Evolution of urban heat island circulation for the flat city

• UHIC on the flat terrain is investigated using the city-scale CFD model. • Evolution of UHIC is divided into the transient stage and quasi-steady stage. • The flow patterns of the two stages are analyzed. • Sensitivity analysis is conducted on the factors influencing the UHIC. Under calm and stable weather, an urban heat island circulation (UHIC) is formed driven by the thermal pressure of the urban heat island, which is vital for air change and pollutant transport between the urban canopy layer and the background air. In this paper, the evolution of the UHIC for the flat city is investigated using a city-scale computational fluid dynamics (CSCFD) model. The CSCFD model can consider the atmospheric compressibility and thermal stratification in the basic equations. The simulation results show that the evolution can be divided into the transient and quasi-steady stages. During the transient stage the flow pattern of the UHIC is mainly in the form of the thermal convection, while the flow pattern of the UHIC during the quasi-steady stage is the city-scale closed circulation. The nondimensional start time of the city-scale closed circulation shows an inversely proportional function to Fr ( tN = 2.172/ Fr ), and the nondimensional mixing height of the UHIC shows a positively proportional function to Fr ( z ic / D = 2.80 Fr ). The sensitivity analysis reveals that the factors influencing the start time of the city-scale closed circulation include, in order of significance, urban diameter > urban heat flux > potential temperature lapse rate, while the factors influencing the mixing height in the quasi-steady stage include, in order of significance, potential temperature lapse rate > urban diameter > urban heat flux. This study provides theoretical bases for the control of industrial activities during critical meteorological periods, and the operation of water tank experiments.