Similarity theory and nocturnal locally scaled turbulence variances in the tropical urban roughness sublayer

Abstract

We assessed the potential of the “local” Monin–Obukhov similarity theory to predict the relationship between scaled turbulence standard deviations (σi; i = turbulence quantity) and atmospheric stability (ζ) in the nocturnal urban roughness sublayer using data collected in a tropical city of which data and research have been lacking as most works previously reported mainly concentrated on temperate mid-latitude regions. The site was LCZ 5B in the Local Climate Zone classification system with open midrise buildings with scattered trees. Unstable (convective) atmospheric conditions persisted through the night with infrequent and intermittent stable conditions. Nighttime turbulence statistics were lower than daytime turbulence statistics due to the weak (<1 m/s) and variable mean wind speeds. Using local scales, the Monin–Obukhov similarity theory failed to predict the trend in the standard deviation of vertical wind velocity (σw) scaled by local friction velocity (u*l) with local atmospheric stability (ζl) but the standard deviation of temperature (σT) scaled to the local friction temperature (T*l) fared better. However, we found that by using the local free convection velocity (uf, l) as the normalising scale with σW, the trend was better described with ζl using simple power law relations for both unstable and stable conditions, but this situation was limited to neutral conditions.