Interaction of multiple urban heat island circulations under idealised settings

Urban heat island circulation (UHIC), commonly established under calm background conditions, is important for understanding the accumulation of pollutants and heat in a city. In a city cluster in which multiple cities exist in proximity, the resulting multiple UHICs can interact. As many city clusters continue to grow in size and in number, particularly in rapidly developing Asia, it is necessary to understand the interactions of multiple UHICs. In this study, the development of a single UHIC, interaction between two identical UHICs, and that of three different UHICs were investigated by water tank modelling experiments, UHIC is characterised as convergent inflow at lower levels, upward flow over the urban area and divergent outflow at upper levels. Stagnant zones were found between two adjacent cities due to competition between their inflows. If the vertical sizes (mixed heights) of two adjacent UHICs are different, the outflow of the smaller UHIC will be lower than that of the larger one and will join the inflow of the larger one; thus, the pollutants will be transported from the smaller to the larger UHIC. Because the outflow of the larger UHIC is higher than that of the smaller one, the outflow of the larger UHIC can shelter the smaller one and thus limit the vertical pollutant dispersion within the smaller one. These findings are useful to explain the haze accumulation phenomenon in major haze episodes such as those occurring in the Beijing-Tianjin-Hebei region in China.