Abstract
The numerical simulation of the nocturnal urban boundary layer (UBL) was conducted byusing the second order turbulence closure model in order to clarify the formation mechanism of thenocturnal UBL. The mechanical turbulence generation by the urban buildings was incorporated into the model equations by the form drag concept (building surface area density). Simulation results werecompared with the field observation data at Sapporo, Japan. The numerical simulation showed that i) the nocturnal UBL was generated by the downward transport of sensible heat from the elevatedinversion due to the mechanically-generated turbulence of the urban building rather than the artificialheat emission, ii) the average elevated inversion base height Zh was approximately twice the averagebuilding height and the height Zh was proportional to the building area density, iii) above the Zh, thetypical temperature “cross-over” region was formed when the urban area exists even if there is noartificial heat emission, iv) the Reynolds stress ‹uw› shows the nearly uniform profile, while, sensibleheat flux ‹wθ› had the negative peak at the inversion base height. This negative value of‹wθ›at the elevated inversion base can be considered as a heating source for the formation of the nocturnal UBL.
