Amplitude modulation of turbulence intensities and fluxes in urban convective boundary layers

Abstract

Despite extensive research on the effect of atmospheric stability, the influence of convective rolls and thermal plumes in convective boundary layer (CBL) on urban winds and turbulence remains merely understood. Using amplitude modulation (AM), we examine the multi-scale interaction among turbulence intensities, fluxes, and large-scale CBL structures in unstable urban boundary layers (UBLs). Nine sets of large-eddy simulation (LES) are conducted, contrasting the CBL behavior over idealized urban morphology from neutral to free convection. This study, comparing the stability-dependent AM coefficient, reveals a tight modulation of small-scale turbulence and fluxes at urban canopy layer (UCL) by different types of characteristic CBL structures. Wavelet analysis corroborates their interactions in the time-frequency domain. Moreover, the correlation between the AM and the coherent CBL structures in the mixed layers is unveiled. Additionally, building-induced secondary flow structures are observed to dominate the modulation of small-scale turbulence residing within. UCL building-scale turbulence exhibits a higher susceptibility to modulation compared with intermediate scales. Notably, strong convection can invert the phase relationship between large scales and UCL turbulence. These results signify the crucialness of considering CBL coherent structures in the development of land-surface-parameterization schemes as well as the cautious implementation of predictive models in urban CBLs.