡献。以高精度大涡模拟数据为研究资料，通过傅里叶变换、本征正交分解和经验模态分解3种滤波方法，分离组织化对流和背景湍涡，计算与两者相关的非局地和局地动量通量，发现与组织化对流相关的非局地动量通量是总通量的重要组成部分，并主导混合层中的垂直动量输送。而后，基于协谱和相位谱分析，探究组织化对流的空间结构对动量传输的影响，发现在热力主导的不稳定环境中，单体型环流结构对动量的传输效率较低。而在风切较强的近中性环境中，滚涡型组织化结构可使垂直和水平流向扰动速度的相位差减小，从而提升动量传输效率。研究结果表明，边界层方案需要包含非局地动量通量项，其参数化应考虑整体稳定度对传输效率的影响。

Non-local mixing of momentum in the daytime convective boundary layer

Thermally-driven organized convection is the most prominent structural feature of the daytime convective boundary layer (CBL). Different from inertial-subrange turbulent eddies, organized convection is vertically coherent at the boundary layer scale, and can contributes to nonlocal vertical transport of mass and energy within the CBL. In this study, we investigate turbulent mixing of momentum in the CBL, with a focus on the role of organized convection in momentum transport. Based on modeled CBL with high resolution large-eddy simulations, organized convection and background turbulence are separated by using Fourier transform, Proper Orthogonal Decomposition and Empirical Mode Decomposition. The nonlocal and local fluxes associated with organized convection and background turbulence are calculated. It is found that nonlocal flux constitutes a significant portion of the total momentum flux, and dominates the transport of momentum in the mixed layer. The mechanism behind organized convective structures affect the transport of momentum is investigated based on momentum co-spectrum and phase-spectrum. Analyses reveal that convective cells generated under strong buoyancy forcing are inefficient at transporting momentum. In the near neutral environment with strong wind shear, convection organizes into horizontal roll vortices, which reduce the phase difference between vertical and horizontal streamwise perturbation velocities, thus improving the efficiency of momentum transport. This study suggests that planetary boundary layer schemes must include nonlocal momentum flux, whose parameterization should consider the influence of bulk CBL stability on momentum transport efficiency.