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

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.