Different Turbulent Regimes and Vertical Turbulence Structures of the Urban Nocturnal Stable Boundary Layer

Abstract

Turbulence in the nocturnal boundary layer (NBL) is still not well characterized, especially over complex underlying surfaces. Herein, gradient tower data and eddy covariance data collected by the Beijing 325-m tower were used to better understand the differentiating characteristics of turbulence regimes and vertical turbulence structure of urban the NBL. As for heights above the urban canopy layer (UCL), the relationship between turbulence velocity scale (V_TKE) and wind speed (V) was consistent with the “HOckey-Stick” (HOST) theory proposed for a relatively flat area.

Four regimes have been identified according to urban nocturnal stable boundary layer. Regime 1 occurs where local shear plays a leading role for weak turbulence under the constraint that the wind speed V<V_T (threshold wind speed). Regime 2 is determined by the existence of strong turbulence that occurs when V>V_T and is mainly driven by bulk shear. Regime 3 is identified by the existence of moderate turbulence when upside-down turbulence sporadic bursts occur in the presence of otherwise weak turbulence. Regime 4 is identified as buoyancy turbulence, when V>V_T, and the turbulence regime is affected by a combination of local wind shear, bulk shear and buoyancy turbulence.

The turbulence activities demonstrated a weak thermal stratification dependency in regime 1, for which within the UCL, the turbulence intensity was strongly affected by local wind shear when V<V_T. This study further showed typical examples of different stable boundary layers and the variations between turbulence regimes by analyzing the evolution of wind vectors. Partly because of the influence of large-scale motions, the power spectral density of vertical velocity for upside-down structure showed an increase at low frequencies. The upside-down structures were also characterized by the highest frequency of the stable stratifications in the higher layer.

摘要

夜间稳定层结湍流特性的研究一直是大气边界物理研究的热点和难点, 目前的天气预报模式或者气候模式模拟的夜间边界层结果和观测值差异较大. 本文利用中国科学院大气物理研究所北京 325m 气象塔从 2017 年 11 月至 2018 年 1 月连续三个月的观测数据, 对城市夜间稳定边界层中湍流的垂直结构和特征分区进行研究. 结果表明, 在城市冠层高度之上, 湍流强度随风速的分布和其他作者基于平坦下垫面上得到的“HOckey-Stick”(HOST)分布一致, 即当风速小于阈值风速时, 局地剪切对湍流产生起主导作用; 当风速超过阈值风速时, 强风引起的总体剪切产生强湍流活动; 由湍流动能向下传输导致产生中等强度湍流活动. 弱湍流活动相对于强湍流活动而言, 受温度层结的影响更小. 在城市冠层之内, 湍流活动强烈地受到局地剪切影响, 当风速超过阈值风速时, 冠层内湍流活动受到局地剪切、 总体剪切以及浮力湍流的共同作用. 通过分析风矢量, 本文给出了不同湍流分区之间的转变和演变过程. 当湍流动能向下传输时, 垂直速度功率谱的低频部分谱密度有所提升, 表明了大尺度运动的影响, 并且在塔的高层出现了更多的稳定层结. 本文获得的结果对提升城市夜间边界层湍流机理的认识和改进湍流过程的参数化具有科学参考意义.