A study of city–scale atmospheric circulations, and their transition from plume to bubble*

The Atmospheric Boundary Layer (ABL) structure and turbulence over urban and rural areas are mainly modulated by the wind speed and the Urban Heat Island intensity. The balance between the mean wind advection and thermal upward convection is believed to determine the large-scale flow pattern over the city. When the advection of the wind is the dominant factor, a plume of air rises from the city, and is advected away to the downwind areas. In contrast, when the ABL is dominated by the convection from the hotter city surface, the urban plume is recirculated into the city, forming a bubble shape circulation. Here, we use large eddy simulations to probe these different possible flow regimes. We reduce the problem using dimensional analysis, and investigate how the circulation regime is influenced by two non-dimensional parameters: (i) the ratio of convective velocities that contrasts urban and rural buoyancy fluxes, and (ii) the ratio of the surface friction velocity and the thermal convection velocity over the city. Finally, we examine how the turbulence characteristics shift when the circulation regime transitions from plume to bubble.