Inversion breakup over different shapes of urban areas

Abstract

<p>Severe haze episodes and extreme heat events are both related to the strong inversion in the atmospheric boundary layer. Inversion breakup is the main mechanism to alleviate the accumulation of heat or pollutants, especially in urban areas. However, the physical process and turbulent flow structures of inversion breakup remain unclear. In this study, the characteristics of mean flow structures and turbulence statistics of inversion breakup at the two-dome stage were investigated by conducting reduced-scale water tank experiments. A two-layer density profile (i.e. a lower-level stable layer and an upper-level neutral layer) was created to simulate the surface-based inversion covered by a neutral residual layer. Various shapes of urban areas were tested. The results showed that the lower-level dome in the stable layer (Dome 1) and the upper-level dome in the neutral layer (Dome 2) are both substantially affected by the urban shape. In Dome 1, the convergent inflow is mainly along the angular bisector across the urban corners, whereas the divergent outflow is perpendicular to the edges. In Dome 2, the divergent outflow is on the same vertical plane with the convergent inflow (i.e. along the angular bisector). Furthermore, the turbulence production and momentum transfer are mainly induced by the thermal plume structures near the urban centre, indicating the strong influences of buoyancy. In divergent outflow regions, both shear and buoyancy play important roles.</p>