Plume or Bubble? Transitions in City-Scale Atmospheric Circulation with the Richardson Number

The atmospheric boundary layer (ABL) structure and turbulence over metropolitan and surrounding rural areas are mainly modulated by the wind speed and the urban heat island (UHI) intensity. Their relative magnitudes, which can be used to construct a Richardson number (Ri), are believed to determine the large scale flow pattern over the city. For low Ri, a warm polluted plume of air, develops as urban emissions are advected downwind; however, for high Ri where the ABL structure is mainly thermally driven, the rising warm plume of air can form a circulation bubble and trap pollutants and heat in the city. In this study, we use large eddy simulations to probe these different possible flow regimes. First, we examine how different cases can be scaled based on the wind speed and heat flux difference between the urban and rural area and how these modulating parameters should be combined to form an Ri. Then, we investigate how the ABL circulation and turbulence are altered as Ri increases from 0 to very large values. Implications for urban environmental quality are then discussed.